Decreased surface and bottom salinity and elevated bottom temperature in the Northern Baltic Sea over the past six decades

Abstract

Temperature and salinity are key factors in controlling marine habitats and gas fluxes. Finnish and Swedish temperature and salinity monitoring data from the northern Baltic Sea since the 1960s, and Argo buoy data from the eastern Gotland Basin and the Bothnian Sea from 2012 to 2021 were examined using linear trend analysis. Since the 1960's near-bottom temperature has increased by 0.75-2.9 °C (0.013-0.115 °C/a) and surface salinity declined by 0.31-1.14 units (0.005-0.019/a). Surface temperature trends at monitoring stations were negative (16 cases out of 33) but deemed unreliable. Near-bottom salinity has declined by 0.35-1.45 units (0.007-0.025/a), except in the northern Baltic Proper and the central-eastern Gulf of Finland. Most rapid increases in near-bottom temperature have occurred after 1993, especially in the northern Baltic Proper and the Gulf of Finland. Argo data corroborated declining surface salinity in the eastern Gotland Basin, increasing deep-water temperature in the eastern Gotland Basin and the Bothnian Sea and increasing deep-water salinity in the eastern Gotland Basin. Argo data from 2013 to 2021 indicated deep-water temperature increase in the Gotland basin was more rapid than the concomitant salinity increase and is probably related to global change.

Keywords: Baltic Sea; Global change; Monitoring; Salinity; Temperature.