The hydrographic-hydrochemical state of the Baltic Sea in 2021

The article summarizes the hydrographic-hydrochemical conditions in the western and central Baltic Sea in 2021. Based on the meteorological conditions, the horizontal and vertical distribution of temperature, salinity, oxygen/hydrogen sulphide and nutrients are described on a seasonal scale.

After the record warm winter 2019/20 (cold sum 0 Kd) a “cold sum” of 32.7 Kd was recorded for wintertime 2020/21 at station Warnemünde. It is classified as a mild winter on 17^th position of warm winters over the past 73 years (1948-2021). The summer “heat sum” of 284.7 Kd is far above the long-term average of 159.7 +/- 75.1  Kd and above the previous year 2020 of 234.3 Kd.

In the course of the year 2021 no larger inflow events occurred, but the deepwater of the Arkona Basin and Bornholm Basin was ventilated by weak inflow events.

The oxygen decline since the last Major Baltic Inflow (2014-2016) generally continued in the central Baltic Sea deep basins. However slight improvements in certain years occured, but didn't change the general bad situation and the trend. So the situation in the deep water of the Fårö Deep was a little better in 2020 with regard to present hydrogen sulphide and the western Gotland Sea (Landsort and Karlsö Deep) somewhat better in 2021. However, in 2021 a pulse of oxygenated water brought some oxygen to the southern Gotland Sea in the halocline range that was distributed further North.

The winter nutrient concentrations of phosphate and nitrate in surface water were clearly higher in 2021 as in a couple of previous years and moved further away from HELCOM target values that appeared within reach for nitrate in recent years. For the Gotland Sea the winter nutrient concentrations of surface water in January/February 2021 were within the range of recent years. Only for the Karlsö Deep a clearly higher nitrate concentration in surface water is documented for winter 2021 in surface water.

In the deep water, a worsening of the situation of the major nutrients phosphate and ammonium occured in the year 2021 in agreement with the development of negative oxygen (hydrogen sulphide). Highest values for 2021 were determined for phosphate in the deep water of the Gotland Deep (5.4 µmol/l), Fårö Deep (4.4 µmol/l), and Karlsö Deep (4.0 µmol/l) since 2017. Nitrate was depleted in euxinic condition and ammonium accumulated to maxima in 2021 in the Bornholm Deep (4.0 µmol/l), Gotland Deep (22.8 µmol/l), Fårö Deep (12.2 µmol/l) and Landsort Deep (10.3 µmol/l) at least since 2017.  

The concentrations of particulate organic carbon (POC) and nitrogen (PON) were in surface waters notably increased in March and July induced by photosynthetic primary production. Especially at coastal and shallow stations like at Warnemünde, at the Darss Sill and Fehmarn Belt this signal was measured down to the mixed deep-water from March to July. The ratio of particular C/N were significant below the long-term means at the deep-water and surface layer, whereas POC concentrations were lowered only in surface waters.

This report summarizes surface water concentrations determined during the Baltic Sea environmental monitoring in January/February 2021 for the chlorinated hydrocarbons (CHC) dichlorodiphenyltrichloroethane (DDT) and its metabolites, polychlorinated biphenyls (PCB_ICES) and hexachlorobenzene (HCB), as well as polycyclic aromatic hydrocarbons (U.S. EPA PAH). Seawater samples were collected in study areas from Kiel Bight to the Gotland Sea by transect sampling during the expedition EMB256.

Concentrations ranging from 4.0 to 14.7 pg/L (median: 5.8 pg/L) were determined for DDT and its metabolites (ΣDDT_sum); the highest concentration of 14.7 pg/L for the Pomeranian Bight. As in previous years, too, concentrations of the long-lived degradation product p,p'-DDE were higher as compared to p,p'-DDT. This implies that there are currently no significant new DDT inputs. Concentrations ranging from 2.4 to 7.5 pg/L (median: 2.8 pg/L) and from 5.1 to 8.1 pg/L (median: 5.6 pg/L) were detected for PCBs (ΣPCB_ICES,sum) and HCB (HCB_sum). Highest concentrations were detected for the Pomeranian Bight. Surface water PAH concentrations (ΣPAH_sum) ranged from 3665 pg/L to 6939 pg/L (median: 5414 pg/L). In contrast to the CHC, highest PAH concentrations were determined in the areas of the Central Baltic Sea (6639 pg/L) to the Eastern Gotland Sea (South: 6939 pg/L, North: 6000 pg/L). The data obtained for the Pomeranian Bight point to the river Odra as a source of the studied pollutants, in particular for the particle bound ones (ΣDDT_part: 7.2 pg/L, ΣPCB_ICES,_part: 4.4 pg/L, HCB_part: 1.3 pg/L, ΣPAK_part: 1872 pg/L).

Analysis of time series data show continuing decreasing trends for concentrations of PCB_ICES as well as DDT and its metabolites.

The evaluation of the determined data on the basis of the environmental quality standards (EQS) of the Water Framework Directive shows that especially the concentrations of the high molecular PAH benzo(b)fluoranthene are of concern for the marine organisms, mainly in the areas from the Pomeranian Bight to the Gotland Sea. Here the annual average EQS values are exceeded.