Performance of a new phytoplankton composition metric along a eutrophication gradient in Nordic lakes
A new phytoplankton metric is presented, which is developed from a large dataset of Norwegian lakes (>2,000 samples from >400 lakes). In contrast to previous metrics, this index is not built on selected ‘indicative’ taxa, but uses all available taxonomic information at genus and species level. Taxa optima with respect to lake trophic status (derived from total phosphorus concentrations) are used to calculate a phytoplankton trophic index (TI) for each sample. Analysis of the TI shows that phytoplankton communities exhibit highly non-linear responses to eutrophication in Norwegian lakes. Reference lakes are characterized by very similar TIs despite having considerable variation in total phosphorus and chlorophyll a concentrations. TI exhibits a non-linear distribution along the eutrophication gradient which separates unimpacted from impacted sites in the study area. We further show that TI exhibits smaller seasonal variations than chlorophyll a, making it a more reliable indicator for lake monitoring. Implications for its applicability within the WFD are discussed.
KeywordsWater Framework Directive Phytoplankton Indicators Eutrophication Biodiversity Threshold Lakes
We thank Susanne C. Schneider, Laurence Carvalho and an anonymous reviewer for comments on the manuscript. Funding by European Commission (res. project REBECCA, SSPI-CT-2003-502158) and Norwegian Research Council (res. project BIOCLASS-FRESH, 184002) are acknowledged.
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