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)
Dr. Norbert Wasmund, Dr. Falk Pollehne, Dr. Lutz Postel, Dr. Herbert Siegel, Dr. Michael L. Zettler
In the frame of the HELCOM monitoring, data on species composition and biomass or abundance of phyto- and zooplankton as well as macrozoobenthos from Kiel Bight, Mecklenburg Bight and the Arkona Sea (Figure 1) were gathered in 2011 in order to continue the time series which exists since 1979. Data from sediment traps deployed in the Arkona Sea give information on particle dynamics. Also satellite data are used to trace the phytoplankton bloom development.
Quantitative information on the species composition and succession of the phytoplankton was gathered from water samples, taken during the cruises and analysed microscopically in the lab. In this report, we concentrated mainly on mixed samples from 0 - 10 m depth. Gaps owing to the low sampling frequency could be closed by additional information from the coastal monitoring of the IOW in from of Heiligendamm,
to be found on http://www.io-warnemuende.de/algenblueten-vor-heiligendamm-2011.html.
The 10 most important phytoplankton species of each season in each sea area are compiled in Table 1. The ranking according to their biomass in 2011 is also given.
Spring bloom: In the investigated area, stretching from Kiel Bight to the Arkona Sea, the known retardation of the spring bloom in easterly direction was noticed, indicated by an increased biomass in Kiel Bight and Lübeck Bight in February 2011 already (Figure 2 a, b). The spring bloom was established in the whole investigation area by the end of March (Figure 2, 3), which was relatively late because of the severe winter. The high share of dinoflagellates in Kiel Bight and Mecklenburg
Bight was unusual. The silicate consumption of 10 - 13 µmol/l in these regions indicates, however, that a diatom bloom has grown there. Kiel Bight and Mecklenburg Bight were obviously in a later successional state in comparison with Arkona Sea. In Kiel Bight and Mecklenburg Bight, flagellates appeared, which would have been counted to Dictyocha speculum in previous years but were specified as Verrucophora farcimen now. As already done
with Dictyocha speculum, they are counted to Chrysophyceae in Figure 2. Besides of Verrucophora farcimen, the spring bloom was formed by the dinoflagellate Peridiniella danica. Both species occur only in Kiel Bight and Mecklenburg Bight and are rather new and uncommon for us. Also the most abundant diatom, Achnanthes taeniata, is new as bloom-forming species in these waters; it was dominant in the Baltic Proper in the 1980s and almost disappeared there.
The succession from diatoms to dinoflagellates (Peridiniella danica) became clear at station OMBMPM2 (Figure 2 c). Therefore we expect that the diatoms in the Arkona Sea will be followed by dinoflagellates, which could not be recorded because of lacking samplings in April. In May, the spring bloom was over and Mesodinium rubrum became dominant.
Summer bloom: The samples from summer are based on only one cruise and cannot reflect the total diversity. Dinoflagellates were dominating, represented mainly by Alexandrium protogonyaulax, which was found for the first time in this area in 2010 and has increased to rank 10 in 2011). Typical abundant diatoms (in this case Cerataulina pelagica) were found in August 2011 only at stations OMBMPN3 and OMBMPM2. Blooms of nitrogen-fixing cyanobacteria (Aphanizomenon sp., Nodularia
spumigena) are unusual in Mecklenburg Bight, but surprisingly their highest biomass was found in Mecklenburg Bight (incl. Lübeck Bight) and not in the Arkona Sea.
Autumn bloom: Usually a bloom of diatoms and/or dinoflagellates occurs in autumn. During the cruise at the end of October 2011, the dinoflagellate Ceratium tripos dominated, but only in the western Baltic. East of station OMBMPM2, the phytoplankton biomass was unusually low. The typical diatom, Coscinodiscus granii, only appeared at station OMBMPK4.
Chlorophyll: The maxima of concentrations of chlorophyll are found in spring with a peak value of 16.02 mg/m³
Sedimentation: Over the year 2011 sedimentation records of organic matter in the Arkona Basin displayed a seasonal pattern of export production with distinct maxima for the major algal groups in spring and autumn. Contrary to the previous year, cyanobacteria displayed high summer fluxes and resuspension events were of minor importance. With the exception of cyanobacteria, the seasonal succession of microalgal diversity was comparable to the previous year on the family- and often on the species level.
Key species of all major groups displayed distinct seasonal patterns of occurrence.
Due to the reduced influence of resuspension the total annual flux for single elements returned to a "standard level” of 515 mmol C, 59 mmol N, 78 mmol Si and 3.1 mmol P//m²/a at a mass flux of 57 g dry mass/m²/a. The different contribution of fresh pelagic and older resuspended material between the years 2010/2011 resulted for the whole year in an increase of a (mass weighted) C/P-ratio from 120 to 165 and of the C/Si ratio from 2.0 to 6.6 and a reduction of the 15N signature of the material from 5.6 to 4.9 ‰, whereas C/N-ratio (8.5) and delta 13C (-25 ‰) did not mirror the change in input sources. Within the seasonal succession of pelagic growth phases all ratios varied accordingly and isotopic composition of the organic matter reflected the different sources of nitrogenous nutrients.
In 2011, the zooplankton assessment within the German EEZ was based on 83 WP-2 net samples. Since the beginning of the BSH monitoring program in IOW, fifty-six mesozooplankton taxa were observed. The maximum was found in the last year as a consequence of salt water influxes. It was indicated by marine species
such as the calanoid copepods Calanus spp., Centropages typicus, Paracalanus parvus, Microsetella spp., developmental stages of Crangon crangon as well as hydromedusae such as Euphysa aurata, Obelia geniculate, Rathkea octopunctata, Sarsia tubulosa west of Darss sill. Additionally larvae of Asterias spp., Carcinus maenas, Galathea spp., of ophiurids and of the trumpet worm Pectinaria spp. sporadicly
occurred in the western Baltic Sea and partly east of Bornholm Island. There were no new invasive species and no Mnemiopsis leidyi found in the samples in 2011.
A decreasing tendency in total maximum abundances since the 1990's was generally observed as before. However, the sum of maximum concentrations of different taxonomic groups doubled in comparison to 2010 due to a moderate increase of rotifers and a significant rise of polychaete larvae. The larvae of polychaetes increased especially below the halocline at Darss sill and Kadet Furrow. It obviously resulted in coinciding periods of simultaneous reproduction and sampling.
The total abundance of adult calanoid copepods did not change much in the last sixteen years before 2010. It remained at about 30.000 ind./m³ with proportions varying between the species. However, in 2011 the maximum concentration dropped by 50 % in contrast to the previous year. In respect to potential effects on higher trophic levels, it would be important to know if this might be a result of generally decreasing eutrophication.
The macrozoobenthic community at 8 offshore stations within south-western Baltic waters in 2011 is described. Following water bodies were sampled: Kiel Bight (OMBMPN3), Fehmarnbelt (OMBMPN1), Mecklenburg Bight (OMBMPM2 and OM18), the Darss Rise (OMBMPK8), Arkona Sea (OMBMPK4) and the Pomeranian Bay (OMBMPK3 and OM160). At each station the species composition, their abundance and their biomass were determined. Whereas 6 stations were sampled during the last decades, 2 additional stations were included
within the monitoring programme since 2006. One "new" station is situated within the Kiel Bight with high salinity (22 psu) and showed with 74 species the highest taxonomical diversity. Another one represents the central Pomeranian Bay with its low salinity (7 psu) and was colonised by 24 species. The 124 species for all stations found in the macrozoobenthos in 2011 mark relatively high
during the last 20 years at the monitoring stations sampled by the IOW. If we compare the median values of the long term data we observe that in 2011 in almost all water bodies the species diversity was the same or higher as the median.
The abundances varied in respect to the area and depending on specific environmental parameters between 388 and 7.400 ind./m². The highest values were observed in the Kiel Bight and in the northern Pomeranian Bay. Due to the different salinity regimes at the different monitoring stations the dominant taxa changed drastically. For example, whereas the Kiel Bight (OMBMPN3) was dominated by the bivalve Kurtiella bidentata and the polychaetes Dipolydora
quadrilobata and Scoloplos armiger with more than 50 %, the northern Pomeranian Bay (OMBMPK3) were most represented by the polychaetes Pygospio elegans and Marenzelleria viridis.
The biomasses (ash free dry weight) ranged from 0.8 g/m² in the central Arkona Basin (OMBMPK4) to 51 g/m² in the Kiel Bight (OMBMPN3). In most cases bivalves like Arctica islandica, Astarte borealis, Mytilus edulis and Mya arenaria reached the highest percentages from the total weights at stations.
At the 8 monitoring stations altogether 17 species of the Red List could be observed. As example, the bivalve Macoma calcarea, critically endangered in German waters, was found in the Kiel Bight in low abundance. Three indigenous species were observed during 2011 sampling. On the one hand the since centuries established species like Balanus improvisus (Cirripedia) and Mya arenaria (Bivalvia) and at the other hand the recently introduced species (since 1990's) Marenzelleria
Figure 1: The station grid for biological sampling in the Baltic Sea, with depiction of the border of the exclusive economic zone of Germany
Stations in the main map represent Kieler Bight (Station OMBMPN3), Fehmarnbelt (Station OMBMPPN1), Mecklenburg Bight (Station OMBMPPM2), Arkona Sea (Station OMBMPPK4), and Pomeranian Bight (Station OM160).
Figure 2: Seasonal variation of phytoplankton wet weight, split into main taxonomical groups, in Kiel Bight (a), Lübeck Bight (b) and Mecklenburg Bight (c, d) in 2011.
Figure 3: Seasonal variation of phytoplankton wet weight, split into main taxonomical groups, in the Arkona Sea (a - c) in 2011.
PDF-Datei, 276 MB
Table 1: The 10 most abundant phytoplankton taxa (percentage of total phytoplankton biomass) in the different sea areas (upper 10 m): averages from the three cruises February - May as well as cruises from July and October 2011.
The mean phytoplankton biomass (in µg/L) is given on the top of each block. "Unidentified", "Gymnodiniales" and "Peridiniales" were redeleted from the list if the specimen accounted for less than 10 % of the biomass in the sample.