The biological state of the Baltic Sea 2021

In 2021 a total of 165 phytoplankton species were recorded on 5 annual monitoring cruises. Mean annual biomass of phytoplankton in the study area was, at 928 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. In spring 2021, chlorophyll-a (Chla)-concentrations ranging from ~1 to ~10 µg l^-1 reflected phytoplankton biomass poorly, due to high representation of non-diatom and mixotroph taxa such as Mesodinium rubrum or diverse flagellates. These included the ichthyotoxic invasive Raphidophyte Heterosigma akashiwo, blooming in Kiel Bight in February. Diatoms were abundant only locally in March, in the Arkona and Bornholm Basins. By May, Chla and biomass had declined in the south, but were still increasing towards the north (~150 µg l^-1 in the south to ~1500 µg l^-1 wet weight in the north), reflecting the typical latitudinal delay of the spring bloom in the Baltic Sea. The summer phytoplankton composition and production was largely shaped by diatoms in the southern Baltic, which is unusual in the open sea areas. Dactyosolen fragilissimus constituted 80 to > 90 % of the biomass in the Bay of Mecklenburg in July. Total phytoplankton biomass of 3000 to nearly 8000 µg l^-1 was measured in the western Baltic during the July cruise. This was contrasted by cyanobacteria dominated communities in the central Baltic, which, however, only amounted to 5 % of the biomass produced by the diatoms in the south. In November, diatoms - mostly Coscinodiscus spp. and Cerataulina pelagica - dominating the phytoplankton community throughout the study area (80 to 90 % of total biomass). In 2021 harmful Nodularia spumigena and Aphanizomenon made 10 % - 50 % of the biomass in the Arkona, Bornholm and Gotland Basins, as typical for the summer season. The 2021 diatom to dinoflagellate ratio and cyanobacteria biomass were in the same range of interannual variation as in previous years. The 2021 annual phytoplankton biomass level was slightly higher than the 20-year mean, as was the diatom to dinoflagellate ratio, reflecting the diatom dominance of summer and autumn communities.  

In 2021, maximum zooplankton abundance achieved 8.0 x 10⁴ ind. m^-3. This represents on average 33 % of the long-term mean of the stocks since 2000 and only a minor recovery from the long-term low of the zooplankton stocks observed in 2021. Although cladocerans showed a slight increase, their abundance together with that of rotifers, Copelata, cyclopoid and calanoid copepods, and polychaete larvae was considerably below their long-term means. Only bivalve and gastropod larvae achieved their long-term average concentrations. Cladocerans also dominated the species composition, mainly due to a small recovery of the summer peaks in the genus Bosmina in the Arkona Basin. Copepods were still abundant, but the contribution of the cyclopid copepod Oithona spp. was unusually low, so that calanoid copepods, escpecially Acartia spp., dominated. In addition, rotifers remained considerably below their historical concentrations. The seasonal development was early, and the zooplankton stocks achieved their maximum already in May in the Belt Sea. Except the calanoid copepod Acartia tonsa, no non-indigenous species were observed.

The 124 species found in the macrozoobenthos in 2021 mark a medium diversity. The species number found at the eight monitoring stations ranged between 20 and 67. In all regions, the oxygen supply in bottom waters in the current year was always higher than 2 ml l^-1. While over the last years, occasional population collapses caused by a lack of oxygen could be observed (especially in the Fehmarnbelt and in the Mecklenburg Bay), as far as we can see from our data the consistently good oxygen conditions in 2021 led to a recovery of the stocks. Except for the southern Mecklenburg Bay with its decline, the diversity at all stations was similar or slightly increased compared to the recent years. Depending on the region, the abundances ranged from 254 to 7687 ind. m^-2, and the biomass (ash free dry weight) from 0.8 g m^-2 to 32 g m^-2. Altogether fifteen species of the German Red List (Categories 1, 2, 3 and G) were observed at the eight monitoring stations. For the first time, the long-term data were used to calculate the Benthic Quality Index (BQI) and thus also the ecological status stationwise. Half of the stations were in “good” condition over the years 2006 - 2021. Two have become predominantly “poor” over the years, and two others are often in “good” condition, although according to the threshold rules they still fall under “poor” if you sum up all relevant assessment years. Especially for the southern Mecklenburg Bay (OM18) we see a negative trend comparing the three different evaluation periods. No trend was observed for the other stations or sea areas. With six, the number of invasive species in 2021 was low, as expected. They were all already known from previous years. Rhithropanopeus harrisii, originally from North America, could be observed in low densities on the Oderbank since 2006. Only the spionid polychaetes Marenzelleria viridis and M. neglecta reached noteworthy abundances in the Pomeranian Bay. Mya arenaria and Amphibalanus improvisus have been common faunal elements in the southern Baltic Sea for more than a hundred years. Finally, the ascidian Molgula manhattensis was observed as a cryptic neozoa species in the Bay of Kiel.

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