The biological state of the Baltic Sea 2011

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 2011.

The phytoplankton spring bloom developed later than usual after the cold winter; it reached its peak at the end of march/beginning of April 2011 in the western Baltic. At least in the Arkona Sea, it was a classical diatom bloom, but formed by the unusual species Achnanthes taeniata. In Kiel Bight, we found abundant dinoflagellates (Peridiniella danica) and Dictyochophyceae (Verrucophora farcimen). Also these species are rather new and unusual in this area. In the central Baltic, Mesodinium rubrum formed a bloom in May. The cyanobacteria developed already in the first half of July and reached the maximum extent on 12 July 2011. A typical summer diatom bloom could not be found during the cruise in August 2011. In autumn 2011, dinoflagellates were dominating in the western part of the investigation area and diatoms in the Baltic Proper.

The chlorophyll a concentrations were highest in spring and confirm that the spring bloom peak was nearly met.

Sedimentation records of organic matter in the Arkona Basin over the year 2010 displayed sedimentation maxima in spring and autumn. Due to the low input of resuspended material the total annual flux for the single elements were in the normal range and amounted to 515 mmol C, 59 mmol N, 78 mmol Si and 3.1 mmol P m^-2 a^-1 at a mass flux of 57 g dry mass m^-2 a^-1.

The total number of mesozooplankton taxa amounted to 56 from the beginning of the BSH-financed monitoring, with most taxa found in 2011 because of salt water inflow. New neozoans were not found and even no Mnemiopsis leidyi. The sum of maximum concentrations of the taxonomic groups doubled from 2010 to 2011, but the tendency of mesozooplankton abundance since the early 1990s is still declining. Especially larvae of polychaetes increased below the halocline at Darss sill and Kadet Furrow. The concentration of calanoid copepods was about 30.000 ind.*m^-3 until 2010, but dropped by 50% in from 2010 to 2011.

The 124 species found in the macrozoobenthos in 2011 mark a medium to high diversity in comparison with previous years, because a long-lasting oxygen deficit did not occur in contrast to the years 2002, 2005, 2008 and 2010. The abundances varied in respect to the area between 388 and 7.400 ind./m². The biomass (ash free dry weight) ranged from 1 g/m² to 51 g/m². At the 8 monitoring stations altogether 17 species of the Red List could be observed. As example, the bivalve Macoma calcarea, threatened with extinction in German waters, was found in the Kiel Bight. 3 neozoan species were observed during the 2011 sampling, e.g. the polychaete Marenzelleria viridis in Pomeranian Bight with abundances of 300 – 1200 ind/m².

Dr. Norbert Wasmund, Dr. Falk Pollehne, Dr. Lutz Postel, Dr. Herbert Siegel, Dr. Michael L. Zettler