The hydrographic-hydrochemical state of the Baltic Sea in 2022

The article summarizes the hydrographic-hydrochemical conditions in the western and central Baltic Sea in 2022. Based on the meteorological conditions, the horizontal and vertical distribution of temperature, salinity, oxygen/hydrogen sulphide and nutrients are described on a seasonal scale.

The winter season 2021/22 shows a “cold sum” of 15.9 Kd recorded at station Warnemünde, which is ranked on 6^th place of warm winters in comparative data from 1948 to date. The summer “heat sum” of 281.4 Kd is far above the long-term average of 161.4 Kd and only slightly below the previous year 2021 (284.7 Kd). In the course of 2022, the Baltic Sea experienced again no strong inflow activity. Only two smaller inflow pulses occurred with total volumes of 140 km³ and 158 km³ all over the year which ventilated the Arkona Basin. During December occured a stepwise sealevel rise of three small pulses which led to an total inflow volume of 141 km³ and salt mass import of 1.15 Gt.

The overall decreasing trend of the oxygen concentration, in fact, the increase of hydrogen sulphide in deep waters of Baltic Sea deeps was basically ongoing in 2022. Recently, it became obvious that also shallow areas of the Baltic Sea are more often subjected to saisonal low oxygen values. The degradation of large amounts of organic matter that is produced because of ongoing eutrophication, causes a high oxygen demand impacting especially the seafloor. However, in February an oxygenated bottom layer in the western and central part of the Arkona Basin with a concentration of 310-335 µmol l^-1 oxygen was formed that only temporarily and locally improved the conditions. The oxygenated water passed the Bornholmsgatt and was mixed with Bornholm Sea deep water. The newly formed water mass contained 130-270 µmol l^-1 oxygen. It was further transported eastward and reached the southern part of the Eastern Gotland Basin in spring and summer 2022 with oxygen concentration of about 45 µmol l^-1 at a depth of 100 m.

The selected stations in the regions west of Darss Sill (Fehmarn Belt, Lübeck and Mecklenburg Bights) showed relatively high winter nitrate concentrations in surface water in comparison to recent years, which might hint to an influence of North Sea water. Whereas the stations in the Baltic Proper were characterized by low nitrate values in February, perhaps originating from an increasing oxygen debt and nitrate consumption in the deep water body. The relatively low winter nitrate concentration of 3.4 µmol l^-1 in Gotland Deep surface water and 2.9 µmol l^-1 at Bornholm Deep station, caused an early exhaustion of nitrate already at low temperatures. This likely enabled availability of phosphate almost throughout the summer half-year in 2022. Only in early August, the value was below the detection limit. The annual mean phosphate concentration in the Bornholm Sea deep water depicted a considerable interannual variability since 2018 on a high level of 4.2 ±0.9 µmol l^-1 at the reference depth. Whereas, the Landsort and Karlsö Deeps with 3.7 ±0.4 µmol l^-1 and 3.8 ±0.3 µmol l^-1, repectively, in the selected reference depths were more stable. The Gotland Deep showed the strongest accumulation of phosphate to 6.1 µmol l^-1 and ammonium of even 27.9 µmol l^-1 in 2022. Thereby, the phosphate and ammonium concentrations increased in the Fårö Deep to 4.7 µmol l^-1 and 15.9 µmol l^-1, respectively, in the reference depth. This increase was interrupted only in 2020 by slightly lower annual averages. In the Bornholm Deep, the ammonium concentration scattered around relativ low values of 2.2 ±1.4 µmol l^-1 in 2022 at high interannual variability.

In 2022, POC and PON concentrations were elevated in the surface water of the Baltic Sea in March, July, and partially May, induced by primary production during spring and summer blooms. Surface and deep POC and PON or particulate C/N ratios were not significantly different from long term means.

This report summarizes surface water concentrations for chlorinated hydrocarbons (CHC) and surface sediment contents for CHC, polycyclic aromatic hydrocarbons (PAH, U.S. EPA PAH) as well as organotin substances (OT) which were determined during the Baltic Sea monitoring in February 2022. Due to the occurrence of technical problems PAH data for surface water samples cannot be reported. Since 2022 the determination of the pesticide heptachlor and its metabolite heptachlorepoxide (HEPEP) is included in the assessment.

Concentrations of 2.7-9.1 pg l^-1 were determined for DDT and metabolites (ΣDDTsum); highest concentration of 9.1 pg l^-1 for the area Pomeranian Bight. Concentrations of the long-lived degradation product p,p'-DDE were higher compared to p,p'-DDT, indicating no significant new DDT inputs. For ΣPCB_ICES,SUM and HCB_SUM, concentrations of 1.6-13.1 pg l^-1 and 4.8-9.9 pg l^-1 were determined. Highest PCB concentrations were detected in the Kiel Bight/Fehmarn Belt area and for HCB in the Pomeranian Bight, i.e., 13.1 pg l^-1 ΣPCB_ICES,SUM and 9.9 pg l^-1 HCB_SUM. HEPEP was detected in all seawater samples with concentrations ranging from 0.28-0.83 pg l^-1 HEPEP_SUM. Heptachlor was not detected in any of the transect samples.

In the surface sediment samples 29.3 ng g^-1 - 168.7 ng g^-1 TOC were determined for DDT and metabolites (ΣDDT); the highest content of 168.7 ng g^-1 TOC for the Pomeranian Bight. For ΣPCB_ICES and HCB, contents ranging from 63.7 ng g^-1 - 166.0 ng g^-1 TOC and 0.34 ng g^-1 - 44.0 ng g^-1 TOC were determined, highest contents for the site in the Pomeranian Bight, i.e. 166.0 ng g^-1 TOC ΣPCB_ICES and 44.0 ng g^-1 TOC HCB.  PAH contents ranged from 16951.8 ng g^-1 - 40000.0 ng g^-1 TOC ΣPAH; the highest content was obtained for the station in Pomeranian Bight with 40000.0 ng/g TOC. Organotin contents of up to 278.7 ng g^-1 TOC ΣOT (MBT, DBT, TBT, TPhT) were detected. TPhT was not detected at any of the sites and none of the organotin compounds was detected at the station in the Pomeranian Bight.

The assessment of the obtained data was based on the environmental quality standards (EQS) of the Water Framework Directive and HELCOM indicators. All surface seawater concentrations for HEPEP exceed the annual average EQS of 0.01 pg l^-1. The detected surface sediment TBT contents exceed the threshold value of 1.6 µg kg^-1 DW, 5 % TOC of the HELCOM indicator “TBT and imposex”. The anthracene content of 0.3 ng g^-1 DW (≙ 50 µg kg^-1 DW, 5 % TOC) determined for the Pomeranian Bight exceeds the threshold value of the HELCOM indicator “Polycyclic aromatic Hydrocarbons and their Metabolites” of 24 µg kg^-1 DW, 5 % TOC.