Abstract
Biological assessment metrics and water chemistry measurements are used to quantify the link between stressors and their effects on lake ecosystems, for the Water Framework Directive. However, correlations between metrics and water chemistry are often poor. This is seen as major weaknesses of Water Framework Directive-related monitoring and assessment. We analyzed macrophytes, benthic algae, benthic macroinvertebrates, water chemistry and sediment total phosphorus content in the littoral of six lakes in the Western Balkans and used CORINE land use data to estimate nutrient enrichment via runoff from the adjacent land. Lakes with a higher estimated phosphorus runoff from the adjacent land did not have higher littoral water nutrient concentrations, but littoral diatom assemblages indicated more eutrophic conditions. These lakes also had higher abundances of littoral benthic primary producers, which in turn were associated with low concentrations of dissolved nutrients, but only in autumn, not in spring. This is consistent with primary producers taking up nutrients during the summer growth season. In lakes with high abundances of benthic primary producers, it is likely that the littoral vegetation plays a large role in the transfer of nutrients from the water to the benthos. This process impairs correlations between biological metrics and water nutrient concentrations. Our results suggest that CORINE land cover may be more useful to characterize littoral nutrient enrichment than lake water chemistry. Increased benthic primary producer biomasses and “eutrophic” diatom indices may indicate littoral nutrient enrichment even if water nutrient concentrations are low.
Highlights
The Water Framework Directive (WFD; European Commission, 2000) triggered the development of hundreds of biological metrics (Birk et al, 2012), which currently are used for an extensive and harmonized assessment of the ecological status of Europe's surface waters (Carvalho et al, 2019)
We show here that a more direct descriptor of littoral nutrient enrichment, i.e. phosphorus runoff estimated from land use in the nearshore surroundings, is better linked to WFD assessment metrics than water chemistry
This should initially lead to increased littoral water nutrient concentrations (Fig. 3)
Summary
The Water Framework Directive (WFD; European Commission, 2000) triggered the development of hundreds of biological metrics (Birk et al, 2012), which currently are used for an extensive and harmonized assessment of the ecological status of Europe's surface waters (Carvalho et al, 2019). Biological metrics used for status assessment according to the WFD are expected to be based on stressor - response relationships (Lyche-Solheim et al, 2013) This means that member states must show that a stressor (for example nutrient enrichment) leads to a quantifiable and consistent biological response (for example a change in the species composition of aquatic macrophytes). Water nutrient concentrations may be low in lakes with low external nutrient loadings and little benthic primary production, as they are in lakes with higher loading and intense benthic primary production This is relevant, because the poor linkage between stressors and effects on the ecosystem, assessed using water chemistry and biological WFD metrics, is seen as major weaknesses of WFD monitoring and assessment (Carvalho et al, 2019). Phytoplankton metrics developed for assessment according to the WFD were more closely correlated with water chemistry than were the benthic organism groups macroinvertebrates, macrophytes and phytobenthos (Birk et al, 2012)
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