The hydrographic-hydrochemical state of the Baltic Sea in 2023

The article summarizes the hydrographic-hydrochemical conditions in the western and central Baltic Sea in 2023. Based on the meteorological conditions, the horizontal and vertical distribution of temperature, salinity, oxygen/hydrogen sulphide and nutrients are described on a seasonal scale.

A “cold sum” of 30.8 Kd was recorded for wintertime 2022/2023 at station Warnemünde. It is classified as a mild winter on 15^th position of warm winters over the past 75 years (1948-2023). The summer “heat sum” of 288.7 Kd is far above the long-term average of 163.0 Kd and above the previous year 2022 of 281.4 Kd.

In the course of the year 2023, the series of years (2017-2022) showing weak inflow activity into Baltic Sea was interrupted. During December occurred around Christmas intensified inflow activity, which is classified as midsized Major Baltic Inflow (MBI) event after Mohrholz (2018) and imported a salt mass of 1.7 Gt (salinity >15 g kg^-1) into the deep water of the Arkona Basin.

By ongoing accumulation of hydrogen sulphide in deep water of the Gotland Sea, the intensifying of the oxygen deficit and accompanying symptoms basically continued in 2023.  The increase of the oxygen deficit is caused by current eutrophication and produced and subsequently remineralized large amounts of biomass. At Gotland Deep station, the decline of oxygen continued since 2019 and showed an accumulation of hydrogen sulphide equivalent of ‑317 µmol l^-1 oxygen and at Fårö Deep station of -171 µmol l^-1 oxygen at respective deep water reference depths in 2023.

The partly cool and stormy weather in summer 2023 hindered a strong seasonal oxygen deficit in bottom water of shallow Baltic Sea areas. Moreover, cold water from a minor barotropic inflow event of December 2022 entered the Arkona Sea and surpassed and entrained the oxygen bearing warm water in the centre of the Bornholm Sea that resulted in an elevated oxygen concentration of 145-255 µmol l^-1 in March. An isolated parcel of the warm oxygenated water of still 30 µmol l^-1 oxygen in the southern Gotland Sea moved further down the Thalweg until a depth of about 120 m.

The winter surface water nitrate concentration of 2.7 µmol l^-1 at Gotland Deep and at Bornholm Deep stations was measured in 2023 again below the values of previous year. The spring bloom in 2023 likely ended in the central Bornholm Sea end of March, and at the Gotland Deep station end of April. A significant replenishment of nitrate in the surface water did not take place at the Bornholm Deep station before mid-November and at Gotland Deep site end of November. At that time, cooling to about 10 °C at the Bornholm Deep site and 12 °C at the Gotland Deep station enabled wind induced mixing in autumn weather conditions and a supply of nutrients from deeper layers. The distribution pattern of the winter dissolved inorganic nitrogen versus phosphate ratio was similar to the situation in the last year and confirmed again in 2023 that nitrogen was a clearly limiting factor in the Baltic Proper, giving diazotrophic cyanobacteria an advantage compared to primary producers that depend on nitrate. In the Gotland Sea deep water strongly accumulated phosphate increased further to a concentration of 6.2 µmol l^-1 in the Gotland Deep reference depths and to 4.3 µmol l^-1 at the Karlsö Deep, whereas at Fårö Deep and Landsort Deep stations almost the same annual average phosphate concentration of 4.7 µmol l^‑1 and 4.1 µmol l^-1 as last year, was determined. Nitrate was mostly below the detection limit in deep water, which is caused by euxinic (sulphidic) conditions that prevent mineralization of organic matter to nitrate, and instead ammonium is formed. Similarly to phosphate, ongoing accumulation of ammonium in deep water was recorded for Gotland Sea deep water. In the Gotland Deep an extremely high ammonium concentration of 35.4 µmol l^-1, at Fårö Deep ammonium increased to 16.9 µmol l^-1, at Landsort Deep to 12.8 µmol l^-1 and on Karlsö Deep a similar ammonium concentration as last year of 15.9 µmol l^-1 were measured.

In contrast to phosphate, the nitrate winter surface water concentration may reach the HELCOM target values in certain years, but a permanent fulfilment appears unlikely in the near future. It should be noted that the deep water of central basins constituted a strong nitrate sink, because of the current intense euxinia. Thus a certain nitrate decline might not indicate the input reductions alone. However, the amount of a potential contribution of ammonium supply from deep water remains unknown. For phosphate very likely it may need some more decades to reach the targets that were aimed.

Surface seawater samples were obtained in study areas from Kiel Bight to the Gotland Sea by transect sampling during the expedition EMB311. These were analyzed for the chlorinated hydrocarbons dichlorodiphenyltrichloroethane (o,p'-DDT, p,p'-DDT) and the metabolites p,p'-DDE and p,p'-DDD, polychlorinated biphenyls (PCB_ICES), hexachlorobenzene (HCB), heptachlor (HEP) and the metabolite heptachlor epoxide (HEPEP) as well as polycyclic aromatic hydrocarbons (U.S. EPA PAH). In addition, surface seawater samples were taken using a spherical glass sampler and analyzed for the α, ß and γ isomers of hexachlorocyclohexane (∑HCH).

Concentrations determined for DDT and metabolites (ΣDDT_sum) ranged from 2.53 pg l^-1 to 13.08 pg l^-1, with the highest concentration found for the Pomeranian Bight. Lower concentrations of p,p'-DDT compared to the long-lived degradation product p,p'-DDE indicate no recent inputs of DDT.

Concentrations of PCB_ICES and HCB ranged from 1.73 pg l^-1 to 8.52 pg l^-1 ΣPCB_SUM and from 4.30 pg l^‑1 to 11.13 pg l^-1 HCB_SUM. The highest concentration of ΣPCB_SUM was determined for the area Mecklenburg Bight (8.52 pg l^-1), whereas the highest HCB concentration was detected in the southern part of the eastern Gotland Sea (11.13 pg l^-1). HEPEP was detected in the dissolved phase of the surface seawater samples ranging from 0.16 to 1.05 pg l^-1. In 2023 also dissolved HEP was detected with 0.09 pg l^-1 in the Mecklenburg Bight study area.

Obtained concentrations for PAH ranged from 1293 pg l^-1 to 5150 pg l^-1 ΣPAH_sum with the highest concentration found for the area Pomeranian Bight. Most ΣPAH_sum data observed in 2023 were below the 25^th percentile of the data obtained within the investigated period (2003 – 2023).

∑HCH concentrations ranged from 108 pg l^-1 at Kiel Bight (N3) to 161 pg l^-1 in the Arkona Sea (K7). The predominant isomer was ß-HCH. The long term analysis at station K4 (Arkona Sea) with data back to the year 1975 shows continuously reducing concentrations of ∑HCH which indicates no recent HCH inputs.

The assessment of the obtained data was based on the environmental quality standards (EQS) of the Water Framework Directive. All concentrations determined for HEP and HEPEP exceeded the annual average EQS of 0.01 pg l^-1. Determined concentrations for the high molecular weight PAHs BBF, BGHIP and ICDP were above the annual average EQS of 0.00017 pg l^-1 in the areas Mecklenburg and Pomeranian Bight.