Cumulative effects of climate change and land use on the ecological status of Scandinavian lakes show contrasted interactions in different ecoregions: the role of pre-disturbance conditions in assessing ecological status

In this study, we used subfossil chironomids to assess temporal changes in lake ecological status over the last ca. 100 years in 30 lakes spread across different ecoregions in Sweden. By comparing Benthic Quality Index values and their temporal trends, we aimed to quantify the cumulative effects of climate change and land use on lakes and unravel how their effects may vary regionally. Results indicate that land use is the overarching driver of ecological changes in impacted lakes, in line with earlier studies showing that local pressures often suppress climate change effects on freshwaters. Furthermore, the known positive co-tolerance of chironomid species to temperature and eutrophication (e.g., cold stenotherm species also being indicators of oligotrophic condition, and conversely) was anticipated to induce antagonistic effects. However, the cumulative effects of climate change and land use differ across the landscape, being synergistic in the boreal forest ecoregion and antagonistic in the mixed forest ecoregion. We suggest that the pre-disturbance conditions (i.e., species composition and pressure sensitivities) play a key role in regulating the interactions between multiple pressures in freshwaters. Overall, this finding is encouraging as it implies that restoration of lakes that focuses on the most impactful pressure (e.g., nutrient loadings from agricultural fields and urban areas) remains a plausible restoration measure despite lake warming. Results also show that the net effect of climate change on the ecological status of the reference lakes varied regionally, being more pronounced in northern lakes due to the predominance of many cold water species which are more prone to disappear in response to small variations in temperature. As reference conditions are seldom revised, it is of fundamental importance to question whether the existing reference conditions are still applicable or need to be revised due to ongoing and future climate change.