Assessment of recent eutrophication and climate influence in the Archipelago Sea based on the subfossil diatom record

Two short sediment cores from the Archipelago Sea in the northern Baltic Sea were examined for their siliceous microfossils in order to study recent eutrophication in the area. The diatom record was divided into diatom assemblage zones. Locally weighted weighted averaging regression and calibration was applied for the quantitative reconstruction of past TN concentrations. The cores were dated using radiometric ( ²¹⁰Pb, ²²⁶Ra and ¹³⁷Cs) methods. The diatom assemblages in the topmost zones in both cores indicate eutrophication during the last decade. TN reconstructions partially fail to trace the actual measured TN concentrations. Especially from the late 1980s to the mid 1990s, the dominating diatom species are more influenced by other factors, such as the length of the ice season, than by nutrient concentrations. It seems that in the Archipelago Sea, diatom assemblages are principally governed by climatic fluctuations with a moderate influence of eutrophication in recent years. The peak in sedimentation in the early 1990s coincides with the short ice seasons indicating that sedimentation may increase with decreasing ice-cover extent due to warming climate.