The biological state of the Baltic Sea 2023

In 2023, a series of inflow events affected the environmental conditions in the different basins of the western Baltic Sea. Traces of a cold, saline inflow in December 2022 were still visible in the bottom water of the Arkona Basin during February. In early summer, a baroclinic inflow build a thick bottom layer of saline water in the Belt Sea and replaced the cold bottom water in the Arkona Basin by very warm, but not exceptionally saline water. During December 2023 intensified inflow activity was recorded, which was classified as midsized Major Baltic Inflow (MBI). In 2023, the surface waters in the western Baltic Sea (Darss Sill and Arkona Buoy) exhibited temperatures significantly higher than the long-term average. The winter and autumn water temperatures were about 1,5 K above the long-term average. Summer temperatures, however, remained close to the long-term average, with occasional deviations due to cold anomalies in June and August, likely caused by episodic upwelling events. The partly cool and stormy weather in summer 2023 as well as the minor barotropic inflow event of December 2022 resulted in a consistently good oxygen supply from the Belt Sea to the Arkona Sea (bottom water > 2 ml l^-1). The concentration of inorganic nutrients in the surface water was within the range of the previous decade in February 2023. The N/P ratio of the surface water showed a decreasing west-east trend from the Belt Sea to the Bornholm Sea. This pattern was similar to the previous year and confirmed again that nitrogen was a limiting factor in the Baltic Proper, giving diazotrophic cyanobacteria an advantage compared to primary producers that depend on nitrate.

In 2023, 29 phytoplankton samples were collected on 5 monitoring cruises from 6 stations located in the Belt Sea (Kiel Bight and Bay of Mecklenburg) and the Arkona Basin. In 2023, mean annual biomass in the study area was 593,2 µg l^-1. This is significantly lower than in 2022, and below the 20 year mean. Corresponding mean chlorophyll a (Chl a) concentrations of 2,8 µg l^-1 were, however, in the range of previous years. Generally, seasonal Chl a dynamics was uniform and very similar in Belt Sea and Arkona Basin throughout the year, with highest concentrations measured in March and November and lowest in May. The prominent 2023 Chl a peak was detected in March in Kiel Bight and amounted to 13 µg l^-1, capturing a fully developed spring bloom. Accordingly, the highest annual phytoplankton biomass (4636 µg l^-1) for 2023 was detected. Despite quite uniform Chl a succession patterns in both sea areas, seasonal dynamics of phytoplankton biomass composition clearly displayed spatial differences along the monitoring transect, reflecting the marine influence in the Belt Sea and the increasingly brackish character of the Arkona Basin. The diatom spring bloom in the Belt Sea started in the westernmost Kiel Bight in February, when Chl a concentrations of nearly 3 µg l^-1 corresponding to biomass of 362 µg l^-1, were measured, mostly consisting of diatoms, with Rhizosolenia spp. constituting > 70 %. A few weeks later, biomass of diatoms had increased more than 10-fold in the Kiel Bight. In contrast, phytoplankton production was slightly delayed in the Bay of Mecklenburg and generally had significantly lower biomass values. The community here was still dominated by diatoms but with significant contribution of Mesodinium rubrum and its cryptophyte prey Teleaulax. Also, the summer community in the Bay of Mecklenburg was highly dominated by diatoms, though portions of dinoflagellates, Tripos, and cyanobacteria had increased then. Late autumn (November) was the most productive period of the annual phytoplankton cycle in the Bay of Mecklenburg in 2023, as indicated by Chl a concentrations of 4,3 µg l^-1 to 8 µg l^-1 and diatom dominated (> 80 %) biomass concentrations of up to 1390 µg l^‑1. In Arkona Basin, the north-east succession of the spring bloom was well reflected by spatial biomass dynamics. Highest annual Chl a (6,3 µg l^-1 at K5 and 3,6 µg l^-1 at K4) and biomass concentrations (1168 µg l^-1 at K5 and 228,5 µg l^-1 at K4) were measured in the western and central parts of the Arkona Basin in November. Winter and spring Chl a concentrations decreased prominently towards the east as did biomasses. In spring, the Arkona community was generally dominated by Mesodinium rubrum. Dinoflagellates and cyanobacteria had high biomass shares (45% and 20 %) in August. The phytoplankton of the Arkona Basinhad a marine character in November, being dominated by the diatoms Cerataulina bergonii and Rhizosolenia fragilissima.

In 2023, alltogether 141 phytoplankton taxa were recorded in the Belt Sea and the Arkona Basin at the five monitoring cruises with highest number of taxa (89) recorded in November. As in previous years, diatoms of Rhizosolenia spp., Proboscia alata and Cerataulina bergonii were the most important biomass producers in Belt Sea in March, August, and November (50 - 90 %) often accompanied by the dinoflagellates Tripos muelleri and Polykrikos schwartzii. In Arkona Basin the Mesodinium rubrum ranked number 1 (3 for all samples, Appendix) at all three cruises in spring, contributing up to 81 % in February. In summer and autumn, diatoms were unexpectedly prominent. Thus, while the spring community was a typical brackish Baltic one, the summer and autumn aspects had a clearly marine character.

In 2023, the number of potentially toxic or harmful algal taxa recorded in the study area had increased to 15 (compared to xy taxa in 2022). The list now contains several warm-water species such as Karenia mikimotoi, Akashiwo sanguinea and Karlodinium veneficum. Cyanobacteria biomass (Cyabi) of 2023 was, for the first time, considered in a long-term analysisfor the Belt Sea and the Arkona Basin: Whereas in the Belt Sea Cyabi was at the 20 year mean, it was above the 20 year mean at the central and Eastern Arkona stations.

In 2023, zooplankton samples were taken on 38 stations in the Kiel Bight, the Bay of Mecklenburg and the Arkona Basin. In contrast to previous years, the hydrographical situation was influenced by a series of inflow events that elevated the salinity in the water column in the Belt Sea and the bottom water in the Arkona Basin, but caused also very warm water temperatures throughout the area during summer. The inflows had, however, only little influence on the species inventory since the number of taxa were in the range of previous years without large inflow events (52 taxa). While a transport of some stenohaline marine taxa such as Penilia avirostris into the Arkona Basin was observed, their number was low. In addition, several of the marine and brackish species that were present in preceding, non-inflow years were lacking in 2023 and, thus, the taxa number remained on a lower level. Apart from Acartia tonsa, no other species classified as non-indigenous species (NIS) were observed.

The annual average species composition was dominated by copepods in all areas. This is common in the Belt Sea, but unusual in the Arkona Basin where rotifers and cladocerans can show large stocks in spring and summer, respectively. These were lacking in 2023. The decline in their contribution continues a series of similar observations since 2019 and, thus, cannot be attributed to unusual hydrographical situation in 2023. The meroplankton abundance was also rather low in all areas, particularly during summer when bivalve and gastropod larvae are common. In the Belt Sea, they were replaced by polychaete larvae as the most common group.

The seasonal dynamics and the composition of the zooplankton stock in the Kiel Bight and the Bay of Mecklenburg was very similar likely owed to their connection via the Fehmarn Belt. In agreement with winter-spring temperatures that were 2-3 °C above the long-term mean, the zooplankton stock showed an early increase with a doubling of the stock size in March and May, respectively. Copepods, especially the genera Pseudocalanus and Oithona, dominated the zooplankton followed by polychaete larvae and the appendicularian Oikopleura. In summer, the zooplankton stock collapsed to only 15-25 % of the long-term mean. Since the saline and thermal conditions were not exceptional for the area, the decline might be related to a combined stress of very warm temperaturates together with the usual food shortage during the summer period. This is reinforced by the composition of the community that was dominated by Oithona and other, thermophilic species. The stock recovered with the cooling in autumn, but with only little effects on the community composition. Despite the different hydrography with a lower salinity throughout the water column, the seasonal dynamics and stock composition of the zooplankton in the Arkona Basin resembled those observed in the Belt Sea. The spring development was early and was caused by a high abundance of the appendicularian Fritillaria in March. As usual, the increase in the copepods followed from March to May. They dominated the zooplankton in spring because rotifers were rather rare and lacked the sometimes typical  mass concentrations in 2023. Pseudocalanus dominated among the copepods, but other groups were common as well. Again, an unusual collapse of the zooplankton in summer was observed that was associated a with low abundance of copepods and cladocerans. This was likely caused by the high temperatures throughout the water column.

The summer collapse of the zooplankton in all areas had only little impact on the long-term variation of the copepods in the western Baltic Sea. In the Belt Sea and the Arkona Basin, their annual mean abundance was close the long-term mean, which can be attributed to the early stock development and high abundance in spring that partly compensated for the decline in summer. In contrast to the copepods, the cladocerans and, in particular, the rotifers displayed rather low annual mean abundance. In their main area of occurrence (Arkona Basin), their contribution to the stock decreased to only 12-17 % of their long term mean.

In autumn 2023, macrozoobenthos sampling was carried out at all eight stations along the German Baltic Sea coast, starting in the Kiel Bayand 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 138 species found in the macrozoobenthos in 2023 represent a relatively high diversity. The number of species found at the eight monitoring stations varied between 16 and 86. In general, neither a significant increase nor decrease in macrozoobenthos species number was observed in 2023. However, at some stations new observed species (never observed during the last 20 years at these stations) were found, e.g. Philine punctata. At all regions without exception, the oxygen supply in the bottom water in the current year was higher than 2 ml l^-1. Compared to the long-term mean, an average number of species was found at all stations. Only in the Bay of Kiel was an above-average diversity of 86 taxa found. Depending on the region, the abundances ranged from 331 ind. m^-2 to 6775 ind. m^-2 and the biomass (ash-free dry weight) from 1,8 g m^-2 to 25,2 g m^-2.

Using the example of station K4 (Arkona Basin), we carried out a long-term analysis of the last 4 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 Macoma balthica) are used as examples to show what changes have taken place and what influence they may have on the ecosystem. For the second time (after 2021), the long-term data was used to calculate the Benthic Quality Index (BQI) and thus also the ecological status. Half of the stations were 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 19 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 Musculus discors, Aporrhais pespelecani, Tritia reticulata and Scalibregma inflatum should also be emphasised here.

At 10, the number of invasive benthic species was relatively high in 2023. Most of them 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. The amphipod Grandidierella japonica has been known from the southern Baltic Sea since 2016 and was also observed in the Bay of Kiel during the present study. The two polychaetes Alitta succinea and Marenzelleria neglecta have been regularly found during sampling in recent years. The bivalve Ensis leei, a North american species, was recorded for the first time on the Darss Sill (K8). It is normally (albeit sparsely) more common in the western part of the Baltic Sea. The Japanese cumacean Nippoleucon hinumensis has been widespread in the Baltic Sea since 2019. In addition to several detections that we have made in other campaigns in the German Baltic Sea (mainly in estuaries), we found it for the first time this year during monitoring sampling in the Kiel Bay (N3). Over the past ten years, we have found a total of 16 non-indigenous species (NIS) at our eight monitoring stations. The long-term trend over the last 10 years in arrivals was 0,8 NIS per year.

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