The biological state of the Baltic Sea 2013

The time series on species composition and biomass or abundance of phyto- and zooplankton as well as macrozoobenthos in Kiel Bight, Mecklenburg Bight and the Arkona Sea, existing since 1979, was continued in 2013.

The phytoplankton spring bloom was not met during our cruises in the western Baltic, but it was still present in the Baltic Proper in the first half of May with dinoflagellates and Mesodinium rubrum as dominating groups. The unusual species Achnanthes taeniata, Verrucophora farcimen and Peridiniella danica, found in spring 2011, were rare in spring 2013. Cyanobacteria blooms appeared on some days in July near the coast of Mecklenburg-Vorpommern. The autumn blooms with dinoflagellates in the western parts and diatom dominance in the Baltic Proper were welldeveloped at the beginning of November 2014.

The chlorophyll a concentrations were highest in autumn and confirm that the spring bloom peak was not met.

Sedimentation records of organic matter in the Arkona Basin over the year 2013 showed a distinct peak in spring and an elongated period of high flux during summer. Resuspension events occurred in periods of intense winter mixing. The annual flux for single elements in 2013 amounted to 1.23 mol C, 142 mmol N, 394 mmol Si and 6.6 mmol P m^-2 a^-1 at a mass flux of 113 g dry mass m^-2 a^-1.

In 2013, the zooplankton species number was similarly high compared to the preceding years. 55 zooplankton taxa were identified at 65 stations. Cladocera, rotifers and cyclopoid and calanoid copepods dominated the zooplankton composition and were responsible for pronounced maxima in zooplankton stocks in the Arkona- and Bornholm Basins. The calanoid copepod Acartia tonsa and the cladoceran Podon intermedius re-appeared in the plankton and achieved considerably high densities. The non-indigenous cladoceran Cercopagis pengoi occurred in low abundance. In the long term, the abundance of total zooplankton remained low. The decline of zooplankton observed since the beginning of the century continued. While this long-term decline was initially caused by a decrease in cladocera and rotifers, calanoid copepods contribute in recent years.

The 118 species found in the macrozoobenthos mark a moderate diversity, which is an indication of absence of an oxygen deficit in 2013. Depending on the region, the abundances ranged from 314 to 6.400 ind./m² and the biomass (ash free dry weight) from 0.4 g/m² to 100 g/m². Some examples of key species were the gastropod Peringia ulvae, bivalves Arctica islandica, Astarte borealis, Mytilus edulis and Mya arenaria, the polychaetes Scoloplos armiger, Terebellides stroemii and Pygospio elegans and the cumacean Diastylis rathkei. At the 7 monitoring stations altogether 14 species of the Red List could be found. 4 indigenous species were observed during the 2013 sampling.

Dr. Norbert Wasmund, Dr. Jörg Dutz, Dr. Falk Pollehne, Dr. Herbert Siegel, Dr. Michael L. Zettler