The biological state of the Baltic Sea 2024

In 2024, a series of inflow events from the North Sea affected the environmental conditions in the different basins of the western Baltic Sea. In December 2023, there was a medium-sized Major Baltic Inflow (MBI) event, which reached the deep waters of the Arkona Basin in early 2024. This was followed by another inflow in January 2024. In July, a baroclinic inflow transported highly saline water into the Arkona Basin and in December 2024 another barotropic MBI of weak intensity occurred. The oxygen concentration in surface water is in general controlled by the seasonal changing temperature and primary production. The highest average oxygen concentrations in surface waters of the western Baltic Sea were observed in February, March and May and ranged between about 8 - 9 ml l^-1 dissolved oxygen. In the bottom water, the Major Baltic Inflow events of December 2023/January 2024 resulted in a consistently good oxygen supply from Kiel Bight to Bornholm Basin in water depths down to 60 m (> 4.5 ml l^-1) up to May 2024. During the summer months, oxygen concentrations decreased but did not fall below the ecological critical value of 2 ml l^-1. The temperature of the surface waters was higher than the long-term average in the western Baltic Sea. The winter and spring water temperatures were about 2 - 3 K above the long-term average. Summer temperature of the surface water was exceptional warm and reached values above 20 °C from Kiel Bight to Bornholm Basin. Compared to the long-term mean, the SST values were 3 K warmer in the western Baltic Sea. The general temperature distribution in November 2024 reflected the autumnal cooling and the erosion of the seasonal thermocline in the surface layer. From the Kiel Bight to Bornholm Basin surface temperatures were 2 - 2.5 K higher than the long-term mean. Considering the winter nutrient data as reservoir for the phytoplankton growth of the year, neither nitrate nor phosphate data showed a significant trend in surface waters of the western Baltic Sea during the last decade. The February surface concentrations of nitrate and phosphate were 6.43 µmol l^-1 and 0.63 µmol l^-1 respectively in Mecklenburg Bight and thus within the decadal range. The surface water DIN/DIP ratio (sum of ammonium, nitrate, and nitrite concentrations versus the phosphate concentration) ranged between about 10 mol mol^-1 in the western part and 6 mol mol^-1 in the Bornholm Sea in February 2024. The N/P ratio showed a decreasing trend from west to east: Kiel Bight/Mecklenburg Bight 9 - 11 mol mol^-1, Arkona Sea 6 mol mol^-1 and Bornholm Sea 5 mol mol^-1. The distribution pattern is similar to the situation in the previous year and confirmed again that nitrogen was a limiting factor from the western part up to the Baltic Proper, giving diazotrophic cyanobacteria an advantage compared to primary producers that depend on nitrate.

In February, March, May, August, and November 2024, a total of 30 phytoplankton samples were analyzed at six stations in the Belt Sea (Bay of Kiel, Bay of Mecklenburg) and the Arkona Sea. The mean annual biomass in the study area was 1740 µg l^-1. This value was more than 3 times higher than in 2023, and about twice as hight as the 20-year mean. A total of 143 phytoplankton taxa was recorded in the study area, with the highest number of taxa recorded in February (86) and November (87). In particular, the large diatoms Dactyliosolen fragilissimus and Cerataulina pelagica build up a high biomass and were identified as the most dominant species in 2024, followed by the cyanobacteria Nodularia spumigena. The seasonal dynamics of phytoplankton distribution showed typical time shifts of the spring bloom from southwest to northeast and pronounced biomass peaks in spring, summer and autumn in different areas. The spring bloom of phytoplankton was already detected in the beginning of February in the most western part of the study area (Bay of Kiel) and nearly exclusively dominated by diatoms, particularly Skeletonema marinoi. While this bloom already had declined in the Bay of Kiel in March, it reached its peak in the Bay of Mecklenburg at that time. However, the phytoplankton biomass of the spring bloom was much lower in the Bay of Mecklenburg compared to the February values in the Bay of Kiel. Further east, from Darss Sill to the Arkona Basin, spring bloom peak was detected in March, reflecting the typical south-to-north progression of the spring bloom across the Baltic Sea basins. At Darss Sill the mixotrophic ciliate Mesodinium rubrum dominated the spring community, followed by the diatom Skeletonema marinoi. In contrast, dinoflagellates - primarily Peridiniella catenata and members of Gymnodinales - comprised the second most abundant group after M. rubrum in the spring bloom of the central Arkona Basin. Further east in the Arkona Basin, diatoms, particularly Skeletonema marinoi, dominated the spring phytoplankton. In May, phytoplankton biomasshad decreased to lowest values in the entire study area, reflecting the end of the spring bloom period. In August quite high values of phytoplankton biomass were measured across the entire study area from the Bay of Kiel to the eastern Arkona Basin, indicating a summer bloom event. In the Bay of Mecklenburg, the highest phytoplankton biomass values of the entire area and entire year 2024 were measured. This bloom was equally dominated by the cyanobacteria Nodularia spumigena and the diatom Dactyliosolen fragilissimus. In November a pronounced autumn bloom was measured in the eastern part of the study area from Kadet Trench to the eastern Arkona Basin. This autumn bloom was almost exclusively dominated by the diatom Cerataulina pelagica. In 2024, 13 potentially toxic or harmful algal taxa were recorded in the study area (15 taxa in 2023, 8 taxa in 2022). Whereas most of the species occurred in overall low abundances, Prymnesiales reached high abundances at almost all stations and even bloom concentration in May in the Darss Sill area. The values of the diatom-dinoflagellate index exceeded the good environmental status threshold in the Bays of Kiel and Mecklenburg and in the Arkona Sea in spring 2024. The annual cyanobacteria biomass in the study area was assessed in a long-term context at the six monitoring stations in Belt Sea and Arkona Basin. In the Bays of Kiel and Mecklenburg, cyanobacterial biomass was strongly enhanced compared to previous years due to high abundance of Nodularia spumigena during sampling period. Apparently, the sampling in Mecklenburg Bay took place in August 2024, right during a cyanobacteria bloom. In the central and eastern Arkona Basin, in contrast, the mean total cyanobacteria biomass was far below the 20-years mean value in August 2024.

The zooplankton was monitored on 5 stations in the Bay of Kiel, the Bay of Mecklenburg and the Arkona Basin in 2024. Despite the inflows, the number of 58 recorded taxa in 2024 was low compared to historical values during major Baltic inflows (max. 70 taxa). The cold inflows during winter reached the Arkona Sea, but had as expected only little influence on the species number due to the low abundance and low activity of the zooplankton. The spring and summer inflows were largely confined to the Belt Sea, but had a larger impact, particularly in the Bay of Mecklenburg. Here, outflowing brackish water was overlaying the salty bottom water and the resulting mix of brackish and marine species elevated the species number to 32-35. Diverse meroplanktonic larvae, stenohaline copepod species and a number of hydrozoans typically associated with inflows were responsible for this increase. The diversity was reduced in the autumn, particularly in the Bay of Kiel, which likely reflects the high summer temperatures since most of the species encountered were thermophilic. This was associated with an unusual high abundance of Oithona in the Belt Sea. Two of the species encountered during the warm season are classified as non-indigenous species (NIS) – the copepod Acartia tonsa and cladoceran Penilia avirostris. Another NIS – the copepod Acartia hudsonica – was observed as single observation during May in the Belt Sea. The annual mean composition of the zooplankton was dominated by copepods in the Belt Sea. This is a typical situation owed to the low influence of brackish water in the surface. In the Arkona Sea, however, the unusual abundance of copepods during 2019-2023 was reduced in 2024 due to increasing stocks of cladocerans, typically common in the area. The abundance of rotifers, however, remained on a historically low level. The seasonal dynamics and the composition of the zooplankton in the Bay of Kiel and the Bay of Mecklenburg was as usual very similar. The winter and spring stocks were slightly lower than usual and dominated by copepods and meroplankton, which was characterized by a high percentage of bryozoan larvae. In May, the zooplankton increased to a concentration above the long-term mean and consisted largely of copepods of the diverse genera such as Temora, Centropages, Acartia and Pseudocalanus. The latter two genera had a higher contribution to the copepod stock in the Bay of Mecklenburg than in Bay of Kiel. The summer abundance was low, mainly due to low stocks of meroplankton. While copepods typically dominate the fauna, thermophilic species such as Acartia tonsa and Oithona dominated. Oithona achieved an unusual high abundance in autumn. The seasonal dynamics of the zooplankton in the Arkona Sea differed from the Belt Sea because the area was under strong brackish influence in 2024 and the dominance of copepods was less pronounced, which is typical for the brackish water. Winter stocks were lower than usual and apart from copepods, appendicularians and meroplankton showed a larger contribution to the community in winter. In spring, cladocerans and rotifers increased together with the copepod genera Pseudocalanus and Acartia. In August, a typical maximum of the Cladocera Bosmina occurred for the first time since 2022. Oithona was only abundant in autumn, but the unusual maxima occurring in the Belt Sea were lacking. The long-term change in the annual mean abundance of the major groups of the zooplankton – copepods, cladocerans and rotifers – displayed a divergent trend within the three major basins. In the Bay of Kiel and the Bay of Mecklenburg, copepods increased above the long-term mean while they continued to decline in the Arkona Sea. In all areas, however, Cladocera recovered from a series of years with low abundance and occurred at their long-term mean concentrations. In contrast, rotifers seem to disappear in all areas.

In autumn 2024, macrozoobenthos sampling was carried out at all 8 (eight) stations along the German Baltic Sea coast, starting in the Bay of Kiel and extending to the Bay of Mecklenburg, the Darss Sill, the Arkona Basin and the Pomeranian Bay. For most stations, a comprehensive dataset of autumn sampling since 1980 is available for long-term analysis. The total of 145 species found in the macrozoobenthos in 2024 represented a relatively high diversity. The number of species found at each of the eight monitoring stations varied between 9 and 82. Compared to the long-term average, the number of species was significantly reduced in the Fehmarnbelt and Bay of Mecklenburg. Increased diversity was observed in the Bay of Kiel, the Darss Sill, the Arkona Sea and the Pomeranian Bay. New species (which had never been observed at these stations in the last 20 years) were found at some stations. However, no new species was detected for the study area in 2024. The nudibranch Philine punctata appears to be spreading and, since its first detection in the Bays of Kiel and Mecklenburg in the previous year, has now spread as far as the Darss Sill. No oxygen deficiency could be detected in the study area based on the measured values from the 5 measurement campaigns. However, it is suspected that at least the Fehmarnbelt and parts of the Bay of Mecklenburg were affected by a deficit in the period August/September. This was noticeable in the greatly reduced diversity and density of the benthic species encountered. Depending on the region, the abundances ranged from 112 to 8421 ind. m^-2 and the biomass (ash-free dry weight) from 1.3 g m^-2 to 30.1 g m^-2. Using the example of station N1 (Fehmarnbelt), we carried out a long-term analysis of the last 3 decades. The long-term development of species numbers, abundance and biomass is shown. To categorise the results, the oxygen and salinity values at the bottom over this period were also analysed and partially presented. Selected species (Diastylis rathkei and Abra alba) are used as examples to show what changes have taken place and what impact they may have on the ecosystem. For the third time (after 2021), the long-term data were used to calculate the Benthic Quality Index (BQI) and thus also the ecological status. Half of the stations have been in ‘good’ status over the years. Three have always been assessed as ‘poor’ over the years, another one between “poor” and ‘good’ status. A total of 22 species on the German Red List (categories 1, 2, 3 and G) were observed at the eight monitoring stations. In addition to regularly encountered species such as Arctica islandica and species of the genus Astarte, somewhat less frequently observed taxa such as Halitholus yoldiaearcticae and Euchone papillosa should also be emphasised. Aporrhais pespelecani, Tritia reticulata and Scalibregma inflatum also occurred as in the previous year. At 10, the number of invasive benthic species was the same as in the previous year. All species were already known from previous years. Mya arenaria and Amphibalanus improvisus have been common in the southern Baltic Sea for more than a hundred years. Four polychaete species (Alitta succinea, Marenzelleria neglecta, M. viridis and Aphelochaeta marioni) have been regularly found during sampling in recent years. The decapod crustaceans Rhithropanopeus harrisii and Palaemon elegans occur regularly in the sampling campaigns. The bivalve Rangia cuneata, which originally comes from North America and was first observed in our study area in 2017/2018, is mainly found in the brackish waters of Bodden and Haffen, but is also increasingly found on the Oderbank. In the last ten years, we have found a total of 16 non-indigenous species (NIS) at our eight monitoring stations. The long-term trend in arrivals over the last 10 years has been 0.7 NIS per year.

Dr. Michael L. Zettler, Dr. Jörg Dutz, Dr. Anke Kremp, Carolin Paul, Susanne Busch Dr. Michael Naumann Dr. Sandra Kube