The biological state of the Baltic Sea 2022

In 2022 a total of 146 phytoplankton species were recorded on 5 annual monitoring cruises. Mean annual biomass of phytoplankton in the study area was, at 1015 µg l^-1, higher than the 20-year mean. The phytoplankton spring bloom started in early February and advanced quickly in the typical manner from south to north. As in previous years, also in spring 2022, Chla concentrations ranged from ~1 to ~10 µg l^-1. Highest spring concentrations were measured in March. Chla generally reflected phytoplankton biomass poorly, due to high representation of non-diatom and mixotroph taxa such as dinoflagellates and Mesodinium rubrum in all phases of the spring bloom. Phytoplankton biomass ranged from 150 µg l^-1 in the south to ~3000 µg l^-1 in the north, reflecting the typical latitudinal progression of the spring bloom in the Baltic Sea. As during the previous year, the summer phytoplankton composition and production was largely shaped by diatoms in the southern Baltic, which is unusual in the open sea areas, but seems to establish a recurrent pattern. Dactyosolen fragilissimus constituted nearly 90 % of the biomass in the Belt Sea and Arkona Basin in July. Total phytoplankton biomass of 7000 µg l^-1 was measured here during the August cruise. Like in previous years, this was contrasted by cyanobacteria dominated communities in the central Baltic, which, however, only amounted to 5 – 10 % of the biomass produced by the diatoms in the south. In November, a mixed diatom/dinoflagellate community had established in the southern basins by the time of TF1122.

In 2022 harmful Nodularia spumigena and specifically Aphanizomenon reached high biomass concentrations in Arkona, Bornholm and Gotland Basins, as typical for the summer season. A bloom of potentially toxic Pseudo-nitzschia spp. occurred in spring in Kiel Bight, though without causing major harm. In 2022, the diatom to dinoflagellate ratio had increased, probably due to the high Biomass shares of Skeletonema marinoi observed in spring. Cyanobacteria biomass was in the same range of inter-annual variation as previously. The 2022 annual phytoplankton biomass level was slightly higher than the 20- year mean of 800 µg l^-1, as was the diatom to dinoflagellate ratio, reflecting the diatom dominance of summer and autumn communities.

In 2022, 54 zooplankton samples were taken on 39 stations in the Kiel Bight, the Bay of Mecklenburg and the Arkona Basin. Due to bad weather conditions, two stations were cancelled on the return journey and the seasonal cycle of zooplankton is incomplete in the Kiel Bight, but no major implications for the analysis of the biodiversity and seasonal dynamics exist in the other areas. The species inventory was dominated euryhaline and brackish species. Nevertheless, true marine species were observed, primarily in the Belt Sea. These include the cladoceran Penilia avirostris that occurred in high numbers but also species which were generally rare such as calanoid copepod Calanus helgolandicus or the cladoceran Pleopis polyphemoides. In contrast to 2021, larvae of various benthic taxa with a marine affinity were also regularly observed. Apart from A. tonsa, no other non-indigenous species were found. In total, 50 taxa were identified in the samples. The species number showed the usual seasonal variation with a maximum in winter-spring and autumn. The short-term seasonal and the long-term dynamics in the area was characterized by an unusual zooplankton composition. While the Rotifera and the Cladocera are usually dominating by their pronounced maxima in spring and autumn, respectively, their stocks were considerably below the usual values and contributed only 5-11% to the stock. Copepods were largely dominating the community by 59% followed by Copelata with 12%. Due to low abundance of rotifers and cladocera, the stock of zooplankton was low in the Arkona Basin where the two groups are usually very abundant. Therefore, the typical increase in zooplankton abundance from the Kiel Bight to the Arkona Basin was lacking in 2022, as well as the late summer maximum of the zooplankton in the Arkona Basin. Among the group of copepods, a major shift from the dominance of the diverse Acartia species to Pseudocalanus spp. was observed in the Belt Sea, while the composition in the Arkona Basin was as usual. The lack of major contributions of rotifers and cladocerans had further consequences for the long-term abundance of the stock. 2022 was the 11^th year in row with zooplankton stock below the long-term average. The stock of 0.6 x 10⁵ ind. m^-3 was the second lowest value after the all-time minimum observed in 2020 and achieved only 22% of the long-term mean. All major zooplankton groups showed a decline by b 38-95% except the Copelata that increased by 60%. Within the copepods, stocks sizes of most taxa were reduced by 39-75%, while Pseudocalanus spp. showed an increase by 208%. The zooplankton composition and seasonal dynamics in the Belt Sea (Kiel Bight and Bay of Mecklenburg) was homogenous without the large spatial variability that occurs in some years. The seasonal variation was pronounced in 2022 and the start of the season was early since stocks increased already considerably in March. Copepods dominated the community during the first half of the year together with the meroplankton that show peak concentrations in the winter-spring transition. Their decline in summer was moderate, and an unusual high density of Oikopleura dioica (Copelata) was observed. In contrast, meroplankton and especially the bivalve larvae showed a low stock size. The copepods were dominated by unusual high concentrations of Pseudocalanus in winter-spring, while the abundance of Acartia was unusually low. In contrast to the Belt Sea, the seasonal development and composition of zooplankton in the Arkona Basin was as usual except the lack of high densities of the cladoceran Bosmina in summer that shifted the annual zooplankton maximum into spring. The winter-spring increase was dominated by copepods and rotifers. During summer, the dominance of the copepods was replaced by increasing proportions of meroplankton and cladocera. The copepods showed the typical balanced composition of the community with more or less similar stock sizes of the major genera.

In the autumn of 2022, the weather conditions in the southwestern Baltic Sea were so harsh that sampling of macrozoobenthos the stations in the Kiel and Mecklenburg Bays and Fehmarn Belt had to be cancelled. Thus, only a limited data set is available for comparison with previous years. For the station in the Kiel Bight (N3), we were able to maintain the time series with sampling material from colleagues at the SH State Office for the Environment. Unfortunately, this was not possible for the two stations N1 (Fehmarnbelt) and M2 (Mecklenburg Bay). The 106 species found in the macrozoobenthos in 2022 represent a medium diversity, remembering that two stations less could be evaluated. The species number found at the six monitoring stations ranged between 19 and 52. In all regions, the oxygen supply in bottom waters in the current year was mostly higher than 2 ml l^-1. The only exceptions were in August, when we measured 1.7 ml l^-1 in Kiel Bight and 0.9 ml l^-1 in Arkona Basin. In the Kiel Bight we found a significantly lower number of species compared to previous years, but this is probably also due to the fact that we did not carry out the sampling ourselves and, for example, no dredge was taken. In the southern Mecklenburg Bay and also in the Pomeranian Bay, the diversity was somewhat lower than the long-term average. Depending on the region, the abundances ranged from 518 to 6530 ind. m^-², and the biomass (ash free dry weight) from 1.1 g m^-² to 53.8 g m^-². Using the example station K8 (Darss Sill), a long-term analysis of the last 4 decades is carried out. The long-term development of the number of species, abundance and biomass is presented. Selected mussel species (Astarte borealis, Macoma balthica and Mya arenaria) are used to show which exemplary changes have taken place and what influence they can have on the ecosystem. A total of ten species from Germany's Red List (categories 1, 2, 3 and G) were observed at the six monitoring stations. With seven, the number of benthic invasive species in 2022 was low, as expected. They were all already known from previous years. Mya arenaria and Amphibalanus improvisus have been common faunal elements in the southern Baltic Sea for more than a hundred years. Since 2016, the amphipod Grandidierella japonica is known from the southern Baltic Sea and was also observed in Kiel Bight during the present study. The two polychaetes Alitta succinea and Marenzelleria viridis were regularly found during sampling in recent years. Finally, the cryptic neozoan decapod species Palaemon elegans has been established for decades and was found in northern Pomeranian Bay. Another cryptic neozoan is the polychaete Aphelochaeta marioni, which was observed in the Kiel Bight (N3).

Dr. Michael L. Zettler, Dr. Anke Kremp, Dr. Jörg Dutz