Biological state of the Baltic Sea 2004

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 2004.

The phytoplankton spring bloom occurred in Mecklenburg Bight (Thalassiosira nordenskioeldii, Porosira glacialis, Skeletonema costatum, Mesodinium rubrum) and the Arkona Sea (Skeletonema costatum, Mesodinium rubrum) in March 2004, but in the Bornholm Sea (Mesodinium rubrum) and in the eastern Gotland Sea (Mesodinium rubrum, Peridiniella catenata) at the end of April/beginning of May. The summer bloom was dominated by Cerataulina pelagica, Ceratium tripos and Proboscia alata in Mecklenburg Bight. The cyanobacteria growth was only weak in summer 2004. A bloom of Chrysochromulina sp. and Dinophysis norvegica in the eastern Gotland Sea in mid June is worth mentioning. The autumn bloom was dominated in Mecklenburg Bight by Ceratium tripos and in the Bornholm Sea and south-eastern Gotland Sea by Coscinodiscus granii.

The sedimentation maximum in spring was dominated by diatoms (Skeletonema costatum, Thalassiosira levanderi, Chaetoceros sp., Achnanthes taeniata). Like in some former years the sedimentation peak of silica shifted to midsummer due to the appearance of the pennate diatom Nitzschia paleacea on the cyanobacterial aggregates. The cyanobacterial (Aphanizomenon sp., Nodularia spumigena) aggregates dominated the vertical flux of carbon, nitrogen and phosphorus as well, which exceeded the long-term means by far in 2003. Additionally, large amounts of sedimenting manganese oxides and iron phosphates sedimented in summer 2003.

The effect of the saltwater influx from summer 2003 on the mesozooplankton, like increase in diversity and introduction of marine species, abated in 2004 with exception of the Gotland Deep. The maximum abundances of all considered taxa declined to 44% in 2004 in comparison to the first half of the 1990s. Also the maximum abundance of Pseudocalanus spp., an important food of adult herring remained on a level which was five times below the expectations. The very low abundance of Bosmina spp. was caused by the relatively low temperature in June/July, the potential feeding by the carnivore cladoceran Cercopagis pengoi and the too early observation period. The latter is also probable because the temperature increased up to an optimal range in the successive weeks. The maximum abundance of the calanoid copepods were kept on a fairly constant level of about 30,000 ind./m³ throughout the years. Centropages hamatus was the most dominant species, which has an reproduction peak at cooler summers in contrast to Acartia tonsa and Eurytemora affinis which remained at lower quantities in 2004.

The quick recovery of the macrozoobenthos after the oxygen depletions in 2002 in the western Baltic is remarkable. Since the beginning of the macrozoobenthos monitoring in 1991, 164 taxa were proved. The long-term tendencies in species number and Shannon-Wiener-Index since 1991 are displayed at one station (Darsser Schwelle). The development of the abundance and biomass of the bivalve Abra alba in the Fehmarnbelt region from 1991 to 2004 is shown. For the ocean quahog Arctica islandica and the arctic relict bivalve Astarte borealis the population structure at two stations are indicated.

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