Paleogenetic records of Daphnia pulicaria in two North American lakes reveal the impact of cultural eutrophication.

Understanding the evolutionary consequences of the green revolution, particularly in wild populations, is an important frontier in contemporary biology. Because human impacts have occurred at varying magnitudes or time periods depending on the study ecosystem, evolutionary histories may vary considerably among populations. Paleogenetics in conjunction with paleolimnology enable us to associate microevolutionary dynamics with detailed information on environmental change. We used this approach to reconstruct changes in the temporal population genetic structure of the keystone zooplankton grazer, Daphnia pulicaria, using dormant eggs extracted from sediments in two Minnesota lakes (South Center, Hill). The extent of agriculture and human population density in the catchment of these lakes has differed markedly since European settlement in the late 19th century and is reflected in their environmental histories reconstructed here. The reconstructed environments of these two lakes differed strongly in terms of environmental stability and their associated patterns of Daphnia population structure. We detected long periods of stability in population structure and environmental conditions in South Center Lake that were followed by a dramatic temporal shift in population genetic structure after the onset of European settlement and industrialized agriculture in its watershed. In particular, we noted a 24.3-fold increase in phosphorus (P) flux between pre-European and modern sediment P accumulation rates (AR) in this lake. In contrast, no such shifts were detected in Hill Lake, where the watershed was not as impacted by European settlement and rates of change were less directional with a much smaller increase in sediment P AR (2.3-fold). We identify direct and indirect effects of eutrophication proxies on genetic structure in these lake populations and demonstrate the power of using this approach in understanding the consequences of anthropogenic environmental change on natural populations throughout historic time periods.