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. Lutz Postel, Dr. Michael L. Zettler
The year 2010 was the third year of the complex biological monitoring conducted in the German off-shore area (mainly the exclusive economic zone, EEZ) of the North Sea by the Leibniz Institute of Baltic Sea Research Warnemünde (IOW) and commissioned and supported by the Federal Maritime and Hydrographic Agency (BSH).
Phytoplankton (incl. chlorophyll a), meso-zooplankton and macrozoobenthos were investigated at 12 stations (for benthos exceptionally 13 stations, Figure 1). The plankton was sampled at 5 cruises and the zoobenthos at 2 cruises.
The phytoplankton comprised 144 taxa, most of them belonging to the diatoms (Table 1). In comparison to the previous year, 19 taxa disappeared from our list, but 25 were newly added (Table 2). The lowest diversity was found in January (43 taxa) and the highest in August/September (99 taxa). The 5 samples per year and station showed clear seasonal patterns, but did not allow discussions on phytoplankton succession. The highest phytoplankton biomass occurred nearshore
in spring and summer. In the past, we applied two different methods for calculation of chlorophyll a concentrations, which gave deviating results. A modification of the method for "total” chlorophyll a led to data which were comparable with those of the acidification method ("Chl.a-kor"). Therefore it is recommended to use the values "Chl.a-kor" up to year 2009 and the values "Chl.a-tot-NEU" since 2010.
The bloom-forming Phaeocystis sp. was rare in our samples, but the abundancies of the potentially toxic Chrysochromulina spp. were relatively high. Sound information on the duration and magnitude of phytoplankton blooms is not possible because of the low temporal coverage. The most important species in terms of biomass were the large-sized Rhizosolenia imbricate, Noctiluca scintillans, Mediopyxis helysia, Odontella sinensis and Coscinodiscus
In the mesozooplankton, a total of 157 taxa were identified in 2010. Arthropoda was the most frequent group with 80 taxa, followed by cnidarians with 27 taxa, polychaets with 15 taxa, and Echinodermata larvae with 9 taxa. The total exceeded the numbers of the previous year by 14 categories and the results in 2008 by 40 taxa. This is primarily a result of increasing skills for species identification especially in the field of cnidarians and meroplanktonic crustaceans.
The spatial distribution of taxa according to the Margalef index of species richness showed a typical estuarine distribution, from the Elbe estuary at the station nearby Helgoland Island (HELGO) towards the central North Sea (stations NSGR2), with increasing numbers at growing distance to the river mouth. In coastal areas, the region off the North Frisian Islands (HELGO, AMRU2, SYLT1) was characterized by a smaller diversity than the region off the East Frisian Islands. This was reflected in the numbers of Pielous
eveness index and is a result of the different origin of the water bodies. While the conditions of the North Frisian Islands are influenced by discharges of Weser and Elbe, those of the East Frisian region are rather a result of influences from the English Channel. This observation is consistent with the large-scale circulation in the North Sea and was also reflected in the results of a cluster analysis. Interestingly, the ten major taxonomic groups occurred in the course of a year earlier (May) off the North
Frisian Islands than in the East Frisian area (August/September).
Macrozoobenthos was investigated during spring and autumn 2010 at 13 stations within the German offshore area (predominantly EEZ) of the North Sea. In both sampling campaigns altogether 286 species were recorded. Between the stations the species number varied from 16 (off Sylt) to 90 (ENTE3). With 121 species (both campaigns added) the station at the central North Sea (ENTE3) was most diverse. With only two exceptions the species richness
was always higher in autumn than in spring. Similar to the species number the autumn densities were mostly higher than the spring ones (Figure 2).
The seasonal variability was extremely high at the shallow areas off Amrum and Sylt, where in spring only 161 and 49 ind./m² was registered respectively, whereas the values of autumn were 15 to 20fold higher (Figure 3). Partially the dominant species at stations varied significantly. With 60 to 80 % relative abundance Ensis directus, Amphiura filiformis, Nucula nitidosa and Phoronis sp.were dominant at different
The ash free dry weight at stations ranged from 1.27 g/m² at station NSB3 (south eastern White Bank) in autumn to 77,5 g/m² at the central North Sea (Entenschnabel) in spring. The very high value was caused by the occurrence of adults of the ocean quahog Arctica islandica. Biomass dominated groups and species at the sampled stations were very different. Bivalves (Ensis directus), sea urchins (Echinocardium
cordatum) and polychaetes (Lanice conchilega, Ophelia limacina) showed the highest biomasses at the distinct stations. Two distinct groups of stations could be distinguished by cluster analysis. The distinctness was mainly caused by different water depths, different sediment characteristics and also different macrozoobenthic assemblages.
57 species from the Red List (RACHOR et al. in press) could be observed during this study. Four species belong to the category 2 (endangered). These species are Buccinum undatum (common whelk), Alcyonium glomeratum (red sea fingers), Echiurus echiurus (echiurid worm) and Modiolus modiolus (northern horsemussel). Four additional species belong to the category 3 (vulnerable).
Figure 1: The stations of biological monitoring in the German Exclusive Economic Zone (EEZ) of the North Sea. Station ENTE3 is an additional Station for zoobenthos samples.
Figure 2: Species number of macrozoobenthos at 13 monitoring stations during spring (F) and autumn (H) sampling in 2010.
Figure 3: Abundance of macrozoobenthos at 13 monitoring stations at spring (H) in 2010.
PDF-File, 202 MB
Table 1: The most abundant phytoplankton taxa (percentage of total phytolankton biomass) in the upper 10 m during the cruises from 2010.
The mean phytoplankton biomass (in mg/L) is given on the top of each block. "unidentified" were disregarded ithe specimen accounted for less than 10 %.
PDF-File, 247 MB
Table 2: List of taxa which "disappeared" (left column) or "appeared" (right column) from 2009 to 2010. Species which were absent both in 2008 and 2009 are merked by bold letters.