Disentangling the response of lake littoral invertebrate assemblages to multiple pressures

Lakes are affected by a number of anthropogenic pressures, often resulting in biodiversity loss and impairment of ecosystem services. Here we analysed responses of littoral invertebrate assemblages in 589 temperate lakes to five anthropogenic pressures and combined pressure load. Over half of the lakes (n=302) were deemed to be affected by one or more pressures. The most pervasive pressures were forestry (73% of the impacted lakes), agriculture (21%) and acidification (11%). Individually, agriculture and pH explained > 20% of the among-lake variability in littoral invertebrate assemblages. Ranking the importance of the five pressures on littoral invertebrate assemblages, after accounting for covariance with natural factors and other pressures, showed that acidity > agriculture > forestry > urbanization ≥ invasive species. Although about 10% of the lakes were acidified according to chemical criteria, our finding that acidity was ranked first is partly due to the naturally low pH of humic boreal lakes, and not acidification per se. Comparison of species composition to cumulative pressure load revealed no clear patterns. Species composition (CA axis 1 scores) in lakes with one or more pressures differed from sites with no pressures, but cumulative pressure effects were non-significant. A significant three-pressure impact was found for species composition characterized by CA axis 2 scores, related to the combined pressures of agriculture, invasive species and urbanization. Collectively, our results show that littoral invertebrate assemblages respond uniquely to several different pressures, underpinning their use as a biomonitoring tool for assessing the ecological status of lakes. Ranking the effects of individual pressures as done here is a critical step in designing and implementing measures to protect and mitigate anthropogenic effects on biodiversity loss.