Multi-Decadal Trends in Northern Lakes Show Contrasting Responses of Phytoplankton and Benthic Macroinvertebrates to Climate Change.

Three decades of continuous monitoring of 110 lakes across Sweden revealed significant long-term changes in physicochemical habitat and biological assemblages comprising multiple trophic levels related to climate. Mean annual air temperature increased for almost all lakes, with notable increases in the northern region. The environmental variables that showed the strongest temporal patterns were increasing water temperatures and decreasing nutrient (TP) and TOC concentrations for lakes in the north and increasing pH and TOC for lakes in the south. As hypothesized, phytoplankton and benthic macroinvertebrate (littoral and profundal) assemblages tracked climate changes directly (temperature, precipitation) and indirectly (changes in physicochemical habitat), but trends differed among the organism groups. The most pronounced changes in both magnitudes and rates of change (slopes) of the biological trends were found in the northernmost ecoregions. In these nutrient- and species-poor ecosystems, taxon richness and diversity had contrasting patterns: phytoplankton and profundal macroinvertebrates had negative slopes while littoral macroinvertebrates had positive slopes. Total phytoplankton biovolume and littoral macroinvertebrate abundance had positive slopes. Spatiotemporal patterns of phytoplankton and littoral macroinvertebrates were largely correlated with temperature and nutrients but not profundal assemblages. For lakes in the south isolating climate-induced effects was confounded by post-acidification recovery, for example, all three organism groups correlated with pH but not with water temperature. Combined results from all of the study lakes indicated habitat-specific responses of biological assemblages to long-term changes in climate and physicochemical habitat. Climate change coupled with catchment vegetation and post-acidification recovery pose heterogeneous impacts directly (temperature) and indirectly (physicochemical habitat) on lake assemblages. All three organism groups showed trends related to climate and therefore should be considered robust sentinels to gauge climate impacts directly and trophic-level effects indirectly in these climate-vulnerable ecosystems.