The biological state of the Baltic Sea 2009

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 2009.

The phytoplankton spring bloom occurred rather early in Kiel Bight (end of January, Skeletonema costatum). The share of Mesodinium rubrum in the spring bloom increases towards the east. Chrysochromulina sp. establishes, like in 2008, as significant part of the spring phytoplankton. Like in some previous years, dinoflagellates are only weakly developed in spring in the Arkona Sea. In contrast to 2007 and 2008, Dictyocha speculum has significantly diminished. At the end of July, a diatom bloom (Proboscia alata) appeared in Kiel Bight. Cyanobacteria were dominant in the Arkona Sea temporarily. At the cruise from end of October/beginning of November, no bloom was noticed. There are indications that an autumn bloom appears much later. The marine diatoms Phaedactylum triconutum, Chaetoceros lorenzianus and Lennoxia faveolata were noticed for the first time in this area.

Chlorophyll a data revealed still a decreasing trend from1979-2009 in Mecklenburg Bight but an increase in the Arkona Sea if only the spring data were considered. Summer and autumn data showed no trend.

Sedimentation records of organic matter in the Arkona Sea in 2009 displayed sedimentation maxima generated by a diatom spring bloom, a summer maximum of diazotroph cyanobacteria and mixed diatom/dinoflagellate assemblages in autumn. The total annual flux for the single elements amounted to 314 mmol C, 41 mmol N, 74 mmol Si and 1,9 mmol P per m² at a mass flux of 45 g dry mass m^-2a^-1.

The number of mesozooplankton taxa amounted to 41 in the last 10 years, with higher numbers in every second half of the year. In 2009, the number increased by four in comparison to the previous year, indicating a higher water exchange. Neozoans like Mnemiopsis leidyi were still present in 2009. The declining tendency of mesozooplankton abundance since the early 1990s is apparent in the last years. Higher concentrations of Aurelia aurita mainly corresponded with lower winter temperatures because of its importance for initialization of the strobilation process. The negative effect of mild winters on Aurelia biomass concentration could not be fully counterbalanced by the warmer summer. Synchronization in food web was also important in this context.

The 140 species found in the macrozoobenthos in 2009 mark a very high diversity during the last 19 years at the monitoring stations. No defaunation process due to the absence of an oxygen deficit situation was observed in 2009. The abundances varied in respect to the area and depending on specific environmental parameters between 650 and 10.000 ind./m². The biomass (ash free dry weight) ranged from 1 g/m² in the central Arkona Basin (OMBMPK4) to 219 g/m² in the Kiel Bight (OMBMPN3). At the 8 monitoring stations altogether 19 species of the Red List could be observed, e.g the bivalve Macoma calcarea. The ecological status of the 8 stations, derived from the BQI (Benthic Quality Index), ranged between "moderate" and "high".

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