Estuarine circulation reversals and related rapid changes in winter near-bottom oxygen conditions in the Gulf of Finland, Baltic Sea

The reversal of estuarine circulation caused by southwesterly wind forcing may lead to vanishing of stratification and oxygenation of deep layers during the cold season in the Gulf of Finland. Six CTD + oxygen transects (130 km long, 10 stations) were conducted along the thalweg from the western border to the central gulf (21 December 2011–8 May 2012). Two bottom-mounted ADCP, near the western border and in the central gulf were installed. A CTD with dissolved oxygen sensor was deployed close to the western ADCP. Periods with typical estuarine circulation were characterized by strong stratification; high salinity, hypoxic conditions and inflow to the gulf in the near bottom layer. Two circulation reversals were observed, in December–January and February–March. The first well-developed reversal event caused the vanishing of stratification and oxygen concentrations that were almost over 270 μmol L 1 (6 mL L 1) in the entire water column along the transect; and lasted for about 1.5 months. Shifts from estuarine circulation to reversed circulation and vice versa were both associated with strong currents (up to 40 cm s 1) in the deep layer. In the western area of study, near-bottom oxygen conditions strongly depended on salt wedge intrusions (hypoxic water) from the NE Baltic Proper, while in the eastern part good oxygen conditions caused by reversals remained for a few months. Change from oxygenated to hypoxic conditions in the entrance area to the Gulf might occur very rapidly, within less than a day.