National Environmental Research Institute University of Aarhus . Denmark NERI Technical Report No. 683, 2008 Macroalgae and phyto- plankton as indicators of ecological status of Danish coastal waters [Blank page] National Environmental Research Institute University of Aarhus . Denmark NERI Technical Report No. 683, 2008 Macroalgae and phyto- plankton as indicators of ecological status of Danish coastal waters Jacob Carstensen Dorte Krause-Jensen Karsten Dahl Peter Henriksen Data sheet Series title and no.: NERI Technical Report No. 683 Title: Macroalgae and phytoplankton as indicators of ecological status of Danish coastal waters Authors: Jacob Carstensen, Dorte Krause-Jensen, Karsten Dahl & Peter Henriksen Department: Department of Marine Ecology Publisher: National Environmental Research Institute © University of Aarhus - Denmark URL: http://www.neri.dk Year of publication: December 2008 Editing completed: December 2008 Referees: Henning Karup and Jens Brøgger Jensen Financial support: Danish Environmental Protection Agency (EPA) - Water Unit Please cite as: Carstensen, J., Krause-Jensen, D., Dahl, K. & Henriksen, P. 2008: Macroalgae and phytoplankton as indicators of ecological status of Danish coastal waters. National Environmental Research Institute, University of Aarhus. 90 pp. - NERI Technical Report No. 683. http://www.dmu.dk/Pub/FR683.pdf Reproduction permitted provided the source is explicitly acknowledged. Abstract: This report contributes to the development of tools that can be applied to assess the five classes of ecological status of the Water Framework Directive based on the biological quality elements phytoplankton and macroalgae. Nitrogen inputs and concentrations representing reference conditions and boundaries between the five ecological status classes were calculated from estimates of nitrogen inputs from Denmark to the Danish straits since 1900 combined with expert judgement of the general environmental conditions of Danish waters during different time periods. From these calculated nitrogen concentrations and a macroalgal model ecological status class boundaries were established for six macroalgal indicators in a number of Danish estuaries and coastal areas. Furthermore, site-specific correlations between concentrations of nitrogen and chlorophyll a were used to define reference conditions and ecological status class boundaries for the phytoplankton metric ‘mean summer concentration of chlorophyll a’ in several Danish estuaries and coastal areas. Precision of the two different chlorophyll a indica- tors ‘summer mean’ and ‘90-percentile’ was evaluated. The 90-percentile was substantially more uncertain than the mean or median indicators, particularly for small sample sizes but also for large sample sizes. Keywords: Water Framework Directive, phytoplankton, macroalgae, indicators, models, reference condi- tion, status classification. Layout: Anne van Acker Cover photo: Single boulder covered with brown algae at shallow waters NE of Zealand. Photo: Karsten Dahl. ISBN: 978-87-7073-060-0 ISSN (electronic): 1600-0048 Number of pages: 90 Internet version: The report is available in electronic format (pdf) at NERI's website http://www.dmu.dk/Pub/FR683.pdf Contents Summary 5 Sammenfatning 6 1 Introduction 7 2 Boundary values for TN concentration 8 2.1 Establishing reference TN inputs 8 2.2 Boundaries for TN concentrations 10 3 Macroalgae as indicators of water quality 18 3.1 Introduction 18 3.2 Aim 19 3.3 Methods 19 3.4 Results 30 3.5 Discussion 50 3.6 Conclusions 55 4 Assessment of ecological status using chlorophyll a 56 4.1 Boundaries for chlorophyll a 56 4.2 Comparison of boundary values for chlorophyll a and eelgrass depth limits 62 4.3 Evaluation of the precision of the chlorophyll a indicator described as 'summer th mean' and '90 percentile' 65 5 Conclusions and recommendations 69 6 References 71 7 Appendices 75 National Environmental Research Institute NERI technical reports [Blank page] Summary During the implementation of the EU Water Framework Directive, an in- tercalibration of selected metrics of the biological quality elements was undertaken at a limited number of sites. This report describes a method for establishing ecological status classes for phytoplankton in more areas and evaluates several macroalgal indicators and their calculated indica- tor values for ecological status class boundaries. In the first part of the report, estimates of nitrogen inputs from Denmark to the Danish straits since 1900 combined with expert judgement of the general environmental conditions of Danish waters during different time periods were used to establish nitrogen inputs representing reference conditions and boundaries between the five ecological status classes. These reference conditions and class boundaries were transformed into nitrogen concentrations in the water in several fjords and coastal locali- ties by the use of site-specific relations between nitrogen inputs and ni- trogen concentrations. An existing macroalgal model was refined in the second part of the re- port. The model describes the following variables: i) the total algal cover, ii) the cumulative algal cover of the total algal community, opportunistic species or late-successional species, iii) the fraction of opportunistic spe- cies and iv) the number of late-successional species. All macroalgal vari- ables responded to changes in total nitrogen but also to changes in salinity which emphasises the need for setting different targets depending on sa- linity. The strongest responses to changes in nitrogen concentration and the least variability were found for the indicators 'total algal cover', 'number of late-successional species' and fraction of opportunists'. Eco- logical status class boundaries were established for all the macroalgal variables in a number of Danish estuaries and coastal areas. A Spanish macroalgal index based on 'cover', 'proportion of opportunists' and 'species richness' was tested using Danish data. Each component of the index responded to nutrient gradients but the index needs adjust- ment of especially the scoring system in order to be applicable to Danish conditions. In the third part of the report site-specific correlations between concen- trations of nitrogen and chlorophyll a (chla) were used to define refer- ence conditions and ecological status class boundaries for the phyto- plankton metric 'mean summer concentration of chla' in several Danish estuaries and coastal areas. The relationship between chla and nitrogen concentrations varied from site to site and reflected the bio-available fraction of total nitrogen. A relationship was demonstrated between ref- erence conditions and good-moderate boundaries for eelgrass depth limits and the corresponding values for chla. Precision of the two different chla indicators 'summer mean' and '90- percentile was evaluated. The 90-percentile was substantially more un- certain than the mean or median indicators, particularly for small sample sizes but also for large sample sizes. 5 Sammenfatning I forbindelse med implementeringen af det europæiske vandrammedi- rektiv blev der foretaget en interkalibrering af delelementer af de biolo- giske kvalitetselementer i et begrænset antal områder. Denne rapport be- skriver en metode til fastsættelse af miljøtilstandsklasser for kvalitets- elementet fytoplankton i yderligere en række områder samt forslag til indikatorer for makroalger med fastsættelse af miljøtilstandsklasser i en række danske områder. På baggrund af estimater af kvælstofoverskud fra dansk landbrug tilbage til år 1900 beskriver rapportens første del fastsættelsen af tilførsler af kvælstof under referenceforhold samt under forhold, der repræsenterer perioder svarende til forskellige miljøtilstandsklasser for havmiljøet ge- nerelt. Ud fra lokale relationer mellem kvælstoftilførsler og kvælstofkon- centrationer i vandet defineres referencekoncentrationer af kvælstof samt kvælstofkoncentrationer svarende til grænseværdier mellem de fem mil- jøtilstandsklasser for en række danske fjorde og åbne kystområder. I rapportens anden del videreudvikles en makroalgemodel, der beskri- ver i) det totale algedække, ii) det kumulative dække af hele algesam- fundet, opportunistiske arter eller kraftigere langsomt voksende arter, iii) fraktionen af opportunistiske arter og iv) antal kraftige langsomt vok- sende arter. Alle disse variable responderede på kvælstofkoncentratio- ner, men også på salinitet, hvilket understreger nødvendigheden af, at forskellige miljømål defineres for forskellige saliniteter. Det tydeligste respons på kvælstofkoncentrationer og den mindste variation fandtes for de tre indikatorer 'totale algedække', 'antal kraftige langsomt voksende arter' og 'opportunisters andel af den samlede vegetationsdækning'. Ba- seret på kvælstofkoncentrationerne svarende til grænserne mellem miljø- tilstandsklasserne er der for samtlige makroalgevariable beregnet værdier for grænserne mellem de fem miljøtilstandsklasser i en række danske fjorde og kystnære områder. Desuden blev anvendeligheden af et spansk makroalgeindeks baseret på 'algedække', 'fraktion opportunistiske arter' og 'artsrigdom' undersøgt. Det spanske indeks kræver væsentlig modifi- kation, før det kan anvendes under danske forhold. Lokale sammenhænge mellem kvælstofkoncentrationer og koncentratio- nen af klorofyl a, der anvendes som indikator for biomasse, benyttes i rapportens tredje del til at definere afgrænsningen mellem de fem miljø- tilstandsklasser for kvalitetselementet fytoplankton i en række danske fjorde og kystnære