The applicability of macrophyte compositional metrics for assessing eutrophication in European lakes

Abstract

The European Water Framework Directive adopted in 2000, despite being prescriptive, has stimulated the development of a diverse array of biological assessment methods in Europe. The multitude of indicators currently used in biomonitoring lacks consistency and thus constrains the comparability of assessments at an international scale. Therefore, there is an argument to define and validate metrics with more universal application that can be applied EU-wide. We explored two metrics based on macrophyte taxonomic composition, the empirically based Intercalibration Common Metric for lake macrophytes (ICMLM) and the expert-based Ellenberg Index (EI), for their ability to detect eutrophication in different types of European lowland lakes. Data from 1474 unique lake-years from 11 countries were used to explore relationships between these metrics and the seasonal mean concentration of total phosphorus (TP) using linear regression.  ICMLM gave a linear and relatively strong (R=0.72,p<0.0001) response over the entire spectrum of TP concentrations, whereas EI performed best in lakes with the TP concentrations up to 250μgL−1 (R=0.64,p<0.0001) and was largely insensitive to higher phosphorus concentrations. Both metrics performed better in Nordic lakes than in the Central-Baltic ones. The responses of both metrics to TP were not modified or only very weakly modified by altitude, lake size and mean depth but were significantly affected by alkalinity. The ICMLM–TP relationship was stronger in lakes with water alkalinity>0.2meqL−1and significantly weaker in less buffered lakes. EI performed better in lakes with alkalinity<1.0meqL−1, whereas in high alkalinity lakes the response was significantly weaker. In all the lakes and in lakes from all the size, depth and alkalinity types, ICMLM was more strongly correlated with TP than EI and was proportionally less sensitive to alkalinity.  We also tested the effect of including helophytes on the metric response to eutrophication pressure by comparing the strength of the relationships to TP of the Ellenberg Index calculated firstly using only hydrophyte taxa (EI′HYDR) and secondly using all macrophyte taxa including both hydrophytes and emergent vegetation (EI′TOT). The differences in metric performance in all the lakes and all the size, depth and alkalinity types, except for the Nordic lakes, were non-significant. Thus, including helophytes generally did not significantly improve the strength of the EI–TP relationships.