Ecological thresholds of eutrophication for littoral macroinvertebrates: a study on 116 Danish lakes

Benthic macroinvertebrates are widely used as bioindicators in freshwaters, and understanding ecological thresholds linked to major shifts in community composition has become increasingly important for effective ecological assessment and management. However, defining clear and ecologically meaningful thresholds remains challenging due to the multiple interacting water quality and habitat structure parameters influencing macroinvertebrate communities. Here, we analysed littoral macroinvertebrate assemblages across 116 Danish lakes monitored from 2013 to 2023 to identify the main eutrophication-related environmental gradients and quantify associated ecological thresholds shaping community composition and structure. Conductivity was the main factor structuring macroinvertebrate assemblages; therefore, lakes were clustered according to conductivity levels, and eutrophication thresholds were assessed in freshwater lakes (conductivity <112 mS m^-1), which represented the majority of sampled lakes. In freshwater lakes, macroinvertebrate community was mainly influenced by eutrophication gradients as well as pH and substrate characteristics. Threshold Indicator Taxa Analysis (TITAN) identified a threshold of 0.030 mg L^-1 total phosphorus (0.029 - 0.035 mg L^-1) and 13.9 μg L^-1 chlorophyll a (3.4 - 20.3 μg L^-1) associated with a pronounced decline of sensitive taxa, representing early-warning thresholds of biodiversity loss in response to eutrophication. In contrast, tolerant taxa showed more gradual increase at higher thresholds of 0.156 mg L^-1 total phosphorus (0.125 - 0.165 mg L^-1) and 75.3 μg L^-1 chlorophyll a (61.6 - 165.2 μg L^-1). This study identifies clearly defined thresholds of macroinvertebrate compositional changes in response to main eutrophication drivers, and present a novel, transferable analytical approach to identify these drivers and to define ecologically meaningful, evidence-based community thresholds to eutrophication.