Leibniz-Institut für Ostseeforschung, Warnemünde
(IOW) (Baltic Sea Research Institute, Warnemünde)
Bundesamtes für Seeschifffahrt und Hydrographie, Hamburg, Rostock
(Federal Maritime and Hydrographic Agency of Germany, Hamburg, Rostock)
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 2005.
The phytoplankton spring bloom appeared in all sea areas more or less simultaneously (March - April 2005).
In contrast to usual blooms, the diatom bloom (Rhizosolenia setigera) persisted, according to microscopical data, until late May
in Mecklenburg Bight. The spring bloom in the Arkona Sea and Bornholm Sea consisted mainly of Skeletonema sp. and
Chaetoceros spp.. A weak diatom growth started in the southern Gotland Sea (Chaetoceros subtilis) and in the eastern
Gotland Sea (Thalassiosira sp., Skeletonema costatum) at the beginning of April. A strong diatom bloom
(Dactyliosolen fragilissimus) grew in June - July 2005 in Mecklenburg Bight incl.
Darss Sill, but no summer diatom bloom occurred in the eastern part of the Arkona Sea and the regions farther east. The expected
cyanobacteria bloom was found only in the Gotland Sea in July. The autumn bloom in Mecklenburg Bight was dominated by
Ceratium tripos and Ceratium fusus whereas diatoms were infrequent in November 2005 (Table 1).
Data of sedimentation of organic matter in the Gotland Basin from the year 2004 are shown. The spring maximum
was dominated by Skeletonema costatum and Thalassiosira levanderi. The main export of silica occurred in spring
and autumn whereas the contribution of Nitzschia paleacea was lower than in previous years. Aggregates of cyanobacteria
dominated the summer flux of carbon, nitrogen and phosphorus again. The sedimentation peak in late autumn became quantitatively
more important than in the previous year. Mass flux amounted to 101 g/a.
Chlorophyll a spring data revealed a decreasing trend 1979 - 2005 in Mecklenburg Bight but an
increase in the Baltic Proper, whereas summer data did not show any trend.
The mesozooplankton comprised 31 taxa in the samples of 2005. The marine Paracalanus parvus and
Noctiluca scintillans were not found anymore. After the strong salt water influx from 2003, the abundance of the marine
Alaurina composite and Oithona similis decreased. The most striking event in 2005 was the decrease in total
maximum abundance of mesozooplankton to one third of the concentration of the period between 1991 - 1995. It was mainly
a result of the strong reduction of rotifer abundance. The concentrations of the cladocerans (Bosmina spp.,
Evadne nordmanni, Podonidae) increased in comparison to 2004 because of the high temperature in summer 2005, but
accounted only for 1/10 of the extraordinary mass development in 2002.
The 78 species found in the macrozoobenthos in 2005 mark one of the lowest diversities during the last
5 years at the 7 monitoring stations. The benthic communities were affected by severe oxygen deficiency in 2005. Only
few organisms survived this event. At Darss Sill, the species number was fairly stable, but abundance and biomass decreased since
the beginning of the 1990s, mainly due to the disappearance or decline of Hydrobia ulvae and Pygospio elegans. A
slight decrease in biodiversity in the Arkona Sea has been detected for 2 years. The northern Pomeranian Bay shows relatively
stable conditions with respect to colonisation by macrozoobenthos. The Bornholm Sea was characterised by high amounts of hydrogen
sulphide and lack of macrozoobenthos.
Table 1: The 10 most abundant phytolankton species (percentage of total phytoplankton biomass) in the different sea areas
(upper 10 m) in spring, summer and autumn 2005.