The biological state of the Baltic Sea 2007

The time series on species composition and biomass or abundance of phyto- and zooplankton as well as macrozoobenthos in the Belt Sea and the Baltic proper, existing since 1979, was continued in 2007.

The phytoplankton growth started in Mecklenburg Bight in March and reached an extreme peak of Dictyocha speculum on 27.3.2007. This early Dictyocha bloom is unusual. It should appear after the diatom bloom in April. But in 2007 it has suppressed the diatom bloom. Also dinoflagellates were low in 2007. A moderate diatom spring bloom occurred in the Bornholm Sea and Eastern Gotland Sea. A diatom summer bloom was not found. The typical cyanobacteria bloom appeared in the Baltic Proper in July and August (see also satellite images). The typical Ceratium bloom did not develop in the western Baltic in autumn, but diatoms were strong. The autumn bloom in the Bornholm and Gotland Sea was exclusively formed by diatoms.

Data of sedimentation of organic matter in the Gotland Basin from 2006 shown, that the spring sedimentation maximum occurred later than usual and was formed exclusively by diatoms. The main export of silica occurred in spring, whereas it was unusually low in summer and autumn. The total annual flux for the single elements amounted to 270 mmol C, 33 mmol N, 107 mmol Si and 2,04 mmol P per m² and year. With 28.5 g dry material the mass flux was considerable lower than in the previous years.

Chlorophyll a data revealed a decreasing trend from 1979-2007 in Mecklenburg Bight but an increase in the Baltic Proper if only the spring data were considered, whereas summer data did not show any trend.

The number of mesozooplankton taxa amounted to 39 if the last 9 years were considered. The maximum in 2007 was found in July. The occurrence of the marine species Oithona similis below the halocline in the Gotland basin is an indication for higher saline water in combination with oxygenated conditions. A long-term decrease in maximum abundance since the 1990s, especially in rotifers was observed. The maximum abundance of Bosmina spp. was very low due to low summer temperature. The ctenophore Mnemiopsis leidyi over-wintered in Kiel Bight and Mecklenburg Bight with up to 5 ind/m³ and below the halocline in the Baltic proper with less than 1 ind/m³.

The macrozoobenthos species number (= 108) at the 7 traditional monitoring stations in 2007 was the second highest in the data series since 1991. If two new stations, sampled since 2006, are included, species number increased to 133. It showed a quick recovery after oxygen depletions in 2002 and 2005. Some marine species like the bivalve Abra alba and the brittle star Ophiura albida, dominant in the previous year at the westernmost stations, decreased probably due to a decrease of salinity in 2007. With the whelk Buccinum undatum and the common pelican foot Aporrhais pespelecani two very rare species of the Baltic Sea were recorded. In the central Arkona Sea, species number, abundance and biomass decreased in comparison with the previous years. The Bornholm Sea (Stat. 213) was characterised by no oxygen and a complete defaunation in 2007.

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