MURSYS - Baltic Sea





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MURSYS - Baltic Sea

   
 
 

Biological Conditions in the Baltic Sea in 2008 (Summary)

  Leibniz-Institut für Ostseeforschung, Warnemünde
(IOW) (Baltic Sea Research Institute, Warnemünde)
commissioned by
Bundesamtes für Seeschifffahrt und Hydrographie, Hamburg, Rostock
(Federal Maritime and Hydrographic Agency of Germany, Hamburg, Rostock)

(http://www.io-warnemuende.de/)

The original report is published in German with an English summary and :
Wasmund, N., F. Pollehne, L. Postel, H. Siegel und M. Zettler (2009): Biologische Zustandseinschätzung der Ostsee im Jahre 2008. Meereswiss. Ber.,Warnemünde,78, 1-91. (link see below):

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 (Figure 1) were gathered in 2008 together with physico-chemical data in order to continue the time series which exists since 1979. Data from sediment traps deployed in 2007 in the Gotland Basin give information on particle dynamics. Also satellite data are used to trace the phytoplankton bloom development.

Phytoplankton

Spring bloom:
Before the spring bloom, i.e. in February, the phytoplankton biomass was very low. The spring bloom started, as expected, in March. It was formed by Rhizosolenia hebetata f. semispina, Coscinodiscus granii and the naked form of Dictyocha speculum at Station 012 (central Mecklenburg Bay) on 11.3.2008. The extremely early Dictyocha bloom of the year 2007, related with a reduced diatom growth, repeated in 2008 in a more moderate form. At station 046 (eastern Mecklenburg Bay) and in the Arkona Sea (Station 030 and 113). Mesodinium rubrum and Skeletonema costatum were important at the end of March/beginning of April, as usual. Besides of this, Chrysochromulina (probably Chrysochromulina polylepis) developed after it has formed an exceptional bloom already in winter 2007/2008 in the central Baltic. It is mixotrophic and might replace the heterotrophic dinoflagellates, which appeared after the diatom spring bloom in the past. The dinoflagellate-growth was only weak already in spring 2007. Chrysochromulina grew in May 2008 and was still present until summer. It was strongest in the Baltic Proper with abundances of 15 Mill. cells/l and a biomass of 2069 mg/m³ at Station 271 (eastern Gotlandsea) on 3.5.2008. At that situation, also a bloom of Peridiniella catenata (1841 mg/m³) occurred. Again, there was no diatom spring bloom in the Bornholm and Gotland Seas. However, already during the cruise from 29.4. - 7.5.2008, accumulations of Aphanizonenon were noticed at the water surface at calm situations, especially from the southern Gotland Sea (Station 253, 2.5.2008) to the Arkona Sea (Station 113, 6.5.2008).

Summer bloom:
After the spring bloom, the biomass of dinoflagellates (mainly unidentified Gymnodiniales) stayed exceptionally low in Mecklenburg Bight, while the uncommon Chrysochromulina biomass stayed until July at least at Station 012 (central Mecklenburg Bay). By the 30.7.2008, a diatom bloom (Proboscia alata, Dactyliosolen fragilissimus, Cerataulina pelagica) developed at station O22 (Lübeck Bight), while the total biomass stayed low, without significant diatoms, at Stations 012 and 046 (central and eastern Mecklenburg Bay). Probably, diatoms develop there in the following month, which cannot be proved because of lacking samples. Ceratium tripos started to grow slowly. Nitrogen-fixing cyanobacteria were insignificant in Mecklenburg Bight, but dominant in the Arkona, Bornholm and Eastern Gotland Seas at the end of July.

Autumn bloom:
After very small Ceratium biomass in autumn 2007, the ratio of dinoflagellates and diatoms regained balance in autumn 2008 in Mecklenburg Bight. The dinoflagellates are in general represented by Ceratium tripos, Ceratium lineatum and Ceratium fusus in autumn. The dominating diatom species are more variable and diverse (Coscinodiscus granii, Pseudo-nitzschia pungens, Cerataulina pelagica, Chaetoceros convolutus). The autumn-peak was reached in Mecklenburg (incl. Lübeck) Bight on 21.10.2008, but in the Arkona and Bornholm Seas only in November. The autumn bloom in the Arkona and Bornholm Seas was almost exclusively made by Coscinodiscus granii.
The 10 most important phytoplankton species of each season in each sea area are compiled in Table 1.

Chlorophyll:
If the seasons were analysed separately, the spring data for chlorophyll a revealed decreasing trends in Mecklenburg Bight but increasing trends in the Arkona Sea and the Bornholm Sea from 1979 to 2008. Summer and autumn data increased in the eastern Gotland Sea.

Sedimentation:
Data of sedimentation of organic matter in the Gotland Basin in 2007 indicate a normal year in terms of major element export. Due to clogging of the funnel from April to July, temporal resolution of spring bloom and summer sedimentation could not be resolved properly. Bulk sedimentation of organic matter throughout the spring phase, however, exceeded that of the previous year by 40 %. Summer sedimentation of material based on nitrogen fixation decreased in comparison to the long term values whereas the autumn rates showed a comparative increase. The spectrum of phytoplankton species did not differ from the previous years except for the colony-forming cyanobacteria, which were represented by just one group (Aphanocapsa) in 2007. The absolute amount of material exported from the mixed layer was about twice as much as in the preceding year and considerably more than the long-term mean. The total annual flux for the single elements amounted to 585 µmol C,  µmol N, 221 µmol Si and 2.3 µmol P per and year. With 43.9 g/a dry material the mass flux was considerably higher than in the previous year (28.5 g/a).

Mesozooplankton

Sampling and sample analysis:
The assessment based on 105 WP-2 net samples (100 µm mesh size, TSK-flow meter, towing velocity 0.75 m/s) in 2008, mainly collected on eight stations between Kiel Bight and the Eastern Gotland Basin, with some regional restrictions. In maximum, three depth levels were considered depending on vertical thermohaline stratification. Mostly, sampling was performed two times per cruise on the same stations with regard to the short-term variability, especially of rotifers and cladocerans. Laboratory analysis covered sub-sampling techniques and a final check of the entire sample in order to find rare specimen. Sub-sample volumes ranged between 1/1 und 1/769 of the total sample, on average 1/19 ±1/6. Between 1 and 816 individuals per taxon were counted, on average 25 ±58.

Species spectrum:
Forty one taxa were recorded in total during the last ten years with higher amounts in the second halves of the years. The maximum in 2008 was observed in July/August. The following groups were principally to distinguish:

  • (1) Species which were permanently present in the assessment period and (nearly) round the year, like Acartia bifilosa, Acartia longiremis, Centropages hamatus, Oithona similis, Pseudocalanus spp., Synchaeta spp., Temora longicornis, and others (in alphabetical order);
  • (2) Taxa with typical seasonal patterns, like Bosmina spp., Acartia tonsa and some meroplanktonic larvae, and
  • (3) Occasionally occurring taxa of marine and limnetic origin, depending on inflow and outflow conditions. These representatives like Calanus finmarchicus and Limnocalanus macrurus were missing in 2008, which is an indication for a restricted horizontal water exchange.

Situation in the Eastern Gotland Basin after the last salt water influx in 2003:
Living condition below the halocline decreased five years after the last medium sized deep water renewal. The occurrence of the marine species Oithona similis, an indicator for higher saline water, declined to nearly zero abundances, caused by a narrowing of its habitat layer of oxygenated water with adequate salinity from 160 m to 30 m. The importance of the status in plankton community development in the source water was discussed in relation for an effective recolonization of temporarily anoxic layers.

Smaller maxima of mesozooplankton abundance since the 1990s:
The negative tendency in the maximum values of the total abundance observed in previous years continues. It was especially caused by a decrease in the maximum abundance of rotifers, a taxon which stands for eutrophication. It is an indication for an improvement of water quality in the open Baltic Sea.

Bosmina spp. is an important constituent of the food spectrum of small pelagic fishes. Normally, mass occurrences were observed at sea surface temperatures between 18 and 22 °C. Currently, concentrations remained lower than expected by one order of magnitude in years with Cercopagis pengoi occurrences in the open Baltic Sea. This effect became likely at least in 1999, i.e. seven years after invasion of this carnivorous water flea in the Gulf of Riga. Its introduction means feeding competition for herring and sprat as well as prolongation of the food chain.

Mnemiopsis leidyi and other invasive species:
Besides Centropagis pengoi, which was imported in 1992 from Ponto-Caspian provinces, Mnemiopsis leidyi , an invasive comb jellyfish was recorded for the first time in the Baltic Sea in 2006. It is the fifth species of an organism group which never played a key role in the Baltic Sea ecosystem. It's spreading into the entrances of the Finnish and Bothnian Gulfs was documented in 2007. However, lobate comb jellies in these areas were identified as Mertensia ovum by genetic analysis in 2009. If confirmed, it could be another example of glacial relict fauna because this species normally inhabits the Arctic Seas. In the future, the spreading of Penilia avirostris could become an interesting feature in the western Baltic Sea. It is an herbivorous water flea of subtropical origin meanwhile dominating the summer plankton of the North Sea and currently observed in the Belt Sea and the Sound.

Macrozoobenthos

Macrozoobenthos analyses comprised species composition, their abundance and their biomass. The present study describes the macrozoobenthic community of 8 offshore stations within southwestern Baltic waters. Following sea areas were sampled: Kiel Bight (station 360), Fehmarnbelt (station 010), Mecklenburg Bight (station 012 and 018), the Darss Rise (station 030), Arkona Sea (station 109) and the Pomeranian Bay (station 152 and 160). Whereas 6 stations were sampled during the last decades, 2 additional stations were included in the monitoring programme since 2006. One "new" station is situated within the Kiel Bight (station 360) with high salinity (21 psu) and showed with 61 species the highest taxonomical diversity. Another one represents the central Pomeranian Bay (station 160) with its low salinity (8 psu) and was colonised by 23 species. The 109 species for all stations found in the macrozoobenthos in 2008 mark a medium diversity during the last 18 years at the monitoring stations sampled by the IOW. The re-deficit of oxygen (also in 2002 and 2005), which effected the deep areas of the Fehmarnbelt (station 010) and central Mecklenburg Bight (station 012), caused a decrease of species number and abundances. At the Darss Sill (station 030) the species number was comparable with the years before.

The strong negative trend in abundance and biomass observed until 2004 which caused by the disappearance or decline of Hydrobia ulvae and Pygospio elegans, changed completely. Since 2005 an increase both in total abundance and biomass could be observed. Especially the snail Hydrobia ulvae reached origin values of the 1990ies. In the central Arkona Sea the values (species number, abundance and biomass) seem to be stabilized. The northern Pomeranian Bay (station 152) shows since several years relatively stable conditions in respect to colonisation by macrozoobenthos. Slight increase of abundance and slight decrease of biomass could be observed. The conditions in the central Pomeranian Bay have to be described as stabile and relatively robust within the time considered. Only the abundance increased and showed a value comparable to the end of the 1990ies. At the 8 monitoring stations altogether 23 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 Bight in low abundance.

In respect to the Water Framework Directive and the Marine Strategy Frame Directive the ecological status of the 8 stations, deviated from the BQI (Benthic Quality Index), ranged between "poor" and "high". Due to the oxygen demand in the summer the benthic communities of the stations 010 (Fehmarnbelt) and 012 (central Mecklenburg Bight) has to be characterized as "poor". A "moderate" value was observed at the Kiel Bight (station 360), southern Mecklenburg Bight (station 018), central Arkona Sea (station 109) and the northern Pomeranian Bay (station 152). The ecological status of the community of the central Pomeranian Bay was "good" and that of the Darss Rise has to be described as "high".

Figure 1: The station grid for biological sampling in the Baltic Sea. , GIF-Graphik: 61 KB

Figure 1: The station grid for biological sampling in the Baltic Sea.
Stations in the main map represent Mecklenburg Bight (Station 012), Arkona Sea (Station 113), Bornholm Sea (Station 213), eastern Gotland Sea (Station 271), and Pomeranian Bight (Station 160).

PDF-Datei, 54 kB

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 February-May as well as cruises from July/August and November 2008.
The mean phytoplankton biomass (in  µg/l) is given on the top of each block. "Unidentified", "Gymnodiniales" and "Peridiniales" we redeleted from the list if the specimen accounted for less than 10 %.

The complete report with an English summary can be found on the Internet:
(http://www.io-warnemuende.de/tl_files/forschung/meereswissenschaftliche-berichte/mebe78_2009-zustand-bio.pdf)

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 © 2016 Bundesamt für Seeschifffahrt und Hydrographie Last Update: 19.04.2013 18:00:14  
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