Long-term variability of near-bottom oxygen, temperature, and salinity in the Southern Baltic

Abstract Bottom conditions in deep sea basins are strongly connected to the health of marine ecosystems. Due to its enclosed location and shallow bathymetry, the Baltic Sea is very sensitive to eutrophication and climate change. The primary objective of this study is to describe long-term variability of near-bottom oxygen concentration, salinity, and temperature in three basins of the Southern Baltic: the Bornholm Basin (BB), Slupsk Furrow (SF), and Gdansk Deep (GD) based on the analysis of historical hydrographic data, and to demonstrate the impact of inflow events on deep ventilation in those basins. Mean bottom oxygen concentration in the period 1946–2016 was very similar in the deep basins (1.2–1.4 ml l−1), and much higher in the shallower SF (3.2 ml l−1) due to the water mass modification occurring between BB and SF. SF was found to contribute to the ventilation of GD with a 1–3-month lag. The results indicate that vertical mixing in SF and the eastward advection of ventilated waters towards GD directly influence bottom water properties in GD. Therefore, next to Major Baltic Inflows (MBIs), also weaker barotropic and baroclinic inflows are important for the bottom ventilation in this area. The long-term trends are very similar in all of the basins. Near-bottom oxygen gradually decreased from 1946, reaching a minimum in 2000. This coincided with salinity and temperature minimums and was linked to reduced frequency and volume of barotropic inflows due to their decadal variability. Oxygen concentrations have been slowly recovering in all three basins since 2000. At the end of the record, their 5-year mean values were approximate to the levels from the 1970's. In the period 1946–2016, hypoxia (oxygen concentration below 2 ml l−1) occurred more frequently in BB (78%) than in GD (73%). Although the results show slow recovery of dissolved oxygen concentrations based on 5-year long mean values, the occurrence of hypoxia increased over the last two decades, reaching 85% in BB and GD and 24% in SF. Mean and decadal variability of the seasonal cycle of near-bottom water properties were analysed. Mean annual cycles of bottom oxygen in three basins are approximately sinusoidal, with the highest values occurring in March, and lowest in September. The range of the mean oxygen seasonal cycle decreases eastward from 1.6 ml l−1 in BB, through 1.3 ml l−1 in SF, to 1.2 ml l−1 in GD. Furthermore, between 1946–1980 and 1995–2016, the range of the mean DO seasonal cycle decreased by 0.2 ml l−1 in BB, 0.6 ml l−1 in SF, and 0.4 ml l−1 in GD. In BB and SF, minimum bottom temperatures occur in summer, and maximum in winter. The strongest annual temperature variation (3 °C) occurs in SF, and is smaller in BB (1.5 °C) and GD (1 °C). The amplitude of the seasonal salinity variation is approximately 0.5 psu in all basins. In BB and GD, a drop in salinity is observed in autumn.