Hydroclimatic variations over the last millennium in eastern Finland disentangled by fossil Cladocera

Future climate warming is expected to alter atmospheric moisture balance and therefore paleoclimatic archives of past moisture and hydrological patterns may provide understanding of the magnitude, speed, and ecosystem responses to the regionally altered climatic regimes. The present study aimed at disentangling millennial variation in hydroclimatic parameters, including summer effective moisture (lake levels) and ice-free season by using sedimentary cladoceran fossils. We used a site-specific cladoceran-based inference model to reconstruct lake level fluctuations in a 1500-yr sediment record from Lake Pieni-Kauro, eastern Finland in the context of previously established paleoclimatic data of summer temperature and precipitation. In addition, we applied cladoceran planktonic/littoral ratio, mean body size, and abundance of fossil ephippia as supplementary proxies for paleoclimatic interpretations. The results showed that lake level was lower during the pre- and early Medieval Climate Anomaly (MCA, around 500-1000 AD) than at present and started to rise toward the culmination during the Little Ice Age (LIA, around 1600–1700 AD) when the lake level was higher than at present. Pelagic cladoceran body size was larger and abundance of ephippia increased during the LIA suggesting significant reduction in the length of the ice-free season and lower water temperatures. Comparison of the lake level reconstruction with the previous paleoclimatic data suggested that effective moisture has had a long-term negative relationship with temperature and positive relationship with summer precipitation in eastern Finland. The results indicated that the pre- and early MCA was warm and dry (long ice-free season, low summer effective moisture) and the culmination of the LIA was cool and moist (reduced ice-free season, increased summer effective moisture). Unraveling the seasonality of the multicentury hydroclimatic patterns in eastern Finland may offer important insights in linking the large-scale atmospheric circulation patterns to seasonal climate events.