The biological state of the Baltic Sea 2001

The biological state of the Baltic Sea in 2000 Species composition and biomass or abundance of phyto- and zooplankton as well as macrozoobenthos from the western part of the Baltic Sea to the eastern Gotland Sea in 2001 were studied and discussed in comparison with satellite- and ship-based physico-chemical data. Comparisons with previous years were made and trends were checked.

Information based on sedimentation traps (from 2000) completed the seasonal reflections of the phytoplankton development. Data from the five regular monitoring cruises were complemented by data gathered from additional phytoplankton samples taken from project and training cruises and from monitoring cruises of the National Environmental Research Institute Roskilde (Dänemark) and Landesamt für Natur und Umwelt of the German country Schleswig-Holstein. By this strategy, up to 21 samples per station and year were available.

Species composition and biomass or abundance of phyto- and zooplankton as well as macrozoobenthos from the western part of the Baltic Sea to the eastern Gotland Sea in 2001 were studied and discussed in comparison with satellite- and ship-based physico-chemical data. Comparisons with previous years were made and trends were checked. Information based on sedimentation traps (from 2000) completed the seasonal reflections of the phytoplankton development. Data from the five regular monitoring cruises were complemented by data gathered from additional phytoplankton samples taken from project and training cruises and from monitoring cruises of the National Environmental Research Institute Roskilde (Dänemark) and Landesamt für Natur und Umwelt of the German country Schleswig-Holstein. By this strategy, up to 21 samples per station and year were available.

The spring bloom started already in the second half of February in Mecklenburg Bight , but still not in the Arkona Sea , Bornholm Sea and eastern Gotland Sea. In Lübeck Bight, the bloom might have reached its peak in the mid of March (8 g m^-3 wet weight), whereas the nutrients were not used up yet in Mecklenburg Bight until 23.3.01. Therefore we assume that the peak of the bloom might reach 5 g m^-3 there at the end of March. This biomass level was measured in the central Arkona Sea on 2.4.01. Surprisingly, the nutrient concentrations decreased quicker in the Arkona Sea than in the central Mecklenburg Bight. This is a sign that the earlier findings of a retardation of the blooms from west to east is not true in every year. Obviously, the inflow of water from the Pomeranian Bight has a modifying effect. The timing of the bloom in the Bornholm Sea might not deviate from that of the Arkona Sea as nitrate was exhausted there already on 1.4.01. As phytoplankton biomass was still low in the eastern Gotland Sea at the end of March , the biomass peak was expected in April, there. The coastal station OB in the Pomeranian Bight deviates from the stations of the open sea. It crops a phytoplankton biomass of over 7 g m^-3 on 7.5.01 . The phytoplankton growth in spring is limited by phosphorus (in contrary to the open Baltic), but by nitrogen in summer (as usual in the Baltic proper).

The spring bloom in 2001 was mainly formed by the diatom Skeletonema costatum. In Lübeck Bight, Chaetoceros sp., Thalassiosira nordenskioeldii and Rhizosolenia setigera were the most abundand representatives. The Skeletonema bloom was followed by Dictyocha speculum in the central Mecklenburg Bight (Stat. 012), but by Mesodinium rubrum in the central Arkona Sea (Stat. 113) and the Pomeranian Bight (Stat. OB). Dinoflagellates, which were expected in the late phase of the spring bloom, developed only weakly in the Arkona Sea, Bornholm Sea and Pomeranian Bight. Insofar, the spreading of dinoflagellates, noticed since 1989, might have stopped (since 2000). They remain abundand, besides of Mesodinium rubrum, which is increasing since 1999, only in the spring bloom in the eastern Gotland Sea . The consumption of silicate in spring shows, however, that the diatoms grew also there.

The expected summer bloom of Dactyliosolen fragilissimus was found in the Mecklenburg Bight and the western Arkona Sea (Stat. 030) in August, while a cyanobacteria bloom was not present. An accumulation of Aphanizomenon sp. was noticed at Stat. 030 at a depth of 15 m. Satellite data revealed that the cyanobacteria bloom started already at the beginning of June northwest of Gotland at a temperature as low as 12 °C, which is in contrast to previous reports. The bloom (Aphanizomenon sp.) has spread over the Gotland Sea in June and reached its maximum on 4.7.01, stretching from the Gulf of Finland to the Arkona Sea. It is less abundand in coastal waters. The cyanobacterial blooms disappeared as from the end of July.

Small autumn blooms of Ceratium tripos, accompanied by diverse diatoms in changing abundances, were present in the Mecklenburg Bight. The frequent occurrence of Ceratium spp. up to Stat. 030, as well as the extension of the summer blooms of Dactyliosolen fragilissimus, indicate a biological border in the Arkona Sea between stations 030 and 113. The eastern areas from the Pomeranian Bight to the eastern Gotland Sea were charcterized by autumn blooms of Coscinodiscus granii. This species spread even to the Mecklenburg Bight at the end of October due to outflowing water.

The 10 most important phytoplankton species of each season in each sea area are compiled. A complete species list of the year 2001, incl. a seasonal indicator, is given.

The sedimentation of organic matter in the eastern Gotland Sea in 2000 agreed with the standard pattern characterized by two peaks in late spring and late summer. Contrary to the long-term trend, the absolute amount of sedimenting carbon was divided equally between material derived from spring diatoms and from summer cyanobacteria. Fluxes of biogenic silica were concentrated in the spring period and rates were higher than the long-term mean, so that a smaller carry-over of dissolved sillica in the mixed layer from spring to summer has to be assumed for this year. Total fluxes of the different elements were 490 mmol C, 50 mmol N, 170 mmol Si and 5 mmol P m^-2 a^-1. The mass flux amounted to 40 g dry matter per m² and year in 2000. Although the seasonal pattern of sedimentation differed from the mean of the previous years, the absolute sedimentary carbon input was well within the relatively constant range of 4-6 g C m^-2 a^-1.

The seasonal development of phytoplankton biomass was reflected also by the chlorophyll a concentration. The measured chlorophyll a and phaeopigment a data, integrated over the upper 20 m of the water column, are shown. Seasonal and annual mean values since 1994 indicate a slight decrease of chlorphyll a concentrations in the Baltic proper, especially because of a decrease of the summer values.

If long-term data from 1979 to 2001 were considered, there is still a significant (p=0.01) increase in chlorophyll a concentrations in the Arkona Sea . The increasing tendencies in chlorophyll a concentrations in the Bornholm Sea and the eastern Gotland Sea are not significant for p=0.05. Chlorophyll a data from Mecklenburg Bight show a significant decrease.

The shift from diatoms to dinoflagellates in the spring blooms, noticed since 1989 or 1990 in the southern Baltic proper, inverted in 2000 and 2001. Spring diatoms developed stronger in 2001 in Bornholm Sea, Arkona Sea and Mecklenburg Bight, while biomass of spring dinoflagellates was low. Even in the eastern Gotland Sea, there are indications of a slight diatom growth in spring 2001. Mesodinium rubrum, which was strongly dominant in winter and spring 1999, reduced since 2000. Coscinodiscus granii, which is bloom-forming in autumn in the Baltic proper, spreads to summer and especially with outflow situations into the western Baltic Sea.

Trend analyses on the basis of HELCOM data and additional data from 1979 to 1999 are extensively discussed in Chapter 5.2. Three statistical methods were applied. The decrease of diatoms and increase of dinoflagellates in spring are confirmed. Moreover, there is an increase in diatoms at station 046 and in dinoflagellates at station 271 in autumn. In summer, dinoflagellates developed differently: downwards trend in the eastern Arkona Sea , upwards trend in Mecklenburg Bight. The decrease in summer biomass of cyanobacteria in the Bornholm Sea results from extremely high biomass at the beginning of the 1980s.

Zooplankton samples had been taken by WP-2 net in changing depth ranges according to the hydrographical situation at the specific station. The 100 µm mesh size of the WP-2 net does not quantitatively consider the smaller groups, like most of the nauplia stages, the larvaceans, rotifers, and tintinnids. In comparison to the previous year, Limnocalanus macrurus had been detected once, during spring, in the eastern Gotland Sea, indicating the influence of low saline water of northern origin. Fresh-water run-off is responsible for the occurrence of cyclopoids, other than Oithona similis, in the Pomeranian Bay. In 2000, such indication was limited to the winter / spring period, whereas it happened during all sampling times throughout 2001.

The average zooplankton abundance amounted to approximately 30.000 Individuals m^-3 in the surface layer of the central Arkona Sea between 1979 and 2001. It declined to 18.000 Ind. m^-3 below the pycnocline. Maxima of 160.000 Ind. m^-3 were recognized in the upper layer and of the same magnitude in the deeper one. The minima were 400 times smaller in both water bodies. In 2001, the maximum abundances were two- to tenfold smaller in comparison to the long-term mean. The minima behaved opposite. Consequently, the conditions in 2001 met the average situation. There were no statistical significant differences between the upper and the lower depth range in 2001, but over the entire period by a factor of about 2.

The abundance of the main taxonomic categories of zooplankton varied partly significantly in 2001 in comparison to the previous year and to the first half of the last decade. According to the maximum occurrence the rotifers decreased to almost ¼ if compared to the 1991 to 1995 period, which is a sign of the rather late spring in 2001. The maximum was to be found in the surface layer sample of the eastern Gotland Sea in May. Remarkable is the high amount of cladocerans (Bosmina spp.), detected during July in the surface layer in the Bornholm Sea. It indicates warmer surface water. Further, the increase in the abundance of bivalvia and gastropod larvae is notable, both detected in the near surface layers of the Belt Sea in July 2001. The larvaceans increased in 2001, probably as a result of the lesser occurrence of the rotifers, which compete for a similar food spectrum.

The species number of macrozoobenthos, amounting to 97, has increased in comparison with previous years. Hence, the increase in diversity, noticed since the beginning of our investigations in 1991, continued. This is caused by inflow of saline bottom water, which stabilized the oxygen conditions. In the Bornholm Sea, however, continued oxygen depletion led to a complete loss in macrozoobenthos for many years. At the other stations, the number of taxa in 2001 amounted to 60-70 % of the total number of taxa found at that station since 1991. Macrozoobenthos abundanceand biomassdecreased in comparison to the year 2000 at almost all stations. Only at station 010 (Fehmarnbelt), biomass increased due to growing abundance of adult Astarte islandica. Some taxa could be found for the first time, like the polychaetes Glycera alba, Mediomastus fragilis, Pseudopolydora antennata and Sphaerodorum flavum. Other species have been recovered in the investigated area, partly after they were absent for decades. Examples are Astarte montagui (Bivalvia), found at Darss Sill, and Monoporeia affinis (Amphipoda), found in the southern Arkona Sea.

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