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 2010 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. The
phytoplankton data are supplemented by data from the State Agency for Agriculture, Environment and Rural Areas Schleswig-Holstein (LLUR), National Environmental Research Institute (NERI).
A general overview on the spatio-temporal development of the phytoplankton was garthered from chlorophyll maps, derived from satellite data of MODIS and MERIS sensors. The spring bloom reached high chlorophyll concentrations at the beginning if March in Kiel Bight, but not si in the Arkona Sea. The maximum of the spring bloom shifted to the northern Arkona and Bornholm
Sea by the beginning of April. First cyanobacteria filaments were seen in the Gotland Sea since 25.6.2010 and in the Pomeranian Bight and northern Bornholm Sea on 4./5. Juli 2010. On 16.7.2010, the whole Arkona Sea was weakly affected, but the cyanobacterial development ceased by the end of July already.
Spring bloom: The diatom bloom started rather early in Kiel Bight (27.1.2010: phytoplankton biomass of about 1 mg/L) and it increased to a peak value of almost 7 mg/L. This peak was almost exclusively formed by Rhizosolenia setigera. In the central Mecklenburg Bight and Lübeck Bight, the bloom reached biomass values higher than 3 mg/L in the mid of March, mainly
composed of Rhizosolenia setigera and Skeletonema costatum. By the end of March, Skeletonema costatum disappeared but dinoflagellates (unidentified Gymnodiniales, Protoperidinium pellucidum) appeared.
In eastern direction, the share of Rhizosolenia setigera increased but that of Mesodinium rubrum decreased. Thalassiosira spp. and Skeletonema costatum were the dominating diatoms in the spring bloom of the western and central Arkona Sea (Stat. OMBMPK8, OMBMPK5), while the bloom in the eastern Arkona Sea consisted almost exclusively of Coscinodiscus radiatus and some Thalassiosira spp. In Mai 2010, the spring bloom was over; Mesodinium
rubrum became dominant at that time.
Chrysochromulina spp. and Dictyocha speculum, which formed high biomass in some previous years, were of low biomass in 2010.
Summer bloom: A diatom summer bloom, found in previous years, was not observed in 2010. Only in Kiel Bight, Dactyliosolen fragilissimus occurred in higher biomass on 16.7.2010. Ceratium species were unexpectedly rare. Dinophysis norvegica appeared in mid of July in higher biomass below the pycnocline. Aphanizomenon sp. and Nodularia spumigena dominated in July in the Arkona Sea but did not form blooms in the German EEZ.
Autumn bloom: Strong diatom blooms were observed in mid of November 2010 at all stations, but only to less degree on Stat. OMBMPK4. They were composed of Pseudosolenia calcar-avis in Kiel Bight and Mecklenburg Bight (Figure 2a-c, 3a) and of Coscinodiscus granii in the Arkona Sea. An unusually low biomass of dinoflagellates was found in Kiel Bight and Mecklenburg Bight, mainly represented by Ceratium tripos, Ceratium fusus and Polykrikos schwartzii.
The 10 most important phytoplankton species of each season in each sea area are compiled in Table 1.
Invasive phytoplankton species: Chaetoceros socialis, which was insignificant in the past, dominated the autumn bloom at our coastal station Heiligendamm on 5.10.2010. Another component of the autumn bloom, Pseudosolenia calcar-avis, was new in our list. Also the species Chaetoceros circinalis and Chaetoceros pseudobrevis (both in Kiel and Mecklenburg Bight in March 2010) and Alexandrium pseudogonyaulax (in summer) were new for us, but they are already included
in the list of HÄLLFORS (2004. Checklist of Baltic Sea Phytoplankton Species. Baltic Sea Environment Proceedings 95).
Chlorophyll: Extensive comparisons of different methods for chlorophyll a analysis were carried through in 2010. These were an "old" method for "uncorrected" total chlorophyll a (Chl.a-tot-ALT), the traditional acidification method leading to a value corrected for pheopigments (Chl.a-kor) and a new fluorometric method (Chl.a-tot-NEU). It turned out that the values "Chl.a-tot-ALT" were clearly too high, but the values "Chl.a-kor" and "Chl.a-tot-NEU"
agreed very well. For storage in national and international data banks (BSH, ICES), only our values Chl.a-kor should be used until 2009. Since 2010, the values Chl.a-tot-NEU have to be used. A break in the data series does not occur by the change of the two recommended methods.
Sedimentation: Sedimentation records of organic matter in the Arkona Basin over the year 2010 displayed a seasonal pattern of export production with maxima preferentially in spring and autumn. Summer fluxes were comparatively low. Due to the dynamic physical environment and the restricted size of the basin this basic pattern was however superimposed by large peaks of resuspended matter during stormy periods in early spring and late autumn. During the growth season the phytoplankton assemblage was much
more divers than in the previous year and key species of all major groups shifted their seasonal patterns of occurrence.
Mainly due to the high input of resuspended material the total annual flux for the single elements exceeded the values of the previous year by far and amounted to 628 mmol C, 68 mmol N, 228 mmol Si and 6.7 mmol P/m²/a at a mass flux of 133 g dry mass m²/a. Particularly the high mass flux and silica values point to the benthic origin of a large part of the material. Omission of data from periods of heavy resespension events (Jan/Feb 2010 and Nov 2010) reduced the annual pelagic flux to more probably 157 mmol C, 17 mmol N, 39 Si and 0.9 P/m²/a. The isotopic composition of the organic matter reflects different sources of nitrogen nutrients over the course of the growth season.
Scince the beginning of BSH monitoring program in IOW, a total of 41 mesozooplankton taxa were abserved, with most taxa in 1998 and in 2010. The net increase by six taxa in 2010 based on marine species like Calanus finmarchicus, Centropages typicus, Paracalanus parvus, Noctiluca scintillans and Phoronis muelleri. This
indicated salt water intrusions.
There were no new invasive species in 2010. Mnemiopsis leidyi seems to be established in the western Baltic Sea while the cladoceran Penilia avirostris which was abundant in Little Belt in 2008 was not observed during the monitoring program neither in 2009 nor in 2010. According to the literature, the carnivorous cladoceran Cercopages pengoi invaded in Pomeranian Bay in 2006 already. This should be considered in herring larvae studies in Greifswald Bodden.
Decreasing tendency in total maximum abundances since the 1990s was further observed in the last year. Except of some high concentrations in 1991 - 1995 and in 2002, the total abundance decreased from 500.000 to 150.000 ind./m³ within the last decade which was a drastic decrease by 50 % in comparison to 2009 and by 2/3 in the entire period. The decrease was especially caused by microphagous rotifers which indicate a decreasing eutrophication.
The heavy decline in 2010 was probably additionally caused by the strong and long lasting winter in case of rotifers and by the too early sampling time in July in relation to the expected cladoceran maximum in August.
The total abundance of adult calanoid copepods did not change much in the last sixteen years. They remained at about 30.000 ind./m³ at varying proportions between the species.
The decreasing trend in concentration of microphagous organisms and the rather stable conditions in calanoid copepods let expect a shift within food web from more "microbial loop" basing on degradation of organic material to more classical food chain. This is field of future research.
The present study describes the macrozoobenthic community of 8 offshore stations within south-western Baltic waters in 2010. Following water bodies were sampled: Kiel Bight (N3), Fehmarnbelt (stn. N1), Mecklenburg Bight (M2 and OM18), the Darss Rise (K8), Arkona Sea (K4) and the Pomeranian Bay (K3 and OM160). At each station the species composition, their abundance and their biomass were determined. Whereas 6 stations were sampled during the last decade's, 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 73 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 118 species found in the macrozoobenthos in 2010 mark a very low diversity within the 20 years of zoobenthos monitoring conducted by the IOW. The species diversity in the Fehmarnbelt in 2010 is currently lower than the median values of the long term data. In the other water bodies the recent values are more or less the same as the long term median values. In 2010 a deficit of oxygen was observed, which affected the deep areas of the
Fehmarnbelt and central Mecklenburg Bight in a similar way as in 2002, 2005 and 2008.
The abundances varied in depending on the area and the specific environmental parameters between 91 and 7.000 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, Pygospio elegans and Scoloplos armiger with more than 50 %, the northern Pomeranian Bay (OMBMPK3) were most represented by polychaete Pygospio elegans. Some examples of key species were the bivalves Arctica islandica, Astarte borealis, Mytilus edulis and Mya arenaria and the polychaetes Lagis koreni, Pygospio elegansand Nephtys ciliata.
The biomasses (ash free dry weight) ranged from 1 g/m² in the central Arkona Basin (OMBMPK4) to 91 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 19 species of the Red List could be observed. As example, the bivalve Macoma calcarea, threatened with extinction in German waters, was found in the Kiel and Mecklenburg Bight in low abundance. Four indigenous species were observed during 2010 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 viridis (Polychaeta) and Rhithropanopeus harrisii (Decapoda).
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 (Stat. ..PN3), Fehmarnbelt (Stat. ..PN1), Mecklenburg Bight (Stat. ..PM2), Arkona Sea (Stat. ..PK4), and Pomeranian Bight (Stat. OM160).
Figure 2: Seasonal variation of phytoplankton wet weight, split into main taxonomical groups, in Lübeck Bight (a) and Mecklenburg Bight (b, c) in 2010. The asterisks mark columns which are based on data of the LLUR.
Figure 3: Seasonal variation of phytoplankton wet weight, split into main taxonomical groups, in Lübeck Bight (a) and Mecklenburg Bight (b, c) in 2010. The asterisks mark columns which are based on data of the LLUR.
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Table 1: The 10 most abundand phytoplankton taxa (percentage of total phytoplankton biomass) in the different sea areas (upper 10 m): averages from the three cruises January-May as well as cruises from July and November 2010.
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.