Records of environmental change and sedimentation processes over the last century in a Baltic coastal inlet

Abstract: The marine ecosystem in the Baltic Sea is affected by multiple stress factors, e.g. eutrophication and deoxygenation, overfishing and anthropogenic pollution. Here we evaluate the short-term trends of dissolved oxygen in bottom waters and changes in the primary productivity within a coastal inlet in the Baltic Sea. Undisturbed sediment cores (mean sedimentation rate 0.5 cm yr-1) were retrieved from the inner and outer part of a coastal inlet, Gropviken, and dated using 210Pb and 137Cs. A multi-proxy approach (grain-size analysis, organic carbon and nitrogen content, biogenic silica and elemental analysis) has been used to provide a better understanding of the environmental change within the inlet. Four significant changes were observed in oxygen deficiency, sedimentation rate and phytoplankton composition during the last century. The deeper area in the inlet has naturally low oxygen conditions due to slow water exchange, but high values of the redox proxies can be observed already in the beginning of the 1930's, with an increase to a maximum in the 1980's. The contributing factor could be an increase of eutrophication from expanded farmland in the area. Signs for increased eutrophication are also seen in higher biogenic silica and organic carbon content beginning in the 1920's . A switch in the phytoplankton composition is observed from the late 1960's onwards and due to the change in nutrient ratios and lower secchi depth. Increased precipitation during this event could have changed the sedimentation rate within the inlet, with a decrease in the shallower area and an increase in the deeper part. As a result of increased precipitation, the riverine input increased and brought more nutrient and weathered particles to the open waters. These four sediment cores will contribute to an increased understanding of short-term environmental changes and sedimentation processes taking place in a coastal inlet of the Baltic Sea.