The biological state of the Baltic Sea 2002

In the frame of the HELCOM monitoring, data on 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 (Fig. 1) were gathered in 2002 and discussed in comparison with satelliteand ship-based physico-chemical data. Information on sedimented material (from 2001) is also given. Comparisons with previous years were made and trends were checked. Data from the five regular monitoring cruises were complemented by data gathered from additional phytoplankton samples taken by Landesamt für Natur und Umwelt of the German country Schleswig-Holstein and from monitoring cruises of the National Environmental Research Institute Roskilde (Dänemark). By this strategy, up to 20 samples per station and year were available (Table 1).

The spring bloom was formed by the end of March 2002 in Mecklenburg Bight (Fig. 4a, c) and the Arkona Sea (Fig. 2) and started to decline at the beginning of April. It was still increasing until early April in the Bornholm Sea (Fig. 5c). In the eastern Gotland Sea, a peak was measured only on 8 May (Fig. 6b). In Lübeck Bight, the real peak of the bloom (5.4 g m-3) might have just been met on 27.3.02 (Fig 4b). Being influenced by riverine water, station OB in Pomeranian Bight (Fig. 6a) is strongly deviating from the open sea. In this coastal water, a spring bloom of Diatoma elongatum was registered on 4.5.02. Of course, the true peaks of the blooms were not met because of too long periods between the samplings. Nutrient decrease in the water serves, however, as a useful indicator for preceding algal growth.

The early stage of the spring bloom, formed by the diatom Skeletonema costatum, was followed by Dictyocha speculum in the central Mecklenburg Bight (stat. 012) and Lübeck Bight (stat. O22), Dinophysis baltica in the eastern Mecklenburg Bight (stat. 046), and Mesodinium rubrum in the central Arkona Sea (Stat. 113) and the Pomeranian Bight (Stat. OB). Dinoflagellates, which were 9 expected in the late phase of the spring bloom in the Baltic proper, developed only weakly in the Arkona Sea (Fig. 5a, b) and Bornholm Sea (Fig. 5c). Insofar, the spreading of dinoflagellates, noticed since 1989, might have stopped (since 2000). They remain abundand, besides of Mesodinium rubrum, only in the spring bloom in the eastern Gotland Sea (Fig. 6b). The consumption of silicate in spring shows, however, that limited amounts of diatoms grew also there.

The expected summer bloom of Dactyliosolen fragilissimus was found in Mecklenburg Bight (Fig. 4a-c) and the western Arkona Sea (Stat. 030, Fig. 5a) in August, while a cyanobacteria bloom was not present. Slight surface blooms of cyanobacteria were noticed only at low wind in the northern Arkona Sea, northern Bornholm Sea as well as in the northern and south-eastern Gotland Sea. From satellite data we concluded that the cyanobacteria bloom started at the end of June north-west of Gotland and spread over the Gotland Sea and finally to the Bornholm Sea and Arkona Sea by the mid of July. Its peak was already over during our summer cruise (Fig. 3).

The autumn blooms in Mecklenburg Bight were dominated by Ceratium tripos, in Lübeck Bight also Prorocentrum minimum, and by diverse diatoms (Coscinodiscus granii, Cerataulina pelagica) in changing abundances. The frequent occurrence of Ceratium spp. up to Stat. 030 indicates a biological border in the Arkona Sea between stations 030 and 113 (Fig. 21). The typical autumn blooms of Coscinodiscus granii were only weakly developed.

The 10 most important phytoplankton species of each season in each sea area are compiled in Table 3. A complete species list of the year 2002, including a seasonal indicator, is given in Table 4.

The sedimentation of organic matter in the eastern Gotland Sea in 2001 could only be measured correctly in spring and autumn, due to a technical problem (clogging) in late spring and summer. Only a residue of the summer material could be recovered in one combined sample, so that the flux during summer and therefore the annual integrated value is faulty and appears lower than in previous years. A fast sedimentation of the first part of the spring bloom could, however, be seen in all variables. A further high sedimentation pulse of primarily diatoms seemed to have occurred in late spring and led to the clogging of the funnel inlet (Fig. 7a). The typical high contribution of diazotrophic cyanobacteria during summer could not be detected microscopically, as the material, due to lack of preservation, did not retain its original form (Fig. 7c, d). The low delta 15N values in the residual summer material, however, points towards the standard high carbon flux by nitrogen fixing cyanobacteria (Fig. 8). Frustules of pennate diatoms, which have been found to be associated with the aggregates of cyanobacteria in this period could not be detected. This can be explained by the intense growth of spring diatoms and the corresponding uptake of the available dissolved silica already in spring with a lower amount left for summer diatom growth.

The increase in delta ¹⁵N in the material in autumn indicates convective processes in the water column, which supply isotopically heavier nitrate as well as silicate to the surface layer. This results again in a high vertical flux of diatoms, which is corroborated by the maxima in particulate silica in the trapped material (Fig. 11). The trend of decreasing importance of diatoms in the central Baltic, which was observed in previous years, seems to reverse in 2001.

For 2001, total annual sedimentation rates amount to 216 mmol C, 25 mmol N, 43 mmol Si und 0.88 mmol P per m² and year with a total flux of 15 g dry matter m^-2 a^-1. These rates might be 10 underestimated due to the probably high losses in the summer sample. The measured sedimentary carbon flux of 2.6 g C m^-2 a^-1 is much lower than the relatively constant values of the previous years (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 in Table 5. A comparison of seasonal and annual mean values especially with the two preceding years indicates an increase of chlorophyll a concentrations in spring and summer and a decrease in autumn in the Baltic proper (Table 6).

If long-term data from 1979 to 2002 were considered, there is still a significant (p=0.01) increase in chlorophyll a concentrations in the Arkona Sea (Fig. 19b). The increasing tendencies in chlorophyll a concentrations in the Bornholm Sea (Fig. 20a) and the eastern Gotland Sea (Fig. 20b) are not significant for p=0.05. Chlorophyll a data from Mecklenburg Bight show a significant decrease (Fig. 19a).

The dominant group in mesozooplankton was the cladocerans, with 0.5*10⁶ ind. m^-3 in the upper 8 m in the Bornholm Sea in August 2002. This is five times more than in previous years. The dominant species in 2002 was Acartia bifilosa, followed by Temora longicornis. Pseudocalanus spp. were of minor importance. Two species of calanoid copepods, Centropages hamatus and Temora longicornis, were present on all sampling dates and at all stations. Marine species indicated that salt water has entered the Arkona and Bornholm Sea via the Sound im October 2002. Meroplankton, which is important for the colonisation of the benthal, was in contrast to previous years decreasing, mainly due to a decrease in bivalvia larvae.

Long-term analyses (1979-2002) in the Arkona Sea (stat. 113) and the eastern Gotland Sea (stat. 271) showed: Two third of the organisms are concentrated in the upper mixed layer, irrespective of the water depth at that station. In comparison with the long-term mean of about 30 000 ind m^-3 in the upper mixed layer, the abundance was 40 % higher at station 113 and 50 % higher at station 271 in 2002. The same tendency was already found in 2001 at station 271. In the water column below the thermocline, the abundance in less variable in the Arkona Sea than in the eastern Gotland Sea. The seasonal pattern shows two prominent peaks in the upper mixed layer but an additional autumn peak in the deeper layers, most outstanding in the deep water of the eastern Gotland Sea.

The species number of macrozoobenthos decreased to 56, whereas it was 97 in 2001 (Tab. 11, Fig. 24). This reduction is caused by the oxygen deficit especially in the Fehmarnbelt (stat. 010) and Mecklenburg Bight (stat. 012). These areas are normally rich in species (Fig. 25). Long-term trends (1983-2002) in abundance, biomass, diversity and dominance at stat. 012 are shown in Fig. 28. The eastern stations on Darss Sill and in the Arkona Sea did not change substantially in comparison to 2001 concerning oxygen conditions, species numbers, abundances and biomass (Figs. 26, 27). Continued oxygen depletion led to a complete loss of macrozoobenthos in the Bornholm Sea for years (Fig. 25-27). The development in abundance of Hydrobia ulvae and Macoma balthica at selected stations from 1991 to 2002 is shown in Figs. 29-30. The amphipod Monoporeia affinis, which was decreasing or vanishing for decades, is still found in the southern Arkona Sea.

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