Evidence for antifouling biocides as one of the limiting factors for the recovery of macrophyte communities in lakes of Schleswig-Holstein

Abstract

Abstract Background The European Water Framework Directive (WFD) has been implemented to achieve a good ecological status in European water bodies requiring macrophyte community assessment as one of the biological quality elements (BQEs). While in several lakes in Schleswig-Holstein (Germany) different BQEs improved within recent years, no recovery of macrophyte communities in some lakes could be achieved, despite the reduction of nutrient input and eutrophication. Due to the fact that no impairment of phytoplankton could be observed, toxic stress due to sediment contamination was hypothesized as a possible limiting factor of macrophyte community recovery. Results Sediment toxicity was investigated by performing an extensive chemical screening of sediment contamination and a risk assessment based on toxic unit (TU) summation, using equilibrium water concentrations and algal toxicity as surrogates for lacking data on macrophyte toxicity. Possible indirect risks via toxic pressure on grazer were assessed via TUs based on crustaceans. The study revealed algal TUs of more than one order of magnitude below chronic toxicity thresholds in lakes with high and good status of the macrophyte community and increasing concentrations and frequency of exceedance of toxicity thresholds for lakes with moderate-to-bad status. The antifouling biocides irgarol and diuron were identified as major risk drivers. In addition, PAHs and glyphosate could not be ruled out to contribute to toxic pressure on macrophytes. Despite exceedance of toxicity thresholds for crustaceans, no connection of the ecological status of the macrophyte communities with toxic risks to grazers could be observed. Conclusions Our study suggests that in a multiple pressure situation the toxic pressure created due to the contamination of sediments with antifouling biocides is one of the limiting factors for the recovery of macrophyte communities in impaired lakes of Schleswig-Holstein. This finding is in agreement with a Europe-wide survey on almost 47,000 sites suggesting that no good ecological status can be observed at sites with contamination exceeding toxicity thresholds. Similar to the survey, our study indicates additional stressors preventing the achievement of a good quality status of the lake ecosystems.