Abstract
Reference conditions and boundary values between Water Framework Directive status classes were estimated for phytoplankton biomass from empirical relationships relating: (1) nitrogen inputs from land to total nitrogen (TN) concentrations and (2) TN concentrations to chlorophyll a (chl a) concentrations. Different periods during the last >100 years were used to characterise hypothesised ecological status, and a hind-casted time series was used to define boundary values for nitrogen inputs. Nitrogen levels in 35 coastal water bodies around Denmark were significantly related to inputs from land to various degrees (factor of 50) reflecting gradients from open coastal to freshwater-influenced estuaries. Significant differences in the relationship between chl a and TN across sites were found, suggesting that previous response models have been too simple and uncertain. Reference and boundary values for chl a, estimated with a relative uncertainty of 5–20%, varied substantially between sites, and the boundary value between good and moderate status was 6–81% higher than the reference condition with an average of 28%. Differences in bioavailability of nutrient sources and grazing pressure are important factors controlling site-specific phytoplankton biomass, and models for predicting phytoplankton responses to nutrient reductions must account for these. The boundary setting must be adaptive to incorporate improved quantitative knowledge and effects of shifting baselines.
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Acknowledgements
This study used data collected within the National Danish Aquatic Monitoring and Assessment Programme, and financial support was given by the Danish Environmental Protection Agency. Henning Karup and Jens Brøgger Jensen are acknowledged for constructive discussions of the approach and comments to the manuscript. This research is a contribution to the Thresholds Integrated Project, funded by Framework Program 6 of the European Commission (contract # 003933-2).
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Guest editors: P. Nõges, W. van de Bund, A. C. Cardoso, A. Solimini & A.-S. Heiskanen
Assessment of the Ecological Status of European Surface Waters
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Carstensen, J., Henriksen, P. Phytoplankton biomass response to nitrogen inputs: a method for WFD boundary setting applied to Danish coastal waters. Hydrobiologia 633, 137–149 (2009). https://doi.org/10.1007/s10750-009-9867-9
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DOI: https://doi.org/10.1007/s10750-009-9867-9