SEDIMENT EVIDENCE OF LIMNOLOGICAL AND ENVIRONMENTAL CHANGES AT LAKE KORTTAJÄRVI BETWEEN AD 520 AND 1874 WITH SPECIAL REGARD OF THE MEDIEVAL CLIMATE ANOMALY

Annually laminated (varved) sediments of Lake Korttajärvi, Finland, were studied for varve structure, pollen, diatoms, and phosphorus fractions in a section that spans the time period between AD 520 and AD 1874. The main aim of the investigation was to examine the effects of the Medieval Climate Anomaly (MCA), an analogue for future climate warming, on the lake. Eutrophication was emphasized because models predict climate warming increases the risk of internal loading in some lakes. Varve data and pollen analysis results, combined with previously published tree ring-based climate reconstructions, suggested a warm MCA period indeed occurred between AD 980 and 1250. The risk of internal loading increased during the MCA because sediment deposited at that time contains high amounts of phosphorus in forms that may become liberated in reducing conditions. Based on stable diatom-inferred lake water total phosphorus concentrations, however, no internal loading event took place. Constrained and partially constrained ordinations suggest that growing season temperature has an effect on diatom assemblages. The results lend support to earlier observations of a 20-year lag in the response of boreal forest lakes to summer air temperature.