Late 20th century shifts in cladoceran community structure and reproduction in an acidified boreal lake

We examined subfossil Cladocera (crustacean) assemblages and ephippia in a high-resolution sediment core from a small acidified boreal lake, Lake Pieni Majaslampi, in southern Finland, with a focus on community and reproductive responses during a period of environmental disturbances. The results showed a significant community shift similar to 1970-1980 AD as planktonic taxon Eubosmina, together with littoral chydorids Acroperus harpae, Alonella excisa, and Alona rustica increased and populations of Alonella nana and Alona affinis declined. In addition, a concurrent and significant change in reproductive patterns occurred, as sexual reproduction of Eubosmina, A. affinis, and A. harpae increased. It appears likely that these ecological shifts were a response to multiple environmental stressors related to severe acidification that took place in this sensitive lake and culminated during the 1980s. Acidification of Lake Pieni Majaslampi resulted in dramatic changes in the food-web structure; perches (Perca fluviatilis) were extirpated and invertebrate predation predominated from the 1980s onwards and after this Eubosmina, which is known to be a desirable prey item for perches in acidic lakes, was apparently favored in the fish-free ecosystem. Furthermore, the observed reproductive shifts were perhaps adaptive responses to either increased invertebrate predation or to increased toxicity of the environment. As the analysis of subfossil ephippia allowed us to detect behavioral shifts in the cladoceran community related to lake acidification, ephippium analysis can provide a powerful tool to integrate ecology and paleoecology, and to enhance our understanding of changes in aquatic ecosystems determined from paleolimnological studies.