Liming-induced taxonomic homogenization of chironomid assemblages in Scandinavian lakes as unraveled by paleolimnological reconstructions

In the 1980s, liming became a large-scale, governmentally supported restoration program implemented by many countries to mitigate the effects of acidification of freshwaters. Despite some 50 years of liming of thousands of lakes and streams, its efficacy remains largely debated. This study is the first of its kind to use paleolimnological reconstructions using both subfossil chironomid assemblages and their carbon stable isotopic composition to compare the ecological trajectories of limed and control (unlimed) lakes over the last 100 years in order to unravel the effects of liming on Scandinavian lakes. Our study revealed strong effects of liming on lake water chemistry and sedimentary characteristics of limed lakes. Overall, higher present-day pH values were observed in lake water of the limed lakes (average pH of 6.7 ± 0.2) than in control lakes (average pH of 5.9 ± 0.5), and the pH variability was much lower among limed lakes. A decrease in the dissimilarity of the chironomid assemblages among limed lakes was also observed as liming progressed, and the degree of dissimilarity after liming was largely lower than pre-acidified levels. This pattern suggests that liming favoured the selection of similar chironomid taxa in limed lakes, thus providing complementary empirical evidences about the effects of liming on aquatic communities through taxonomic homogenization of chironomid assemblages. As the ecological trajectories of limed and control lakes strongly differed, our study also questions the impact of liming on the resilience of Scandinavian lakes and, importantly, their ability to cope with ongoing and future climatic and environmental changes. Finally, isotope-based results indicated that liming masks potential complex effects on carbon flows at the base of the food web by shifting algal carbon isotopic baselines. The use of carbon stable isotope analysis of chironomid head capsules to study the effects of liming on energy flows within lake food webs is, therefore, not suitable.