The hydrographic-hydrochemical state of the Baltic Sea in 2019

For the southern Baltic Sea area, the Warnemünde station recorded in the winter 2018/2019 a “cold sum” of the air temperature of 18.3 Kd leading to a classification of a mild winter season and a ranking as the seventh warmest winter since the beginning of the record in 1948. The summer “heat sum” of 283.1 Kd ranks on the 24th position over the past 71 years and is far below last year’s record of 394.5 Kd. The long-term average is 158.6 +/- 68.9 Kd.

The situation in the deep basins of the Baltic Sea was mainly characterized by stagnation and widespread hypoxic to euxinic areas. The baroclinic inflows of the record warm summer 2018 reached the deep-water of the central Baltic Sea. The Gotland Deep showed record high 8.6 °C at the bottom. Additional inflow pulses of warm water arrived in March and April 2019, but salinity values and concentrations of dissolved oxygen stayed mainly unchanged. Three smaller barotropic inflows occurred from autumn 2018 to December /January 2019 and imported 3 Gt of salt into the western Baltic Sea. In the course of the year 2019 another four barotropic inflows of weak intensity occurred (April, June, September, December). The last one imported 1 Gt of salt. These events propagated from the Arkona Basin up to southern parts of the eastern Gotland Basin. The bottom salinity values stayed on the high level caused by the several Major Baltic Inflows in the time span 2014-2016, so that the 2019 events of weak intensity could not ventilate the bottom near water body in the central basins.

Anoxic and euxinic conditions in the deep waters intensified in 2019. This determined also the nutrient situation in the deep water of the northern and western Gotland Basin. In the eastern Gotland Sea, phosphate and ammonium concentrations were increasing since 2017. In 2019, oxygen was zero in all deep water reference depths. An exception reflected the intermittently reoxygenated Bornholm Sea deep water with about 3 ml/l oxygen in March, leading to an annual average of about 1.0 ml/l oxygen, thus, showing a clearly increased nitrate concentration to 6.8 μmol/l in 2019, a relatively low ammonium concentration of 1.5 μmol/l, and an annual average phosphate concentration of about 3.8 μmol/l.

This report summarizes obtained data for chlorinated and polycyclic aromatic hydrocarbons (CHC, PAH) for Baltic Sea surface water from the January/February 2019 observation in areas of the Kiel Bight/Fehmarn Belt up to the Gotland Sea. Overall, concentrations for CHC and U.S. EPA PAH decrease from areas of the western Baltic Sea to the Gotland Sea with the exception of the Pomeranian Bight. There, highest concentrations for PAH and CHC were observed (ΣPAH_sum: 15,000 pg/l, ΣCHC_sum: 31.5 pg/l). Among the analysed CHC highest concentrations were observed for HCB (HCB_sum: 6 to 8 pg/L) followed by DDT/metabolites (ΣDDT_sum: 3 to 7 (15)pg/L) and PCB_ICES (ΣPCB_sum: 2 to 9 pg/L) with the exception that at the Pomeranian Bight concentrations for DDT/metabolites were highest. The data depict high contaminant pressure for the areas Pomeranian Bight and the Kiel Bight/Fehmarn Belt, which indicates higher contaminant sources at these sites. Assessment of the results on the basis of the EQS of the Water Framework Directive shows that concentrations of a number of high molecular weight PAH might be of concern for marine organisms, particularly at the Pomeranian Bight.