Long-term changes in a zooplankton community revealed by the sediment archive

To reconstruct changes in zooplankton communities in response to past anthropogenic perturbations, one possibility is to use the sedimentary records. We analyzed the sediments at a coastal site in the Northern Baltic Sea to relate changes in the zooplankton community to anthropogenic eutrophication and the invasion of a predatory cladoceran, Cercopagis pengoi. We sampled 30-cm laminated sediment cores and dated the sediment layers back to the 1950s. From each 1-cm layer, we measured eutrophication indicators (delta C-13, delta N-15, TC, TN, TP) and identified and counted zooplankton resting eggs (cladoceran, calanoid copepod, rotifer). In addition, we estimated the abundance of the cladoceran Bosmina (Eubosmina) maritima by counting subfossils (carapaces, headshields, and ephippia) and estimated the experienced stress as the relationship between sexual and asexual reproduction. Using redundancy and variance partitioning analyses, we found similar to 16% of the variation in the zooplankton community to be explained by eutrophication, and 24% of the variation in B. (E.) maritima abundance and reproduction mode to be explained by eutrophication and the introduction of the alien predator. Our results show a long-term shift from calanoid copepods and predatory cladocerans toward small-sized zooplankton species, like rotifers. Furthermore, the results indicate that the invasion of C. pengoi induced a short-term increase in sexual reproduction in B. (E.) maritima. The results indicate that anthropogenic eutrophication since the 1950s has altered the zooplankton community toward smaller species, while the invasion of the predatory cladoceran had only a transitory influence on the community during its expansion phase.