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Water clarity and eelgrass responses to nitrogen reductions in the eutrophic Skive Fjord, Denmark

  • WATER BODIES IN EUROPE
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Abstract

Eelgrass depth limits and water clarity in the Skive Fjord estuarine system have not improved despite nutrient input reductions of 30%. Long-term monitoring data (1989–2010) were used to investigate the underlying causes. Dissolved inorganic and organic nitrogen concentrations decreased significantly over time, whereas particulate organic nitrogen concentration, assumed to consist primarily of phytoplankton and phytoplankton detritus and calculated as a proportional factor to chlorophyll a, did not change. Total organic carbon, mostly of autochthonous origin, remained constant despite reduced nitrogen concentrations, resulting in an increasing C:N ratio of the organic material in the water column. Phytoplankton primary production also remained constant suggesting that phytoplankton growth was only limited by nitrogen to a minor degree. Alleviated grazing pressure caused by a reduction in the blue mussel standing stock and a pelagic food web dominated by jellyfish may have contributed to the constantly high phytoplankton levels. Particulate inorganic matter, likely reflecting sediment resuspension, increased over time, most probably in response to removal of blue mussels and declining eelgrass cover. The Skive Fjord estuarine system is affected by multiple pressures—nutrient enrichment, mussel dredging and climate change that must be addressed together for water clarity to improve and eelgrass to recover.

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Acknowledgments

This research is a contribution to the WISER project (contract #FP7-226273), funded by the European Commission, and the REELGRASS project (contract #09-063190), funded by the Danish Agency for Science, Technology and Innovation. Additional founding was provided by the Danish Council for Strategic Research to the project IMAGE (Integrated Management of Agriculture, Fishery, Environment and Economy, Grant No. 09-067259). We are grateful to the Danish Environmental Centers responsible for data collection under the Danish Nationwide Aquatic Monitoring and Assessment Program. This is a contribution from the Baltic Nest Institute.

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  1. Department of Bioscience, Aarhus University, Frederiksborgvej 399, 4000, Roskilde, Denmark

    Jacob Carstensen, Stiig Markager & Karen Timmermann

  2. Department of Bioscience, Aarhus University, Vejlsøvej 25, 8600, Silkeborg, Denmark

    Dorte Krause-Jensen & Jørgen Windolf

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  1. Jacob Carstensen
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Correspondence to Jacob Carstensen.

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Guest editors: C. K. Feld, A. Borja, L. Carvalho & D. Hering / Water bodies in Europe: integrative systems to assess ecological status and recovery

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Carstensen, J., Krause-Jensen, D., Markager, S. et al. Water clarity and eelgrass responses to nitrogen reductions in the eutrophic Skive Fjord, Denmark. Hydrobiologia 704, 293–309 (2013). https://doi.org/10.1007/s10750-012-1266-y

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