Reduced Cover of Drifting Macroalgae Following Nutrient Reduction in Danish Coastal Waters
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Based on a large dataset from the national Danish monitoring programme, we analysed the temporal variability of drifting algae cover in shallow (1–2 m) water during a period of reduced nutrient loadings. Algal cover was analysed both in absolute terms and relative to eelgrass, Zostera marina, cover to test the hypotheses that (1) the cover of drifting algae and the relative dominance of algae versus eelgrass in shallow waters have declined in parallel to reductions in nutrient levels during the last decades, and (2) spatio-temporal differences in algal cover can be related to differences in nutrient conditions and environmental characteristics. The cover of drifting algae was positively related to total nitrogen concentration and Secchi depth but negatively related to exposure, salinity and mean summer temperature. The cover of drifting macroalgae showed significant declines over the past two decades, paralleling the reduction in nutrient concentrations. The present cover of drifting algae is low (<10 %) and probably pose little threat to the general distribution of eelgrass in shallow Danish waters though local accumulations may still be harmful. However, the ratio between drifting algae cover and eelgrass cover showed no significant trend, reflecting that eelgrass cover had not increased despite the reduced levels of nutrients and drifting algae. This ratio also showed no consistent relationship to water quality probably because different regulation mechanisms govern drifting algae and eelgrass, and feedback mechanisms may delay eelgrass recovery. Reduced drift algal cover may be an early sign of improved ecological status, while further improvements in terms of recovery of eelgrass meadows have longer perspectives.
KeywordsDrift algae Eelgrass Zostera marina Nitrogen Nutrients Relative exposure index
This project was funded by the Danish Agency for Science, Technology and Innovation (REELGRASS; contract #09-063190), the European Commission (WISER contract #FP7-226273 and DEVOTES contract #FP-308392) and the project WATERS funded by the Swedish Environmental Protection Agency. We thank Michael Stjernholm, Bioscience, Aarhus University, for initial discussions on exposure calculations and Per-Olav Moksnes, Department of Biological and Environmental Sciences, University of Gothenburg, Sweden, for information on the distribution of algae and eelgrass along the Swedish west coast.
- Andersen, J., S. Markager, and G. Ærtebjerg. 2004. NOVANA; Tekniske anvisninger for marin overvågning. Ch. 3.1. In Danish. http://bios.au.dk/videnudveksling/fagligt/fagdatacentre/fdcmarintny/tekniskeanvisningernovana20042010
- Boström, C., S.P. Baden, and D. Krause-Jensen. 2003. The seagrasses of Scandinavia and the Baltic Sea. In World atlas of seagrasses, ed. E.P. Green and F.T. Short, 27–37. Berkeley, USA: University of California Press.Google Scholar
- Boström, C., S. Baden, A.-C. Bockelmann, K. Dromph, S. Frederiksen, C. Gustafsson, D. Krause-Jensen, T. Möller, S.L. Nielsen, B. Olesen, J. Olsen, L. Pihl, and E. Rinde. 2014. Distribution and function of Nordic eelgrass (Zostera marina) ecosystems: implications for coastal management and restoration. Aquatic Conservation: Marine and Freshwater Ecosystems. doi: 10.1002/aqc.2424.Google Scholar
- Gutiérrez, J.L., C.G. Jones, J.E. Byers, K.K. Arkema, K. Berkenbusch, J.A. Commito, C.M. Duarte, S.D. Hacker, J.G. Lambrinos, I.E. Hendriks, P.J. Hogarth, M.G. Palomo, and C. Wild. 2011. Physical ecosystem engineers and the functioning of estuaries and coasts. Treatise on Estuarine and Coastal Science 7: 53–81.CrossRefGoogle Scholar
- Hansen, J. W. (Ed.) 2013. Marine områder 2012: NOVANA. Tilstand og udvikling i miljø-og naturkvaliteten. Aarhus University; DCE - Danish Centre for Environment and Energy. Scientific report no. 77, 162 pp. http://dce2.au.dk/pub/SR77.pdf. In Danish.
- Kaas, H. and S. Markager. 1998. NOVA; Tekniske anvisninger for marin overvågning. Ch. 12. In Danish. http://www2.dmu.dk/1_Om_DMU/2_tvaer-funk/3_fdc_mar/programgrundlag/tekanv/tekniskanv.asp#teknisk
- Laursen, J. S., D. Krause-Jensen, and S. E. Larsen. 2000. Interkalibrering af metode til undersøgelser af bundvegetation i marine områder. 329: Ministry of Environment and Energy, Denmark.Google Scholar
- Nielsen, R., A. Kristiansen, L. Mathiesen, and H. Mathiesen. 1995. Distributional index of the benthic macroalgae of the Baltic Sea area. Acta Botanica Fennica 155.Google Scholar
- Risinger, B. 2012. God havsmiljö 2020. Göteborg: Havs-och vattenmyndigheten. In Swedish.Google Scholar
- SAS Institute Inc. 2008. SAS/ETS (C) 9.2 user's guide. Cary, North Carolina: SAS Institute Inc.Google Scholar
- SAS Institute Inc. 2009. SAS/ETS (C) 9.2 user's guide, 2nd ed. Cary, North Carolina: SAS Institute Inc.Google Scholar
- Schmidt, A.L., J.K.C. Wysmyk, S.E. Craig, and H.K. Lotze 2012. Regional scale effects of eutrophication on ecosystem structure and services of seagrass beds. Limnology and Oceanography 57: 1389–1402.Google Scholar
- Sokal, R.R., and F.J. Rohlf. 2012. Biometry: the principles and practice of statistics in biological research. New York: W.H. Freeman.Google Scholar
- Waycott, M., C.M. Duarte, T.J.B. Cattuthers, R.J. Orth, W.C. Dennison, S. Olayarnik, A. Calladine, J.W. Fourqurean, K.L. Heck, A.R. Hughes, G.A. Kendrick, W.J. Kenworthy, F.T. Short, and S.L. Williams. 2009. Accelerating loss of seagrasses across the globe threatens coastal ecosystems. Proceedings of the National Academy of Sciences of the United States of America 106: 12377–12381.CrossRefGoogle Scholar