Cladocera and geochemical evidence from sediment cores show trophic changes in Polish dystrophic lakes

Change in the trophic state of lakes is a topic of primary interest for limnologists and paleolimnologists, but also for governments in many countries. These changes can be the result of the natural evolution of lake ecosystems, but nowadays are most often connected with human activity influencing water bodies. In this article, we reconstruct changes in the lake productivity and trophic state in three dystrophic (humic) lakes located in Northern Poland. Sediments from these lakes, which are part of a national park, were submitted to Cladocera and chemical composition analyses. Currently, the trophic state of these lakes has been described based on the water's chemical composition, and they have been classified as undisturbed ecosystems with a stable trophic state. The main objective of this study was to evaluate whether these lakes have been stable and undisturbed ecosystems during the past centuries and therefore whether they can be classified as natural and pristine. The results of subfossil Cladocera analysis and sedimentary geochemical analysis confirmed the specific nature of studied lakes. However, our results were surprising and showed that during the last 200 years two of the three lakes have undergone distinct trophic changes, while one of them has barely changed at all.