The biological state of the Baltic Sea 2012

The time series on species composition and biomass or abundance of phyto- and zooplankton as well as macrozoobenthos in Kiel Bight, Mecklenburg Bight, Arkona Sea, Bornholm Sea and Gotland Sea, existing since 1979, was continued in 2012.

The phytoplankton spring blooms developed in Kiel and Mecklenburg Bights after mid of February, in the Arkona Sea by the end of March and in the Eastern Gotland Sea by beginning of May 2012. A strong diatom development occurred in the western parts, but the share of Mesodinium rubrum increased into easterly direction. Dictyocha speculum could be found in significant amounts only in Lübeck Bight. The rather unusual species Achnanthes taeniata, Verrucophora farcimen and Peridiniella danica, which were important in spring 2011, were rare or absent in 2012. The cyanobacteria reached their maximum at the end of July. The autumn 2012 was usual, with dinoflagellate blooms in the western parts and diatom dominance in the Baltic Proper. However, Mesodinium rubrum appeared unusually frequent in autumn 2012. The chlorophyll a concentrations were highest in autumn and confirmed that the spring bloom peak was not met.

In comparison to previous years a clear reduction in the diversity of diatoms, cyanobacteria and flagellates was recorded in the sinking material, whereas dinoflagellate species number remained on the same level. Cyanobacterial summer flux was reduced. The spring bloom contributed most to the particle export. Elemental fluxes of C, N, P and Si were slightly lower but still in the range of the long term rates known from the central Baltic basins.

In the year 2012 the mesozooplankton consisted of 55 taxa. The neozoan calanoid copepod Eurytemora carolleeae was recorded in few samples. The invasive cladoceran species Penilia avirostris was observed only in one sample. Massive occurrence of the invaded ctenophore Mnemiopsis leidyi was not detected. The sum of maximum concentrations of the taxonomic groups was similarily low as in 2010. Since the early 1990s, the mesozooplankton abundance tends to decline. The maximal concentration of calanoid copepods was about 11.000 ind.*m^-3, which is very low in comparision to the total mean of all years with 25.000 ind.*m^-3. In comparision of all maximum abundances of the most important taxonomic groups at all stations the share of Copepoda was low, whereas Cyclopoida and larvae of bivalvia had a high share, which decreased from west to the Darss sill. Rotatoria increased, but was constant in the Arkona Sea. Towarts east the high abundance of Cladocera was reduced.

The 142 species found in the macrozoobenthos mark a very high diversity, which is an indication of an absence of an oxygen deficit in 2012. Depending on the region, the abundances ranged from 278 to 12113 ind./m² and the biomass (ash free dry weight) from 2.9 g/m² to 81 g/m². Both in respect to abundance and biomass the dominant species varied at the different stations. Some examples of key species were the gastropod Peringia ulvae, bivalves Arctica islandica, Astarte borealis, Mytilus edulis and Mya arenaria, the polychaetes Scoloplos armiger and Pygospio elegans and the cumacean Diastylis rathkei. At the 8 monitoring stations altogether 20 species of the Red List could be observed. 4 indigenous species were observed during the 2012 sampling.

Dr. Norbert Wasmund, Dr. Christina Augustin, Dr. Falk Pollehne, Dr. Herbert Siegel, Dr. Michael L. Zettler