1.8 Transitional waters and coastal waters 1.8 Transitional waters and coastal waters 1.8.1 Odense Fjord Northern Belt Sea Figure 1.8.1 ODF22B Map of Odense Fjord Odense Fjord is a shallow (average depth 2.25 indicating the bound- m) mesohaline estuary located in the northern aries of the inner fjord part of Fyn (Figure 1.8.1). The submerged area (Seden Strand) and outer fjord. The three encompasses 63 km2. The fjord can be subdivided monitoring stations, into a smaller inner fjord, Seden Strand, which SS8 in Seden Strand, has an average depth of 0.8 m and accounts for a ODF17 in the outer little less than 1/4 of the submerged area, and an part of the fjord and E g outer part which has an average depth of 2.7 m e n ODF22B in the boun- s G OTTERUP e a D b and accounts for the remaining approx. 3/4. e Odense e dary zone outside the e t p Fjord fjord are indicated. By far the major source of riverine runoff to the Outer fjord is the River Odense, which fl ows into the fjord inner part of Seden Strand. The water exchange ODF17 between the fjord and the open sea (Northern MUNKEBO Belt Sea) takes place via Gabet, the narrow mouth KERTEMINDE Seden Ø Strand e g i of the fjord. Using a hydrodynamic model it has t S SS8 l been calculated that the residence time of River Depth na Ca 30-40 m se n l R Odense water is short, around a yearly average 20-30 m de a n i O a v 10-20 m C e ld r of 17 days for the fjord as a whole and 9 days for 6-10 m O O d 4-6 m e n Seden Strand. 0 5 km 2-4 m s ODENSE Odense Harbour e Nutrient loading of the fjord from anthropo- 0-2 m genic sources is high due to the large size of the catchment (1 060 km2), which corresponds to one third of the area of Fyn and is characterized by intensive agricultural production and a high fl uorescence are measured electronically every population density (see Section 1.1). Thus in 2001, 10–20 cm along the water column; the oxygen for example, total loading from the catchment and concentration is alternatively measured using the the atmosphere amounted to approx. 32 g N (or Winkler technique. Transparency is measured 2.3 mol N) and approx. 0.88 g P (or 0.03 mol P) using a Secchi disc. per m2 of fjord, with atmospheric deposition only Nutrient concentrations – total nitrogen, am- accounting for 5% or less. Given this high level of monium, nitrate (+nitrite), total phosphorus, loading, Odense Fjord can be characterized as a orthophosphate and silicate – are measured in eutrophic water body. surface water samples and in the bottom water at Both national and local initiatives initiated in the deeper stations. the late 1980s have helped reduce this high level Phytoplankton conditions are evaluated by of loading, and the point-source dominated P load measurement of chlorophyll a at several depths has since been reduced by a factor of 6–7. The N and of primary production (C-14 method). At load, which is predominantly attributable to dif- ODF17 in addition, carbon biomass is measured fuse loading from farmland, has only been reduced at species and taxa level. by around 1/3 though, (see also Section 3). A number of other variables are monitored concomitantly with these pelagic activities, but Monitoring programme with varying frequency. These are: Benthic in- Pelagic variables are monitored weekly at three vertebrate number, biomass and fi ltration capac- stations representative of the fjord and its border ity; macroalgal and rooted macrophyte species zone: SS8 (depth 0.8 m) in the shallow inner composition, coverage and (to a lesser extent) fjord, Seden Strand, ODF17 (depth 8.5 m) in the biomass; nutrient and oxygen exchange across outer deep part of the fjord, and ODF22B (depth the sediment-water interface; continual hydro- 17 m) in the border zone outside the fjord (Figure graphic measurements for use in a deterministic, 1.8.1). hydrodynamic 3D-model, which is coupled to a The physical variables monitored are salinity, dynamic eutrophication model; and the content temperature and light attenuation, which togeth- and effects of hazardous substances in water, er with oxygen concentration and chlorophyll sediment and biota. Odense PRB Odense Pilot River Basin 55 1.8 Transitional waters and coastal waters Hydrographic conditions and nutrient condi- due to the high runoff, and are highest (median tions in Odense Fjord 3 500–4 000 µg N/l) in the inner fjord near the The seasonal variation in the salinity of the fjord main freshwater outfl ow, decreasing towards the and border zone is illustrated in Figure 1.8.2 open sea, i.e. approx. 1 400 µg N/l in the outer based on data for the past seven years. The sa- fjord and approx. 350 µg N/l outside the fjord. In linity in Seden Strand varies during the year be- the summer the concentrations of total nitrogen tween approx. 10 and 15 PSU (monthly median), are lower, the median values being around 650, being lowest in winter/spring when riverine run- 450 and 250 µg N/l, respectively. off is highest. The salinity is higher in the outer Corresponding estuarine winter gradients also fjord (16–21 PSU), and highest outside the fjord occur for total phosphorus (Figure 1.8.4). In this (16–24 PSU). The water temperature at the three case the winter median is approx. 110, 50 and 30 stations varies between approx. 0 and 25ºC. µg P/l in the inner fjord, outer fjord and border The estuarine gradient in nutrient concentra- zone, respectively. A special feature of the nutri- tions is illustrated in Figure 1.8.3 (nitrogen) and ent cycle in the fjord is the almost bell-shaped 1.8.4 (phosphorus). The concentrations of total course of phosphorus concentration over the nitrogen are generally higher in the winter period summer, which also comprises the yearly maxi- Salinity, surface Total N, surface PSU µg N/l Figure 1.8.2 (left) 90% quantile SS8 7 000 90% quantile SS8 Surface water salinity 24 Median 1996-02 Median 1996-02 at three stations inside 10% quantile 6 000 10% quantile and outside Odense 20 Fjord (see Figure 1.8.1) 5 000 16 for the period 1996– 4 000 2002. The median and 12 10% and 90% quan- 3 000 8 tiles are shown together 2 000 with all the data. 4 1 000 0 0 JFMAMJJASONDJ JFMAMJJASONDJ PSU µg N/l Figure 1.8.3 (right) Surface water con- 90% quantile ODF17 90% quantile ODF17 28 Median 1996-02 Median 1996-02 4 000 centration of total 10% quantile 10% quantile nitrogen at three 24 stations inside and 3 000 outside Odense Fjord 20 (see Figure 1.8.1) for 2 000 the period 1996–2002. 16 The median and 10% 1 000 and 90% quantiles are 12 shown together with all the data. 8 0 JFMAMJJASONDJ JFMAMJJASONDJ PSU µg N/l 90% quantile ODF22B 700 90% quantile ODF22B 32 Median 1996-02 Median 1996-02 10% quantile 600 10% quantile 28 500 24 400 20 300 16 200 12 100 8 0 JFMAMJJASONDJ JFMAMJJASONDJ Odense 56 PRB Odense Pilot River Basin 1.8 Transitional waters and coastal waters Total P, surface mum. The highest median concentrations are µg P/l found in the inner fjord, reaching a maximum Figure 1.8.4 90% quantile SS8 Surface water con- of approx. 200 µg P/l as compared with approx. 500 Median 1996-02 centration of total 100 µg P/l in the outer fjord. The concentration 10% quantile phosphorus at three 400 peak is primarily attributable to release from stations inside and the sediment iron-bound phosphorus pool and outside Odense Fjord 300 (to a lesser extent) point-source discharges of (see Figure 1.8.1) for phosphorus and poorer water exchange during 200 the period 1996–2002. the summer. The phosphorus peak is not seen in The median and 10% the surface water outside the fjord; on the other 100 and 90% quantiles are hand, oxygen defi cit-dependent accumulation of shown together with all phosphorus (and nitrogen) often occurs through- 0 the data. out the summer and autumn in the bottom layer JFMAMJJASONDJ of these stratifi ed open marine waters. µg P/l During the growth season, nutrient concentra- 90% quantile ODF17 tions in the fjord are potentially limiting for the 160 Median 1996-02 phytoplankton, although exhibiting temporal 10% quantile and spatial variation (data not shown). Potential P limitation generally exists in the spring, and 120 potential N limitation in the summer. Moreover, nutrient limitation of the phytoplankton is gen- 80 erally more pronounced in the outer fjord (where silicate can also be potentially limiting for dia- 40 toms) than in Seden Strand. Not surprisingly, the greatest potential nutrient limitation exists at the 0 considerably lower nutrient levels found in the JFMAMJJASONDJ open marine waters outside the fjord. µg P/l 90% quantile ODF22B Biological conditions in Odense Fjord 50 Median 1996-02 Despite the high nutrient loading of the fjord, 10% quantile pelagic phytoplankton biomass, which is domi- 40 nated by diatoms, is relatively low (data not shown). At the same time, the seasonal devel- 30 opment of the phytoplankton biomass is very 20 dynamic, with rapidly shifting concentrations of chlorophyll a.