Nutrient and Eutrophication in Danish Marine Waters

NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS

Gunni Ærtebjerg, Jesper H. Andersen & Ole S. Hansen Editors

Ministry of the Environment Danish Environmental Protection Agency & National Environmental Research Institute

NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . RESPONSES AND ADAPTIVE MANAGEMENT 85 (Blank page) NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS

2 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . BACKGROUND, DEFINITION, CAUSES AND EFFECTS NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . BACKGROUND, DEFINITION, CAUSES AND EFFECTS 3 (Blank page) NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS

A CHALLENGE FOR SCIENCE AND MANAGEMENT

Gunni Ærtebjerg, Jesper H. Andersen & Ole S. Hansen Editors

Ministry of the Environment Danish Environmental Protection Agency & National Environmental Research Institute

4 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . BACKGROUND, DEFINITION, CAUSES AND EFFECTS NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . BACKGROUND, DEFINITION, CAUSES AND EFFECTS 5 PREFACE

Nutrients and Eutrophication in Danish Marine Waters The objective of this assessment report The assessment focuses on factors and A Challenge for Science and Management is to describe and document the effects parameters that cause, control or re- and degree of nutrient enrichment and spond to eutrophication. Special atten- Edited by Ærtebjerg, G., Andersen, J.H. & Hansen, O.S. eutrophication status in all Danish ma- tion is put on ecological status and rine waters by addressing the follow- temporal trends. Seasonal variations National Environmental Research Institute © ing questions: and more system-orientated descrip- • What is nutrient enrichment and tions of the fluxes and turnover of ISBN: 89-7772-728-2 eutrophication? nutrients have been mitigated. The • What are the causes and actual assessment is not a comprehensive See data sheet on page 126 for details. effects? assessment of the health of the marine • Temporal trends: what is natural environment in Denmark or a text- variation and what is due to hu- book in marine ecology. The assess- man activities? ment is more or less an extended sum- • What has been done so far in Den- mary of more than 13 years of moni- mark to reduce eutrophication in toring and subsequent production of Danish marine waters? different assessments reports on the • How can the fi ndings be used and state of the marine environment within transformed into an informed the framework of the Danish National management strategy? Monitoring and Assessment Pro- gramme (1988-2003). The assessment is written in order to fulfi l the Danish obligations in relation CHAPTER 1 to the OSPAR Common Procedure. How- presents background information, de- ever, the assessment covers not only fi nitions and descriptions of the cause- the OSPAR areas: the North Sea, Ska- effect relationships as well as a brief gerrak and Kattegat, but all Danish reference to the Danish National Moni- marine waters, including the transi- toring and Assessment Programme, tional waters (the Sound and Belt Sea) which is the major source of data for between the Kattegat and the Baltic this assessment. Sea, as well as the western parts of the Baltic Sea. This is because: CHAPTER 2 1) the outfl ow from the Baltic Sea has includes the technical and scientifi c a large infl uence on the Kattegat – assessment of the eutrophication Belt Sea ecosystems, and status of the Danish marine waters 2) the eutrophic state and develop- and is structured according to the ment of the Kattegat and Belt Sea principles and guidelines adopted by runs in parallel and is interrelated. OSPAR. Focus is on the state of the

marine waters compared to the eco- waters and discusses possible future Preface ...... 3 logical quality objectives. The tempo- actions. Contents ...... 5 ral trend is also addressed, both with respect to the observed data and in- The assessment includes a glossary 1 Background, defi nition, causes and effects ...... 7 dices corrected for variations in cli- and a list of acronyms in order to reach 1.1 Defi nition ...... 8 mate (run-off, temperature, insola- readers without a professional back- 1.2 Causes...... 10 tion etc.). ground in marine ecology or oceano- 1.3 Effects and consequences ...... 12 graphy. CHAPTER 3 2 Technical-scientifi c assessment ...... 19 describes existing national strategies Suggestions for further reading as 2.1 Climate...... 24 and measures implemented to abate well as links to relevant web-sites on 2.2 Hydrography...... 26 nutrient enrichment and eutrophica- eutrophication and the health of the 2.3 Inputs from land-based sources, tion. marine environment can be found at the atmosphere and adjacent seas ...... 28 the end of the report. 2.4 Nutrient concentrations, nutrient CHAPTER 4 ratios and nutrient limitation...... 36 summarises the fi ndings of sections 1, A map of Danish marine waters men- 2.5 Phytoplankton and harmful algal blooms...... 46 2 and 3, assesses the overall eutrophi- tioned in the assessment can be found 2.6 Zooplankton...... 54 cation status of the Danish marine at page 122. 2.7 Oxygen depletion...... 58 2.8 Degradation of organic matter in estuarine sediments ...... 62 2.9 Submerged aquatic vegetation...... 68 2.10 Soft-bottom macrozoobenthos ...... 76 2.11 Fish kills in coastal waters ...... 80

3 Responses and adaptive management ...... 85 3.1 Principles ...... 86 3.2 Nutrient reduction strategies in Denmark...... 88 3.3 International co-operation to abate marine eutrophication ...... 96

4 Summary, conclusions and the future...... 101

Glossary and abbreviations ...... 111

Where can I read more?...... 116

List of contributors...... 117

References...... 118

Annexes ...... 123

For more than 30 years, nutrient en- discharged or transported to the sea All people in richment has been one of the major their inherent characteristics as plant Denmark have threats to the health of marine ecosys- nutrients affect and modify the struc- less than 55 km tems and resources (Ryther & Dunstan ture and function of the ecosystem. The to the sea. 1971, Danish EPA 1984, Nixon 1995, response to nutrient enrichment is Elmgren 2001). When nutrients are called eutrophication. 1 Photo: CDanmark Photo: NERI/Peter Bondo Christensen Photo: NERI/Jan Damgaard

6 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . BACKGROUND, DEFINITION, CAUSES AND EFFECTS NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . BACKGROUND, DEFINITION, CAUSES AND EFFECTS 7 & Jørgensen point out that when we water and to the quality of the water DEFINITION speak of eutrophication it is cultural concerned” (EU 1991). The Nitrates 1.1 eutrophication or that, which is caused Directives defi nition is almost identi- by anthropogenic activities, which is cal, except that it is restricted to eutro- of interest. phication from agriculture (EU 1991). The defi nition of eutrophication by The differences between the vari- OSPAR is: “Eutrophication means the ous definitions leave the definition enrichment of water by nutrients caus- open for interpretation. However, this ing an accelerated growth of algae and is not critical, as long as there is a Eelgrass covered higher form of plant life to produce an common understanding of the effects with fi lamentous undesirable disturbance to the balance and agreement upon the acceptable green algae, of organisms present in the water and levels of deviations from a healthy a sign of nutrient to the quality of the water concerned, marine environment. Eutrophication enrichment. and therefore refers to the undesirable should be seen both as a process and effects resulting from anthropogenic as a continuum, since the background enrichment by nutrients” (OSPAR values may vary from area to area due 1998). A number of EU Directives also to natural causes. For example, the defi nes eutrophication. In the Urban productivity in the open Baltic Sea is Wastewater Treatment Directive eu- relatively low compared to the south- trophication means: “The enrichment ern and eastern parts of the North Sea. of water by nutrients, especially com- Therefore, when speaking of eutrophi- pounds of nitrogen and/or phospho- cation, both the initial process and

Photo: Fyn County/Nanna Rask rus, causing an accelerated growth of direct effects (sensu Nixon) and the algae and higher forms of plant life to derived primary and secondary There is no single and globally accep- nutrient enrichment into account, and produce an undesirable disturbance to effects should be taken into account, ted defi nition of marine eutrophica- it is diffi cult to make fully operational the balance of organisms present in the cf. box 1. tion. The word “eutrophication” has its since the majority of existing marine roots in Greek where “eu” means “well” monitoring programmes seldom in- and “trope” means “nourishment”. clude all the variables needed to esti- BOX 1 Defi nition of eutrophication Nixon (1995) defi nes marine eu- mate the total supply of organic matter trophication as “an increase in the to a given body of water. Eutrophication is the enhanced inputs of nutrients and organic matter leading to changes in primary pro- supply of organic matter”. The supply Gray (1992) focuses on the direct duction, biological structure and turnover and resulting in a higher trophic state. The causative factors are: is not restricted to pelagic primary effects of nutrient enrichment on pro- elevated inputs of nutrients from land, atmosphere or adjacent seas, elevated winter DIN- and DIP concen- production, but also includes bacterial ductivity, the secondary effects where trations, and increased winter N/P-ratios compared to the Redfi eld Ratio. In the case of marine waters, the production, primary production of the produced organic material is not primary or direct effects include: increased primary production, elevated levels of biomass and chlorophyll submerged aquatic vegetation, inputs consumed by grazers, and the ex- a concentrations, shift in species composition of phytoplankton, and shift from long lived macroalgae to of organic matter from land via rivers treme and ultimate effects, which in- short lived nuisance species. The secondary or indirect effects include increased or lowered oxygen concen- and point sources as well as the net cludes the growth of macroalgae, oxy- trations, and changes in species composition and biomass of zoobenthos. Low oxygen concentrations in advection from adjacent waters. The gen depletion and mortality of spe- the bottom water (oxygen depletion, hypoxia) can further affect the fi sh, benthic invertebrates and plants. advantage of this defi nition is that it is cies. Richardson & Jørgensen (1996) Total oxygen depletion (anoxia) can result in the release of hydrogen sulphide from the sediment, causing short, simple and does not confuse focus both on the process, the associ- extensive death of organisms associated with the sea fl oor. As only a few species can survive these extreme causes and effects. The limitations of ated effects of nutrient enrichment conditions, and as it takes time for plants and animals to recolonise damaged areas, eutrophication can the defi nition are 2 fold. It does not take and natural versus cultural caused result in impoverished biological communities and impaired conditions. structural or qualitative changes due to eutrophication. Prudently, Richardson

8 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . BACKGROUND, DEFINITION, CAUSES AND EFFECTS NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . BACKGROUND, DEFINITION, CAUSES AND EFFECTS 9 1) Inputs from the German Bight to have already received high inputs of Figure 1.1 CAUSES the waters along the west coast of nutrients. The input from the Baltic The nitrogen cycle in 1.2 Jutland. Sea differs from the two fi rsts be cause the aquatic environ- 2) Inputs of inorganic nutrients from it actually dilutes bioavailable nitro- ment. All relevant Skagerrak to the Kattegat bottom gen concentrations in the Danish sources are shown. water. Straits and the Kattegat. The median The major difference 3) Inputs of surface waters from the winter surface concentrations of ni- between the nitrogen Baltic Sea to the Danish Straits and trate (1986–1993) in the water from the and phosphorus cycles Kattegat. Baltic is 4.6 µM compared to the me- is that phosphorus is Liquid manure dian concentrations in the waters in not emitted to the air spread on fi eld All three sources have infl uence on the the Danish Straits and Kattegat, which and that there conse- in February. marine environment by supplying are 7.3 (6.5–8.8) µM and 7.4 (6.0–9.7) quently is no anthropo- additional nutrients. The waters from µM, respectively. genic deposition. The the German Bight and the Baltic Sea main anthropogenic sources of organic matter are discharges from wastewater treatment plants and industries as well as losses from aquaculture. From Atmospheric Danish EPA (2000). deposition

Combustion Photo: Biofoto/Bent Lauge Madsen

Mariculture The essential nutrients causing eutro- 1) agriculture, Ammonia phication are nitrogen in the form of 2) discharges from urban wastewater volatilization Sludge nitrate or ammonium and phospho- treatment plants, and Commercial and rus in the form of phosphate. In addi- 3) separate discharges from indus- animal fertilizer Industry Combustion tion, inputs of bioavailable organic tries, phosphorus and nitrogen can cause the first being the most important eutrophication, as bacteria can miner- diffuse source. Discharges from point Fodder alise the organic phosphorus to phos- sources, losses from agriculture and Sparsely bulit-up phate and the organic nitrogen to atmospheric deposition are monitored area Town ammonium, which is further oxidised as an integrated part of the National Combustion Freshwater Surface runoff to nitrite and nitrate. Aquatic Monitoring and Assessment Stormwater fishfarms outfall Marine waters receive dissolved Programme and reported annually and particulate nutrients and organic (Conley et al. 2002). matter from land via rivers and direct The inputs from adjacent seas are discharges, from the atmosphere and also monitored within the national mo- Storage in aquifer Plants Algae from adjacent seas (see Figure 1.1). In nitoring programme. There are three Drain Denmark the most important direct major avenues of advective transport sources are: of nutrients that must also be consid-

ered: Groundwater

10 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . BACKGROUND, DEFINITION, CAUSES AND EFFECTS NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . BACKGROUND, DEFINITION, CAUSES AND EFFECTS 11 PHYTOPLANKTON PRIMARY 3) Non-siliceous phytoplankton spe- EFFECTS PRODUCTION AND BIOMASS cies dominate versus diatom species. 1.3 Primary production is most often 4) Gelatinous zooplankton (jellyﬁ sh) limited by the availability of light and dominate versus crustacean zoo- AND CONSEQUENCES nutrients. Nutrient enrichment will plankton. therefore increase phytoplankton primary production. Consequently there LIGHT AND SEDIMENTATION will be an increase in phytoplankton The Secchi depth, a measure of the biomass. Elevated phytoplankton pro- turbidity and light penetration in the Growth of the duction and biomass will increase se- water column, is negatively affected nuisance algae dimentation of organic material. Chan- by chlorophyll. Increased trends in Ulva sp. is stimulated ges in the pelagic ecosystem could inputs of nutrients increase phyto- by high nutrient con- enhance the sedimentation. See micro- plankton biomass and reduce the centrations. bial loop below. Secchi depth. This decreases the colonisation depths of seagrasses and MICROBIAL LOOP macroalgae. AND THE PELAGIC SYSTEM The microbial loop may be enhanced OXYGEN CONCENTRATIONS by changes in the species composition Increased animal and bacterial acti- and functioning of the pelagic food vity at the bottom due to increased web when growth of small ﬂ agellates amounts of organic matter settling to rather than diatoms is stimulated. The the bottom increases the total oxygen Phytoplankton

Photo: Fyn County/Nanna Rask shift in phytoplankton cell size leads demand. The increase can lead to respond rapidly to

to lower grazing by copepods and oxygen depletion and release of H2S changes in nutrient Overloading with nitrogen, phospho- CHANGES IN N:P:SI RATIO possibly an increased sedimentation. from the sediment. This will induce concentrations and rus and organic matter can result in a The optimal DIN:DIP ratio (N/P-ratio) The smaller cells will on the other changes in community structure or can be used as an series of undesirable effects. The ma- for phytoplankton growth is 16:1 (ba- hand increase the relative importance death of the benthic fauna. Bottom indicator of eutrophi- jor impacts of eutrophication include sed on molar concentrations) and called of grazing by ciliates and heterotro- dwelling fi sh may either escape or die. cation status. changes in the structure and function- the Redfi eld ratio. Signifi cant lower de- phic dinofl agellates. A larger fraction ing of marine ecosystems and reduc- viations of the N/P-ratios indicate po- of the primary production is conse- tion of biodiversity. tential nitrogen limitation and higher quently channelled through the proto- 50 µm N/P-ratios potential phosphorus limi- zooplankton before it is available to NUTRIENT CONCENTRATIONS tation of phytoplankton primary pro- the copepods. Eutrophication as nutrient enrichment duction. Deviations from the Redfi eld The general responses of pelagic means elevated and/or increased ratio limit phytoplankton primary pro- ecosystems to nutrient enrichment trends in inputs of nutrients from land, duction, and affect phytoplankton bio- can in principle be a gradual change atmosphere or adjacent seas. And mass, species composition and conse- towards: rensen consequently elevated dissolved inor- quently food web dynamics. Redfi eld 1) Increased planktonic primary pro- ø ganic nitrogen (DIN) and dissolved ratios for diatoms for Si:N and Si:P duction compared to benthic pri- inorganic phosphorus (DIP) concentra- ratios are 1:1 and 16:1, respectively (ba- mary production. tions during winter. sed on molar concentrations), and the 2) Microbial food webs dominate ver- abundance of silicate relative to nitro- sus linear food chains. rhus County/Helene Munk S gen and phosphorus effect the growth Å

of diatoms. Photo:

12 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . BACKGROUND, DEFINITION, CAUSES AND EFFECTS NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . BACKGROUND, DEFINITION, CAUSES AND EFFECTS 13 25 14 SEASONAL SIGNALS SEDIMENTS to a further reduction in species diver- 12 Many of the eutrophication effects as In the Danish marine areas a signiﬁ - sity, and mass mortality of most or- )

20 3 10 well as some of the driving forces have a cant portion of the primary produc- ganisms, especially due to production m 6 C), salinity º 15 8 pronounced seasonal variation. Fresh- tion during the spring sediments to of H2S in sediments. 10 6 water run-off, temperature and salin- the sea bottom. An increase in primary Salinity 4 5 Runoff (10 ity have a strong seasonal signal. The production means that the sediment SOCIAL CONSEQUENCES Temperature 2

Temperature ( Runoff same is the case for inorganic nutrient will experience elevated inputs of orga- Reductions in dermersal fish and 0 0 Jan Feb Mar Apr May June July Aug Sept Okt Nov Dec concentrations, phytoplankton pri- nic material. This leads to increased shellfi sh due to oxygen depletion and mary production, chlorophyll a con- bacterial activity, hence an increase in harmful algal blooms will reduce har- 9 DIN 0,7 centrations, phytoplankton biomass oxygen demand. vests. In the case of commercial fi sher- DIP 8 0,6 and oxygen concentration in bottom ies these changes have large economic 7 0,5 6 water. The seasonal variations are SUBMERGED AQUATIC VEGETATION implications. The increased risk of toxic 5 0,4 illustrated in Figure 1.2. This assess- Eutrophication in general affects sub- and harmful blooms will also affect 4 0,3 DIP (µM) DIN (µM) 3 ment report will, as mentioned in the merged vegetation in two different mariculture, which can also be infl u- 0,2 2 preface, neither analyse the role of ways. Reduced light penetration and enced by oxygen depletion. Another 0,1 1 eutrophication on the seasonal succes- shadowing effect from phytoplankton consequence of toxic algal blooms is the 0 0,0 Jan Feb Mar Apr May June July Aug Sept Okt Nov Dec sion in these parameters nor the chan- can reduce the depth distribution, bio- risk of shellfi sh poisoning of humans ges in turn-over or fl uxes of nutrients mass, composition and species diver- by algal toxins. The recreational value ) -1 900

d during spring, summer and autumn. sity of the plant community. Increased of beaches especially for swimming is -2 800 nutrient levels favour opportunistic reduced due to reduced water quality 700 600 macroalgae species. The stimulated induced by discoloration and foam 500 growth of filamentous and annual formation by algal blooms or decaying 400 300 nuisance species at the expense of pe- rotting macroalgae. This could particu- 200 rennials will result in a change in ma- larly impact tourism at beaches. 100 0 croalgae community structure with

Primary production (mg C m Jan Feb Mar Apr May June July Aug Sept Okt Nov Dec reduced species diversity and reduced nurseries for ﬁ sh. The dominance of 200 5 filamentous macroalgae in shallow ) -1

4 ) sheltered areas will increase the risk 150 -1 of local oxygen depletion. 3 100 2 BENTHIC FAUNA 50 Chlorophyll 1 The increased load of organic material Carbon Chlorophyll (µg l

Carbon biomass (µg C l 0 0 Figure 1.2 to the bottom affects the macrozoo- Jan Feb Mar Apr May June July Aug Sept Okt Nov Dec Seasonal variation at the station “Gniben” benthic community. The enrichment located in south-western Kattegat. will enhance growth and increase spe-

) 7 -1 l Monthly averages during the period cies diversity and biomass. A change 2 6 1998-2001. in community structure will follow 5 favouring suspension and burrowing 4 A – Runoff, salinity and temperature detritus feeders. Reduction in species 3 B – N- and P-concentrations diversity and biomass will follow at 2 C – Primary production 1 progressively higher levels of organic 0 D – Chlorophyll a + Carbon-biomass load, and opportunistic species will be

Oxygen concentration (ml O Jan Feb Mar Apr May June July Aug Sept Okt Nov Dec

E – Oxygen concentration in bottom water favoured. Oxygen depletion will lead Photo: NERI/Britta Munter

14 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . BACKGROUND, DEFINITION, CAUSES AND EFFECTS NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . BACKGROUND, DEFINITION, CAUSES AND EFFECTS 15 CONCEPTUAL UNDERSTANDING Figure 1.3 OF EUTROPHICATION Conceptual model of marine eutrophication with lines indicating interactions between the dif- Figure 1.3 illustrates in a simpliﬁ ed way ferent ecological compartments. A balanced system in Danish marine waters is supposedly the effects and consequences of nutri- characterised by: ent enrichment and eutrophication in 1) A short pelagic food chain (phytoplankton → zooplankton → ﬁ sh) the marine environment. 2) Natural species compositions of planktonic and benthic organisms 3) A natural distribution of submerged aquatic vegetation Nutrient enrichment results in changes in the structure and function of marine ecosystems as

indicated with bold lines. Dashed lines indicate release of hydrogen sulphide (H2S) and phosphorus, which is positively linked to oxygen depletion. Based on OSPAR 2001 and Rönnberg 2001.

Atmospheric deposition

Runoff and Nutrients Phytoplankton Zooplankton Fish direct · Elevated winter DIN · Increased production and biomass ·Changes in species composition ·Changes in species composition discharges and DIP concentrations · Changed in species composition ·Increased biomass ·Less fish below the halocline · Changed N:P:Si ratio · Increased bloom frequency ·Mass death due to oxygen depletion · Elevated DIP concentrations · Decreased transparency and light or release of hydrogen sulphide due to release of nutrients availability from sediments due to · Increased sedimentation of Inputs oxygen depletion organic matter from adjacent seas

Submerged aquatic vegetation Macrozoobenthos Oxygen ·Changes in species composition ·Changes in species composition · Increased oxygen consumption ·Reduced depth distribution due to shading ·Increased biomass of benthic animals due to increased production ·Growth of epiphytes and nuisance macroalgae on shallow bottoms above the of organic matter ·Mass death due to release of hydrogen sulphide halocline due to increased · Oxygen depletion sedimentation · Formation or release of ·Mass death due to oxygen depletion hydrogen sulphide or release of hydrogen sulphide

Oxygenated sediments Anoxic sediments

16 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . BACKGROUND, DEFINITION, CAUSES AND EFFECTS NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . BACKGROUND, DEFINITION, CAUSES AND EFFECTS 17 TECHNICAL-SCIENTIFIC ASSESSMENT

This assessment is based on data from tant that Den mark and the neighbour- The health of the Nationwide Aquatic Monitoring ing countries (Germany, Norway, and the Danish marine Programme (1988–1997) and the Natio- Swe den) have almost harmonised environment is nal Aquatic Monitoring and Assess- ways of assessing the eutrophication of great public ment Programme (1998–2003) (See status of common waters. Such com- interest. Danish EPA 2000 for details). The data mon understanding of principles and used for this assessment covers the criteria are dis cussed and agreed with- period 1989–2001 (in some cases only in OSPAR and HEL COM. 1989–2000). In a limited number of ana- The OSPAR Strategy to Combat lyses other data have been included Eutrophication is together with the for support. monitoring activities among2 the main The national monitoring program- drivers of the assessment process me is run in collaboration between the (OSPAR 1998). The strategy has the Danish Counties and NERI. The coun- aim of identifying the eutrophication ties are responsible for the activities status of all parts of the convention within the coastal waters. NERI carries area by the year 2002. The Common out most of the monitoring of the open Procedure for the Identiﬁ cation of the marine waters, co-ordinates the pro- Eutrophication Status of the Maritime gramme, runs the national marine da- Area is a main element of the strate- tabase and produces annual reports on gy. The Common Procedure in clu d es the state of the marine environment. a checklist of qualitative assess ment Marine eutrophication is an inter- criteria to be used when assessing national problem and can only be sol- eutrophication status. In addition, a ved by co-ordinated national and in- set of quantitative criteria has been ternational efforts. The same principle developed in order to assist a harmoni- applies for assessment of eutrophica- sed assessment (OSPAR 2001). These tion status of marine waters receiving assessment criteria fall into the follow- and transporting nutrients from dif- ing four categories:

Photo: CDanmark ferent countries. Therefore, it is impor-

18 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 19 I: Degree of nutrient enrichment. etc.). The on-going work with regard have been national assessment of ecological structure and function II: Direct effects of nutrient en rich- to classifi cation of ecological quality criteria since the mid-1980s. The before the enrichment took place is ment. shows: OSPAR criteria are: < 2 mg l-1 and subject to an element of uncertainty. III: Indirect effects of nutrient en- • The acceptable deviation should be 2-6 mg l-1. These criteria are not ap- Box 3 summarises the current under- richment. 15-20%, cf. Henriksen et al. 2001. plicable for Danish marine waters, standing of the reference conditions in IV: Other possible effects of nutrient • A generally acceptable deviation of where the oxygen concentrations Danish marine waters. enrichment. 25% will result in a limited number due to natural reasons may be 5-6 The implementation of the EU of false positive situations when mg l-1. Such values are region spe- Water Framework Directive and the The harmonised assessment criteria compared to the existing Danish cific and due to strong stratifi- development of Ecological Quality and the assessment levels are sum- assessments principles, cf. Krause- cation in summer and autumn, Objectives (EQOs) defi ning the eco- marised in Box 2. Jensen et al. (subm.). which prevents the saline, cold logical status of all European coastal Assessments of eutrophication sta- • The Swedish assessment criteria bottom waters from being mixed waters will represent a major step tus of the marine environment should (Swedish EPA 2000) for macroalgae with the brackish and oxygenated forward. The descriptions of ecologi- be based on knowledge of how the will be met almost every year, even surface waters. The Danish assess- cal status will be based on commonly situation would be without any an- in wet years with high anthropo- ment criteria match the natural agreed defi nitions of reference condi- thropogenic infl uence. Two factors are genic inputs of nutrient, to the ma- effect criteria. Fish will try to avoid tions. The basis for assessing the eco- important when assessment criteria rine environment, cf. Henriksen et waters with oxygen concentrations logical status will change from expert are developed: al. 2001. below 4 mg l-1. Fish and benthic judgements to operational and nu- • What are the reference conditions • A 50% deviation from reference invertebrates can only survive con- meric quality classes. for the given parameter? conditions for macrovegetation at centrations below 2 mg l-1 for a An EU funded R&D project “Char- • What are an acceptable deviation reefs in Kattegat suggests that the limited time. acterisation of the Baltic Sea Ecosys- from reference conditions? reefs are not subject to eutrophica- tem: Dynamics and Function of Coas- tion at all, cf. Henriksen et al. 2001. Box 2 summarises the OSPAR eutro- tal Types” (CHARM) will be a major The principle of assessing the actual phication quality objectives (EQO- contribution to this in the Baltic Sea/ state of the environment in relation to The OSPAR eutrophication assess- eutro) and the Danish criteria used in Kattegat Area. One of the products of the reference conditions is important. ment criteria have therefore been ad- this assessment. the CHARM project, which runs for However, the critical factor is not the justed at a national level. The Danish Background data describing refer- the years 2002-2004, is descriptions of defi nition of background conditions assessment criteria are: ence conditions when anthropogenic reference conditions for nutrients, phy- but the decision on acceptable devia- • The acceptable deviation for win- inputs of nutrients were at natural toplankton, submerged aquatic veg- tion from background values. The ter DIN and DIP concentrations is levels are scarce. Hence descriptions etation and macrozoobenthos. OSPAR assessment criteria ack now- 25%, cf. Henriksen et al. 2001. ledge this and give the countries the • The acceptable deviation for maxi- opportunity to establish national, re- mum and mean chlorophyll a is gional or even site-specifi c criteria. 25%, cf. Henriksen et al. 2001. The Recent work in relation to the na- growing season covers the period tional implementation of the Water March – October. Framework Directive and the Habitat • The acceptable deviation for ma- Directive has focused on reference crophytes including macroalgae is conditions and classifi cation of ecolo- 25%, cf. Krause-Jensen et al. (subm.) gical quality for eelgrass and mac- and Henriksen et al. 2001. roalgae and the parameters control- • The Danish oxygen depletion cri- ling the growth of submerged aquatic teria are: severe acute oxygen de- -1 vegetation (chlorophyll, Secchi depth, pletion: 0-2 mg O2 l , oxygen deple- -1 Photo: NERI/Britta Munter runoff, nutrient concentrations, depth tion: 2-4 mg O2 l . These values Photo: NERI/Britta Munter

20 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 21 BOX 2 The OSPAR eutrophication quality objectives (ECO-eutro) BOX 3 Background or reference conditions and the Danish criteria used in this assessment

OSPAR EAC Danish ECO-eutro See page Nutrients Background winter concentrations for DIN in the Kattegat, the Skagerrak and the North Sea and in the I: Degree of nutrient enrichment (causative factors) Wadden Sea has within OSPAR provisionally been estimated to 4-5, 10, and 6.5 µmol l-1, respectively. -1 1 Riverine total N and total P inputs and direct discharges (RID) Winter DIP concentrations have been estimated to 0.4, 0.6 and 0.5 µmol l , respectively. Elevated inputs and/or increased trends (compared Phytoplankton biomass and production with previous years) 25 % 28–35 Chlorophyll a background concentration for offshore areas in the Skagerrak has within OSPAR been esti- 2 Winter DIN- and/or DIP concentrations mated to <1.25 µg l-1, and for the North Sea coast to 2-10 µg l-1. The phytoplankton primary production in Elevated level(s) (defi ned as concentration > 50 % above salinity the Kattegat and Belt Sea has increased from 80-100 mg C m-2 year-1 in the 1950s – 1960s to 120-290 mg -2 -1 related and/or region specifi c background concentration) 25 % 36–41 C m year in the 1970s – 1980s (Ærtebjerg Nielsen et al. 1981; Richardson & Christoffersen 1991; Heil- 3 Increased winter N/P ratio (Redfi eld N/P = 16) mann et al. 1994, Richardson & Heilmann 1995). Elevated cf. Redfi eld (>25) 43–44 Oxygen From scattered measurements of oxygen concentrations in Danish waters from the period 1902–1975 and II: Direct effects of nutrient enrichment (growing season) more systematic measurements since then it seems that the major decrease took place from the 1960s to the 1 Maximum and mean Chlorophyll a concentration late 1980s. In the Kiel Bight the bottom water oxygen concentration in July-August decreased from about 8 -1 -1 in March–October mg O2 l in the late 1950s to about 4 mg O2 l in the late 1980s (Babenerd 1991). Trend analysis of bottom Elevated level (defi ned as concentration > 50 % above water oxygen concentrations in late summer – autumn in the Kattegat and Belt Sea area generally shows a decrease of 1.5-2.2 mg O l-1 during the period from mid 1970s to about 1990 (Ærtebjerg et al. 1998). spatial (offshore) / historical background concentrations) 25 % 46–51 2 2 Region/area specifi c phytoplankton indicator species Eelgrass 2 Elevated levels (and increased duration) 41–53 In 1900, eelgrass was widely distributed in Danish coastal waters, and covered approximately 6726 km or 3 Macrophytes including macroalgae (region specifi c) 1/7 of all Danish marine waters. In the 1930s, the world wide wasting disease substantially reduced eelgrass Shift from long-lived to short-lived nuisance species (e.g. Ulva) 68–76 populations, especially in north-west Denmark. In 1941, eelgrass covered only 7% of the formerly vegetated areas, and occurred only in the southern, most brackish waters and in the low saline inner parts of Danish estuaries. Analyses of aerial photos from the period 1945–1990s, reveal an initial time lag of more III: Indirect effects of nutrient enrichment (growing season) than a decade before substantial re-colonisation of the shallow eelgrass populations began. The photos also 1 Degree of oxygen depletion show that large populations had recovered in the 1960s. Today eelgrass again occurs along most Danish Decreased levels (<2 mg O l-1: acute toxicity; 2 coasts but has not reached the former area extension. Comparisons of eelgrass area distribution in two 2-6 mg O l-1: defi ciency) < 2 and 2–4 mg l-1 58–76 2 large regions, the Sound and Limfjorden, in 1900 and in the 1990s, suggest that the present distribution 2 Changes/kills in zoobenthos and fi sh kills area of eelgrass in Danish coastal waters constitutes approximately 20-25% of that in 1900. Reduction in Kills (in relation to oxygen depletion, H2S and/or toxic algae) area distribution is partly attributed to loss of deep populations. In 1900 colonisation depths averaged 5-6 Long term changes in zoobenthos biomass and species composition 76–79, 80–83 m in estuaries and 7-8 m in open waters, while in the 1990s the colonisation depths were about halved to 3 Organic Carbon/Organic Matter (in sediments) 2-3 m in the estuaries and 4-5 m in open waters. Elevated levels (in relation to III.1) (relevant in sedimentation areas) 62–67 Macrozoobenthos There have been changes of macrozoobenthos communities in the Kattegat area over the last 100 years. IV: Other possible effects of nutrient enrichment (growing season) The reference material is mainly the large-scale mapping performed by C.G.J. Petersen at the end of the 1 Algal toxins (DSP/PSP mussel infection events) 19th and the beginning of the 20th century. Comparisons in the 1980s and 1990s indicate that biomass, Incidence (related to Category II.2) 52–53 and then probably secondary production, has increased with at least a factor of 2. Main contribution to the increase in deeper waters is from the suspension-feeding brittle star Amphiura fi liformis and some polychaetes, whereas some amphipod crustaceans have decreased in importance. In shallower waters it is likely that biomass of bivalves have increased as has been documented from the eastern and southern Baltic Sea. Local reductions of benthic faunal biomass due to hypoxia seems to have occurred at times in recent decades both in the southern Kattegat, the Belt seas and some Danish estuaries.

22 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 23 CLIMATE 14 2.1 12 10 ) 6 8 x 10

Runoff 6 (m 4 Figure 2.1 Annual runoff from Denmark to the Katte- 2 Climate inﬂ uences gat – Belt Sea area 1975-2001. (Data from 0 1977 1980 1983 1986 1989 1992 1995 1998 2001 the ecological NERI). response to nutrient enrichment. 9 8 7 6 5 C) º

( 4 Figure 2.2 3

Annual mean air temperature measured at Mean temperature 2 Sprogø/Risø 1978-2001. (Data from Sund og 1 Bælt Holding and Wind Energy Department, 0 1980 1983 1986 1989 1992 1995 1998 2001 Risø National Laboratory). Photo: High-lights 250 The Danish marine waters are continu- between a minimum of 4.84 km3 in 200 ously affected by short term variations 1976 to a maximum of 12.52 km3 in Figure 2.3 Mean summer (May-Aug.) solar radiation in freshwater runoff, air temperature, 1994. Runoff was generally low in the ) 150 wind forcing and solar radiation, gov- mid 1970s, high during the 1980s, measured at Højbakkegård/Risø. (Data from -2 100 erning the nutrient load, water tempera- 1994–95 and 1998–2000, and very low Laboratory for Agrohydrology and Bioclima- (W m ture, water exchange and stratiﬁ cation in 1976, 1989 and 1996–97 (Figure 2.1). tology, Department of Agricultural Sciences, 50 as well as irradiance available for pri- The annual mean air temperature at The Royal Veterinary and Agricultural Uni- Mean summar solar radiation mary production. Large inter annual Sprogø in the middle of the Great Belt versity, and Wind Energy Department, Risø 0 variations are also present in these varied from 5.8°C in 1987 to 8.3°C in National Laboratory). 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 driving forces, especially concerning 1990 (Figure 2.2). The mean summer runoff. However, no general trend is (May–Aug.) solar radiation measured 7,5 detected in the time series over the last close to Copenhagen varied from 175 7 -2 25 years. to 244 W m in 1987 and 1976, respec- 6,5

The climatic driving forces are used tively (Figure 2.3). 6 )

in later chapters in an attempt to cor- The mean annual wind speed meas- -1 5,5 rect the chemical and biological indi- ured 70 m above sea level at Sprogø in Figure 2.4 (m s 5 -1 Mean annual wind speed at Sprogø/Risø

cators for natural climatic induced va- the Great Belt varied from 6.3 m s in Mean wind speed riations. 1985 to 7.1 m s-1 in 1994 (Figure 2.4). 1978-2001. (Data from Sund og Bælt Hold- 4,5 Freshwater runoff from Denmark ing and Wind Energy Department, Risø 4 1980 1983 1986 1989 1992 1995 1998 2001 to the Kattegat and Belt Sea varied National Laboratory).

24 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 25 linity increases northward along the summer. The variation in the bottom HYDROGRAPHY Danish coast as it is mixed with central water is from 4°C to 11°C. The annual 2.2 North Sea water. The salinity distribu- mean surface temperature in Kattegat tion in the surface of the open Danish has increased signifi cantly from 8.5 °C waters is shown in Figure 2.6. to 9.6°C in the period 1970–2000 The salinity in the estuaries and (Figure 2.7). fjords depends on the amount of freshwater received, residence time and the Figure 2.5 salinity of the coastal water outside the 10 The Danish marine estuary. Most of the Danish estuaries Surface water Bottom water Median monthly areas can be are shallow with only periodic stratifi - 15 salinity in the open illustrated by the cation due to infl ow of saline bottom 20 Kattegat and Belt Sea picture of Denmark water or establishment of a thermo- surface water (0-10m) Salinity 25 and its islands cline during calm and warm periods. and bottom water situated in a river Some fjords, such as Mariager Fjord, 30 (20-40m) 1975-2000. mouth with the Flensborg Fjord and the deep open 35 (After Henriksen et al. 1975 1980 1985 1990 1995 2000 Baltic Sea as the Åbenrå Fjord have permanent halo- 2001). river. clines. 11 The seasonal amplitude of the sur- y = 0,0353x - 61,095 r2 = 0,2128 face temperature in open waters goes 10 from 1-4°C in winter to 15–20°C in Figure 2.7 9 The development of

Photo: High-lights annual mean surface 8 Temperature (ºC) (0-10 m) temperature

The Danish marine area is very diverse halocline in 13-15 m depth, which is 7 in Kattegat 1970- and includes many semi-enclosed estu- re-enforced by a thermocline during 1965 1970 1975 1980 1985 1990 1995 2000 2005 2000. aries and fjords, open estuaries and summer. The salinity in the Kattegat – 33 bights, narrow straits, semi-enclosed Belt Sea area has a general seasonal 34 Skagerrak 32 seas as the Baltic and Kattegat and variation with the highest salinity in 32 30 34 open shelf seas as the Skagerrak and the surface and lowest in the bottom 33 30 32 Sweden North Sea (see page 122). water during winter (Figure 2.5). 34 30 29 Kattegat 26 The annual freshwater net surplus The German Bight annually re- 33 32 30 of about 475 km3 from the Baltic Sea ceives about 37 km3 freshwater from 26 24 34 22 passes through the Danish straits the rivers Elbe and Weser (mean 1980– North Sea

(Sound and Belt Sea) and Kattegat to 86), and the southern North Sea Bight 33 The Denmark Sound

30 24 the Skagerrak/North Sea. The salinity about 81 km3 from the Rhine (Gerlac 34 of the outﬂ owing Baltic water is about 1990). The Jutland Coastal Current an-

3 33 8 and forms a brackish surface layer in nually transports about 1,500 km 32 24 12 30 10 the transition area with the salinity water from the German Bight to the

24 12 Baltic

28 22

increasing to 25-30 at the Skagerrak Skagerrak. The runoff to the Southern 20 10 Sea Figure 2.6 14 12 33

border due to mixing with saline bot- North Sea causes a salinity in the Jut- 32 Interpolated distribu- 30 28 20 16 tom water. High saline Skagerrak water land Coastal Current to be about 28– 33 24 18 tion of surface salinity ﬂ ows as bottom water into the Kattegat 30 when it enters the Danish coastal Germany (mean 1-10 m) in and Belt Sea. This creates a strong waters in the German Bight. The sa- February 2002.

26 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 27 1980s – early 1990s has reduced the 20 Figure 2.8 ) INPUTS FROM LAND-BASED point source P load to surface waters 3 16 Annual freshwater m (fresh and marine) by nearly 90%. This 6 runoff, and the total 2.3 12 has reduced the overall P load and N annual nitrogen and SOURCES, THE ATMOSPHERE 8 load to marine waters by 60% and 14%, phosphorus load to AND ADJACENT SEAS respectively, compared to 1990, and Runoff in (10 4 Danish coastal waters today the diffuse P load is higher than 0 in the period 1989- the point source P load (Bøgestrand 81-88 89 90 91 92 93 94 95 96 97 98 99 00 01 2001, divided 2001, Ærtebjerg et al. 2002). between diffuse load, 180 Freshwater runoff The diffuse P and N loads follow Direct discharges point sources to is the most impor- the runoff creating large seasonal and 150 Point sources to freshwater freshwater and direct Diffuse load tant factor affecting inter-annual variations. For example, 120 point sources to nutrient concentra- the N load might be twice as large in 90 marine waters. An tions in Danish wet years (1994-1995) as in dry years 60 estimate of the aver- coastal waters. (1996-1997) (Figure 2.8), which masks 30 age annual load in Nitrogen (tonnes x 1000) long term trends in loads. However, 0 the 1980s is shown during the period 1990-2001 the diffuse 81-88 89 90 91 92 93 94 95 96 97 98 99 00 01 for comparison. N load corrected for inter-annual varia- From Bøgestrand 10 tions in runoff has decreased about 21% Direct discharges (2001). due to reduced use of N in agriculture 8 Point sources to freshwater Diffuse load (Bøgestrand 2001, Ærtebjerg et al. 2002). 6 The nutrient and BOD loads to 5 4 Photo: Biofoto/Svend Tougaard different Danish coastal areas in 2001 are shown in Table 2.1, and source ap- 2

The main increase in nutrient loads estimated an increase in atmospheric N portionment for 2000 is shown in Table Phosphorus (tonnes x 1000) 0 81-88 89 90 91 92 93 94 95 96 97 98 99 00 01 from land and atmosphere took place deposition of 80% and 100% from the 2.2. The diffuse background load was long before comprehensive load com- late 1940s to the 1980s for the North Sea about 10% of the total load for both N pilations were initiated in the late 1980s. and Baltic Sea, respectively. In recent and P in 2000. Agriculture accounted for Kronvang et al. (1993) recorded a 3.7% years the load to Danish waters has de- 88% of the N load and 45% of the P load annual increase in the export of nitrogen creased, especially for P, but also for N to fresh- and marine waters. Direct point (N) during the period 1967–1978 in 6 from land and atmospheric sources. sources to marine waters accounted for Danish rivers draining mainly agricul- 4% and 17% of the N and P load, respec- tural catchment areas. They also esti- LAND-BASED SOURCES tively. mated that the annual riverine N load Detailed Danish nutrient load compila-

to Danish coastal waters in the late 1960s tions were initiated in 1988. The devel- Sea area Drainage area Runoff Nitrogen Phosphorus BOD5 was only about 60% of that in the 1980s. opment up to 2001 is shown in Figure km2 mm 106 m3 tonnes tonnes tonnes From long term time series of N and 2.8. Nitrogen (N) comes primarily from North Sea 10,860 449 4,852 17,500 530 6,500 Table 2.1 phosphorus (P) concentrations in a few leakage from agricultural soils, while Skagerrak 1,098 420 462 2,300 100 1,500 Runoff, total nitrogen, North European rivers the EEA (2001) point sources play a minor and de- Kattegat 15,852 347 5,490 28,100 810 12,700 total phosphorus and

estimated that the N load has at least creasing role. Earlier, phosphorus (P) Belt Sea 12,346 241 3,061 20,400 670 10,500 BOD5 load from Den- doubled from the 1950s to the 1980s in came mainly from point sources, e.g. The Sound 1,709 170 292 2,200 190 1,300 mark to the main sea the North Sea – Baltic Sea region, and domestic and industrial wastewater. Baltic Sea 1,206 221 266 2,300 50 600 areas in 2001. Com- the P load increased fourfold from the However, improving sewage plants Total 43,070 335 14,423 72,800 2,340 33,200 piled from Ærtebjerg 1940s to the 1970s. The EEA (2001) also with P (and N) removal during the late et al. 2002.

28 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 29 Table 2.2 Figure 2.9 > 2.1 Source apportion- Sources Nitrogen Phos- BOD Nitrogen Phos- BOD The atmospheric wet 5 5 1.9 _ 2.1 _ ment of the Danish phorus phorus 1.7 1.9 and dry deposition of nitrogen (N), phos- t yr-1 t yr-1 t yr-1 % % % 1.4 _ 1.7 inorganic nitrogen _ phorus (P) and BOD Riverine inputs: 1.2 1.4 (NO +NH ) in tonnes 5 _ x x load to coastal waters A Diffuse load 0.9 1.2 N km-2 at Danish sea 0.7 _ 0.9 in 2000. Compiled • Background 8,500 290 7,700 10.2 11.4 21.2 areas in 2001. Grid < 0.7 from Bøgestrand • Agriculture 73,700 1,150 9,200 88.4 45.1 25.3 size 30 km x 30 km. (2001) and Danish • Settelments 1,000 220 3,850 1.2 0.3 10.6 (From Ærtebjerg et EPA (2001). B Point sources to freshwater al. 2002). • Sewage plants 2,475 250 1,800 3.0 9.8 5.0 • Industry 25 5 35 - 0.2 0.1 • Rainwater overﬂ ows 590 150 1,700 0.7 5.9 4.7 • Freshwater aquaculture 2,390 90 3,400 2.9 3.5 9.4 C Retension in freshwater -8,880 -35 -85 -10.7 -1.4 -0.2 Total riverine load 79,800 2,120 27,600 95.7 83.1 75.9

Direct point sources: • Sewage plants 2,180 295 1,500 2.5 11.6 4.1 • Industry 870 55 4,900 1.0 2.2 13.5 • Rainwater overﬂ ows 170 45 500 0.2 1.8 1.4 • Mariculture 325 35 1,850 0.4 1.4 5.1 Total direct load 3,545 430 8,750 4.3 16.9 24.1 Total load: 83,345 2,550 36,350 100 100 100

INPUT FROM THE ATMOSPHERE About 40% of the N deposition on 3,5 3,5 The atmospheric deposition of inor- all Danish marine waters in 2000 were AnholtAmmonium Keldsnor Ammonium ganic nitrogen (NO and NH ) is impor- in the reduced form (NH ), which stems Nitrate Nitrate x x x 3,0 Ammonia 3,0 Ammonia tant on large sea surfaces, but insigniﬁ - from ammonia evaporation from agri- cant in small estuaries, compared to cultural husbandry. The other 60% were Figure 2.10 2,5 2,5

other N sources. For the Kattegat and in the form of oxidised nitrogen (NOx) ) ) Annual means for -3 -3 Belt Sea the atmospheric N deposition from the combustion of fossil fuels. the period 1989-2001 2,0 2,0 makes up about 30% of the total N load About 11% of the total deposition on of concentrations of (µg N m (µg N m from surrounding land and atmos- Danish marine waters came from Dan- ammonia (green), 1,5 1,5 phere. The distribution of the deposition ish emissions, varying from 3% in the particle bound ammo- in 2001 on the Danish seas is shown in southern Belt Sea to 20% in the Skager- nium (red) and sum-

Figure 2.9. During the period 1989– rak. Emissions from shipping made up Concentration 1,0 Concentration 1,0 nitrate (blue) at Anholt 2001 there is a decrease in the air concen- 7% of the total deposition (Ellerman et in the middle of Kat- tration of N bound to particles and a ten- al. 2001). 0,5 0,5 tegat and Keldsnor in dency to a decrease in deposition of the southern Belt Sea. about 15% (Figure 2.10) (Ellerman et al. 0 0 (From Ellermann et al. 1989 1991 1993 1995 1997 1999 2001 1989 1991 1993 1995 1997 1999 2001 2001; Ærtebjerg et al. 2002). 2002).

30 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 31 NUTRIENT INPUT FROM with P, some of it permanently, but land, the excess P coming from the to their respective riverine and point ADJACENT SEAS AND much of it in labile pools. Especially sediment pools. In the latest years the source loads to determine the direct NEIGHBOURING COUNTRIES large pools of phosphate are released P export from the estuaries generally loads from the surrounding countries. The exchange of water and nutrients from sediments to the overlying water equals the load from land (Henriksen The gross advective transports from between the Baltic Sea and Skagerrak column during summer and autumn, et al. 2001). the Baltic Sea and the Skagerrak in the through the Kattegat and the Belt Sea when the sediments become partly period 1974–1999 were used instead is intense. The annual freshwater net anoxic (Rasmussen et al., in press). NUTRIENT BUDGET FOR of the net transports given in Table 2.3 surplus of about 475 km3 from the The Danish nutrient load to the THE KATTEGAT – BELT SEA AREA (Ærtebjerg et al. 2002). Determined Baltic Sea passes through the Danish North Sea, Skagerrak and Baltic Pro- An average annual nitrogen nutrient from the supply of total-N the Danish straits and Kattegat to the Skagerrak/ per is small (Table 2.3.) compared to budget has been established for the contribution to the Kattegat – Belt Sea North Sea. The average annual net the advective transports and load Kattegat – Belt Sea area to evaluate the area amounts to 12% (Table 2.4). transports of N and P in the period 1974- from other countries. For example the signifi cance of the different contribu- The bio-availability of the nitrogen 1999 are shown in table 2.3 together Danish nitrogen load to the North Sea tions (Figure 2.11, Table 2.4). In the in the different sources was calculated with the loads from the atmosphere (including estuaries and the Wadden budget the land based nitrogen loads from measured concentrations of in- and surrounding countries in the pe- Sea) is 10–15% of the riverine load to from Denmark, Sweden and Germany organic nitrogen and nitrogen built riod 1989–1996. The gross transports the German Bight. Much of this is given in Table 2.3 are used. However, into phytoplankton, and compared to are much larger, especially at the Ska- transported north along the Danish instead of the atmospheric deposition experimental results (Kaas et al. 1994). gerrak border. Here an infl ow of deep coast with the Jutland Coastal Cur- in the period 1989-96 the average Including the bio-availability of the water rich in inorganic nutrients enters rent, which annually transports about deposition 1999–2001 was chosen, as nitrogen sources in the budget in- the Kattegat bottom water, and eventu- 160,000 tonnes dissolved inorganic these estimations are more reliable. creased the Danish contribution to ally is mixed to the surface water and nitrogen (DIN) from the German Bight The sources of emissions to the atmos- 25% of the gross supply to the area re-exported to the Skagerrak, either in to Skagerrak. Episodically some of this phere were identifi ed, and the contri- (Figure 2.11, Table 2.4). However, some inorganic or organic form, dependent water and associated nutrients might butions to the deposition from Den- of the nitrogen supplied from the on the season. Due to shallow sills the enter the Kattegat in some years, but it mark, Germany and Sweden added Skagerrak actually originates from the infl ow from the Baltic Sea to the Danish is generally exported to the North straits is Baltic surface water, which is Atlantic with the Norwegian Coastal low in bio-available nitrogen, due to Current. the long residence time (about 25 years) Load from the surrounding land of the Baltic Sea. dominates the nutrient budgets in the Table 2.3 Evidently, there is a substantial net Danish estuaries. Dependent on the The average annual supply of both N and P to the Kattegat residence time, 12% to 95% of the N net supplies of N and – Belt Sea area of on average about received, is exported from the estuar- P to Kattegat and the 125,000 tonnes N and 5,800 tonnes P ies to the open coastal areas. In the Belt Sea in 103 tonnes per year. Some of the nitrogen is re- beginning of the 1990s, after reduction year-1. Negative moved by denitrifi cation. The rest is of the P-load, the estuaries exported values mean trans- accumulated in the sediments together more P than they actually received from port out of the area. Based on Rasmussen et al. (in press). rtebjerg Denmark Sweden Germany Atmosphere Baltic Sea Skagerrak Sum Æ Nitrogen 60 26 12.5 44.5 150 -165 128 Phosphorus 3.38 0.54 0.35 11.44 -9.88 5.83 Photo: NERI/Gunni

32 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 33 Kattegat and some is removed by de- from the Baltic Sea and the Skagerrak nitriﬁ cation or exported to the Bal- to 14% and 19% respectively, and the tic Sea. Taking this into account in- contributions from other European creases the Danish contribution of bio- countries via the atmosphere to 13% available nitrogen to 32%, the direct (Figure 2.11, Table 2.4) (Ærtebjerg et contributions from Sweden and Ger- al. 2002). many to 11% each, the contributions

Total Bio-available Corrected for Percentage nitrogen nitrogen re-circulation contributions Denmark 70 64 64 32 % Sweden 28 23 23 11 % Germany 24 23 23 11 % Other countries 26 26 26 13 % via the atmosphere Table 2.4 Skagerrak 223 89 32 19 % Nitrogen sources to Baltic Sea 217 28 28 14 % the Kattegat and Belt Total 588 253 203 100 % Sea in 1,000 tonnes Danish contribution 12 % 25 % 32 % per year. Period covered: see text.

Denitrification

26 Sweden Denmark Atmosphere Germany land + land + other land + atmosphere atmosphere countries atmosphere

23 6426 23 The Skagerrak The Baltic Sea

66 28 Photic zone +17 Figure 2.11 Transport of biological active nitrogen in 16 102 the transition area 55 63 39 between the Skager- rak and the Baltic. Bottom water 56 10 +11 Loss to the bottom 34 includes denitriﬁ - 24 cation (30,000 tons) 30 47 and permanent burial (47,000 tons). Period Denitrification Permanent burial

Photo: CDanmark covered: see text.

34 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 35 NUTRIENT CONCENTRATIONS, DIN (µmol l-l) < 10 2.4 10-25 NUTRIENT RATIOS 25-50 50-100 AND NUTRIENT LIMITATION Skagerrak > 100 Sweden

A fraction of Kattegat the water samples are analysed for The inorganic nutrients, Denmark Sound total nitrogen North Sea and phosphate and silicate. Baltic Sea

Germany Photos: NERI/Jens Christian Pedersen

-l According to the deﬁ nition given in Baltic Sea, and the highest in open DIP (µmol l ) < 0.5 Chapter 1, eutrophication is caused by waters are found in the German Bight 0.5-0.7 enrichment of the water by inorganic of the North Sea. In the estuaries and 0.7-0.9 nutrients. In the Baltic water entering coastal waters the concentrations vary 0.9-1.1 Skagerrak the Danish straits both winter nitrate from lower concentrations similar to > 1.1 and phosphate concentrations have what is found in open waters to high Sweden about doubled from 1970 to the mid levels in estuaries with long residence

1980s (Nausch et al. 1999). The same times and high nutrient loads (Figure Kattegat is the case for water entering from the 2.12). The geographical variation of German Bight to Danish North Sea inorganic nitrogen nutrients (DIN =

waters with the Jutland Coastal Cur- NO3-N + NO2-N + NH4-N) is much The Denmark Sound rent (Hickel et al. 1995). In the Kattegat larger (range factor 70) than for phos- North Sea surface water the winter nitrate and phate (DIP = PO4-P) (factor 20). phosphate concentrations have in- The water in the Danish North Sea Figure 2.12 creased about 40% in the period 1971– is essentially a mixture of two water Baltic 1990 (Andersson 1996). masses: freshwater from the rivers to Sea Mean winter con- the southern North Sea and German centrations of DIN NUTRIENT CONCENTRATIONS Bight with high nutrient concentra- and DIP, 1998–2001, The lowest nutrient concentrations in tions, and central North Sea water in Danish marine Germany Danish waters are observed in the with high salinity (34–34.5) and low in waters.

36 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 37 nutrients. Therefore, the winter nutri- small compared to the Baltic outfl ow BOX 4 Indices for annual mean levels of nutrients ent concentrations generally show an to infl uence the salinity signifi cantly in inverse linear correlation to the salin- the Belt Sea and Kattegat. In Skagerrak ity (Figure 2.13). Likewise, the water many different water masses and mix- Annual mean levels of nutrient concentrations were calculated by means of 3-fac- in the Kattegat and Belt Sea essentially tures between them may be present: tor analysis-of-variance (ANOVA) after log-transformation. The 3-factor ANOVA is a mixture of Baltic Sea surface water Jutland Coastal Current water from the described variations between individual stations, years and months as categorical with low salinity (~8) and nutrient German Bight (salinity 30–33), central factors. The assumptions of the ANOVA were that seasonal and interannual vari- concentrations, and Skagerrak water North Sea water, North Atlantic water ations in nutrient concentrations were similar at all stations, deviating only by a with high salinity (34–35) and higher (salinity ~35), Kattegat surface water scaling factor. nutrient concentrations. However, the and locally infl uenced coastal waters The marginal distribution of yearly means were calculated and back-trans- winter nutrient concentrations in the (Figure 2.13). The water in the estuaries formed into original scale to provide yearly indices for the mean annual nutrient Kattegat and Belt Sea show a positive is generally a mixture between local concentration of all stations included in the analysis. The marginal yearly means deviation from a linear relationship in freshwater runoff and coastal seawater correspond to yearly averages if monitoring data was balanced, that is equal the salinity interval 10–25, due to local from outside the estuary. number of observations for each month, each year at each station. The yearly supplies of nutrients in the Belt Sea For assessing the development nutrient mean levels were calculated by means of PROC GLM in SAS/STAT. and Kattegat. Freshwater runoff is too of nutrient concentrations in Danish

40 12 waters indices for mean annual con- 80 10 centrations of DIN, TN, DIP and TP 30 60 8 in the upper mixed layer were devel- (µM) (µM) 40 20 (µM) 6 oped for estuaries-coastal waters and DIN DIN 4 the open Kattegat–Belt Sea, respec- 20 10 2 tively (see box 4). North Sea Skagerrak - Kattegat N Kattegat - Belt Sea 0 0 0 The indices for DIN and TN gener- 25 26 27 28 29 30 31 32 33 34 35 29 30 31 32 33 34 35 36 510152025 30 35 Salinity Salinity Salinity ally ﬂ uctuated around a constant level during the period 1989–2001, in both the Figure 2.13 50 30 estuaries–coastal waters and the open 80 25 Relations of DIN, DIP 40 Kattegat-Belt Sea (except for a weak 60 20 and DIN:DIP ratio to 30 decreasing tendency for DIN in open 15 salinity in February 40 waters). However, low concentrations

20 DIN:DIP DIN

DIN:DIP 10 2002 in the North DIN:DIP 20 were observed in the very dry years 10 5 Sea, Skagerrak and North Sea Skagerrak - Kattegat N Kattegat - Belt Sea 1996 and 1997, when the land-based 0 0 0 Kattegat-Belt Sea, 25 26 27 28 29 30 31 32 33 34 35 29 30 31 32 33 34 35 36 510152025 30 35 nitrogen load was about half of normal. respectively. The lines Salinity Salinity Salinity The indices for DIP and TP decreased in the North Sea plots signiﬁ cantly in the estuaries–coastal 1,6 1,2 1 are linear regression waters, but was less pronounced in the 1 lines, while the lines 1,2 0,8 0,8 open Kattegat–Belt Sea through the in the Kattegat-Belt 0,6 period 1989-2001 (Figure 2.14).

(µM) 0,8 0,6 (µM) Sea plots are theoreti- (µM) 0,4 Freshwater runoff is the most im- DIP 0,4 DIP cal mixing lines for 0,4 DIP 0,2 0,2 portant factor affecting nutrient con- mixtures of Baltic North Sea Skagerrak - Kattegat N Kattegat - Belt Sea centrations in Danish waters. In estu- 0 0 0 water and Skagerrak 25 26 27 28 29 30 31 32 33 34 35 29 30 31 32 33 34 35 36 510152025 30 35 aries and coastal waters the correla- Salinity Salinity Salinity water. tions between the indices for annual

38 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 39 Figure 2.14 14 14 1 1 Indices for annual 12 12 0,8 0,8 mean concentrations 10 10 8 0,6 0,6 of DIN, TN, DIP, TP and 8 6 6 0,4 0,4 the DIN:DIP ratio (left (µM) DIP DIN (µM) DIN 4 4 column), as well as the 2 0,2 0,2 2 Normalised DIP (µM) Normalised DIN (µM) DIN Normalised same indices corrected 0 0 0 0 for runoff variations 1989 1991 1993 1995 1997 1999 2001 1989 1991 1993 1995 1997 1999 2001 1989 1991 1993 1995 1997 1999 2001 1989 1991 1993 1995 1997 1999 2001 (right column). 95% conﬁ dence limits are 2 2 2,5 2,5 given for the uncorrec- 2 2 1,5 1,5 ted indices. Estuaries 1,5 1,5 and coastal waters 1 1 1 1 TP (µM) TP are shown with blue (µM) DIN 0,5 0,5 circles, and open 0,5 0,5 Normalised TP (µM) Normalised DIN (µM) DIN Normalised Kattegat-Belt Sea with 0 0 0 0 red triangels. 1989 1991 1993 1995 1997 1999 2001 1989 1991 1993 1995 1997 1999 2001 1989 1991 1993 1995 1997 1999 2001 1989 1991 1993 1995 1997 1999 2001

60 60 30 30

50 50 25 25 40 40 20 20

30 30 15 15

TN (µM) TN 20 20 10 10

10 Normalised TN (µM) 10 5 5 Normalised DIN:DIP ratio DIN:DIP Normalised 0 0 ratio DIN:DIP annual Mean 0 0 1989 1991 1993 1995 1997 1999 2001 1989 1991 1993 1995 1997 1999 2001 1989 1991 1993 1995 1997 1999 2001 1989 1991 1993 1995 1997 1999 2001

14 1989 0,8 1990 mean DIN and TN concentrations and and TP. However, all the annual mean 12

runoff were highly signiﬁ cant for the nutrient indices were corrected for 10 1998 0,6 Figure 2.15 8 1999 1991 (µM) period 1989–97 (Figure 2.15). The runoff variations according to the 2000 (µM) 0,4 Annual mean indices 6 1998 1999 DIP

DIN 2001 years 1998–2001 deviate from this, as correlations observed. 4 2001 2000 for DIN and DIP in 0,2 2 y = 0,0006x + 1,7292 y = 8E-06x + 0,3395 the DIN and TN concentrations in In the estuaries and coastal waters R2 = 0,9724 R2 = 0,4148 estuaries-coastal 0 0 Danish streams decreased during this the DIN and TN indices normalised 0 5000 10000 15000 20000 25000 0 5000 10000 15000 20000 25000 waters (upper row) period (Bøgestrand 2001). Likewise, to mean runoff show a significant Runoff (106m3) Runoff (106m3) and open Kattegat- signiﬁ cant correlations were found for decrease in the later years 1998–2001, Belt Sea (lower row) 1,6 0,35 DIP and TP in estuaries and coastal and the normalised DIP and TP indi- 1990 against runoff. The 1,4 0,30 waters for the period 1992–1997. The ces have reached a constant low level 1,2 1989 years 1998–2001 are 1998 0,25 1,0 1991 years 1989–1991 were omitted as sig- in the same years. In the open Katte- 1999 0,20 marked with green 0,8 2000 1998 (µM) niﬁ cant reductions in the phosphorus gat–Belt Sea the normalised indices 2001 (µM) 0,15 2000 squares, and the 0,6 2001 DIP DIN 0,10 point source load took place in these for DIN, DIP and TP show a slow de- 0,4 1999 years 1989–1991 y = 1E-06x + 0,1504 0,2 y = 7E-0,5x + 0,5005 0,05 2 years. In the open Kattegat–Belt Sea crease over the whole period 1989– R2 = 0,3482 R = 0,0103 are marked with red 0 0 significant correlations were also 2001 (Figure 2.14). 0 2000 4000 60008000 10000 12000 14000 0 2000 4000 60008000 10000 12000 14000 triangles in the DIP 6 3 6 3 found for DIN and TN, but not for DIP Runoff (10 m ) Runoff (10 m ) diagrams.

40 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 41 NUTRIENT RATIOS In the North Sea the N/P-ratio is ge- the open Kattegat-Belt Sea no general In estuaries phytoplankton pri- The optimal DIN:DIP ratio (N/P-ra- nerally high ranging between 25 and development is observed in the an- mary production is often potentially tio) for phytoplankton growth is 16:1 60, except in the saline central North nual DIN:DIP indices during the peri- limited by low phosphate concentra- (based on molar concentrations) and Sea. In the Skagerrak N/P-ratios are od 1989–2001. tions early in the productive season. is called the Redﬁ eld ratio. Signiﬁ cant also high at salinities lower than 33 In 2000 potential phosphate limita- deviations from 16 at low N/P-ratios (Figure 2.13 and 2.16). Generally the NUTRIENT LIMITATIONS tion in 6 estuaries extended from about might indicate potential nitrogen limi- winter N/P-ratio in estuaries is high A simple first order approach to 1 month in Horsens Fjord and Odense tation and at high N/P-ratios poten- (>25) to very high (>100) (Figure assess potential nutrient limitation is Fjord to 3 months in Skive Fjord and tial phosphorus limitation of phy- 2.16). to examine for time periods when Limfjorden and up to 6 months in Ring- toplankton primary production. This N/P-ratios based on annual mean nutrient concentrations are below the købing Fjord. While no potential phos- might affect the biological state of the DIN and DIP indices in the estuaries- theoretical half-saturation constant phate limitation was observed in Ros-

ecosystem, in particular the phyto- coastal waters show an increase from (Ks) for uptake and to compare the kilde Fjord. Potential co-limitation by plankton biomass, species compo- 1989 to 1998 parallel to the reduction stoichiometry to expected Redﬁ eld ra- low concentrations of both DIN and sition and eventually food web dy- in phosphorus load, and then a de- tios. While crude, this has been found phosphate was most pronounced in namics. crease to 2001 parallel to the decrease to be a robust approach to determine Horsens Fjord for a period of about 2 In the open Belt Sea and Kattegat in nitrogen load per unit runoff. In the which nutrient is most limiting. The months. The estuaries, except Ringkø-

the N/P-ratio based on winter DIN very dry years of 1996-97 with low ni- Ks values used are 2 µM for DIN, 0.2 bing Fjord, were potentially nitrogen and DIP concentrations is generally trogen load the uncorrected N/P-ratio µM for DIP and 2 µM for DSi (Fisher limited in late summer for one to two between 10 and 20 and thus does not indices were much lower than in et al. 1992) and Redﬁ eld ratio is 16 for months (Figure 2.17A) (Henriksen et al. deviate much from the Redﬁ eld ratio. neighbouring years (Figure 2.14). In DIN:DIP. 2001). The number of days with poten-

DIN:DIP < 25 25-50 50-100 100-200 Skagerrak > 200

Sweden

Kattegat

The Denmark Sound North Sea

Baltic Figure 2.16 Sea Monitoring of the Mean winter DIN:DIP open marine waters ratio 1998–2001 in Denmark is made in Danish marine on board R/V Gunnar Germany waters. Photo: NERI Thorson.

42 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 43 tial phosphate limitation has increased CONCLUSION 300 350 Co-limitation Co-limitation signiﬁ cantly in the estuaries since the Improvement of the ecological state of 250 Only DIP limitation Only DIP limitation early 1990s parallel to the decrease in the Danish marine waters calls for fur- Only DIN limitation Only DIN limitation 200 300 phosphorus load from land (Figure ther reduction in the nitrogen load, as 150 2.17). the primary production in the open 100 250 Both nutrient concentrations and Kattegat and Belt Sea is mainly nitro-

50 nutrient ratios suggest that DIN con- gen limited and the production in the 200 0 tinues to be the nutrient potentially estuaries is most often nitrogen lim- Days per year in 1989-1999 (mean) 250 most limiting to phytoplankton bio- ited during late summer and autumn. 150 mass in the Kattegat and Belt Sea Further reduction in the phosphorus

200 Number of days (Figure 2.17B). Phytoplankton were load to the open Kattegat and Belt Sea 150 100 mostly potentially co-limited by DIN will probably have insigniﬁ cant effect 100 and DIP concentrations or limited by as phosphorus is seldom the most limit- 50 DIN concentrations (Ærtebjerg et al., ing nutrient, and because the phos- Days per year in 2000 50 0 1998). Redfield ratios suggest that phorus contributions from internal Roskilde Odense Horsens Ring- Skive Lim- Fjord Fjord Fjord købing Fjord fjorden when phytoplankton were co-limited processes and neighbouring seas are St. 60 St. 17 St. 6489 Fjord St. 3727 St. 3702 0 St.1 1990 1992 1994 1996 1998 by DIN and DIP, that DIN is most of- large compared to the load from land ten the most potentially limiting nutri- (Rasmussen et al., in press). However, Figure 2.17 A Figure 2.17 B ent. Although phosphate was poten- in the estuaries phosphorus limitation The number of days with potential nutrient The number of days with potential nutrient tially limiting by itself only for limited of the primary production in spring limitation of the phytoplankton primary limitation in the productive season in the periods each year, the periods that the and early summer is pronounced and production given as mean for the period years 1989–1998 at station 925 in the open sea areas are co-limited by low has increased after the substantial 1989–1999 (upper panel) and in 2000 south-western Kattegat. (From HELCOM concentrations of DIN and DIP has phosphorus load reductions in the (lower panel) in 6 Danish estuaries: Roskilde 2002). signiﬁ cantly increased with time (Fig- early 1990s. Further reduction of the Fjord, Odense Fjord, Horsens Fjord, Ringkø- ure 2.18). Dissolved silicate concentra- phosphorus load to the estuaries bing Fjord, Skive Fjord and Limfjorden. tions are occasionally low enough (< 2 might further improve the environ- (From Henriksen et al. 2001). µM) to limit diatom populations. mental state.

100 100

80 80

60 60

40 40

20 20 Potential P-limitation (%) Potential N-limitation (%) 0 0 1989 1991 1993 1995 1997 1999 2001 1989 1991 1993 1995 1997 1999 2001

Figure 2.18 Indices for potential N limitation (left) and P limitation (right) of the phytoplankton primary production in the productive period March-September calculated as the probability of

obtaining DIN or DIP concentrations below the theoretical half-saturation constants (Ks) for uptake. Indices for estuaries-coastal waters shown with blue circles, and for open Kattegat- Belt Sea with red triangels.

44 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 45 2001 and with a long-term sampling This pattern of dominance has, how- PHYTOPLANKTON record of more than fi ve years. In 12 out ever, changed over time, in particular 2.5 of 17 areas the average phytoplankton in open sea areas. Here diatoms ac- AND HARMFUL ALGAL BLOOMS biomasses in 2001 were lower (8–61%) counted for <20% up to 40% of the than the long-term averages (Table total biomass from 1979 until 1998 2.5). while dinofl agellate contribution to bio- In 1999 and 2000 diatoms gener- mass increased from <23% during 1979 ally dominated the phytoplankton to 1985 to 28–65% after 1986. The in- carbon biomass with important but creasing importance of dinoflagel- Diatoms dominate somewhat smaller contributions from lates has been accompanied by redu- the phytoplankton dinoflagellates (especially in 1999) ced contributions from other groups, biomass with im- and other organisms (Figure 2.19). mainly nanofl agellates. Table 2.5 portant contribu- Year 2001 average tions from dino- phytoplankton bio- fl agellates and Geographical area Period 2001 biomass 2001 relative to masses and biomasses other organisms. µg C l-1 long- term average relative to long–term Sønderho, east 1990–2000 147 37 averages for the given Ringkøbing Fjord 1989–2000 45 8 time periods at all Nissum Bredning 1985–2000 117 25 Danish stations with Løgstør Bredning 1985–2000 133 4 quantitative phyto- Skive Fjord 1985–2000 255 10 plankton sampling Ålborg Bugt 1989–2000 67 43 and a history of more

Photo: Biofoto/N.T. Nicoll Photo: Biofoto/N.T. Hevring Bugt 1989–2000 147 15 than ﬁ ve years of Århus Bugt 1989–2000 113 10 sampling. Figures Phytoplankton are the base of pelagic tions under pristine conditions. There- Mariager Fjord 1989–1996 230 25 show percentage food webs in aquatic systems. In ad- fore present levels are generally com- Horsens Fjord 1989–2000 138 13 increase or decrease in dition, sedimentation of phytoplank- pared with background concentra- Vejle Fjord 1989–2000 176 14 biomass relative to ton provides an essential nutritional tions in offshore areas. For Danish wa- Kolding Fjord 1989–2000 405 10 long-term average. input to the benthic fauna. With gen- ters the offshore Skagerrak chlorophyll Little Belt, northern part 1989–2000 112 20 eration times ranging from <1 day to a concentration of 1.25 µg l-1 can be Gniben 1979–2000 41 61 Increase a few days phytoplankton respond applied. However, this concentration Little Belt, southern part 1989–2000 121 14 Decrease rapidly to changes in nutrient concen- represents the maximum concentra- Roskilde Fjord 1992–2000 44 40 trations. Therefore, phytoplankton tions for the growing season (spring- The Sound, northern part 1979–2000 35 31 Figure 2.19 have been included in the Danish mo- late summer) (OSPAR 2001). The OS- Contributions (%) nitoring programme since 1979 as PAR assessment criteria are not opera- of phytoplankton an indicator of the eutrophication sta- tional, and the Danish assessment has groups to annual tus. Phytoplankton are quantiﬁ ed as therefore been based on mean chlo- 100 100 average biomasses Open sea stationsDiatoms Estuaries and coastal stations carbon biomass determined from mi- rophyll a concentrations in March- 80 80 Dinoflagellates at sampling stations Other groups croscopy or indirectly as the concen- October. 60 60 in open sea areas tration of chlorophyll a, a pigment (left) and estuaries (%) 40 (%) 40 found in all autotrophic phytoplank- PRESENT LEVELS and coastal areas ton organisms. In 2001 the average phytoplankton 20 20 (right) of the Danish No data exists on phytoplankton biomass varied from 35 to 405 µg C l-1 0 0 waters in the Baltic Contribution to carbon biomass Contribution to carbon biomass 1980 1983 1986 1989 1992 1995 1998 2001 1980 1983 1986 1989 1992 1995 1998 2001 biomass or chlorophyll a concentra- at the 17 Danish stations sampled in entrance area.

46 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 47 BOX 5 Indices for phytoplankton parametres BOX 6 Adjustment of indices to climatic conditions

For each sampling station an average value is calculated for individual years and a Indices of chlorophyll a concentrations, primary production and Secchi depth were global average of all annual averages. The annual indices are calculated as: correlated with indices of climatic conditions (e.g. runoff, wind, irradiance and tem- • Index (year, station) = average (year, station) x 100/global average (station) perature) by multiple linear regression. The model was optimised with respect to • Subsequently a national index for a given year is calculated as the average of indices of climatic conditions (e.g. lag phase for runoff, irradiance during seasons indices for all stations that year. where nutrients were expected not to limit primary production) and time period • Only stations with a minimum of 5 years sampling with a frequency of >5 included in the regression. samples year-1 were included. Subsequently, indices adjusted for variations in climatic conditions were found as: Adjusted index = measured index – modelled value + 100. The adjusted indices represent predicted indices under identical climatic condi- In estuaries and coastal waters dia- of diatoms in these areas was 50% of tions and thus serve to illustrate variations over time due to other factors than dif- toms prominently dominated the phy- the biomass in the early 1980s. In estua- ferences in climatic conditions. toplankton (35–78%) since sampling ries and coastal areas no long-term was initiated in 1989 (Figure 2.19). The trend in diatom biomass was found contributions from dinoﬂ agellates have (Figure 2.20). varied between 16% and 46% and the Primary production in the open importance of other groups, mainly na- sea areas has shown an overall decline noﬂ agellates, increased from 1989 to with major year-to-year variations from 160 160 A D 1998. 1977 to 1997 (Figure 2.21). The subse- 140 140 Because diatoms dominate the quent increase in primary production 120 120 spring bloom, of which a large fraction during 1998–2001 may be due to a sediment to the bottom, they are im- reduction in the number of monitor- 100 100 Figure 2.21 Clorophyll index portant for the transfer of organic ing stations and changes in the sam- 80 Clorophyll index 80 Indices for Secchi

matter of pelagic origin to the benthic pling strategy in 1998. During 1993- 60 60 depth, chlorophyll fauna. In addition, the input of organic 2001 primary production was signiﬁ - 19771980 1985 1990 1995 20011977 1980 1985 1990 1995 2001 concentrations and matter to the water mass below the cantly correlated with runoff, wind annual primary pro- 160 160 duction in open sea pycnocline will affect the subsequent and temperature. The primary pro- B E development in bottom water oxygen duction index adjusted to changes in 140 140 areas. Indices are given for measured conditions. climatic conditions shows a lower pro- 120 120 The biomass of diatoms has de- duction during the 1990s than during values (A, B and C) 100 100 creased signiﬁ cantly over the past 20 the 1980s. Despite the apparent de- and adjusted for vari- years in the open Kattegat and Belt Sea. crease in primary production from the 80 80 ations in climatic con- Primary production index During 1995–2000 the average biomass 1980s to the 1990s, the annual prima- 60 Primary production index 60 ditions (D, E and F). 19771980 1985 1990 1995 20011977 1980 1985 1990 1995 2001 Adjustments for climatic variations are 400 400 160 160 Estuaries and coastal areas Open sea areas C F developed on data 140 140 from time periods 300 300 Figure 2.20 120 120 represented by Annual averages of 200 200 hatched lines and up 100 100 diatom biomass in till 2001. Subse- 100 100 80 80 Secchi depth index Danish waters. Bio- Secchi depth index quently the adjust- Diatom biomass (index) Diatom biomass (index) masses are given as 0 0 60 60 ments were applied 19771980 1985 1990 1995 20011977 1980 1985 1990 1995 2001 indices (see box 5). 19801983 1986 1989 1992 1995 1998 2001 19801983 1986 1989 1992 1995 1998 2001 on data in all years.

48 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 49 ry production in the Kattegat area has presumably due to reduced phospho- correlated signifi cantly with primarily a correlated signifi cantly with runoff increased 2- to 3-fold from the 1950s rus loading to the estuaries through irradiance during early spring and during 1993–2001. When adjusted for to 1984–1993, apparently as a result of the establishment of sewage treatment autumn. Chlorophyll a index values variations in climatic conditions, chlo- eutrophication (Richardson & Heil- plants in the late 1980s and early 1990s adjusted for variations in climatic rophyll a index values showed a very mann 1995). and subsequent reduction in the nitro- conditions were very variable prior to consistent decline from 1993 to 2001. In estuaries and coastal areas pri- gen load both from point and diffuse 1990 and showed a general slight in- In open sea areas the Secchi depth, mary production has decreased from sources. crease for the period 1990-2001. Since a measure of water transparency, has 1980 to 1997 and subsequently in- Chlorophyll a concentrations have 1980 the concentrations of chlorophyll increased since the mid-1980s (Figure creased during 1998-2001 (Figure 2.22). decreased in the open sea areas since the a in Danish open sea areas (Hansen et 2.21). The Secchi depth adjusted for For the period 1993-2001 primary pro- 1980s (Figure 2.21). However, mainly al. 2000) have been >50% above back- variations in climatic conditions has duction was signifi cantly correlated during the 1980s the year-to-year varia- ground concentrations (1.25 µg chl a l-1) increased despite a similar increase in with runoff, irradiance and tempera- tions have been substantial and pos- given by OSPAR (2001). chlorophyll concentration. In estuaries ture. Index values adjusted for varia- sibly related to the lack of standardised Chlorophyll a concentrations have and coastal areas the decline in chloro- tions in climatic conditions showed a sampling strategies and methods for decreased in estuaries and coastal phyll concentrations since the mid- very consistent decline after 1993. This analyses prior to 1989. For the period waters since the mid 1980s (Figure 1980s has been accompanied by in- decline in primary production was 1990-2001 chlorophyll a concentrations 2.22). The concentrations of chlorophyll creased Secchi depth (Figure 2.22).

160 160 A D 140 140

120 120

100 100

80 80 Chlorophyll index Chlorophyll index 60 60 19771980 1985 1990 1995 20011977 1980 1985 1990 1995 2001 Figure 2.22 Indices for Secchi 160 160 depth, chlorophyll B E 140 140 concentrations and annual primary pro- 120 120 duction in estuaries 100 100 and coastal waters. 80 80 Indices are given for Primary production index 60 Primary production index 60 measured values (A, 19771980 1985 1990 1995 2001 19771980 1985 1990 1995 2001 B and C) and adjust- Satellite image of ed for variations in 160 160 chlorophyll in Danish C F climatic conditions marine waters, the 140 140 (D, E and F). Adjus- North Sea and the 120 120 tements for climatic Baltic Sea 16 July variations are devel- 100 100 2002. Red colours oped on data from 80 80 Secchi depth index indicate high Secchi depth index 1993–2001, but concentrations of 60 60 applied on data in 19771980 1985 1990 1995 2001 19771980 1985 1990 1995 2001

chlorophyll. by GRAS A/S, Denmark Image: SeaWiFS Orbimage processed data from all years.

50 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 51 EXCEPTIONAL AND HARMFUL the commercial bivalves fishermen Species Year Month(s) Geographical Maximum Effects ALGAL BLOOMS and the mussel industry are undertak- area abundance Fish kills Several potentially toxic and harmful ing monitoring of toxic phytoplankton or biomass species of algae are common minor and algal toxins in bivalve shellfi sh in Chattonella spp. 1998, 2000, 2001 Jan-May Århus Bugt, 23 mill. l-1 Fish kills components of the phytoplankton com- all areas where shellfi sh are harvested. Danish west coast, munities in Danish waters. In some Areas may be closed for fi shing of shell- Skagerrak, Kattegat years, however, they form conspicuous fi sh if toxic algae are found in concen- Chrysochromulina 1988, 1992 May Kattegat, Belt Sea 100 mill. l-1 Fish kills, dead benthic blooms that may have severe effects trations above given limits (Bjergskov spp. invertebrates, macrophytes on other organisms or on the tourist et al. 2001) or if algal toxins are detect- affected and fishing industries (Table 2.6). ed in shellfi sh in concentrations above Dictyocha 1983, 1999 May-Jun Little Belt, Alssund 25 mill. l-1 Fish kills in maricultures Blooms of some species, e.g. Chryso- the limit for human consumption. Most speculum associated with bloom chromulina polylepis, have occurred registrations of shellfi sh containing Gymnodinium 1999 Aug-Dec Nissum Bredning 15 mill. l-1 None reported only once, while blooms of other spe- algal toxins in concentrations above chlorophorum W Kattegat cies are recurring, e.g. Karenia miki- limits are from the 1980s and the early Karenia mikimotoi 1 1968, 1981, 1997 Aug-Oct Skagerrak 8.6 mill. l-1 Dead benthic invertebrates motoi and Chattonella sp. The latter was 1990s and from the eastern coast of Danish west coast, found in high concentrations for the Jutland (Figure 2.23) where the fi shed Kattegat, Belt Sea fi rst time in 1998. shellfi sh amounts to only one third of Nodularia 1975, 1976, 1992, Jul-Aug Århus Bugt, 869 µg C l-1 Dogs dying after swimming Potentially harmful species are re- the catches in the Limfjorden in north- spumigena 2 1994, 1995, 1997, W Kattegat, Belt in the water gistered and quantifi ed in the national ern Jutland. 1999, 2001, 2002 Sea, W Baltic monitoring programme. In addition, Phaeocystis sp. Yearly, 2000 May-Jun Danish west coast Foam on beaches Prorocentrum 1983, 1984, 1987, Jul-Oct Most Danish 53 mill. l-1 None reported minimum 1989, 1992, 1993, estuaries and 1994, 1995, 1996, coastal areas Skagerrak 1997, 1999 DSP -1 PSP Pseudo-nitzschia 1992, 1999, 2000 May-Nov Kattegat, Belt Sea 346 µg C l None reported ASP Figure 2.23 1987 1987 spp. SW Kattegat Registered accumu- 1988 Danish west coast lations in shellfi sh of 1989 algal toxins in con- 1987 Kattegat centrations above 1988 Table 2.6 limits for human con- 1990 Blooms of potentially toxic phytoplankton 1 = Gyrodinium aureolum sumption. DSP, 1997 Denmark species or exceptional blooms in Danish 2 = Maximum biomass given was total bio- 1986 Diarrhetic Shellfi sh 1988 waters. Years with major blooms are given, mass of all cyanobacteria including Apha- Poison; PSP, Paralytic 1989 The and the most signifi cant ones marked in nizomenon sp. and Anabaena sp. registered 1996 1992 Sound Shellfi sh Poison and bold. Months indicate the most frequent in the national monitoring programme. North Sea 1992 1991 1989 ASP, Amnesic Shell- 1994 time of year for blooms. Maximum abun- Locally, much higher biomasses may have 1993 fi sh Poison. ASP 1991 1989 dance and effects refer to the most extreme occured in accumulations along the shore. 1994 detected in 1993 was 1994 1992 episodes registered. analysed by a single 1991 1992 1992 1992 laboratory and has 1994 Baltic 1999 Sea not been confi rmed 1992 by other laboratories. (From Bjergskov et al. 2001).

52 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 53 of protozooplankton, rotiferans, mero- monitoring period several estuaries ZOOPLANKTON plankton and cladocerans is highest in like Roskilde Fjord, Kertinge Nor and 2.6 the estuaries and decreases gradually Limfjorden have suffered from occa- towards the open sea. Conversely, the sional blooms of gelatinous zooplank- importance of copepods increases ton such as the jellyfi sh Aurelia aurita towards the sea. (Frederiksborg Amt & Roskilde Amt During the period 1983-96 the 2002, Fyns Amt 2002). The jellyfi sh winter biomass of mesozooplankton graze on the mesozooplankton, which was generally low (2-5 mg C m-3) in in turn leads to lowered grazing on Copepods have an the open waters, increasing to a sea- the phytoplankton. In Kertinge Nor it increased impor- sonal summer peak of about 30-40 mg has been shown that high abundance tance towards the C m-3. Inter-regional differences were of jellyfi sh reduces the importance of sea while the rela- minor, also with respect to species zooplankton as grazers on phytoplank- tive importance of composition. An analysis of copepod ton to almost negligible levels (Peter- protozooplankton biomass data revealed a signifi cant sen et al. 1999). Moreover, jellyfi sh are and meroplankton but only slight reduction in the meso- suspected to affect the recruitment of is highest in the zooplankton biomass from 1989 to fi sh negatively, both by grazing on fi sh estuaries. 1997 at two out of three Kattegat sta- eggs and larvae and by affecting the tions (Ærtebjerg et al. 1998).

In the estuaries the composition 750 and seasonal succession of the zoo- Ciliates

plankton community is much more 500 -2

Photo: Biofoto/N.T. Nicoll Photo: Biofoto/N.T. variable than in the open sea. Both seasonal successions and time trends mg C m 250 The zooplankton are an important link phication, zooplankton have been given differ markedly among the estuaries

between the phytoplankton and higher relatively low priority in the Danish monitored. In the Limfjord the meso- 0 trophic levels such as fi sh. Zooplank- monitoring programme with respect zooplankton biomass has increased JFMAMJJASONDJ ton constitute a diverse group of organ- to the number of sampling stations signifi cantly since 1985, while the bio- 3000 isms with respect to size, life cycle and and sampling frequency. From 1998 mass of protozooplankton has decreas- Heterotrophic dinoflagellates 2500 behaviour. The smallest size class, zooplankton was given even lower pri- ed, though not signifi cantly. In Ring- 2000 nanozooplankton (2-20 µm), are single ority in open waters, but is still inclu- købing Fjord the biomass of mesozoo- -2 celled organisms. The larger size class, ded in the monitoring of many coastal plankton has decreased signifi cantly 1500 the microzooplankton (20-200 µm) are waters. since 1989 while the protozooplank- mg C m 1000 dominated by ciliates and heterotro- In general the diversity of the zoo- ton biomass has increased, though not 500 Figure 2.24 0 phic dinofl agellates, small metazooans plankton community decreases with signifi cantly. In Roskilde Fjord no time JFMAMJJASONDJ Seasonal variation in as rotiferans and the earliest develop- salinity from the North Sea to the trend was observed in the biomass of biomass (mg C m-2) mental stages of copepods. The largest Baltic Sea and from the open sea to the either meso- or protozooplankton. The 2000 of ciliates, hetero- Mesozooplankton Copepods size class, mesozooplankton (> 200 low salinity estuaries. However, diver- lack of general inter-fjord time trends Cladocera trophic dinofl agel- 1500 lates and mesozoo- µm), consist of copepods, cladocerans sity is high in the Kattegat and the Belt is probably due to the effect of local -2 and larvae of benthic invertebrates.The Sea due to the infl uence of both brack- conditions. 1000 plankton in the

more conspicuous jellyfi sh often domi- ish and Atlantic species as a result of Eutrophication is believed to cause mg C m southern part of nates the gelatinous plankton. the infl ow of water from the Baltic an increase in the relative importance 500 Kattegat. (Modifi ed

Due to the doubtful usefulness of Proper and the Skagerrak-North Sea, of gelatinous zooplankton vs. crusta- 0 from Nielsen & zooplankton as an indicator of eutro- respectively. The relative importance cean zooplankton. Throughout the JFMAMJJASONDJ Hansen 1999).

54 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 55 feeding conditions for fi sh larvae and zooplankton biomass, but due to par- (Turner et al. 1988) and thus plays a corded in Koster Fjorden (Sweden) in planktivorous fi sh by causing a de- thenogenesis they can increase rapidly different role in the pelagic food web 1997. In 2001 P. avirostris was observed crease in the abundance of prey organ- in numbers when growth conditions than the other marine cladoceran in relatively low abundances of about isms such as copepods. Jellyfi sh them- are favourable and occasionally they species. Due to this P. avirostris may 100 individuals m-3 in the Kattegat re- selves are in a sense a trophic dead become quantitatively important graz- be an important link between bacte- gion, but in August-September 2002 end. Energy and organic matter that ers. The two dominating genera Evadne rioplankton and higher consumers it was among the dominating meso- could otherwise be channelled into and Podon are believed to graze main- because of its predation on bacterivo- zoopankton species in the Sound with harvestable organisms is turned into ly on large phytoplankton cells and on rous fl agellates. P. avirostris was re- abundances up to 4000 individuals non-utilisable jelly. protozooplankton (Egloff et al. 1997). ported in the North Sea as early as m-3 (Per Juel Hansen, pers comm). P. In the autumn 2001 a new clado- 1948 and since 1999 it has been a avirostris has the capacity to quickly THE CLADOCERAN PENILIA ceran species Penilia avirostris was steady component of the zooplankton build dense populations and signifi - AVIROSTRIS: AN ADDITION observed in plankton samples from community in the southern and east- cantly infl uence the food web struc- TO THE DANISH FAUNA Århus Bight and Kattegat (Ærtebjerg ern parts of the North Sea, typically ture and the fate of the primary pro- Cladocerans have their main distribu- et al. 2002). This species has its main in September/October. Since the fi rst duction. Therefore it is important to tion in freshwater habitats and the distribution in tropical and subtropi- record in the North Sea it has spread follow closely the occurrence and de- number of species in Danish marine cal seas, where it at times dominates progressively northward. For example velopment of this new, invasive spe- water is limited. Usually they consti- the mesozooplankton biomass. It feeds it was found around Helgoland (Ger- cies which is probably well establis- tute only a minor fraction of the meso- mostly on nanoplankton (2-20 µm) many) in the early 1990s and was re- hed in Danish waters.

Figure 2.25

The size of zooplank- ): MBL/Per Juel Hansen The cladoceran ton varies from single Penilia avirostris, cell microzooplankton which is probably to large organisms as Penilia avirostris now established Photo:High-light jelly ﬁ sh. Photo: Ocean Photo, Norway/Erling Svensen Photo ( in Danish waters.

56 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 57 tions. Therefore, it is difﬁ cult to assess, runoff and load as in 1996-1997 cor- OXYGEN DEPLETION if a particular oxygen depletion inci- responded to higher oxygen levels 2.7 dent is due to increased consumption than modelled (Figure 2.27). Hansen et rate or decreased supply rate of oxy- al. (1995) made a scenario analysis of gen, and thus to relate the oxygen con- effects of reduced nitrogen input on centrations to nutrient loads. oxygen conditions in the Kattegat and

Area Period Annual change Reference The “cloud” is sul- Kattegat 1971-82/90 -0.05-0.1 ml l-1 Andersson & Rydberg 1988, 1993 phur in the water Kattegat NE 1970-95 -0.2% saturation HELCOM 1996 due to hydrogen 1975-88 -1.16% saturation Agger & Ærtebjerg 1996 sulphide released Kattegat S 1975-88 -1.17-1.41% saturation Agger & Ærtebjerg 1996 Table 2.7 from sediments The Sound 1970-95 -0.5% saturation HELCOM 1996 Published statistical during a period of 1975-88 -1.10% saturation Agger & Ærtebjerg 1996 signiﬁ cant results oxygen depletion. Great Belt 1974-95 -0.7% saturation HELCOM 1996 of analyses of the Little Belt S 1976-97 Decrease Ærtebjerg et al. 1998 development in Fehmarn Belt 1975-97 Decrease Ærtebjerg et al. 1998 bottom water oxygen 1979-93 -0.5% saturation HELCOM 1996 concentrations late 1975-88 -2.97% saturation Agger & Ærtebjerg 1996 summer – autumn in Kiel Bight 1957-86 -0.10-0.11 ml l-1 Babenerd 1991 the Kattegat-Belt Sea 1976-90 -0.15 ml l-1 Weichart 1992 from the 1970s to

Photo: Fyn County/Nanna Rask late 1980s/1990s.

Nutrient enrichment/eutrophication In the period 1989–2001 no general Therefore, oxygen level in the bot- Belt Sea. They concluded, that the na- may give rise to an increased rate of development in the summer-autumn tom layer (20–40 m) in the southern tionally and internationally agreed ni- oxygen consumption, decreased oxy- bottom water minimum oxygen con- Kattegat and Belt Sea was modelled trogen load reductions would signifi - gen concentrations and an increased centration was observed. However, a as a function of a temperature depen- cantly improve the oxygen situation, frequency of oxygen depletion. In Den- tendency for a rise in minimum oxy- dent gross oxygen consumption rate, but to reach the oxygen level in the mark oxygen depletion is defi ned as gen concentration in spring (April- water temperature and residence time. 1950s it would probably be necessary oxygen concentrations below 4 mg l-1 June) has been found (Hansen et al. The later defi ned as the time since the also to lower the atmospheric nitrogen (2.8 ml l-1), and severe (acute) oxygen 2000, Ærtebjerg et al. 2002). bottom water advected below the pyc- deposition. depletion as below 2 mg l-1 (1.4 ml l-1). Oxygen depletion only occurs in nocline. The residuals between mod- Most of the Danish estuaries-coa- The observed oxygen concentrations in stratifi ed water columns where strati- elled and observed oxygen saturation stal waters are shallow, and the wind September 2001 and 2002 are shown in fi cation prevents oxygen-rich surface was interpreted to arise from inter- can easily mix the water column to the Figure 2.26. waters to mix to the bottom. The oxygen annual variations in the availability of bottom. Stratifi cation only occurs perio- Analyses of long term develop- concentration close to the sea bottom organic matter for respiration (Hen- dically during calm and warm periods ment in bottom water oxygen concen- in stratifi ed waters depends on two riksen et al. 2001). In the period 1982– building up a thermocline, or by infl ow trations during late summer – autumn processes each varying in time and 2000 the residuals (mean May–Sept.) of saline bottom water creating a tem- in the Kattegat-Belt Sea from the 1970s space: the consumption rate, which is correlated signifi cantly to the runoff porary halocline. In a detailed study to late 1980s/1990s (Table 2.7) show mainly dependent on the supply of and N-load in the previous hydrologi- of a shallow Danish estuary (Skive signifi cant decreases in all areas with organic matter and the temperature, cal year (June–May). High runoff and Fjord/Lovns Bredning) it was possible a stratifi ed water column, especially and the oxygen supply rate, which is load generally corresponded to lower to separate the effect of meteorological from the mid 1970s to the late 1980s. mainly dependent on wind condi- oxygen levels than modelled, and low forcing (wind, solar radiation) and

58 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 59 nutrient load on oxygen depletion in 10 years meteorological data and an A September 2001 bottom water. During summer peri- empirical model, it was calculated that No oxygen depletion ods of stratiﬁ cation oxygen saturation a 25% reduction in nitrogen loading 2-4 mg l-1 0-2 mg l-1 could be attributed to the time elapsed would reduce the number of days after the onset of stratiﬁ cation and the with severe oxygen depletion (<15% accumulated nitrogen loading 10 saturation) by more than 50% (Møh- Skagerrak months prior to measurement. Using lenberg 1999). Sweden 20 20 1996 2 1996 2 1997 R = 0,26, P<0,03 1997 R = 0,32, P<0,02 15 15 Kattegat 10 10 5 5 North Sea 1994 1994 The 0 0 Denmark Sound -5 -5 1988 1988 1987 -10 -10 1995 1989 1995 -15 Residuals (% saturation) -15 Residuals (% saturation) 4500 6500 8500 10500 1250025.000 45.000 65.000 85.000 Runoff, mio. m3/hydrological year TN load, tonnes/hydrogical year Baltic Sea Figure 2.27 Sulphur formed by Residuals between modelled and obser- hydrological year (June-May). The two oxidation of hydro- ved oxygen saturation (mean May-Septem- hydrological years with lowest and the gen sulphide released Germany ber per year) as a function of (A) the runoff, three hydrological years with highest from the sediment in and (B) the N-load during the previous runoff are indicated. Odense Fjord.

B September 2002 No oxygen depletion 2-4 mg l-1 0-2 mg l-1

Skagerrak

Sweden

Kattegat

North Sea The Denmark Sound Figure 2.26 Oxygen depletion in Danish and neighbouring waters Baltic within September Sea 2001 (A) and 2002 (B). Swedish and German monitor- Germany ing data are included. Photo: Fyn County/Søren Larsen

60 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 61 OXYGEN NITROGEN DEGRADATION OF ORGANIC The sediment oxygen consumption The fl ux of nitrogen (and of phospho- 2.8 describes a typical bell-shaped curve, rus, for that matter) between the ma- MATTER IN ESTUARINE SEDIMENTS the lowest values being found in the rine sediment and the bottom water winter months and maximum values is governed by differences in concen- from May to September (Figure 2.28). trations between pore water (within This variation corresponds closely to the sediment) and water column. the variation in water temperature. Thus, sediments will always take up + - An analysis of the correlation be- NH4 or NO3 when water-column con- Photosynthetic tween temperature and oxygen concentrations of the respective N com- microalgae living sumption shows that temperature pounds are greater than porewater in sunlit surface explains 55% of oxygen consumption concentrations, and accordingly re- sediments can (R2=0.5467, n=644, p<<0.0001). For lease nitrogen to the bottom water produce small the remaining part, the explanation when nitrogen porewater concentra- oxygen bubbles. probably lies in the amount of organic tions are higher. matter reaching the sea fl oor during spring and summer. Fig 2.28 Box-Whiskers plot of the seasonal variations in sediment oxygen consumption/respira- 100 Oxygen consumption tion expressed as the numerical magnitude of the O2 fl ux measured in darkness. The

-1 75 Photo: NERI/Peter Bondo Christensen d dataset is a compilation of observations -2

m 50 2 from 6 estuaries each represented with 3 In marine sediments, organic matter the exact amount of oxygen that stations sampled 16 times over a two-year 25 is mainly degraded through bacterial would have been used if the entire mmol O period. Median (line), 25% and 75% quan- Sheets of white processes, by which N and P bound degradation process had been aerobic. 0 tiles (boxes) and 10% and 90% quantiles sulphur bacteria in organic compounds as a result of Therefore, a measurement of the J FMAM J J A S OND (bars) are shown. on a mussel bed. primary production are released once amount of oxygen consumed within again. the sediment in darkness will cor- Up to half of the bacterial degrada- respond quite closely to the total tion taking place in the sediments metabolism of organic matter going proceeds through oxygen respiration on within the sediment, since this (i.e. aerobically). The remaining de- measurement is the sum of aerobic gradation takes place anaerobically and anaerobic degradation. through respiration of nitrate, iron, manganese or sulphate (Jørgensen 1996). + Besides the release of CO2, NH4 3- and PO4 , anaerobic degradation results in the formation of waste products (such as hydrogen sulphide from sulphate respiration) that are ultimately oxidised, thereby consuming Photo: Fyn County/Nanna Rask

62 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 63 Figure 2.29 4 From January to May, estuarine se- The fl ux of nitrogen from the sedi- PHOSPHORUS NO - flux A Box-Whiskers plot of 3 diments take up dissolved inorganic ment to the water column continues Through mineralisation, phosphorus the seasonal variations -1 2 nitrogen (DIN=NH ++NO -), primarily into autumn; but gradually NO - be- is released as inorganic phosphate d 4 3 3 - -2 - 3- in the sediment NO3 0 in the form of NO3 (Figure 2.29). During comes dominating in the release of (PO4 ) (Eq. 1). Contrary to nitrogen + fl ux (A), NH4 fl ux (B) this period, the water-column concen- nitrogen (Figure 2.29 A and B). During compounds, a considerable amount of 3- -2 - and PO4 fl ux (C). mmol N m tration of NO3 is high. Therefore, the autumn, the sediment oxygen demand phosphate is bound more or less per- Positive values indicate sediments NO - uptake may be a result decreases (Figure 2.28), and this causes manently within the sediment, while -4 3 net release, and neg- J FMAM J J A S OND of a lower pore-water concentration of the oxygen content of the sediment to a smaller fraction is found dissolved - ative values indicate NO3 , perhaps governed by denitrifi ca- increase. This means that nitrifi cation in pore water. consumption. The + - NH4 flux B tion activity and a high NO3 assimila- gains more and more importance, and 3 + dataset is a compilation by active benthic microalgae. Not that more and more NH4 is oxidised Eq. 1: tion of observations -1 until the summer months (June, July to NO - (nitrifi ed) within the sediment. d 1 3 -2 + 3- from 6 estuaries each and August) does the estuarine sedi- As benthic microalgae only assimilate 106CO2 + 16NH4 + PO4 + 106H2O represented with 3 -1 ment release nitrogen in the form of insignifi cant amounts of NO - during 3 Primary Bacterial + stations sampled 16 mmol N m NH (Figure 2.30). Low water-column autumn because of lowered activity production -3 4 degradation - - times over a two-year NO3 concentrations and a high sedi- (see Figure 2.29 A), NO3 accumulates J FMAM J J A S OND + 3- period. Median (line), ment oxygen demand resulting in less within the sediment, even though (CH2O)106(NH4 )16 (PO4 )+53O2 25% and 75% quan- favourable oxygen conditions cause some denitrifi cation is taking place, 1.5 tiles (boxes) and 10% 3- both nitrifi cation and denitrifi cation to and eventually diffuses from the sedi- It is the pool of oxidised iron (Fe ) PO4 flux C ox and 90% quantiles decline markedly or stop altogether. At ment, since water-column NO - con- that binds part of the phosphate found 1 3 (bars) are shown. -1 the same time, NO - assimilation ceases. centrations, at least until November, in estuarine sediment (Jensen & Tham-

d 3 -2 0.5 + Thus, in the summer months, NH4 are low. drup 1993). Good oxygen conditions constitutes the major part of the DIN 0

mmol P m ﬂ ux from sediment to water column. -0.5 J FMAM J J A S OND Figure 2.31 Figure 2.30 Internal phosphorus Internal nitrogen sup- 10 5 supply in 2000 at 17 ply in 2000 at 17 sta- Annual flux (2000) 4 Annual flux (2000) stations representing Summer flux (June-August) Summer flux (Juni-August) tions representing 6 3 6 types of estuaries, types of estuaries, 5 Roskilde Fjord, 2 Roskilde Fjord, Odense Odense Fjord, Hor- 1 ) ) -2 Fjord, Horsens Fjord, -2 sens Fjord, Ringkøbing 0 0 Ringkøbing Fjord, Lim- Fjord, Limfjorden and (g P m (g N m -1 fjorden and Skive Skive Fjord, shown -2 Skive Fjord Fjord, shown for the Skive Fjord for the entire year -5 r Bredning) ø entire year (annual r Bredning) -3 (annual ﬂ ux) and for gst ø bing Fjord (Sed2) bing Fjord (Sed2) bing Fjord (Sed1) ø bing Fjord (Sed6) ø ø Roskilde Fjord (60) Roskilde Fjord (65) Roskilde Fjord (60) Roskilde Fjord (65) ø ø gst -4 Odense Fjord (804) Odense Fjord (803)

ﬂ ux) and for the ø the months of June, Horsens Fjord (6089) Horsens Fjord (6592) Horsens Fjord (5790) Horsens Fjord (6592) Roskilde Fjord (8500) Roskilde Fjord (8500) bing Fjord (Sed1) Odense Fjord (804) bing Fjord (Sed6) Odense Fjord (803) ø ø

-10 Odense Fjord (6910008)

-5 Ringkj Ringkj months of June, July Ringkj July and August Ringkj Horsens Fjord (6089) Limfjorden (Nibe Bredning) Limfjorden (Nibe Bredning) Limfjorden (Kaas Bredning) Limfjorden (Kaas Bredning) and August (summer Ringkj (summer ﬂ ux). Posi- Ringkj Limfjorden (Thisted Bredning) Limfjorden (Thisted Bredning) Limfjorden (L Horsens Fjord (5790)

ﬂ ux). Positive values Limfjorden (L tive values indicate indicate net release, net release, and ne- and negative values gative values indicate Odense Fjord (6910008) indicate consumption. consumption.

64 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 65 + within the sediment increases the Feox NH4 release to the water column, and, a net release of phosphorus took place tion in the sediment (the internal sup- 3- pool, which is able to retain either as described above, the release of PO4 during the summer months at all stati- ply) may stimulate primary produc- phosphate or hydrogen sulphide is caused by poor oxygen conditions ons except three (Figure 2.31), and, on tion in the estuaries at this time of (through a reaction forming ferrous within the sediment that stimulate average, more than 75% of the total an- year. 3- sulphide; FeS – see Eq. 2). On the other hydrogen sulphide production, so to nual PO4 release to the water column The internal and external supplies - + hand, poor oxygen conditions cause speak, leading to increased consump- took place during this period. of nitrogen (NO3 +NH4 ) and phos- 3- the pool of oxidised iron to grow tion of oxidised iron. It is in the summer months that phorus (PO4 ) to four estuaries repre- gradually smaller. As the Feox concen- water-column concentrations of nitro- senting four different types of area, tration decreases, phosphate is re- INTERNAL NUTRIENT LOAD gen and phosphorus are low. Thus, Roskilde Fjord, Odense Fjord, Hors- leased from the sediment to the water Nation-wide observations of nutrient growth of algae within the estuaries ens Fjord and Ringkøbing Fjord, in the column. fl uxes in Danish estuaries from Ros- depends on a steady supply of nutri- period June-August 2000 are shown in kilde Fjord in the east to Ringkøbing ents to the water column from land Table 2.8, assessed as the total supply Eq. 2: Fjord in the west show that during the (streams etc.), atmosphere, adjacent to the entire estuary. It is evident that summer of 2000 (June-August) nitro- seas or through degradation of or- fl uxes from the sediment of both nitro- 3- 3- Feox -PO4 + H2S FeS +PO4 gen was released from the sediment at ganic matter in the water column or gen and phosphorus contributed signi- 14 of 17 stations (Figure 2.29 C). At the sediment. In June, July and August fi cantly to the nutrient supply reaching In the “typical” estuary the flux of almost half of the stations an annual 2000, the total nutrient supply from the estuaries in the summer of 2000. 3- PO4 is insignifi cant in spring, autumn net release of nitrogen from the sedi- land constituted 10% of the annual In this period, between 36 and 93% of and winter (Figure 2.31). Signifi cant ment to the water column was seen in supply. Thus, a relatively low external the supply of phosphorus to the estu- 3- PO4 release from the sediment to the 2000, and the fl ux of nitrogen during nutrient supply during summer means aries came from the sediment, and in water column is seen only in June, July the summer months contributed most that release of nitrogen and phospho- the case of nitrogen, the internal sup- and August. This period coincides with to the total net fl ux that year. Likewise, rus through organic matter degrada- ply was almost as large (10-78%).

Table 2.8 Internal vs. external supply of N and P to four types of estuary in the period June- August 2000

Nitrogen Phosphorus Internal External Total Internal Internal External Total Internal tonnes tonnes tonnes % tonnes tonnes tonnes % Roskilde 387,1 107,8 494,9 78% 81,7 8,3 90,0 91% Odense 17,3 152,6 169,9 10% 7,2 6,5 13,7 53% Horsens 39,7 108,0 147,7 27% 43,1 3,2 46,3 93% Ringkøbing 263,8 884,8 1.148,7 23% 10,3 18,4 28,7 36% Photo: NERI/Ole Schou Hansen

66 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 67 photos from the archives of KMS (The sion (Olesen 1993, Boström et al. in SUBMERGED National Survey and Cadastre Agency) press). Comparisons of eelgrass area 2.9 covering the period 1945–1990s and distribution in two large regions, Øre- AQUATIC VEGETATION data from the national Danish moni- sund and Limfjorden, in 1900 and in toring programme on marine vegeta- the 1990s, suggest that the present tion covering the period 1989–2001. distribution area of eelgrass in Danish coastal waters constitutes approxi- LONG-TERM CHANGES mately 20–25% of that in 1900. The area IN EELGRASS DISTRIBUTION distribution of eelgrass in Limfjorden Seagrasses In Denmark, records of eelgrass distri- was estimated at 345 km2 in 1900 and macroalgae bution date back to around 1900, and (Ostenfeld 1908) and only at 84 km2 in enhance bio- provide a unique opportunity to de- 1994 (based on aerial photography data diversity by scribe long-term changes. In 1900, eel- from the Limfjord counties). In Øre- providing habitat grass was widely distributed in Danish sund, eelgrass covered about 705 km2 and shelter for coastal waters, and covered approxi- in 1900 (Ostenfeld 1908) and only many species. mately 6726 km2 or 1/7 of all Danish about 146 km2 in 1996–2000 (Krause- marine waters (Petersen 1901, 1914) Jensen et al. 2001). Differences in me- (Figure 2.32). The standing crop made thodology may, however, influence up almost 1 kg dw m-2 in the densest these comparisons. stands and the total annual eelgrass The large reduction in area distribu- production was estimated at 8 million tion of Danish eelgrass meadows is tonnes dry weight (Petersen 1914). In partly attributed to loss of deep popula- Photo: NERI/Peter Bondo Christensen Photo: Biofoto/S.E. Sørensen the 1930s, the world wide wasting ditions. In 1900, colonisation depths ave- Photo: NERI/Peter Bondo Christensen sease substantially reduced eelgrass raged 5–6 m in estuaries, and 7–8 m in Eelgrass (Zostera marina) is the most reducing particle loads and absorbing populations, especially in north-west open waters while in the 1990s, coloni- widely distributed marine angiosperm dissolved nutrients; they stabilise se- Denmark where salinity is highest sation depths were reduced about 50% in shallow Danish coastal waters. On diments and infl uence global carbon (Blegvad 1935). In 1941, eelgrass cov- to 2-3 m in estuaries and 4–5 m in open hard substrates the vegetation is domi- and nutrient cycling (see Hemminga ered only 7% of the formerly vegetated waters (Figure 2.33) (Boström et al. in nated by macroalgae. Growth of short- & Duarte 2000). The many ecosystem areas and occurred only in the south- press). The deep populations are most lived nuisance species of macroalgae services provided by eelgrasses have ern, most brackish waters and in the likely lost as a consequence of eutrophi- is a problem in inner parts of some created a concern for the response of low saline inner parts of Danish estuar- cation. Increased nitrogen concentra- estuaries. this species to eutrophication. This ies (Lund 1941, Rasmussen 1977)(Figure tions stimulate phytoplankton growth concern has been intensifi ed over the 2.32). No national monitoring existed and thereby reduce the transparency of EELGRASS IN COASTAL WATERS last few decades where large-scale between 1941 and 1989 but analyses of the water column and restrict the co- Eelgrass and seagrasses in general reductions in seagrass meadows have aerial photos from the period 1945– lonisation depth (Nielsen et al. 2002). often represent a large standing bio- been reported in response to eutrophi- 1990s, reveal an initial time lag of more mass and a high primary production cation from many areas (see Short & than a decade before substantial recolo- RECENT INTER-ANNUAL FLUCTUA- that influence the overall functio- Wyllie-Echeverria 1996). nisation of the shallow eelgrass popu- TIONS IN EELGRASS DISTRIBUTION ning of coastal ecosystems. Seagrass- The following paragraphs sum- lations began. The photos also show Since 1989, the Danish Aquatic Moni- es enhance biodiversity by providing marise the development of Danish that large populations had recovered toring and Assessment Programme habitat and shelter for many species; eelgrass meadows during the last cen- in the 1960s (Frederiksen et al. subm.). has included annual surveys of coloni- they are nursery and foraging areas tury. The presentation is based on Today eelgrass again occurs along sation depth and cover of eelgrass for commercially important species of early reports on eelgrass distribution most Danish coasts (Figure 2.32) but along depth gradients in a wide range fi sh; they improve water quality by covering the period 1900-1940, aerial has not reached the former area exten- of estuaries and coastal waters.

68 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 69 1901 1941 1900 1900 Figure 2.33 Skagerrak Skagerrak 30 30 Maximum colonisation depth of Danish eel- 20 20 Sweden Sweden grass patches along 10 10 open coasts and in

Frequency (%) Frequency (%) estuaries in 1900 and 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 123456789101112131415 16 1996/1997. Based on Kattegat Kattegat Colonisation depth (m) - along open sea coast Colonisation depth (m) - in estuaries data from 18 sites North North Sea Denmark Sea Denmark along open coasts and The The 199750 1997 12 sites in estuaries Sound Sound 30 40 investigated by Osten-

30 feld (1908) in 1900 20 and by the Danish 20 Baltic Baltic 10 Aquatic Monitoring

Sea Sea Frequency (%) 10 Frequency (%) and Assessment Pro- Germany 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 123456789101112131415 16 gramme in 1996/1997 Germany Colonisation depth (m) - along open sea coast Colonisation depth (m) - in estuaries (Boström et al. in press). 1933 1994 Skagerrak Skagerrak The colonisation depth of eelgrass Open coasts reﬂ ects differences in water quality 120 and physical setting along estuarine Sweden Sweden 110 gradients. In the inner parts of estuar- 100 ies, the annual mean colonisation depth ranged between 2.9 and 3.5 m, in outer 90 Depth limit index (%) Kattegat Kattegat parts between 3.3 and 4.3 m, and along 80

North North open coasts between 4.7 and 5.8 m du- 1989 1991 1993 1995 1997 1999 2001 Denmark Denmark Sea Sea ring the period 1989–2001 (Figure 2.34). Figure 2.34 The The Outer estuaries Sound Sound Eelgrass colonisation depth shows no Index of maximal eel- signiﬁ cant trend between 1989 and 120 grass colonisation 2001 and does not reﬂ ect the sligh ame- 110 depths (± s.e.) during lioration of water clarity observed the period 1989– Baltic Baltic 100 Sea Sea through the same period (Ærtebjerg et 2001 in inner estuar- 90 al. 2002). Other regulating factors may ies (lower panel), Depth limit index (%) 80 outer estuaries (cen- Germany Germany blur the relationship between light and colonisation depth so that more 1989 1991 1993 1995 1997 1999 2001 tral panel) and along Figure 2.32 marked changes in light climate are open coasts (upper Map of eelgrass area distribution in Danish coastal waters in 1901 (redrawn after Petersen needed before colonisation depths in- Inner estuaries panel). Index values 120 1901), 1933 (redrawn after Blegvad 1935), 1941 (redrawn after Lund 1941) and 1994 crease. For example, eelgrass sudden- represent colonisation 110 (coarse map based on visual examination of aerial photos and data from the national Danish ly disappeared from several sites dur- depths of a given year monitoring programme, produced by Jens Sund Laursen). Green areas indicate healthy ing the warm summers of 1992 and 100 relative to average eelgrass while orange areas (on the 1933 map) indicate where eelgrass was affected by the 1994 possibly due to combined expo- 90 colonisation depths wasting disease but still present in 1933. The arrow shows the location of Limfjorden. of the period 1989– sure to anoxia, sulphide and extreme Depth limit index (%) 80 (Boström et al. in press). temperature (Goodman et al. 1995, 2001 (Ærtebjerg et al. Terrados et al. 1999). 1989 1991 1993 1995 1997 1999 2001 2002).

70 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 71 Figure 2.35 showed no signiﬁ cant trend in eelgrass SHORT-LIVED Figure 2.36 0 300 Eelgrass cover as a cover at water depths above 2 m, but NUISANCE MACROALGAE 0-1 m Cover index of loose 2 250 function of depth. the cover of shallow populations was Short-lived nuisance macroalgae are lying nuisance macro- 4 200 Data represent signiﬁ cantly reduced in inner estuaries favoured by large supplies of nutrients algal species in the 6 150 average cover of 276 and along open coasts (Henriksen et al. (Pedersen 1995). Together with ma- inner parts of estu- 8 100 Cover index (%) depth gradients Cover index 2001). We have found no obvious ex- roalgal species numbers and domi- aries during the 10 50 monitored under planation for this pattern. nance patterns, the cover of nuisance period 1993–2001. 12 0 the national Danish 0 5 10 15 20 25 30 35 In conclusion, eelgrass responds to species is therefore used as an indicator 1993 1995 1997 1999 2001 Data represent the Aquatic Monitoring Cover (%) several types of disturbances: changes of the state of macroalgae in coastal depth intervals 0–1 m, and Assessment in energy input (light), physical dis- areas in the Danish Aquatic Monitor- 300 1–2 m and 2–4 m. 1-2 m Programme in 1994. Eelgrass displays a bell-shaped di- turbances (e.g. wind, waves, extreme ing and Assessment Programme. 250 The cover for each depth interval in a stribution pattern along the depth temperature, ice), chemical disturban- Information on these algae is rela- 200 given year has been gradient with maximum abundance at ces (e.g. anoxia, sulphide) and biologi- tively scarse and analyses of trends are 150 intermediate depth and lower abun- cal disturbances (e.g. the wasting dis- therefore only possible for the inner indexed relative to 100

dance in shallow and deep water (Fi- ease). When eelgrass is used as a moni- part of estuaries at depth intervals of Cover index (%) average cover of the 50 gure 2.35). Exposure, desiccation and toring parameter to reﬂ ect changes in 0–1, 1-2 and 2–4 meter. During the mo- depth interval during 0 ice-scour act to reduce eelgrass abun- light climate due to eutrophication we nitoring period (1993–2001) average 1993 1995 1997 1999 2001 the period 1993– dance in shallow water and render the should therefore be aware that other relative cover varied from 1–20% at 2001 (Ærtebjerg et populations extremely dynamic and factors may affect the response. As the 0–1 meter depth, 1–25 % at 1–2 meter 300 al. 2002). 2-4 m unpredictable. In the deeper, more intensity of physical disturbances de- depth and 0–20% at 2–4 meter depth. 250 protected waters, reductions in eel- cline with depth, eelgrass colonisation There were no significant changes 200 grass abundance towards the lower depth and abundance from intermedi- during the period 1993 to 2001 (Kend- 150 depth limit correlate with light attenua- ate depths towards deeper waters are alls-tau, p > 0,05) (see ﬁ gure 2.36). 100

tion (Sand-Jensen et al. 1997, Krause- therefore likely to be better response Cover index (%) 50

Jensen et al. 2000) and are therefore parameters to eutrophication than the MACROALGAE 0 more directly coupled to changes in eu- abundance of shallow populations. ON REEFS IN OPEN WATERS 1993 1995 1997 1999 2001 trophication. The period 1989–2001 As with seagrasses the macroalgae on the reefs in the open waters enhance biodiversity by providing habitat and shelter for many species. Furthermore they constitute a great part of the biodiversity in marine vegetation. The vegetation in the open waters consists of a multilayer of red and brown algal vegetation at water depths of 10–12 meters. At depths greater than 12–14 m total cover of upright algae decreases to a single layer with coverage less than 100% and cover decreases further with increased depth. In addition to upright forms, crust forming macroalgae cover stones and shells. The cover of crust forming algae is Photo: NERI/Karsten Dahl Photo: NERI/Karsten Dahl large, even at 24–25 meters depth.

72 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 73 Macroalgal vegetation is monitored at ter inputs) and total cover of macro- benthic vegetation and the pelagic verage was low in years with rela- 9 stone-reefs and rocky bottom areas algae at deep stations in the Kattegat parameters, Secchi depth and chloro- tively high runoff and high in years evenly distributed in the Kattegat, during the period 1994 to 2000 (Hen- phyll concentration. This supports the of low runoff. Accordingly there was with one station located in the north- riksen et al. 2001). This means that hypothesis that the supply of nutri- no overall trend in the cover of mac- ern Belt Sea. The total relative cover high supplies of inorganic nutrients ents infl uences light penetration to the roalgae at the monitored reef stations of the upright forms is described or freshwater leads to a reduced de- bottom and thereby the environmen- (Figure 2.37). along with specifi c cover of individual velopment of benthic vegetation. Ex- tal quality of the individual reef lo- species including crust forming algae. ceptions are stations with an intense cations. It has been shown that a good grazing pressure from the sea urchin The total cover of the upright vege- empirical correlation exists between Strongylocentrotus droebachiensis. tation tended to increase in 2001 rela- the supply of the inorganic nutrients There is a corresponding signifi - tive to the mean of the period 1994– (nitrogen and phosphorus in freshwa- cant correlation between the cover of 2001 (Table 2.9). In general, algal co-

Figure 2.37 50 Year Month Number of Macroalgae coverage P The relative deviation 40 observations with reference to in macroalgal cover in average coverage 1994-2001 30 relation to mean 1994 June 4 – 20 values for individual August 11 – stations and time of 10 1995 June 11 – investigation. Devia- 0 August 12 – tions are given in 2 1996 June 9 – -10 meter depth intervals. August 9 * -20 Stations where algal Relative deviation (%) 1997 June 11 *** abundance was -30 August 11 *** limited by sea urchin -40 1998 June 10 * 12-14 m depth 16-18 m depth m depth grazing are left out. 14-16 m depth 18-20 m depth m depth August 10 ** -50 1999 June 10 * 1994 1995 1996 1997 1998 1999 2000 2001 2002 August 11 *** 2000 June 10 – August 12 – 2001 June 11 – August 11 –

Table 2.9 Macroalgal cover at a number of stonereefs in Kattegat relative to the average cover in the period 1994–2001. means that the majority of examined stations has a more developed vegetation cover. means that the majority of examined stations has a less developed vegetation cover. means that there is an equal number of stations with a less respectively more developed vegetation cover. A signtest shows whether cover has increased or decreased relative to the mean for the

period 1994-2001. * = P < 5%, ** = P < 1%, *** = P < 0,1%. Photo: NERI/Karsten Dahl

74 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 75 and in the middle of the 1990s (Figure However, correcting for the linear SOFTBOTTOM MACROBENTHOS 2.38). While the ﬁ rst peak was domi- effects of runoff, the general pattern in 2.10 nated by both polychaetes and crusta- abundance is still there, suggesting the ceans, the peak in the 1990s was only importance of other factors hitherto not dominated by polychaetes. Within the identified. Some evidence indicates taxonomic groups different species that reduced nitrogen nutrient concen- dominated in different peaks and at trations and possibly reduced diatom different stations. Biomass did not abundance, when corrected for runoff, show the same clear pattern as abun- may have reinforced the decrease in Filter feeding dance. Biomass and total abundance benthos stocks in recent years. bivalves are an showed the lowest values in the pe- important link riod 1998–2001 since the measure- COASTAL AREAS between the ments started. From 1998 the monitoring strategy for pelagic and the Over the two last decades changes estuaries and coastal areas was chan- benthic environ- in total abundance and biomass of ged from measurements on single ment. macrobenthos are below the halocline stations to measurements on grids of in Danish open sea areas. Correlation stations in each area. Benthic fauna data analyses performed between the bio- sampled with consistent methods are logical variables and two environmen- now available from 25 different estua- tal variables related to climate, the rine and bay areas from the period North Atlantic Oscillation index (NAO- 1998 to 2001. Unlike the open Kattegat index) and runoff of freshwater from and the Belt Sea, where faunal changes

Photo: Biofoto/Michael Jensen Denmark, showed signifi cant positive in total abundance and biomass to a correlation with a 1 or 2 years time lag, large degree are synchronised between Figure 2.38 Benthic macrofauna in soft sediments cluded in the Danish Monitoring Pro- indicating infl uence of climate on ben- stations, changes in coastal areas do not Temporal develop- plays an important role in the degrada- gramme since 1979 and is measured thic variations (Henriksen et al. 2001). seem to be synchronised to the same ment of abundance tion of organic matter produced in the with replicated quantitative sampling In particular winter nutrient input, degree. This is exemplifi ed in Table 2.10 at 3 stations of 17-55 pelagic zone and several species serve at fi xed stations. Number of individu- and the spring phytoplankton bloom, where total number of species in the 4 m water depth, in the as food for demersal fi sh. Since it is als and biomass (wet weight or dry most likely infl uences the abundance years are shown for each coastal area. Kattegat-Sound-Belt composed of many different species (in weight) are measured for each species. of benthic macrofauna. The number of areas with increasing Sea area partitioned the Kattegat there are more than 500 Sampling occurs once a year typically on major taxonomic species), benthic macrofauna further- in May/June. The number of fixed groups. Bristleworms 6000 more constitute a great part of the bio- stations regularly visited at present Crustacea (Polychaeta), Molluscs diversity in the water surrounding include 24 stations in open sea areas Echinoderm. (Mollusca), Echino- 5000 Mollusca Denmark. The macrofauna integrates mainly in the Kattegat and station Polychaeta derms (Echinoder-

)

environmental changes in the benthic grids in 22 estuaries and near coastal -2 4000 mata) and Crusta- environment and is a highly cost effec- areas around Denmark. ceans (Crustacea). tive parameter to sample. Moreover, 3000 Remaining taxa had since this fauna often is limited by OPEN SEA AREAS a negligable part of

Abundance (m 2000 food, such as sedimenting phytoplank- General trends in macrozoobenthos the total abundance. Note the dramatic ton biomass, macrozoobenthos is a abundance in the Sound, the Kattegat 1000 useful parameter to describe changes and the Belt Sea follow a bimodal decrease in numbers in eutrophication. pattern over the last 20 years with 0 of crustaceans over 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 Benthic macrofauna has been in- peaks in the beginning of the 1980s the 2 decades.

76 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 77 species numbers are about the same KILLS OF MACROZOOBENTHOS the last century. However, the area in present. A well known example is from as the decreasing ones. Reasons for lack The increase in oxygen consumption the southern Little Belt where macro- 1986 when fi shermen caught dead Nor- of synchrony between coastal areas due to enhanced loads of organic mat- benthos suffer from oxygen depletion wegian lobsters (Nephros norvegicus) in is probably local factors like oxygen ter to the bottom can lead to oxygen is in recent years fi ve times larger than the Kattegat. It is also likely that 2002 defi ciency. For example, in Mariager depletion and death of benthic fauna. 100 years ago (Marin ID 1988). Large will be well-known for the extended Fjord fauna was more or less extermi- Though these events are not part of the scale oxygen depletion events with oxygen depletion and resulting kills nated by oxygen defi ciency in 1997 and monitoring programme, many of the kills of macrobenthos belong to the of macrobenthos. thereafter a succession of species have events are still recorded (see Table 2.11). taken place reflected by increasing There are examples from Danish number of species. In conclusion, there marine areas, like the southern Little are no general synchronous changes in Belt, where severe acute oxygen de- abundance, biomass or number of pletion and probably kills of macro- Year Place Cause species in coastal areas over the last 4 benthos have been an almost regular 1981 North Sea, Hevring Bugt, Århus Bugt, Oxygen depletion years. yearly event since the beginning of Pæregård Strand, Dalby Bugt, Kalø Vig 1983 Lillebælt, Kieler Bugt Oxygen depletion 1985 Kattegat Oxygen depletion Coastal area m2 1998 1999 2000 2001 1986 Kattegat, Kieler Bugt Oxygen depletion Roskilde Bredning 0.51 29 29 23 30 1987 Sejrø Bugt, North coast of Sjælland, southern Lillebælt Oxygen depletion Horsens Fjord 0.61 – 24 24 40 1988 Skagerrak, Kattegat Harmful algal bloom Vejle Fjord 10.07 – 52 36 55 1988 Southern Kattegat, Limfjord, Århus Bugt, Oxygen depletion Kolding Fjord 5.54 – 57 36 61 1989 Horsens Fjord Oxygen depletion Ringkøbing Fjord 7.15 20 22 22 17 1990 Århus Bugt, Northern Lillebælt Oxygen depletion Nissum Fjord 6.44 30 29 33 28 1991 Haderslev Fjord Oxygen depletion Hevring Bugt 5.54 76 69 87 91 1992 Haderslev Fjord Oxygen depletion Øresund 6.44 68 51 54 52 1993 Vejle Fjord, Southern Lillebælt Oxygen depletion Køge Bugt, central part 6.44 26 30 28 35 1994 Roskilde Fjord, Limfjord, Vejle Fjord, Nørrefjord, Oxygen depletion Odense Fjord 6.45 66 78 57 57 Flensborg Fjord, Sydfynske Øhav, Isefjord, Århus Bugt, Ringgårdbassin 5.81 27 28 25 19 Åbenrå Fjord, Genner Fjord, Flensborg Fjord, Lillebælt Roskilde Fjord, northern part 3.58 33 30 31 37 1995 Åbenrå Fjord, Roskilde Fjord Oxygen depletion Table 2.10 Isefjord 3.15 24 36 19 15 1996 Limfjorden, Vejle Fjord, Lillebælt Oxygen depletion Changes in macro- Kattegat 2.86 63 51 61 66 1997 Mariager Fjord, Limfjorden, Nakkebølle Fjord, Oxygen depletion fauna species num- Lillebælt 5.54 35 50 51 27 South and east of Møn, Isefjord bers 1998-2001 in Karrebæksminde Bugt 6.57 40 28 37 31 1998 Southern Lillebælt, Åbenrå Fjord Flensborg Fjord Oxygen depletion 25 coastal areas in Skive Fjord 6.44 31 36 24 41 1999 Limfjord, Kalø Vig, Knebel Vig, Århus Bugt, Oxygen depletion Denmark. The fi rst Nissum Bredning 6.44 33 31 49 34 Horsens Fjord, Vejle Fjord, Flensborg Fjord, coloumn denotes Løgstør Bredning 6.44 42 34 32 35 sydlige Bælthav, Sydfynske Øhav total sampling area, Wadden Sea, northern part 26.66 43 43 41 47 2000 Langelandssund, Ebeltoft Vig, Århus Bugt, Oxygen depletion red denotes a Århus Bugt 6.03 62 46 54 57 Odense Fjord, Limfjorden decrease compared Mariager Fjord 5.54 17 14 28 26 2001 Limfjorden, Århus Bugt Oxygen depletion Table 2.11 to previous sampling, Flensborg Fjord 5.41 66 30 30 17 2002 Limfjorden, Århus Bugt, Hevring Bugt, Mariager Fjord, Oxygen depletion Recorded macro- green denotes an Wadden Sea, southern part 1.98 41 36 43 40 Vejle Fjord, Ålborg Bugt, Kalø Vig, Åbenrå Fjord, zoobenthos kills increase and Nivå Bugt 3.58 – 62 65 57 Flensborg Fjord, coastal areas north and south of Fyn, and their causes blue denotes no Average species number 41.524 39.84 39.6 40.6 Lillebælt in Danish marine difference. waters 1981–2002.

78 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 79 and June 1988 were due to a large bloom of oxygen poor and H2S containing FISH KILLS of the fl agellate Chrysochro mullina poly- bottom water. 2.11 lepis (Lancelot et al. 1989). In 1997, al- The ongoing national monitoring IN COASTAL WATERS most all fi sh were killed in the inner programme does not include fi sh mo- part of Mariager Fjord due to oxygen nitoring. However, fi sh kills have been

depletion and release of H2S in the reported annually since 1989 as an entire water column. In October 2002 integral part of the National Aquatic fi sh kills occurred several places along Monitoring and Assessment Program. the Jutland east coast due to upwelling Table 2.12 summarises the recorded Fish kills are not the most common effect of eutrophication but illustrate Year Place Cause severe effects of 1981 Hevring Bugt, Århus Bugt, Pæregård Strand, Dalby Bugt, Oxygen depletion oxygen defi ciency Båring Vig, Vejle Fjord, Limfjorden, including Thisted and harmful algal Bredning, Visby Bredning, Bjørnsholm Bugt, rensen ø blooms. Risgårde Bredning, Skive Fjord and Lovns Bredning 1981 Fjaltring Strand, Glyngøre Harbour Harmful algal bloom 1982 North Sea, Køge Bugt, Holckenhavn, Kalø Vig, Isefjord Oxygen depletion 1983 Lillebælt, Als Sund, Langelandssund Harmful algal bloom 1987 Sejrø Bugt Oxygen depletion rhus County/Helene Munk S Å 1988 North coast of Sjælland, Sejrø Bugt, Ringkøbing Fjord Oxygen depletion Photo: Photo: Bent Lauge Madsen 1988 North Sea, Skagerrak, Kattegat, and the northern part Harmful algal bloom of Øresund Normally two types of events related wind directions and forces, trapping 1989 Ringkøbing Fjord Oxygen depletion to eutrophication can cause fi sh kills. the fi sh in these water masses with no 1990 Århus Bugt Oxygen depletion It is either toxic algae or low oxygen chance to escape. 1990 Djurslands Østkyst Oxygen depletion or con cen tra tions. Low oxygen concen- The Danish EPA published the harmful algal bloom trations in the bottom waters result fi rst comprehensive national assess- 1991 Lillebælt in the release of hydrogen sulphide ment of the effects of oxygen deple- 1992 Lillebælt Possible harmful

(H2S) from the sediments. The H2S is tion and harmful algal blooms in 1984 algal bloom lethal to most animals and the result (Danish EPA 1984) This report as- 1994 Årgab, Hovvig, Mariager Fjord, Vejle Fjord, Isefjord, Roskilde Oxygen depletion of the sudden release is often exten- sessed fi sh kills in the Danish waters Fjord, Thisted Bredning, Skive Fjord, Lovns Bredning sive and leads to the immediate death and concluded that fish kills were 1995 Årgab, Hanstholm Oxygen depletion of animals living at or near the sea restricted, and were exceptional, but 1997 Mariager Fjord, Vejle Fjord, Sydfynske Øhav, Korsør Nor, fl oor as well as in the water column. natural events, before the 1970s, and Isefjord Oxygen depletion This immediate effect, therefore, also that the problem increased in the 1970- 1998 North Sea and Skagerrak Harmful algal bloom includes fi sh. However, the most spec- and early 1980s. 1999 Vejle Fjord Oxygen depletion tacular fi sh kills with dead or dying The four best known Danish ex- 2000 Kattegat, Smålandsfarvandet, Ebeltoft Vig, Oxygen depletion fi sh accumulating on the shores, as an amples of fish kills are from 1981, Sydfynske Øhav example in 1981 and 2002, are due to 1988, 1997 and 2002. In 1981 the kills 2001 Vejle Fjord Oxygen depletion Table 2.12

upwelling of oxygen poor or H2S were caused by severe oxygen deple- 2002 Ålborg Bugt, Hevring Bugt, Vejle Fjord, Kalø Vig, Oxygen depletion Recorded ﬁ sh kills in containing bottom water to the sur- tion in most of the Danish waters Øresund, Sydfynske Øhav Danish marine waters face along the coasts due to changing (Danish EPA 1984). Fish kills in May 1981–2002

80 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 81 fi sh kills in Danish marine waters. It • Fishing intensity has had an im- might be argued that the situation has pact on plaice (Pleuronectes plates- not improved since the mid 1980s, sa), which has decreased from the mainly because inputs of nutrients are 1950s to the 1990s. still high, especially in wet years, and • Conversely, the dab (Limanda li- that the events of oxygen depletion manda) has become more frequent that might trigger fi sh kills are tightly and is now the dominating fl at fi sh coupled to meteorological and hydro- in the bay. graphic forcing. • The mean length of dab has de- Regional authorities have carried creased since 1957 and is unam- out studies on fi sh populations and biguously linked to the duration fish kills in Limfjorden, Mariager of oxygen depletion. The growth is Fjord, Ringkjøbing Fjord, Roskilde limited in years with longer peri- Fjord, the area around Rødsand, the ods of oxygen depletion compared Wadden Sea and Århus Bugt. Studies to the situation in the 1950s. Less in Århus Bugt, which covers the that 1% of dabs reach the mini- period 1953–1998 and includes coher- mum size limit of 25 cm. ent data on fi sh, fi sheries, zoobenthos and water quality, are summarised by Future possibilities to assess the health Jensen (1999): of coastal fi sh populations will im- • The occurrence and composition prove from 2004, when monitoring of of fi sh have changed signifi cantly non-commercial fi sh in six representa- during the last 50 years, due to tive coastal waters will be conducted changes in fi shing intensity and on a regular basis within the monito- duration of oxygen depletion. ring and assessment programme. rtebjerg Æ Photo: NERI/Gunni Photo: NERI/Peter Bondo Christensen

82 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . TECHNICAL-SCIENTIFIC ASSESSMENT 83 RESPONSES AND ADAPTIVE MANAGEMENT

Since the mid 1980s high priority has The general objectives for Danish LilleChristiansborg, billedtekst been given to the quality and protec- marine waters are based on the Con- home of the tion of the aquatic environment in Den- solidated Environmental Protection Danish Parliament. mark. The overall goal is to ensure that Act, the 1992 Helsinki Convention on the waters are clean. The endeavours Protection of the Marine Environment in this respect are described in Aquatic in the Baltic Sea Region and the 1992 Environment 1999 (Danish EPA 2000) Convention for the Protection of the and summarised in Conley et al. (2002). Marine Environment of the Northeast These papers state that Danish Atlantic. More specifi c objectives are: administrations shall work towards • Anthropogenic pollution and human ensuring: activities may only insigni3fi cantly • That watercourses, lakes and mari- or slightly affect fauna and fl ora. ne waters are clean and of a satis- • Nutrient levels have to be at or close factory quality as regard to health to natural levels.

Photo: Biofoto/Kjeld Olesen and hygiene. • Clarity of the water has to be nor- • That exploitation of water bodies and mal or close to normal. associated resources takes place in • Unnatural blooms of toxic plank- a sustainable manner. tonic algae must not occur. • That the objectives of relevant inter- • Pollution-dependent macroalgae national agreements will be fulfi lled. must not occur. • Oxygen defi ciency may only occur The central legal instrument to fulfi l in areas where this is natural. these political objectives is the Con- solidated Environmental Protection Commercial exploitation, i.e. fi shery, Act, which aims to safeguard the en- offshore industry, dumping of seabed vironment, to support a sustainable material, recreational activities etc. has social development, and to protect to be conducted in a manner that is fl ora and fauna (Ministry of Environ- sustainable and respects environmen- ment and Energy 1998). tal and natural wealth.

84 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . RESPONSES AND ADAPTIVE MANAGEMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . RESPONSES AND ADAPTIVE MANAGEMENT 85 changes, effects of measures and ef- ple on how the concept of strategic PRINCIPLES fectiveness of the measures etc. environmental planning works. The 3.1 The Danish policies and strategies strategies and measures as well as the to abate eutrophication of the aquatic chronology (1986-2002) are described environment are an illustrative exam- in Chapter 3.2.

The primary means of achieving the quality objectives for surface waters is a reduction in nutrient discharges, losses and emissions.

Photo: Danish Institute of Agricultural Science/Henny Rasmussen Governmental Position Paper on nature and environment (every 4 year) One of the guiding principles in the The principle which is illustrated Danish work on abatement of pollu- in Figure 3.1 shall ensure that the goals Sector or thematic tion in general and nutrient enrich- can either be met with the agreed mea- Action Plans ment and eutrophication in particular sures or – if goals are not met – that (irregular intervals) is “strategic environmental planning”. measures should be revised or even The intentions of the principle is to supplemented with additional meas-

deal with environmental problems in a ures. The overall goal is to ensure that Evaluation Implementation coherent way and that policies are to be environmental policies become more or revision of action plans developed as a process with the follow- preventive, holistic and target orien- of action plans ing elements: tated. • Political agreements on goals. The Danish administration put em- • Implementation of agreed measures. phasis on that the principles of strategic Monitoring of the state of the environment • Continuous monitoring and regular environmental planning are imple- (reporting every year) production of assessment reports, mented at all levels and in all actions aiming at a higher level of under- aiming at a healthier aquatic environ- Figure 3.1 State of the Environment Report standing of the actual problem. ment. Danish policies to improve the (every 4 year) Illustration of • Periodic evaluations and adjustments health of the marine environment have the concept (if relevant) of goals and measures. been and will be based on the available of strategic environmental information on relations between so- Report, document Input Process Influence ciety and environmental responses to planning.

86 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . RESPONSES AND ADAPTIVE MANAGEMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . RESPONSES AND ADAPTIVE MANAGEMENT 87 It should also be noted that losses • Establishment of sufﬁ cient capaci- NUTRIENT REDUCTION of phosphorus from cultivated land ty to store 9 months of manure 3.2 are not included, because of the uncer- production so that manure can be STRATEGIES IN DENMARK tainties that are related to these esti- stored until crop growth season mates. begins. • Establishment of crop rotation and SPECIFIC REDUCTION TARGETS FOR fertilisation plans to ensure that ni- THE AGRICULTURAL SECTOR trogen content of fertiliser is opti- Since the mid 1980s, a number of mally exploited. The Danish action plans and strategies have been • Fields must have green cover dur- Parliament. adopted by the Danish Parliament to ing winter period. regulate development of the agricul- • Manure has to be ploughed in or in tural sector, one of the main sources some other way deployed into the of nutrients to the aquatic environ- soil within 12 hours of application. ment, and its impact on the aquatic • Limits on the amount of livestock environment. The action plans include: manure applied to ﬁ elds. • The 1985 NPo (Nitrogen, phosphorus and organic matter) Action Plan. It soon became clear that it would not • The 1987 Action Plan against Pollu- be possible to attain the reduction tion of the Danish Aquatic Environ- targets by 1993. The measures stipu- ment with Nutrients (Action Plan lated in the Action Plan on the Aquatic on the Aquatic Environment). Environment I were therefore tight-

Photo: Scanpix • The 1991 Action Plan for Sustain- ened in 1991 in the Action Plan for Sus- able Agriculture. tainable Agriculture. The reduction tar- The primary means of achieving the tively. This corresponds to a reduction • Parts of the Governments 10-Point get was maintained but the time frame quality objectives for surface waters is in annual discharges and losses from Programme for Protection of the was extended to the year 2000. The mea- a reduction in nutrient discharges and a level of around 283,000 tonnes N and Groundwater and Drinking Water sures were: emissions. In the January 1987 Action 9,120 tonnes P at the time the plan was from 1994. • Fertilisation accounts so that fertili- Plan on the Aquatic Environment and adopted to a level of ca. 141,600 tonnes • The 1996 Follow-up on the Action ser application can be documented. the April 1987 Report on the Action N and ca. 1,820 tonnes P (see Table 3.1 Plan for Sustainable Agriculture. • More stringent and fi xed require- Plan on the Aquatic Environment the for details). The Action Plan covers the • The 1998 Action Plan on the Aqua- ments on utilisation of the nitro- goal of reducing nitrogen and phos- 3 major sources: agriculture, municipal tic Environment II. gen content of livestock manure. phorus loads to the aquatic environ- wastewater treatment plants and sepa- • All farms must establish suffi cient ment was set to 50% and 80%, respec- rate industrial discharges. Reduction targets for nitrogen and capacity to store 9 months of ma- Tabel 3.1 phosphorus stipulated in the Action nure production. Sector specifi c Plan on the Aquatic Environment I are • A ban on the application of liquid reduction targets for Nitrogen Phosphorus an approximate 50% reduction of ni- manure between harvest time and annual discharges 1987 ÷ Reduction = Target 1987 ÷ Reduction = Target trogen loads and the elimination of the February except on fi elds cultiva- and losses of nitrogen Agriculture 260.000 ÷ 127.000 = 133.000 4.4001) ÷ 4.000 = 400 phosphorus farmyard load to avoid ted with winter rape or grass. (tot-N) and phospho- Municipal WWTPs 18.000 ÷ 11.400 = 6.600 4.470 ÷ 3.250 = 1.220 unintended eutrophication of the aqua- rus (tot-P) to the Separate industrial tic environment. The reduction targets After the Action Plan for Sustainable aquatic environment discharges 5.000 ÷ 3.000 = 2.000 1.250 ÷ 1.050 = 200 were to be attained by 1993 through Agriculture, there have been a number in Denmark Total 283.000 ÷ 141.400 = 141.600 9.120 ÷ 8.050 = 1.820 the following measures carried out by of follow-up plans for reducing the (Danish EPA 2000). 1) Includes the farmyard load, i.e. does not include losses of phosphorus from agricultural fi elds. the agricultural sector: impact of the agricultural sector on the

88 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . RESPONSES AND ADAPTIVE MANAGEMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . RESPONSES AND ADAPTIVE MANAGEMENT 89 aquatic environment, including the cluding financial support to far- 1993 2000 2003 Tabel 3.2 Government’s 1994 10-Point Program- mers willing to utilise sensitive ag- 1. Action Plan on the Aquatic Environment I Summary of measures me for Protection of the Groundwater ricultural areas in a more environ- (1987): and estimated reduc- and Drinking Water in Denmark. mentally sound manner by using Optimal utilisation of livestock manure tions (in tonnes N per The need to further tighten the less fertiliser or by completely refrai- • NPo Action Plan 55,000 year) in nitrogen dis- regulation of agricultural loads of ni- ning from cultivating the land. There • NPo Subsidy Act 5,000 charges and losses trogen has become even more neces- has hitherto been very little interest • Further initiatives 10,000 from agriculture, cf. sary because Denmark must comply in this scheme. Programme for improved utilisation Action Plan on the with the EU Nitrates Directive by the • Improved fodder utilisation and of fertiliser Aquatic Environment I, year 2003. The directive restricts appli- changes in feeding practice. • Systematic fertilisation plans 15,000 Action Plan for Sus- cation of livestock manure to 170 kg • Implementation of stricter harmony • Improved application methods 5,000 tainable Agricultural N hectare-1 yr-1. In the case of some criteria governing livestock density. • Winter green fi elds – catch crops and Development, follow- types of farms this is less than levels cur- • Stricter requirements on utilisation ploughing down of straw 20,000 ups hereto, and Ac- rently permitted. Denmark has sought of the N content of livestock manure. • Winter green fi elds – further initiatives 8,000 tion Plan on the Aqua- permis sion to derogate from the 170 • Converting 170,000 hectares to or- Structural measures 9,000 tic Environment II. kg N hectare-1 rule on cattle holdings ganic farming; catch crops on a fur- Total 127,000 50,000 so as to enable application of up to 230 ther 6% of a farmers land. kg N hectare-1 yr-1 on a small number • Reducing the nitrogen norm by 10%, 2. Action Plan for Sustainable Agricultural of these holdings. e.g. farmers may now only apply ni- Development (1991): In February 1998, the Danish Par- trogen in amounts corresponding • Improved utilisation of livestock manure 20,000-40,000 liament adopted several new instru- to 90% of the economically optimal • Reduction in commercial fertiliser 8,000-15,000 ments aimed at achieving the reduc- level. consumption tion targets stipulated in the Action • Protection of groundwater in particularly Plan on the Aquatic Environment I. As If the measures in the Action Plan on vulnerable areas 1,000-2,000 a supplement to the Action Plan on the the Aquatic Environment II are imple- • Reduction in agricultural acreage 17,000-20,000 Aquatic Environment I, the Action Plan mented as changes in agricultural prac- • Structural development, other measures 15,000 on the Aquatic Environment II will tice, 20 years of nitrate policy (1985- Total 77,000 89.900 reduce nitrogen leaching by a further 2003) is likely to result in a 100,000 ton- 37,000 tonnes N yr-1 so as to enable the nes N yr-1 reduction in leaching from 3. Action Plan on the Aquatic Environment II reduction target of 100,000 tonnes N agricultural land. Moreover, consump- (1998): yr-1 to be achieved no later than the tion of nitrogen in the form of commer- • Wetlands 5,600 end of the year 2003 (table 3.2). The cial fertiliser will decrease from approxi- • Sensitive agricultural areas 1,900 following measures have been imple- mately 400,000 tonnes N yr-1 in 1985 • Afforestation 1,100 mented under the Action Plan on the to approximately 200,000 tonnes N • Improved fodder utilisation 2,400 Aquatic Environment II: yr-1 in 2003 (Iversen et al. 1998). • Stricter livestock density requirements 300 • Re-establishment of 16,000 hectares In connection with the Action Plan • Stricter requirements on utilisation 10,600 of wet meadow to help reduce ni- on the Aquatic Environment I it was of N content of manure trogen leaching to the aquatic en- estimated that nitrogen loads could be • Organic farming 1,700 vi ronment due to their ability to reduced by a total of 127,000 tonnes N • Catch crops on a further 6% of the fi elds 3,000 -1 convert nitrate to N2. yr by 1993. The reduction targets • 10 % reduction in nitrogen standards • Afforestation in Denmark and plan- were 100,000 tonnes N yr-1 for the for crops 10,500 ting 20,000 hectares forest before nitrogen load from fi elds and 27,000 Total 37,100 the year 2002. tonnes N yr-1 for the farmyard load. In Grand total 127,000 127,000 127,000 • Photo: High-light Agri-environmental measures in- the Action Plan for Sustainable Agri-

90 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . RESPONSES AND ADAPTIVE MANAGEMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . RESPONSES AND ADAPTIVE MANAGEMENT 91 culture it was estimated that by the The EU Council Directive 91/271/ 1987 this was considered as low as it ments for industry as for wastewater year 2000, the measures stipulated in EEC of 21 May 1991 concerning Urban is practically possible to reach with bio- treatment plants. Industry was to re- the Action Plan on the Aquatic Envi- Wastewater Treatment as amended by logical nitrogen removal. As regards duce its discharges through application ronment I would only have reduced Commission Directive 98/15/EU of to phosphorus, municipal wastewater of BAT understood as the level of nitrogen loads by 50,000 tonnes N yr-1 27 February 1998 - commonly referred treatment plants exceeding 5,000 PE treatment that is technically attainable and that further measures were there- to as the Urban Wastewater Directive have to remove phosphorus down to an and economically viable for the indus- fore needed to achieve the total reduc- – is one of the most important legal annual average of 1.5 mg P l-1. try in question. tion of 127,000 tonnes N yr-1. documents in the EU legislation on the The reduction target for nitrogen The existing measures and targets aquatic environment. The purpose of of 6,600 tonnes N yr-1 was achieved in REDUCTION TARGETS under the Action Plan on the Aquatic the directive is to protect the environ- 1996, as was the reduction target for FOR OTHER SECTORS AND SOURCES Environment I and the Action Plan for ment against negative effects associ- phosphorus of 1,200 tonnes P yr-1. A number of other sectors and types Sustainable Agriculture were re-eva- ated with discharge of inadequately of sources also contribute to nutrient luated in 1998 in connection with the treated urban wastewater and biologi- SPECIFIC REDUCTION TARGETS FOR loading to the aquatic environment. preparation of the Action Plan on the cally degradable industrial wastewa- SEPARATE INDUSTRIAL DISCHARGES These include freshwater fi sh farms, Aquatic Environment II. It was conclu- ter from enterprises within the food The Environmental Protection Act, the mariculture, transport, combustion ded that by the year 2003, the existing processing industry. According to the EU Directive on Pollution Prevention plants (heat and power production), measures would reduce nitrogen loads directive, wastewater discharges have and Control (IPPC Directive) and de- sparsely built-up areas and stormwa- by 89,900 tonnes N yr-1. Together with to be subjected to a level of treatment rivative statutory orders and offi cial ter outfalls. The Action Plans on the the expected reduction under the Ac- appropriate to the environment at the guidelines regulate separate industrial Aquatic Environment did not speci- tion Plan on the Aquatic Environment place in question and the use to which discharges. fy specifi c reduction targets for these II, it was concluded that nitrogen loads the recipient water bodies in question The IPPC Directive aims at inte- sectors and types of source but in- would be reduced by 127,000 tonnes are put. Denmark implemented the grated prevention and control of pol- stead describes a number of other N yr-1 by 2003. provisions of the directive in Danish lution by major industries. The direc- measures. Not all measures in the Action Plan legislation in 1994. tive specifi cally regulates the energy on the Aquatic Environment II will have The Action Plan on the Aquatic En- industry (power stations and refi ner- FRESHWATER FISH FARMS taken full effect by 2003. The Action vironment’s reduction targets for mu- ies, etc.), production and processing of The Ministry of Environment issued Plan on the Aquatic Environment II nicipal wastewater treatment plants metals, the mineral industry, the che- the Statutory Order on Freshwater Fish also encompasses so-called regional were adjusted in 1990 on the basis of mical industry, waste management Farms on 5 April 1989 to reduce nutri- measures. These represent implemen- the results of the Nation-wide Monito- plus a number of other activities such ent loading. It gives guidelines for the tation of recommendations of the ring Programme (Danish EPA, 1991). as paper manufacturers, textiles pre- County authorities to stipulate the Drinking Water Committee concern- In the case of nitrogen, annual dis- treatment and dyeing, slaughterhous- maximal permitted feed consumption ing the protection of groundwater charges in treated wastewater are to es and dairies, as well as installations at fi sh farms, minimum requirements resources considered particularly vul- be reduced from ca. 18,000 tonnes N for intensive rearing of poultry and pigs as to treatment measures as well as nerable to nitrate pollution. to ca. 6,600 tonnes N. Phosphorus dis- exceeding a certain capacity. The IPPC minimum requirements as to utilisa- charges are to be reduced from ca. Directive contains measures designed tion and quality of feed. SPECIFIC REDUCTION TARGETS 4,470 tonnes P to ca. 1,220 tonnes P. to prevent or, where that is not practi- FOR MUNICIPAL WASTEWATER The reduction in nitrogen discharges cable, to reduce emissions to the atmos- MARICULTURE TREATMENT PLANTS from municipal wastewater treatment phere, water and land from the above (SEAWATER-BASED FISH FARMING) Discharges from municipal wastewa- plants corresponds to all new or up- mentioned activities. In 1987 a moratorium was placed on ter treatment plants are regulated by graded plants exceeding 5,000 PE and Because of large differences be- establishment of new farms and expan- the Environmental Protection Act, the all existing plants ex-ceeding 1,000 PE tween individual enterprises and their sion of existing farms until 1990 when Urban Wastewater Directive and de- having to implement biological treat- discharges of wastewater, the Action Statutory Order No. 640 on mariculture rivative statutory orders and offi cial ment with nitrogen removal down to Plan on the Aquatic Environment I did was issued. This stipulated general -1 Photo: CDanmark guidelines. an annual average of 8 mg N l . In not stipulate general discharge require- regulations on feed quality and con- Photo: Biofoto/Morten Rasmusssen

92 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . RESPONSES AND ADAPTIVE MANAGEMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . RESPONSES AND ADAPTIVE MANAGEMENT 93 sumption as well as consumption of mark has entered into an international that are vulnerable to pollution, and dies in order to ensure that the

feed relative to production. In addition, agreement to reduce emissions of NOx based on its knowledge of the envi- area receiving subsidies matches upper limits were placed on nutrient by 30% over the period 1986-98. How- ronmental state and pollution load on the needs for bread wheat. discharges to the surrounding aquatic ever, the measures needed to meet this the individual recipient waters, has to • Revision of nitrogen standards. environment from each individual goal are inadequate in Europe as re- assign each individual recipient a farm. In 1996 the Danish EPA requested gards acidifi cation and eutrophica- maximal environmentally permissible The amendments to Action Plan II in- the Counties not to issue any permits tion. In June 1999, the EU Commis- level of pollution. clude an over-fulfi lment by 175 tonnes for new sea-based or land-based mari- sion, therefore, issued proposals for N yr-1 compared to the measures agreed culture farms or for extensions of exist- two directives on acidification and RAINWATER OUTFALLS in 1997. ing farms and were urged to assess ozone formation at ground level (Pro- Rainwater outfalls are one of the rea- An Action Plan III on the Aquatic whether environmental or operational posal for a Directive on National Emis- sons that many watercourses and ur- Environment is on the programme of benefi ts could be obtained by moving sion Limits for Certain Polluting Sub- ban lakes as well as some coastal mari- the Danish Government and is planned or merging existing farms. In June 2001 stances and Proposal for a Directive on ne waters fail to meet the agreed qua- to be negotiated in 2003. The parties the 1996 request was lifted and the the Ozone Content of the Air). These lity objectives. Despite the increasing behind Action Plan I and II have agreed Danish EPA published a new strategy two directives stipulate national limits importance, there is a lack of knowedge that Action Plan III shall focus on: • to reduce loads of nutrients and or- for emissions of NH3 and NOx. In the of how to manage rainwater outfl ows. The nitrogen balance of the Danish ganic matter from mariculture. A revi- case of Denmark, the proposed direc- Ongoing work by the Wastewater Com- agricultural sector, in particular on

sion of the existing Statutory Order is tives will limit ammonia (NH3) emis- mittee under the Danish Engineering the magnitude of losses from Dan- being negotiated and is expected to be sions to 71,000 tonnes per year and Association and the Danish EPA is ish agriculture in the mid 1980s. • implemented in 2003. nitrogen oxide (NOx) emissions to intended to result in proposals for General measures to reduce dis- 127,000 tonnes per year from 2010. guidelines that can be incorporated in charges and losses from the agri- EMISSIONS NOX offi cial EPA guidelines. cultural sector. When the Action Plan on the Aquatic SPARSELY BUILD-UP AREAS • Losses of phosphorus from fi elds, Environment was adopted in 1987, the The relative contribution of nutrients FOLLOW-UP ON THE MID-TERM which so far have been excluded Danish EPA was instructed to prepare from sparsely built up areas has in- EVALUATION OF ACTION PLAN II from action plans. a report containing a specifi c reduction creased over the past 10 years because The effects for a number of measures • Possibilities to implement regional

programme for power station NOx emis- of point source reductions from waste- in Action Plan II from 1998 are based measures in order to protect speci- sions. Regulation of NOx emissions in water treatment plants and industry. It on an assumed development in agri- fi c regional waters. Denmark has concentrated on impro- is expected that future improvements cultural practises. Therefore, the plan ved combustion technology and fl ue in treatment of wastewater from spar- was subject to a mid-term evaluation in gas abatement at power stations (e.g. sely built-up areas will occur resulting 2000/2001. This evaluation concluded Statutory Order No. 885 of 18 Decem- from initiatives in connection with the that the realised changes in agricultu- ber 1991 on Limitation of Emissions of 1997 amendment of the Environmen- ral practices could not match the ex- Sulphur Dioxide and Nitrogen Oxides tal Protection Act concerning waste- pected changes. The losses of nitrogen from Power Stations), enhanced use of water treatment in rural areas. Accor- from fi eld were estimated to be 93,000 natural gas and renewable energy (e.g. ding to state instructions to the Coun- tonnes of N, indicating an annual un- the Government’s 1990 Energy Action ties concerning revision of the Regio- der-fulfi lment of 7,000 tonnes of N. Plan) as well as implementation of the nal Plans in 2001, the Countries have The results of the political mid-term requirement for catalytic converters on specifi ed areas in which the treatment evaluation were presented in May cars (e.g. Ministry of Justice Statutory of wastewater from properties in rural 2001. The conclusions included: rdam Olesen rdam ø Order on Detailed Regulations for the areas is to be improved, they must • Changed rules for funding of rees- Motor Vehicle Design and Equipment stipulate quality objectives for indi- tablishment of wetland, in order to from 1990). Under the EEC Convention vidual recipient waters in its Regional make this more attractive. Photo: High-light Photo: NERI/Helge R on Transboundary Air Pollution, Den- Plan, identify watercourses and lakes • Reduction of bread wheat subsi-

94 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . RESPONSES AND ADAPTIVE MANAGEMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . RESPONSES AND ADAPTIVE MANAGEMENT 95 adopted a programme to achieve the ning specifi c objectives that have to be INTERNATIONAL reductions. In 1989, the reduction tar- achieved before the year 2005. 3.3 get was specifi ed in relation to specifi c Comparing reduction targets of the CO-OPERATION TO ABATE sectors. In 1992, it was decided to inte- Danish action plans with agreed reduc- grate the Oslo and Paris Conventions, tion targets in OSPAR and the North both of which aimed to prevent marine Sea Conference, there are some differ- MARINE EUTROPHICATION pollution from dumping and land- ences. The Danish reduction targets based sources of pollution. The objec- make a total reduction of discharge tive of the successor – the OSPAR Con- and losses of nitrogen on the order of The sea is the link vention – is to protect the marine en- 50% and phosphorus on the order of between states, vironment of the Northeast Atlantic 80% from three sectors, agriculture but does not respect region. As a follow-up on the 1988 (only nitrogen), industry, and munici- state borders. decision, the 1998 OSPAR Ministerial pal wastewater plants. The OSPAR and Meeting adopted a strategy to combat North Sea Conference agreed reduc- eutrophication. Included in the strat- tion targets make a total reduction on egy is adoption of achieving a healthy the order of 50 % of the inputs of both marine environment where eutrophica- phosphorus and nitrogen into areas tion does not occur by 2010. where these inputs are likely, directly At a ministerial meeting in Febru- or indirectly to cause pollution. ary 1988, HELCOM adopted a declara- Marine monitoring and assessment tion specifying a 50% reduction target is another important area of interna- for discharges of nutrients etc. over a tional co-operation.

Photo: CDanmark 10-year period. In the Communiqué In connection with adoption of the from the ministerial meeting in 1998, Action Plan for the Aquatic Environ- The bridge Eutrophication of the Danish parts of to reduce the transport of nutrients to the ministers confi rm that they have ment in 1987, a national monitoring in The Sound the Wadden Sea, along the west coast Danish waters. committed themselves to attaining the programme was established to demon- between of Jutland and in open parts of Kattegat At the North Sea Conference in strategic goal from 1988 and to defi - strate the effectiveness of measures Denmark and the Baltic Sea is to a large extent London in November 1987, the coun- and Sweden. caused by inputs from adjacent waters tries of the North Sea adopted the goal and to deposition from the atmos- of reducing nitrogen and phosphorus phere. The nutrients causing eutrophi- inputs by ca. 50% over the period cation originate from discharges, emis- 1985-95 in areas where these inputs are sions and losses in other areas and are likely, directly or indirectly, to cause transported to Danish waters where pollution. At ministerial conferences they add to discharges and losses from in The Hague (1990), Esbjerg (1995) and Danish sources. International co-ope- Bergen (2002), these reduction targets ration is essential to abate the problem were reiterated and the need to take in these areas. The efforts and results action against wastewater discharges due to the Danish Action Plans on the and losses from agriculture was speci- Aquatic Environment, therefore, must fi ed. be supplemented by parallel activities In June 1988, the Paris Commission in neighbouring countries. adopted a 50% reduction target for A number of international initia- nutrient inputs to marine waters sus- tives have been agreed and are likely ceptible to eutrophication and also Photo: Biofoto/Lars Gejl

96 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . RESPONSES AND ADAPTIVE MANAGEMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . RESPONSES AND ADAPTIVE MANAGEMENT 97 contained in the plan. The nation- methods are identical to the extent wide programme was revised in 1992 possible and the monitoring frequency and the latest revision in 1997–1998 of sampling a station are at a level resulted in implementation of the Da- which makes it possible to assess the nish Aquatic Monitoring and Assess- eutrophication status of marine waters ment Programme 1998–2003 common- on a year-to-year and in some cases on ly referred to as NOVA–2003. a season-to-season basis. The monitoring and assessment- The outcome of the Danish moni- programme has been planned and toring and assessment programme is designed on the basis of national and evaluated and reported annually. international obligations and the needs County authorities are responsible for agreed by the Danish Parliament, assessment of local areas and, wher- OSPAR and HELCOM. The existing ever relevant, they include an assess- programme implements all national ment as to what extent the regional or international agreed monitoring and quality objectives for the aquatic en- assessment activities, including para- vironment have been met. Based on meters, numbers of stations, frequen- the regional reports and information cies, quality assurance, data handling from other national and international and reporting. monitoring activities, NERI prepares The Danish monitoring cruises are a nation-wide assessment of dischar- co-ordinated at different levels: ges and environmental state. Oxygen • Between counties. depletion reports in August, September • Between counties and NERI. and October and crosscutting theme • Between NERI and Swedish, Nor- reports supplement the annual assess- wegian and German institutions. ment. The annual assessment reports are also the basis for the Danish con- This added value of co-ordination re- tribution to various international con- duces sailing time in many coastal and ventions and organisations. open waters. The stations sampled and Photo: CDanmark

98 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . RESPONSES AND ADAPTIVE MANAGEMENT NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . RESPONSES AND ADAPTIVE MANAGEMENT 99 SUMMARY, CONCLUSIONS AND THE FUTURE

The summary is based on the eutrophi- improved sewage treatment has also Denmark and most cation status in the years 2000 and reduced the overall N load by about countries bordering 2001, and the development since im- 14%, while decrease in leaching from the North Sea and plementation of the Danish National agricultural soils has reduced the N the Baltic Sea have Monitoring and Assessment Program- load with about 21%. All ﬁ gures are so-far not been me in 1989, the latter being an integral corrected for variations in runoff. able to achieve the part of the Danish Action Plan on the Atmospheric deposition of inor- agreed reductions Aquatic Environment from 1987. ganic nitrogen is important on large sea in the inputs of surfaces as Kattegat and the Belt Sea nutrients to the DEGREE OF NUTRIENT ENRICHMENT where atmospheric N deposition makes marine waters. NUTRIENT LOAD up about 30% of total N load4 from Detailed nutrient load compilations to surrounding land and atmosphere. Danish waters were initiated in 1989, During the period 1989–2001 there was although the main increase in nutrient a decrease in the air concentration of N loads from land and atmosphere actu- bound in particles and a tendency to ally took place long before. As an ex- a decreasing deposition of about 15%. ample, the estimated annual riverine load of N to Danish coastal waters in NUTRIENT CONCENTRATIONS

Biofoto/Jesper Plambech the 1960s was about 60% of that in the The assessment of trends in nutrient 1980s and the P load increased four concentrations in Danish waters is fold in the Baltic Sea, North Sea re- based on indices for mean annual con- gions from the 1940s to the 1970s. A centrations of DIN, TN, DIP and TP growing number of sewage treatment in the upper mixed layer developed plants have reduced the Danish point for estuaries-coastal waters and the source loads of P to surface waters open Kattegat–Belt Sea. (fresh and marine) since the late 1980s The decreasing nutrient load to by nearly 90% and the total land based Danish waters is reﬂ ected in the nu- P load to marine waters has been re- trient concentrations. The nitrogen con- duced by 60% from 1990 to 2001. The centrations in 2001 were the lowest ob-

100 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . SUMMARY, CONCLUSIONS AND THE FUTURE NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . SUMMARY, CONCLUSIONS AND THE FUTURE 101 served during the period 1989-2001 and to have decreased in open sea areas had an abundant cover of nuisance waters and common Danish-Swedish at the same level as in the very dry years since 1980. However, year-to-year va- macroalgae. While there seemed to be and Danish-German waters. In many 1996 and 1997, even though the runoff riations have been substantial. Chloro- a decrease in the relative cover of nui- areas of the southern Kattegat, the was about normal. In the open waters phyll a index values adjusted for va- sance species at 0-1 and 1-2 meter Sound and the Belt Sea oxygen levels of the Kattegat and Belt Sea the run- riations in climatic conditions shows depths since 1997, there was an in- in bottom waters were reduced to a off corrected nitrogen concentrations that chlorophyll a values were very crease in cover from 1998 to 2001 at 2-4 level seldom or never seen before. shows a steady decrease since 1989. In constant in the period 1987-2001. meters. In general, there where no the estuaries and coastal waters, a sig- In estuaries and coastal waters signifi cant changes during the period MACROZOOBENTHOS nifi cant decrease was observed after chlorophyll a concentrations have de- 1993 to 2001. The tendency of a reduc- General trends in abundance of ma- 1997. In the estuaries and coastal waters, creased since the late 1980s. The chlo- tion in relative cover was not true for crozoobenthos in the Sound, Kattegat the phosphorus concentrations have rophyll a index values adjusted for some estuaries where relative cover and Belt Sea follow a bimodal pattern stabilised at a low level after signifi cant variations in climatic conditions for had actually increased. over the last 20 years with peaks in the decreases in the beginning of the 1990s. 1993–2001 were at a consistent and Results from 2001 showed that beginning of the 1980s and in the decreasing lower level than that found total cover of upright vegetation on middle of the 1990s. During the period N/P RATIO in the mid 1980s. reefs in open waters tended to increase 1998–2001 biomass and essentially The optimal N/P-ratio, the Redfi eld according to the mean of the period total abundance of macrozoobenthos ratio, for phytoplankton growth is SUBMERGED AQUATIC VEGETATION 1994–2001. In general, algal coverage in the open waters showed the lowest 16:1. In the open Belt Sea and Kattegat The colonisation depth of eelgrass re- was low in years with relatively high values since the measurements started the winter DIN/DIP-ratio did not de- fl ects differences in water quality and runoff and high in years of low runoff two decades ago. Some evidence indi- viate much from the Redfi eld ratio and physical setting along estuarine gradi- during the period. Accordingly, there cates that reduced N-nutrient concentra- there was no general trend in annual ents. In the deeper, more protected was no over all trend in the distribu- tions, and possibly reduced diatom mean N/P-ratio during the period waters, reductions in eelgrass abun- tion on the monitored reef stations. abundance, when corrected for runoff, 1989–2001. dance towards the lower depth limit may have reinforced the decrease in In estuaries the winter DIN/DIP- correlate with light attenuation and INDIRECT EFFECTS zoobenthos stocks in recent years. ratio is high (>25) to very high (>100). are therefore more directly coupled to OXYGEN CONCENTRATIONS Correlation analyses between biolo- Annual mean N/P-ratios in the estu- changes in eutrophication. During the Analyses of the development in bot- gical variables and the North Atlantic aries-coastal waters showed an in- period 1989-2000 eelgrass colonisation tom water oxygen concentrations dur- Oscillation index and runoff of fresh- crease from 1989 to 1998 parallel to the depth showed no significant trend ing late summer/autumn in the Kat- water from Denmark, showed signifi - reduction in phosphorus load, and then and did not refl ect the slight ameliora- tegat-Belt Sea from the 1970s to late cant positive correlations with 1 or 2 a decrease to 2001 parallel to the de- tion of water clarity observed through 1980s/1990s showed significant de- years time lag, thereby indicating that crease in nitrogen load per runoff. the same period. During the period creases in all areas with a stratified climate also infl uences variations in In the North Sea the N/P-ratio was 1989-2001 eelgrass cover deeper than water column. The decrease was espe- benthic macrofauna. In particular generally high ranging between 25 and 2 meters showed no signifi cant trend, cially pronounced from the mid 1970s winter nutrient input, and likely the 60, except in the saline central North Sea but the cover of shallow populations to the late 1980s. There was no general spring phytoplankton bloom, did in- water. Also in the Skagerrak N/P-ratios was significantly reduced in inner development in the summer/autumn fl uence benthic abundance. were high at salinities lower than 33. estuaries and along open coasts. There bottom water minimum oxygen con- The number of coastal areas with is no obvious explanation for this pat- centration in the period 1989–2001. increasing species diversity were about DIRECT EFFECTS tern. Though eelgrass colonisation However, there was a tendency for an the same as those with decreasing CHLOROPHYL CONCENTRATIONS depth and abundance from intermedi- increase in minimum oxygen concen- diversity during the period 1998–2001. Since 1980 absolute concentrations of ate depths towards deeper waters are trations in spring (April–June). Never- Local factors like oxygen defi ciency is chlorophyll a in Danish open sea areas likely to be better response parameters theless, at the end of August 2002 an a possible reason for these differences. have been >50% above background to eutrophication than the abundance unusually widespread and serious During the same period there was no concentrations given by OSPAR. At of shallow populations. oxygen deficiency was observed in general change in abundance, biomass

Photo: Fyn County/Nanna Rask ﬁ rst chlorophyll a concentrations seem In 2001 many Danish estuaries still large areas of the inner Danish marine or number of species in coastal areas. Photo: NERI/Peter Bondo Christensen

102 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . SUMMARY, CONCLUSIONS AND THE FUTURE NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . SUMMARY, CONCLUSIONS AND THE FUTURE 103 A Coastal areas 1998 1999 2000 2001 Assessment criteria in 2001 B Open sea areas 1998 1999 2000 2001 Asessment criteria in 2001 NC MO BF SAV NM OD NC MO BF SAV NM OD 1. North Sea and 1. North Sea and 2. Skagerrak 2. Skagerrak • North Sea, coastal parts ÷ ÷ ÷ ÷ x x • North Sea, open parts + + + + x • Ringkjøbing Fjord ÷ ÷ ÷ ÷ x x x • Skagerrak, open parts + + + + x • Nissum Fjord ÷ ÷ ÷ ÷ x x x 3. Kattegat • Vadehavet ÷ ÷ ÷ ÷ x x x x • Kattegat, northern open parts +/÷ +/÷ +/÷ +/÷ x x • Skagerrak, coastal parts +/÷ +/÷ +/÷ +/÷ x • Kattegat, central open parts +/÷ +/÷ +/÷ +/÷ x x 3. Kattegat 4. Northern Belt Sea • Isefjord ÷ ÷ ÷ ÷ x x x x • Belt Sea, northern open parts ÷ ÷ ÷ ÷ x x • Limfjorden ÷ ÷ ÷ ÷ x x x x x 5. Little Belt • Mariager Fjord ÷ ÷ ÷ ÷ x x x x x x • Little Belt, northern open parts ÷ ÷ ÷ ÷ x x x x • Randers Fjord ÷ ÷ ÷ ÷ x x x • Little Belt, southern open parts ÷ ÷ ÷ ÷ x x x • Roskilde Fjord ÷ ÷ ÷ ÷ x x x x 6. Great Belt • Kattegat, coastal southern parts ÷ ÷ ÷ ÷ x x x • Great Belt, open parts ÷ ÷ ÷ ÷ x x • Kattegat, coastal western parts ÷ ÷ ÷ ÷ x x x x 7. The Sound 4. Northern Belt Sea • Øresund, northern part ÷ ÷ ÷ ÷ x x x • Horsens Fjord ÷ ÷ ÷ ÷ x x x x • Køge Bugt ÷ ÷ ÷ ÷ x x x • Odense Fjord ÷ ÷ ÷ ÷ x x x x x 8. Southern Belt Sea and • Sejerø Bugt ÷ ÷ ÷ ÷ x x x 9. Baltic Sea • Århus Bugt/Kalø Vig ÷ ÷ ÷ ÷ x x x x x • Southern Belt Sea, open parts ÷ ÷ ÷ ÷ x x x x 5. Little Belt • Arkona Bassin ÷ ÷ ÷ ÷ x x x • Augustenborg Fjord ÷ ÷ ÷ ÷ x x • Baltic Sea, east of Bornholm (÷) (÷) (÷) (÷) x x x • Flensborg Fjord ÷ ÷ ÷ ÷ x x • Kolding Fjord ÷ ÷ ÷ ÷ x x x • Vejle Fjord ÷ ÷ ÷ ÷ x x x • Åbenrå Fjord ÷ ÷ ÷ ÷ x x 6. Great Belt Table 4.1 A + B • Kertinge Nor ÷ ÷ ÷ ÷ x x x Fulfi lment of objectives and assessment • Kalundborg Fjord ÷ ÷ ÷ ÷ x x x criteria in selected coastal areas (A) and • Korsør Nor ÷ ÷ ÷ ÷ x x x selected open sea areas (B) from ÷ ÷ ÷ ÷ • Sydfynske Øhav x x x x 1998–2001. The assessment criteria are: ÷ ÷ ÷ • Karrebæk Fjord +/ +/ +/ x x x x NC – Nutrient concentrations • Karrebæksminde Bugt + + + + x x x x MO – Mass occurrence of algae • Dybsø Fjord + + + + x x x x BF – Benthic fauna 7. The Sound SAV – Submerged aquatic vegetation ÷ ÷ ÷ ÷ • Øresund, northern part x x NM – Annual nuisance macroalgae ÷ ÷ ÷ ÷ • Køge Bugt x x x OD – Oxygen depletion 8. Southern Belt Sea and 9. Baltic Sea EQOs impaired + • Bornholm, coastal waters ÷ ÷ ÷ ÷ x x EQOs close to fulfi lment +/÷ • Hjelm Bugt ÷ ÷ ÷ ÷ x x x x x EQOs fulfi lled + • Præstø Fjord ÷ ÷ ÷ ÷ x x x x

Based on Ærtebjerg et al. 2002. Photo: CDanmark

104 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . SUMMARY, CONCLUSIONS AND THE FUTURE NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . SUMMARY, CONCLUSIONS AND THE FUTURE 105

EFFECTIVENESS OF STRATEGIES The Danish government has in au- EQO-eutro AND MEASURES tumn 2002 taken initiatives to start The national target of an 80% reduc- discussions on new measures to be ta- OSPAR COMP Non-Problem Area Problem Area tion of phosphorus from sewage plants ken to combat eutrophication. The dis- Danish AEPS Compliance Non-Compliance (or Accommodated) and industry to fresh and marine waters cussions are expected mainly to focus EU WFD High Good Moderate Poor Bad was achieved already by 1996. The re- on discharges and losses of nitrogen and EU Habitats Directive Favourable Conservation Status Unfavourable Conservation Status ductions of nitrogen discharges from phosphorus from agriculture. Until EU Urban Wastewater Directive Less sensitive Sensitive areas sewage treatment plants have also recently, phosphorus from agriculture EU Nitrates Directive Less vulnerable zones Vulnerable zones been successful. The remaining and has not been considered an important EU Marine Strategy Compliance with goals Impaired ecosystem health still unsolved problem is related to los- source, as most of the phosphorus was

ses of nutrients from the agricultural considered to be retained in the root

sector in Denmark, even a 30% reduc- zone. However, improved knowledge EQO-eutro tion in losses from the root zone has on the phosphorus cycle indicates a been achieved. However, this is not greater loss of phosphorus from agri- Figure 4.1 only the case in Denmark, as almost culture than taken into calculation in Suggested identity all countries bordering the North Sea the previous action plans. between eutrophi- and the Baltic Sea have so-far not been The strategies and measures to Strategy. The quality objectives should duction in nitrogen (21%) is mainly cation related able to achieve the agreed reductions reach the goal of a healthy marine en- be parallel as illustrated in Figure 4.1. caused by reduced loses from agri- Ecological Quality in the losses from agriculture. vironment must focus on a reduction cultural soils while the reduction in Objectives (EQOs) It has been estimated that the Da- of discharges, emissions and losses for CONCLUSIONS phosphorus is due to extension of se- according to OSPAR nish Action Plans on the Aquatic Envi- all relevant sources and on ecological Eutrophication effects have been docu- wage treatment. Atmospheric nitro- Comprehensive ronment in 2003 will result in a change quality objectives for the marine en- mented in all Danish marine waters gen deposition to Danish marine wa- Procedure (COMP), in agricultural activities, which toge- vironment. With regard to reductions, every year since the beginning of the ters seems to have declined about 15% the existing Danish ther with the reductions of discharges the Danish Action Plans on the Aqua- 1980s. At present it can be concluded since 1989. Aquatic Environmen- from industries and municipal waste tic Environment focus on the major that quality criteria has only been fulfi l- tal Planning System water treatment plants, is likely to resources in order to achieve cost/ef- led in a restricted number of Danish es- NUTRIENT CONCENTRATIONS (AEPS), the Water duce discharges from these sources by fective reductions. However, future tuaries, coastal and open water areas. Nitrogen concentrations in estuarine Framework Directive nearly 50% (Grant et al. 2000). This cor- action plans should include all rele- The few coastal areas that meet the cri- and coastal areas have decreased sig- (WFD), the Habitat responds to a reduction of nitrogen in- vant sources. With regard to ecological teria are generally non-stratifi ed shal- nifi cantly after 1997 and since 1989 in Directive and the puts to marine areas by at least 35%. The quality objectives, the Danish objec- low water areas with a relatively re- the open waters. Concentrations of future EU Marine actual reductions in inputs to marine tives from 1984 are normative. They stricted load from land based sources. phosphorus have stabilised after a sig- Strategy. (Modifi ed waters due to reductions in other sour- have to be operational and expressed The causes for not fulfi lling the criteria nifi cant decrease in the beginning of from OSPAR ETG ces and sectors (atmospheric emissions, as numbers and values. This will be are primarily the effects of nutrient the 1990s. 01/7/1, Annex 6, scattered settlements, stormwater over- done as an integral part of the Danish load and oxygen depletion. 2001; Henriksen fl ows, freshwater aquaculture, and ma- implementation of the Water Frame- With corrections for variations in DIRECT EFFECTS et al. 2001 and riculture) will be somewhat higher. work Directive. This directive is a ma- precipitation, run off and other clima- Water clarity increased and phyto- Pedersen et al. 2001). Despite the achieved reductions in jor player in the years to come. A suc- tic related conditions it can be conclu- plankton biomass and production de- the nutrient inputs to marine waters, cessful implementation is a prerequi- ded that: creased signifi cantly in estuaries and the quality status of the majority of site for an informed management of coastal areas until the mid 1990s. The marine waters still does not fulfi l the marine eutrophication. The imple- NUTRIENT LOADS changes are less pronounced in open general quality objective with no or on- mentation process should not over- Since 1990 land based inputs of nitro- waters, although water clarity was highly a slight impact on fl ora and fauna. look the Habitat Directive and the gen and phosphorus to estuaries and er and diatom biomass as well as phy- However, improvements can be seen work within HELCOM and OSPAR coastal areas have been reduced by toplankton production were lower in

Photo: CDanmark in local areas. together with the future EU Marine 35% and 60%, respectively. The re- the 1990s than in the 1980s.

106 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . SUMMARY, CONCLUSIONS AND THE FUTURE NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . SUMMARY, CONCLUSIONS AND THE FUTURE 107 During the period 1989-2001 eel- environment. But the road to fulfi l- thus be complicated beyond reason. management should make it possible grass cover at water depths above 2 ment of eutrophication quality criteria It is likely that the implementation to calculate backwards from the nor- meters increased in the outer part of is long and winding. The existing crite- of the Water Framework Directive will mative defi nition of good ecolo gical the estuaries, but was signifi cantly re- ria and the acceptable deviations from lead to supplementary measures in status to concentrations, inputs, and duced at shallow depths in inner estu- reference conditions should be regard- catchment areas to specific coastal other human activities affecting eutro- aries and along open coasts. The depth ed as a starting point. The variability waters as vulnerable and sensitive phication status of marine waters. The distribution has not increased, but in found for different parameters is natur- estuaries and enclosed bays. The com- key question in the future is whether the inner estuaries actually decreased, ally large. It might therefore be reason- bination with legally binding quality diffuse sources can and will be ade- possibly due to oxygen depletion. able to develop global or type-specifi c objectives and integrated strategic quately managed. The amount of nuisance macroal- assessment criteria and, taking these as gae has been reduced (though this is a starting point, to develop site-specifi c not always noted in the cover) at shal- assessment criteria. low depths. Macroalgae cover at open Focusing on national reductions in water reefs shows no trends since 1994. inputs is reasonable. But inputs from adjacent seas should also be taken into INDIRECT EFFECTS account. It is likely that these inputs The biomass of macrozoobenthos in will decrease. However, future enlarge- open waters has decreased through ment of the European Union is likely to the 1990s following a reduction in the result in a development of the agricul- biomass of diatoms and phytoplank- tural sector in Poland and the Baltic Sta- ton production. Macrozoobenthic tes. This may result in an increase in los- communities in estuaries and coastal ses form cultivated farmland and even- areas show large inter annual varia- tually an increase in inputs to the Baltic tions, which seem to be caused by Sea, and thus an increase in the inputs to recurrent oxygen depletion. Danish waters from the Baltic Sea. However, nutrient enrichment, eu- THE FUTURE trophication and oxygen depletion are The experience from the years 1996 and naturally occurring phenomena. It is 1997, when precipitation and runoff the extent in space and the duration were very low and the 50% reduction and strength that is affected by human target for nitrogen reduction was ac- activities. These undesirable effects in tually met, clearly shows that the re- our marine waters can not be avoided, duction target of 50% in the input to but the anthropogenically induced the marine area considerably improves strengthening of these phenomena the quality status of the marine envi- should be reduced. ronment. The 50% reduction in nitro- The implementation of the Water gen inputs, therefore, needs to be met Framework Directive has to take the in order to reduce eutrophication prob- Habitat Directive and EU Marine Stra- lems in marine areas. tegy into account, especially with re- Trends in nutrient reductions are gard to the objectives on ecological rtebjerg on the right track, both with respect and conservation status. If not, the Æ to the reduction of discharges, emis- management of the ecosystems and sions and losses of nutrients to marine resources in the Danish marine water

waters and to the quality of the marine will face different protection levels and Photo: NERI/Gunni

108 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . SUMMARY, CONCLUSIONS AND THE FUTURE NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . SUMMARY, CONCLUSIONS AND THE FUTURE 109 GLOSSARY AND ABBREVIATIONS

Advection – the transfer of heat or matter ced where there is a depletion of oxygen, by horizontal movement of water masses. owing to very high organic productivity, and Aerobic – with the presense of oxygen. Is a lack of oxygen replenishment to the water used to describe chemical processes or or sediment, as in the case of stagnation or organisms that require oxygen. See also stratifi cation of the body of water. anaerobic. Aquatic – growing or living in or near water. Algae – a large assemblage of lower plants, ASP – short for Amnesic Shellfi sh Poisoning. formerly regarded as a single group, but The poisoning is caused by intake of shellfi sh now usually classifi ed in eight separate divi- which has accumulated certain algal toxins. sions or phyla, including the blue-green Atmospheric deposition – deposition of algae (Cyanophyta), green algae (Chloro- nutrients, heavy metals and other pollutants phyta), brown algae (Phaeophyta), red from the atmosphere. algae (Rhodophyta), diatoms and (Chryso- Autotroph – an organism which builds com- phyta). Marine macroalgae are commonly plex organic molecules from simple inorganic known as seaweeds. compounds. Also used to name organisms Photo: High-light Ammonia (NH3) – a colourless gas formed using photosynthesis in this process. by decomposition of protein, nitrogenous BAT – Best Available Techiques, means the bases and urea. It is easy soluble in water most advanced activities, processes, and ope- and its pungent smell is well known from rating methods and the methods most effec- ammonia water. tive in preventing or limiting pollution from + Ammonium (NH4 ) – a nitrogen com- a given sector. pound, an ion derived from ammonia. Benthos – those organisms attached to, liv- Anaerobic – without the presense of oxy- ing on, in or near the sea bed, river bed or gen. Is used to describe chemical processes lake fl oor. or organisms that does not require oxygen. Bio-available – matter that is available for See also aerobic. primary production or will become available Anoxic – the state of oxygen depletion with within the residence time of the water in a absence of oxygen. Anoxic sediments and given marine area. anoxic bottom waters are commonly produ- Biomass – the weight of organisms in a

110 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . GLOSSARY AND ABBREVIATIONS NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . GLOSSARY AND ABBREVIATIONS 111 certain area either described with reference free nitrogen (N2) by certain bacteria. Euphotic zone – the upper, illuminated where there is much freshwater input from to volume or area. Deposition – see atmospheric deposition zone of aquatic ecosystems. The zone of rivers producing surface waters of low salin- Bluegreen algae – marine and freshwater Detritus – small pieces of dead and decom- effective photosynthesis. In marine ecosys- ity, a zone where salinity increases rapidly unicellular, colonial or ﬁ lamentous bacteria. posing plant and animal material, i.e. tems it is much thinner than the deeper with depth (the halocline) and a deeper Resembles algae in the way that they have organic material. aphotic zone which is below the level of zone of more saline, denser waters. chlorophyll pigments and can perform pho- Diatom – a unicellular algae with siliciﬁ ed effective light penetration. It typically HELCOM – the Helsinki Commission. tosynthesis. walls. Diatoms often make up the majority reaches 20–30 meters in coastal waters Heterotroph – an organism that cannot Brackish water - water with a salt concen- of the spring bloom phytoplankton biomass. but extending to 100–200m in open ocean synthesise its own food and is dependent tration between 5-18ppt. Diffuse sources – larger geographical area, waters. on complex organic substances for nutri- C – carbon, see carbon biomass. excluding city areas, from which nutrients Eutrophication – eutrophication is en- tion. That is, it eats other organisms.

Carbon biomass – biomass as the amount or contaminants are washed out to the sea hanced inputs of nutrients and organic H2S – hydrogensulfi de. of carbon (C) in a given area or volume. (see also point sources). matter. Eutrophication can be a natural Hypoxia – see oxygen depletion. CHARM – an EU funded project which has DIN – dissolved inorganic nitrogen. The sum process, but is most often caused by ICES – International Council for the Explora- been developed to provide a scientifi c foun- of nitrate, nitrite and ammonium i.e. nitrogen humans. See also box 1. tion of the Sea. dation for fulfi lling the requirements of the EC that can be absorbed by plants. Fauna – animal organisms. Inorganic – a chemichal substance that Water Framework Directive in Baltic coastal Dinofl agellate – any of numerous minute, Flagellates – microscopic unicellular organ- does not involve neither organic life nor the waters. chiefl y marine protozoans of the order Dino- isms in aquatic systems. They move by the products of organic life, i.e. hydrocarbon Chl a – see Chlorofyll a. fl agellata, characteristically having two fl ag- use of one or more fl agella, a string-like groups.

Chlorophyll a – a specifi c plant pigment ella and a cellulose covering and forming extension from the cell. Amongst the fl agel- Ks – half-saturation constant. Nutrient con- essential for photosynthesis. It is quantita- one of the chief constituents of plankton. lates are both heterotophs, autotrophs and centration at which the phytoplankton nutri- tively the most important pigment found in They include bioluminescent forms and mixotrophs. ent uptake rate is reduced to half the maxi- all photosynthetic phytoplankton cells. forms that produce red tides. Flora – plant organisms. mum uptake rate. Chlorophyll – any of several green pigments DIP – dissolved inorganic phosphorus. The Food chain – refers to direct links be- Macroalgae – macroalgae are plants found in the chloroplasts of plants and in chemical form in which phosphorus can be tween organisms that describes how food that lack true roots, stems, leaves, and other photosynthetic organisms. They mainly absorbed by plants. energy is transfered through the ecosystem fl owers. They mostly live attached to a absorb red and violet-blue light energy for DSP – short for Diarrhetic Shellfi sh Poison- from the smallest primary producers to top hard substrate. the chemical processes of photosynthesis. ing. The poisoning is caused by intake of predators. An example from the marine Macrozoobenthos – animals larger than 1 Cladoceran – water fl ea. A small crustacean shellfi sh that has accumulated certain algal ecosystem is planktonic algae → copepods mm living attached to, on, in or near the sea with a carapace that forms a bivalved shield. toxins. → fi sh → seal. bed, river bed or lake fl oor. The cladocerans are suspension feeders and Eelgrass (Zostera marina) – a submerged Food web – a description of who eats who Marine – of, or pertaining to, the sea, the collect food with fi ne bristles on the trunk fl owering plant that grows along the major in an ecosystem. In its most simple form a continuous body of water covering most of appendages. part of the Danish coasts. food chain, but more commonly a net of the earth’s surface and surrounding its land Ciliate – a diverse group of one celled ani- Emission – release of chemicals to the organisms where several organisms are capa- masses. Marine waters may be fully saline, mal like organims. They possess cilia for loco- atmosphere. ble of eating the same food item. brackish or almost fresh. motion and in many species for suspension EQO – Ecological Quality Objective. Grazing – literally to feed on growing Mesozooplankton – animal plankton of feeding. Estuary – the transition area between a river grasses and herbage the term refers to the size 0.2 – 2.0 mm. Contaminants – are substances that are and the sea, i.e. an estuary is a body of water feeding habits of animals in the terrestrial Metabolism – the chemical change, con- toxic to living organisms. Most of them are that is formed when fresh water from rivers environment. In the aquatic environment structive and destructive, occuring in living hard to degrade in a natural environment fl ow into and mixes with salt water from the the analogue is organisms feeding on plant organisms. Copepod – a small free-living or parasitic ocean. In estuaries, the fresh river water is or plant like organisms. An example is cope- Metazooan – multicellular, motile animal crustacean. One of the most abundant blocked from directly entering the open ocean pods that graze on phytoplankton. organisms with cells organized into tissues marine zooplankton organisms. either by the surrounding mainland, penin- Halocline – a zone in which there are rapid, and controlled by a nervous system. Cyanobacteria – see bluegreen algae. sulas, barrier islands, or fringing salt marshes. vertical changes in salinity. The halocline is Microbial loop – a part of the pelagic Denitrifi cation – reduction of nitrates to EU – European Union. usually well-developed in coastal regions planktonic food web consisting of bacteria,

112 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . GLOSSARY AND ABBREVIATIONS NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . GLOSSARY AND ABBREVIATIONS 113 fl agellates and ciliates. Organisms in the mi- essential ones including nitrogen, potas- energy source. Most forms of photosynthe- power. Fine waterborne matter deposited crobial loop transfers energy from dissolved sium, calcium, sulphur and phosphorus. sis release oxygen as a by-product. or accumulated in beds. Includes mobile or organic carbon back to the copepods. Oligotrophic – applied to waters or soils Phytoplankton – the plant plankton and soft substrates such as cobbles, pebbles, Microzooplankton – animal plankton of that are poor in nutrients and have low pri- primary producers (i.e. drifting, more or less sand and mud. the size 0.02 – 0.2 mm. mary productivity. (See also eutrophic). microscopic, photosynthetic organisms) of Stratifi cation – in the sea it is a boundary Mixotroph – an organism which is capable Organic – organic compounds contain the aquatic ecosystems. between two water masses of different spe- of performing photosynthesis as well a living element carbon. Of, relating to, or derived Plankton – free passively fl oating organisms cifi c gravity. The stratifi cation is typically partly as a heterotroph. Mixotrophy is com- from living organisms. (animals, plants, or microbes) in aquatic sys- formed by differences in temperature or monly found among dinofl agellates. The Organism – an individual form of life. An tems. salinity or both. mixotrophy appears to have different func- animal, plant or bacteria. Point source – discharge from one point. Tot-N – see TN. tions in different dinofl agellates; in some OSPAR – the Oslo and Paris Commission. The sources are sewage treatment plants, Tot-P – see TP. primarily phototrophic dinofl agellates feed- Oxygen – a non-metallic element constitut- industrial plants, storm water runoff, fresh TN – total nitrogen, which includes dis- ing appears to be a mechanism for obtaining 21 percent of the atmosphere by vol- water aquaculture and mariculture. solved inorganic nitrogen and organically ing limiting inorganic nutrients, in some pri- ume. Oxygen is produced by autotrophic Population – all the organisms that con- bound nitrogen. marily heterotrophic dinofl agellates photo- organisms and is vital to oxygen breathing stitute a specifi c group or occur in a speci- TP – total phosphorus, which includes diss- synthesis appears to be a mechanism for organisms. fi ed habitat. soleved inorganic phosphorus and organi- supplementing carbon metabolism. Oxygen depletion – a situation where the Predator – an organism that lives by prey- cally bound phosphorus. Molar – designating a solution that contents demand for oxygen has exceeded the supply ing on other organisms. Water column – the open-water environ- one mole of solution per litre. of oxygen leading to low concentrations of Primary production – the production by ment, as distinct from the bed or shore, µ (prefi x) – micro, 10-6. oxygen. Low oxygen concentrations are nor- autotrophs. which may be inhabited by swimming N – see nitrogen. mally found in the water close to the sea Protozooplankton – zooplankton consist- marine or freshwater organisms. (See NAO – North Atlantic Oscillation is an index bottom. In Denmark, concentrations below ing of only one cell. pelagic).

that is based on the difference in atmos- 4 mg O2 per liter are deﬁ ned as oxygen Rotifer – or rotatoria commonly called Wet deposition – deposition of matter by

pheric pressure between the Azores and depletion and concentrations below 2 mg O2 wheel animals. A microscopic aquatic ani- rain. Iceland. per liter are deﬁ ned as severe acute oxygen mal with at ciliated organ called a corona, WFD – EU Water Framework Directive.

Nitrate (NO3) – an important nitrogen con- depletion. which looks like a rotating wheel. Mostly a Zooplankton – small planktonic animals in taining nutrient. The chemical form in which P – see phosphorus. freshwater animal although some marine fresh- or sea water with almost none or no plants uptake most of their nitrogen. It is the Parthenogenesis – litterally it means virgin species exist. swimming capacity. They are, therefore, salt of nitric acid. descent and refers to production og off- Runoff – that part of rainfall that is not transported randomly by water movements. Nitrogen (N) – is a chemical element that spring where the female has not been fer- absorbed in soil but falls on or ﬂ ows directly constitutes about 80% of the atmosphere tilized by a male. into streams and rivers. by volume. Nitrogen is an important part of Pelagic – the open-water environment, or Salinity – a measure of the total quantity of proteins and is essential to living organisms. water column, as distinct from the bed or dissolved substances in water, in parts per

Oxides of Nitrogen (NOx) – chemical com- shore, inhabited by swimming marine thousand (ppt, per mille) by weight, when all pounds, gases, formed by nitrogen and oxy- organisms. organic matter has been completely oxidised,

gen. NOx are formed by the combustion of Phosphate (PO4) – is an important phos- all carbonate has been converted to oxide, oil, gasoline, coal and gas. NOx is soluble in phorus containing nutrient. It is the chemical and bromide and iodide to chloride. The water and reacts with water or substances in form in which plants uptake phosphorus. salinity of ocean water is in the range 33-38 water to form nitrate. Phosphorus (P) – a non-metallic chemical ppt, with an average of 35 ppt. Nutrient – chemical elements which are element. Secchi depth – a measure of the clarity of involved in the construction of living tissue Photosynthesis – the process in green the water. that are needed by both plants and animals. plants and certain other organisms by Sediment – any material transported by The most important in terms of bulk are which carbohydrates are synthesised from water that will ultimately settle to the bot- carbon, hydrogen and oxygen, with other carbon dioxide and water using light as an tom after the water loses its transporting

114 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . GLOSSARY AND ABBREVIATIONS NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS . GLOSSARY AND ABBREVIATIONS 115 WHERE CAN I READ MORE?LIST OF CONTRIBUTORS

RELEVANT LINKS • Ærtebjerg et al. (2002): Marine områder This report is based on contributions from a National Environmental Research Institute and • OSPAR: http://www.ospar.org 2001 - Miljøtilstand og udvikling. Techni- large number of people working at the the Danish Environmental Protection Agency, • HELCOM: http://www.helcom.fi cal report from DMU nr. 419. (In Danish Department of Marine Ecology (ME), Depart- Marine Division (MD). Information on who has • Department of Marine Ecology, with English Summary). ment of Atmospheric Pollution (ATMI) and contributed can be seen below. NERI: http://www.dmu.dk/1_om_dmu/ Department of Freshwater Ecology (FE) at the 2_afdelinger/3_hav/default_en.asp SELECTED PAPERS • Annual assessment reports on the • Cloern, J.E (2001): Our evolving concep- The following authors have contributed to the individual sections of this report: status of the marine environment in Den- tual model of the coastal eutrophication 1 J.H. Andersen, ME, NERI; O.S. Hansen, ME, NERI mark: http://www.dmu.dk/1_Om_DMU/ problem. – Mar. Ecol. Prog. Ser. 210: 223- 2.1 G. Ærtebjerg, ME, NERI; T. Christiansen, ME, NERI 2_tvaer-funk/3_fdc_mar/publikationer.asp 253. 2.2 G. Ærtebjerg, ME, NERI; T. Christiansen, ME, NERI • CHARM: http://charm.dmu.dk • Conley, D.J., H. Kaas, F. Møhlenberg, B. 2.3 G. Ærtebjerg, ME, NERI; T. Ellerman, ATMI, NERI; N.B. Ovesen, FE, NERI • Marine monitoring in Denmark: Rasmussen & J. Windolf (2000): Charac- 2.4 G. Ærtebjerg, ME, NERI; J. Carstensen, ME, NERI; D.J. Conley, ME, NERI http://m-fdc.dmu.dk teristics of Danish Estuaries. – Estuaries 2.5 P. Henriksen, ME, NERI; S. Markager, ME, NERI Vol. 23, No. 6:820-837. 2.6 C.M. Andersen, ME, NERI; T.G. Nielsen, ME, NERI RELEVANT BOOKS AND REPORTS • Nixon, S. (1995): Coastal marine eutrophi- 2.7 G. Ærtebjerg, ME, NERI • Christensen et al. (1996): The Danish cation: a defi nition, social causes, and 2.8 P.B. Christensen, ME, NERI; N. Risgaard-Petersen, ME, NERI; H. Fossing, ME, NERI Marine Environ ment. Have Political Action future concerns. – Ophelia 41: 199-219. 2.9 D. Krause-Jensen, ME, NERI; O.S. Hansen, ME, NERI: K. Dahl, ME, NERI improved its State? Havforskning fra Miljø- 2.10 A.B. Josefson, ME, NERI; J. Hansen, ME, NERI styrelsen, 62. SELECTED READING ON POLICY 2.11 J.H. Andersen, ME, NERI; O.S. Hansen, ME, NERI • HELCOM (2002): Environment of the Bal- AND MANAGEMENT 3 J.H. Andersen, ME, NERI; J. Brøgger Jensen, MD, DEPA; H. Karup, MD, DEPA tic Sea Area 1994–1998. Bal. Sea Environ. OF THE AQUATIC AND MARINE 4 G. Ærtebjerg, ME, NERI; J.H. Andersen, ME, NERI; O.S. Hansen, ME, NERI; H. Karup, Proc. No 82 B. ENVIRONMENT MD, DEPA • Henriksen et al. (2001): Marine områder • Danish EPA (2000): Aquatic Environment 2000 – Miljøtilstand og udvikling. Techni- 1999. Environmental Investigations from cal report from DMU nr. 375. (In Danish the Danish EPA No. 3, 2000. Preface, Where can I read more?, and List of • Ole H. Manscher, Department of Marine with English Summary). • Conley, D.J., S. Markager, J. Andersen, T. contributors, as well as the brief introductions Ecology, NERI for management of MADS, • Jørgensen & Richardson (1996): Eutrophi- Ellermann & L.M. Svendsen (2002): to chapters 1, 2, and 4 have been drafted the National Marine Database System and cation in Coastal Marine Ecosystems. Coastal Eutrophication and the Danish and edited by the editors. retrieval of data, the latter accomplishment Coastal and Estuarine Studies 52. Ameri- National Aqua tic Monitoring and Assess- The annex on the eutrophication status being a trade mark of a true professional. can Geophysical Union. Washington D.C. ment Program. – Estuaries Vol 25, No 4B: in the western Baltic Sea, the Danish Belt Sea, • Associate professor Benni W. Hansen, Ros- • Lomstein (1998): Havmiljøet ved årtusin- 848-861. Kattegat, Skagerrak, North Sea and the Wad- kilde University Centre and senior scientist deskiftet. Olsen & Olsen. (In Danish). • Fyn County (2001): Aquatic Environment den Sea has been compiled by the Marine Daniel J. Conley, Université P. et M. Curie, • OSPAR (2000): Quality Status report 2000, of Funen, Denmark, 1976-2000. 148 p. Division of the Danish Environmental Protec- Paris for helpful and constructive com- Region II – Greater North Sea. OSPAR • Iversen, T.M., R. Grant & K. Nielsen tion Agency. ments to a draft manuscript. Commission. (1998): Nutrient enrichment of European • Colin Stedmon, Department of Marine • Rabalaies & Turner (2002): Coastal inland and marine waters with special ACKNOWLEDGEMENTS Ecology, NERI for linguistic corrections. Hypoxia. Consequences for Living attention to Danish policy measures. – En- The editors would like to thank: • Danish Environmental Protection Agency Resources and Ecosystems. Coastal and vironmental Pollution 102, S1: 771-780. • All the Danish Counties and their dedica- and National Monitoring Co-ordination Estuarine Studies 58. American Geophysi- ted staff who are responsible for data col- Section (NERI) for fi nancial support. cal Union. Washington D.C. lection in Danish coastal waters.

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120 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS 121 ANNEXES Water depth in Danish marine coastal waters MAP OF THE DANISH MARINE WATERS

10 m 20 m The estuaries, coastal waters and open marine waters mentioned in the report are indicated on 2 30 m the map on the opposite page. Names referring to the numbers on the map are listed below. 40 m The translation of the Danish names to English are: “Bredning“ – a broad; “Bugt“ – a bay; 50 m 100 m “Bælt“ – a belt; “Bælthav“– a belt sea; “Fjord“ – an inlet, but often used as a general term for 200 m estuaries and should not be confused with the definition of a classical fjord with a sill; “Nor“ – 300 m a cove; “Sund“– a strait or sound; “Vig“ – a cove; “Øhav“ – archipelago; “Farvand“ – waters. 400 m 500 m 600 m Danish marine Estuaries and 13 Hovvig 26 Limfjorden 40 Risgårde Bredning sub areas coastal waters 14 Isefjord 27 Lovns Bredning 41 Roskilde Fjord 1 North Sea 1 Als Sund 15 Kalundborg Fjord 28 Løgstør Bredning 42 Rødsand 2 Skagerrak 2 Augustenborg Fjord 16 Kalø Vig 29 Mariager Fjord 43 Sejrø Bugt 26 3 Kattegat 3 Bjørnsholm Bugt 17 Karrebæk Fjord 30 Nakkebølle Fjord 44 Skive Fjord 28 47 3 4 Northern Belt Sea 4 Båring Vig 18 Karrebæksminde 31 Nissum Bredning 45 Smålandsfarvandet 3 50 5 Little Belt 5 Dalby Bugt Bugt 32 Nissum Fjord 46 Sydfynske Øhav 40 29 31 27 6 Great Belt 6 Dybsø Fjord 19 Kertinge Nor 33 Nivå bugt 47 Thisted Bredning 44 37 11 7 The Sound 7 Ebeltoft Vig 20 Kieler Bugt 34 Nørrefjord 48 Vadehavet 8 Southern Belt Sea 8 Flensborg Fjord 21 Knebel Vig 35 Odense Fjord 49 Vejle Fjord 32 16 9 Baltic Sea 9 Genner Fjord 22 Kolding Fjord 36 Præstø Fjord 50 Visby Bredning 21 1 52 7 10 Haderslev Fjord 23 Korsør Nor 37 Randers Fjord 51 Åbenrå Fjord 11 Hevring Bugt 24 Køge Bugt 38 Ringgård Bassin 52 Århus Bugt 39 4 33 12 Horsens Fjord 25 Langelandssund 39 Ringkøbing Fjord 12 43 14 41 7 49 15 4 5 22 35 24 19 6 OVERALL CLASSIFICATION 48 23 10 17 KEY TO THE TABLE AT PAGE 124 AND 125 5 34 18 36 9 38 25 6 NI Riverine total N and total P inputs and direct discharges 51 30 45 1 46 2 9 DI Winter DIN and/or DIP concentrations 13 8 NP Increased winter N/P ratio Ca Maximum and mean Chlorophyll a concentration 42 Ps Region/area specific phytoplankton indicator species 8 Mp Macrophytes including macroalgae

O2 Degree of oxygen deficiency Ck Changes/kills in zoobenthos and fish kills Oc Organic carbon/organic matter At Algal toxins (DSP/PSP mussel infection events) + Increased trends, elevated levels, shifts or changes in the respective assessment parameters

122 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS 123 Area Category I Category II Category III and IV Initial classiﬁ cation Appraisal of all relevant information (concerning the harmonised assessment criteria Final Assessment Degree of nutri- Direct effects Indirect effects/ their respective assessment levels and the supporting environmental factors) classiﬁ cation period ent enrichment other possible effects

Baltic Sea NI + Ca + O2 + At ? Problem area Blooms of Nodularia spumigena, Aphanizomenon, oxygen depletion Problem area 1989–2001 DI + Ps + Ck NP + Mp + Oc

Southern Belt Sea NI + Ca + O2 + At ? Problem area Elevated inputs and/or increased trends of nutrients. Elevated concentrations of DIN. Elevated chlo- Problem area 1989–2001 DI + Ps + Ck + rofyll a concentrations, Blooms of Nodularia spumigena, Aphanizomenon, Karenia mikimotoi, NP + Mp + Oc Pseudo-nitzschia, Prorocentrum minimum and Chrysochromulina, decreased depth limit of eelgrass, oxygen depletion

Little Belt NI + Ca + O2 + At + Problem area Elevated inputs and/or increased trends of nutrients. Elevated concentrations of DIN. Elevated chlorofyll Problem area 1989–2001 DI + Ps + Ck + a concentrations, Blooms of Nodularia spumigena, Aphanizomenon, Dictyocha speculum, Karenia miki- NP + Mp + Oc motoi, Pseudo-nitzschia, Prorocentrum minimum and Chrysochromulina, decreased depth limit of eelgrass, oxygen depletion and algae toxins found in mussels in some areas.

Great Belt NI + Ca + O2 + At ? Problem area Elevated inputs and/or increased trends of nutrients. Elevated concentrations of DIN. Elevated chlorofyll a Problem area 1989–2001 DI + Ps + Ck concentrations, Blooms of Nodularia spumigena, Aphanizomenon, Karenia mikimotoi, Pseudo-nitzschia, NP + Mp + Oc Prorocentrum minimum and Chrysochromulina, decreased depth limit of eelgrass, oxygen depletion

The Sound NI + Ca + O2 + At ? Problem area Elevated inputs and/or increased trends of nutrients. Elevated concentrations of DIN. Elevated chlorofyll Problem area 1989 - 2001 DI + Ps + Ck a concentrations, Blooms of Nodularia spumigena, Aphanizomenon, Karenia mikimotoi, Pseudo-nitz- NP + Mp + Oc schia, Prorocentrum minimum and Chrysochromulina, decreased depth limit of eelgrass, oxygen depletion

Northern Belt Sea NI + Ca + O2 + At + Problem area Elevated inputs and/or increased trends of nutrients. Elevated concentrations of DIN. Elevated chlorofyll Problem area 1989–2001 DI + Ps + Ck + a concentrations, Blooms of Nodularia spumigena, Karenia mikimotoi, Pseudo-nitzschia, Prorocentrum NP + Mp + Oc minimum and Chrysochromulina, decreased depth limit of eelgrass, oxygen depletion and algae toxins found in mussels in some areas.

Kattegat NI + Ca + O2 + At + Problem area Elevated inputs and/or increased trends of nutrients. Elevated concentrations of DIN. Elevated chlorofyll Problem area 1989–2001 Coastal areas DI + Ps + Ck + a concentrations, Blooms of Chatonella, Karenia mikimotoi, Pseudo-nitzschia, Gymnodinium chloropho- NP + Mp + Oc rum, Prorocentrum minimum and Chrysochromulina, decreased depth limit of eelgrass, oxygen depletion and algae toxins found in mussels in some areas (Limfjorden).

Kattegat NI + Ca + O2 + At – Problem area Elevated inputs and/or increased trends of nutrients. Elevated concentrations of DIN. Elevated chlorofyll Problem area 1989–2001 Open areas DI – Ps + Ck + a concentrations, Blooms of Chatonella, Karenia mikimotoi, Gymnodinium chlorophorum and Chryso- NP – Mp +/– Oc chromulina. Oxygen depletion.

Skagerrak NI + Ca + O2 – At – Problem area Concentration of N and P elevated due to transboundery input (Jutland Coastal Current from German Problem area 1989–2001 Coastal area DI + Ps + Ck – Bigth). Elevated concentrations of DIN. Elevated chlorofyll a concentrations, Blooms of Chatonella, Kare- NP + Mp + Oc nia mikimotoi and Gymnodinium chlorophorum

Skagerrak NI – Ca + O2 – At – Problem area Elevated chlorofyll a concentrations, Blooms of Chatonella and Karenia mikimotoi. Problem area 1989–2001 Open area DI – Ps + Ck NP – Mp Oc

North Sea NI + Ca + O2 – At – Problem area Concentration of N and P elevated due to transboundery input (Jutland Coasta Current from German Problem area 1989–2001 Coastal area DI + Ps + Ck – Bigth). Blooms of Phaocystis, Pseudo-nitzschia, Karenia mikimotoi and Chatonella. The western limit of NP + Mp + Oc the area to be deﬁ ned.

Central North Sea NI – Ca ? O2 – At – Non problem area No elevated nutrient concentrations. The limit between the open area and the coastal area needs to be Non problem All available DI – Ps ? Ck – speciﬁ ed. A possible potential problem area in between the coast and the open sea area should also be area data NP – Mp ? Oc identiﬁ ed.

Wadden Sea NI + Ca + O2 – At + Problem area Concentration of N and P elevated due to local and transboundery input (Jutland Coastal Current from Problem area 1989–2001 DI + Ps – Ck – German Bigth). Mass occurrence of algae, including annual nuissance macroalge. Algae toxins found in NP + Mp + Oc mussels in some areas.

124 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS 125 DATA SHEET

Title: Nutrients and Eutrophication in Danish Marine Waters Subtitle: A Challenge for Science and Management Editors (2003): Gunni Ærtebjerg, Jesper H. Andersen & Ole S. Hansen Department: Department of Marine Ecology Publisher: Ministry of the Environment, National Environmental Research Institute © Date of publication: March 2003 Editing complete: December 2002 Financial support: Danish Environmental Protection Agency and Environmental Monitoring Coordination Section (NERI) Please cite as 2003: Ærtebjerg, G., Andersen, J.H. & Hansen, O.S. (Eds.): Nutrients and Eutrophi- cation in Danish Marine Waters. A Challenge for Science and Management. National Environmen- tal Research Institute. 126 pp. Reproduction is permitted, provided the source is explicitly acknowledged. Abstract: Eutrophication – or nutrient over-enrichment – of Danish marine waters can to a large extent be attributed to excess nutrients fl owing from upstream watersheds into coastal waters. The nutrient enrichment can result in algae blooms, oxygen depletion, kills of fi sh and organisms living at the seabed as well as other undesired effects. The result is impairment of environmental and habitat quality objectives. This assessment report explains technical aspects of eutrophication and assesses the present situation. In addressing abatement strategies, the assessment discusses the importance of developing normalised indices and setting ecological quality objectives. The assessment also reviews the National Action Plans on the Aquatic Environment from 1987 and 1997 and other policy options for reducing the losses and discharges of nutrients. Keywords: Eutrophication, nutrient enrichment, coastal waters, estuaries, nitrogen, phosphorus, oxygen depletion, phytoplankton, submerged aquatic vegetation, zoobenthos, Action Plan on the Aquatic Environment, environmental protection, management of aquatic resources Layout and drawings: Britta Munter, Grafi sk Værksted, NERI Cover photos: Nanna Rask, Fyn County; Henny Rasmussen, DIAS & Bent Lauge Madsen External referee: Benni W. Hansen, Roskilde University Centre, Denmark Internal referee: Daniel J. Conley, Université P. et M. Curie, Paris, France ISBN: 87–7772–728–2 ISSN (print): 0905–815X ISSN (electronic): 1600–0048 Printed by: Schultz Grafi sk, Certifi ed under ISO 14001 and ISO 9002 Paper quality: Cyclus print Number of pages: 126 Circulation: 800 Price: DKK 250,- (incl. 25% VAT, excl. freight) Internet-version: The report is also available as a PDF-fi le from NERI’s homepage http://www.dmu.dk For sale at: Ministry of the Environment Frontlinien Strandgade 29 DK-1401 Copenhagen K Tel. +45 3266 0200 [email protected]

126 NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS NUTRIENTS AND EUTROPHICATION IN DANISH MARINE WATERS