The hydrographic-hydrochemical state of the Baltic Sea in 2020

The article summarizes the hydrographic-hydrochemical conditions in the western and central Baltic Sea in 2020. Based on the meteorological conditions, the horizontal and vertical distribution of temperature, salinity, oxygen/hydrogen sulphide and nutrients are described ona seasonal scale. For the southern Baltic Sea area, the Warnemünde station recorded in the winter 2019/2020 a“cold sum” of the air temperature of 0 Kd leading to a classification of an extreme mild winter season, setting a new record as warmest winter since the beginning of the time-series in 1948. The summer “heat sum” of 234.3 Kd ranks on the 14th position over the past 72 years and is far below the record of 394.5 Kd during 2018. The long-term average is 159.7 +/- 75.1 Kd. The situation in the deep basins of the Baltic Sea was mainly characterized by stagnation and widespread hypoxic to euxinic areas. In wintertime 2019/2020 three weak inflow phases occurred in November, January and February which dominated the situation in the deep water of Arkona Basin and Bornholm Basin. A next weak inflow pulse occurred from mid to end of October. Comparing of temperature and salinity measurements at the key stations Bornholm Deep and Slupsk Channel, none of these events crossed the Slupsk Sill. The deep water temperature in the Slupsk Sill stayed warm between 9.1 to 9.5 °C whereas an annual mean of 8.4 °C was found the Bornholm Deep, which was influenced by colder water of the winterly inflow pulses. The deepwater at the eastern Gotland Basin was still influenced by the warm inflows of previous years and with bottom temperatures of 7.2 °C above average. The winter nutrient concentrations of nitrate and phosphate in surface water in the western and central Baltic Sea were slightly lower in 2020 compared to 2019 according to 9 reference stations. An exception was the Mecklenburg Bight that showed an elevated nitrate concentration in 2020. However, a clear trend over the last years could not be registered. It may be emphasized that the dissolved inorganic nitrogen/phosphorus ratio in winter in surface water was above 11 in the Mecklenburg Bight and almost 8 in the western Gotland Sea, significantly higher compared torecent years. This might have reduced the cyanobacteria abundance in 2020 in these areas. The euxinic conditions in the deep water of the Baltic proper continued to intensify in the year 2020. This determined the nutrient situation in the Baltic Deeps along the Thalweg. In the Bornholm Deep phosphate and nitrate concentrations still slightly declined. The Gotland, Landsort and Karlsö Deeps showed ongoing accumulation, with a maximum in the Gotland Deep of about 5 μmol/l phosphate and 20 μmol/l ammonium in 2020. A slight improvement was documented for the Fårö Deep that received some oxygenated water. So hydrogen sulphide concentration and nutrient accumulation were reduced there. Under the prevailing euxinic conditions nitrate was depleted in deep waters. An exception was the Bornholm Deep, that showed an annual average of 0.9 ml/l oxygen and consequently a high annual average nitrate concentration of 8 μmol/l. So, no ammonium could be detected and the phosphate concentration was determined at 3 μmol/l, a quite normal value for oxic deep water. This report summarizes surface water concentrations and sediment contents for chlorinated hydrocarbons (CHC) and polycyclic aromatic hydrocarbons (U.S. EPA PAH), as well as surface sediment contents for organotin substances (OT) which were determined during the Baltic Seamonitoring in January/February 2020. Seawater concentration gradients for the analysed contaminants ranging from the western BalticSea in the area of the Kiel Bight/Fehmarn Belt (ΣDDTsum: 5.72 pg/L, ΣPCBICES,SUM: 7.20 pg/L, HCBSUM: 4.90 pg/L, ΣPAKSUM: 4660 pg/L) to the eastern Gotland Sea (ΣDDTsum: 1.96 pg/L, ΣPCBICES,SUM: 1.22 pg/L, HCBSUM: 3.43 pg/L, ΣPAKSUM: 1344 pg/L) with noticeable concentrations in the Pomeranian Bight (ΣDDTsum: 13.06 pg/L, ΣPCBICES,SUM: 5.15 pg/L, HCBSUM: 9.30 pg/L, ΣPAKSUM: 5626 pg/L). The data suggest that the Odra River is a source of pollution in the Baltic Sea, especially for particulate contaminants. The highest CHC and PAH levels in the surface sediment were detected for the Arkona Basin (ΣDDT: 90.7 ng/g TOC, ΣPCBICES: 9 1.3 n g/g T OC, H CB: 5 .0 n g/g T OC, ΣPAH: a bout 37000 ng/g TOC); while highest organotin levels were detected in the Mecklenburg Bight (ΣOT: 275 ng/g TOC). The assessment of the data based on the EQS of the Water Framework Directive shows that a harmful effect on marine organisms must be expected from concentrations of the high molecular weight PAH benzo(b)fluoranthene for the Kiel Bight/Fehmarn Belt, eastern and western Gotland Sea areas. Contents for anthracene in surface sediment exceeded the threshold value of the HELCOM indicator PAH at station N1 in the Fehmarn Belt. Surface sediment contents of tributyltin exceeded the threshold value of the HELCOM test indicator TBT and imposex at all sites studied. Time series data for surface water back to year 2001 show decreasing trends for concentrations of PCBICES and DDT and its metabolites. No trends can be observed for surface sediment timeseries data within the studied time period.