MURSYS - Baltic Sea





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MURSYS - Baltic Sea

   
 
 

Biological Conditions in the Baltic Sea in 2007 (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/)

Figures and tables given in this summary refer to the original report published in German:
Wasmund, N., F. Pollehne, L. Postel, H. Siegel und M. Zettler (2008): Biologische Zustandseinschätzung der Ostsee im Jahre 2007. Meereswiss. Ber.,Warnemünde,74, 1-88. (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 2006 together with physico-chemical data in order to continue the time series which exists since 1979. Data from sediment traps deployed in 2006 in the Gotland Basin give information on particle dynamics. Also satellite data are used to trace the phytoplankton bloom development.

Phytoplankton

Spring bloom:
The development of the spring bloom of the phytoplankton started in Mecklenburg Bight (Station 012), including Lübeck Bight (Station O22) in early March, i.e. later than in the extreme year 2006. This start phase was dominated by Chattonella verruculosa, Dictyocha speculum and Mesodinium rubrum. The bloom reached a peak on 27.3.07 with extremely high biomasses of Dictyocha speculum of 17,893 mg/m³ and 12,842 mg/m³ at Station 012 and O22, respectively. Also in Kiel Bight (Station 360), the bloom was strong at that time, with chlorophyll-a concentrations of 30,6 mg/m³. This early Dictyocha-bloom is unusual.
Normally, Dictyocha appears only after the diatom bloom, which should be strong in the western Baltic. However, the diatom bloom could not develop in 2007, probably because of competition by the flagellates. The decrease in spring diatoms is a phenomenon which was described by WASMUND et al. (1998) for the Baltic Proper in the 1990s. A relatively weak diatom growth occurred after the Dictyocha-bloom.
Also the dinoflagellates were only weakly developed in the western Baltic in 2007; also they did not appear as usual after the diatoms but before the diatoms, together with Dictyocha. This reverse of the common succession cannot be explained recently.
Also in the Arkona Sea, no significant diatom bloom could be found. A spring bloom, confirmed by a chlorophyll-a-concentration of 7,2 mg/m³, occurred in the Bornholm Sea. It was dominated by Mesodinium rubrum, Skeletonema costatum, Chaetoceros cf. wighamii and unidentified Gymnodiniales. Also in the Gotland Sea, a moderate diatom growth could be proved. Hence, the common succession from diatoms to dinoflagellates is still intact in the Baltic Proper.

Summer bloom:
A summer bloom of diatoms, frequently noticed in Mecklenburg Bight in the past, could not be found there in 2007. Only in early August, higher biomass of Proboscia alata occurred for a short while. In the Arkona Sea, the summer phytoplankton was dominated by cyanobacteria (Nodularia spumigena, Aphanizomenon sp., Aphanocapsa sp.) and diatoms (Chaetoceros impressus, Chaetoceros danicus, Actinocyclus sp., Coscinodiscus radiatus, and particularly on 4.8.07 Proboscia alata). In the Bornholm and Gotland Sea, the typical high biomass of cyanobacteria (Nodularia spumigena, Aphanizomenon sp., Cyanodictyon planctonicum, Aphanothece spp., Aphanocapsa spp., Pseudanabaena limnetica) was found in July 2007. Moreover, Actinocyclus spp., Katablepharis remigera, Plagioselmis prolonga, Teleaulax sp. and unidentified Gymnodiniales are worth mentioning.

Autumn bloom:
Diatoms developed in the western Baltic in autumn, especially Proboscia alata, Dactyliosolen fragilissimus, Cerataulina pelagica and Actinocyclus sp. Astonishingly, Ceratium tripos was poorly developed in 2007. Only at station 046 (eastern Mecklenburg Bight), it was dominant on 2.11.07. The strong diatom development in autumn compensated the weak occurrence in spring in the annual balance. Also in the Arkona Sea, diatoms (Dactyliosolen fragilissimus, Cerataulina pelagica, Coscinodiscus granii and Actinocyclus sp.) were dominant in autumn. The high share of cyanobacteria (Nodularia spumigena, Aphanizomenon sp.) and Mesodimium rubrum at Station OMBMPK5 at the end of October was unusual. The autumn bloom in the Bornholm and Gotland Sea was exclusively formed by diatoms (Coscinodiscus granii, Actinocyclus sp., probably Actinocyclus octonarius). At Station 271 (central Arkona Sea), this bloom was fully developed on 30.10.07. The 10 most important phytoplankton species of each season in each sea area are compiled in Table 5 (Annotation: Figure 1 to this summary). A complete species list of the year 2007, including a seasonal indicator, is given in Table 6.

Sedimentation:
The sedimentation of organic material could be recorded in 2006 over the whole year without a loss of samples. A short phase of reduced sedimentation in June could be due to an intermediate deposition of material at the funnel slopes and the consecutive transport into the sample container in a later phase. On the other hand the month of June is generally the period with the lowest rates and the quality of the deposited material in terms of phytoplankton species and elemental composition is so much distinct from preceding and subsequent material, that the rates have to be seen as realistic.
This year again displayed the typical separation in three qualitatively and quantitatively different sedimentation periods related to the seasonal difference in physical and biological drivers for the vertical particle flux. The spring sedimentation maximum (March/April/May) set in later than usual and was formed exclusively by diatoms, where, different from the preceding years, Skeletonema costatum, Thalassiosira levanderi, Achnantes taeniata and species of the genus Chaetoceros occurred in similar amounts.
The main export of silica out of the surface mixed layer was more or less restricted to the spring season, as during this summer the silica fluxes due to the diatom Nitzschia paleacea, that grows on cyanobacterial aggregates were comparatively low. The fraction of diatoms in the autumn maximum and the corresponding vertical silica flux was as well below the long term mean for this season.
Aggregates of cyanobacteria (Aphanizomenon sp., Nodularia spumigena, Anabaena spp.) dominated the vertical transport of carbon, nitrogen and phosphorus in the summer period. The significance of theses nitrogen-fixing organisms was reflected in the extremely low isotopic signal of nitrogen during the main sedimentation phase in July/August.
The sedimentation maximum in late autumn was quantitatively less relevant than in the previous year. It was formed by a mix of different diatoms with Coscinodiscus granii and Actinocyclus octonarius and the genus Chaetoceros dominating the composition. During this phase (October, November) species of the colonial cyanobacteria Aphanothece, Coelosphaerium and Snowella, which were not very abundant over the rest of the year, occurred in higher proportions.
The total annual flux for the single elements amounted to 270 mmol C, 33 mmol N (C/N=8,2), 107 mmol Si and 2,04 mmol P per   and year. With 28.5 g dry material the mass flux was considerable lower than in the previous years.

Chlorophyll:
The chlorophyll a data from the surface water are shown in Table 7. If the seasons (Table 8) were analysed separately, the spring data revealed long-term trends, but not the summer data (Table 9, Figure 14). The spring data were decreasing in Mecklenburg Bight but increasing in the Arkona Sea and the Bornholm Sea from 1979 to 2007.

Mesozooplankton

Sampling and sampling treatment: The assessment based on 121 WP-2 Net samples (100 µm mesh size, TSK-flow meter) in 2007, collected on eight stations between Kiel Bight and the Eastern Gotland Basin. If possible in 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 seldom specimen. Sub-sample volumes ranged between 1/1 und 1/769 of the total sample, in average 1/21 ±1/6. Between 1 and 2588 individuals per taxon were counted, in average 25 ±82. Species spectrum: Thirty nine taxa were recorded in total during the last nine years with higher amounts in the second halves of the years. The maximum in 2007 was observed in July. The following groups were principally to distinguish:

  • Species which were permanently present in the assessment period and round the year,
  • Taxa with typical seasonal patterns, like Bosmina spp. and some meroplanktonic larvae
  • Occasionally occurring taxa of marine and limnetic origin, depending on inflow and outflow conditions, for example Calanus finmarchicus and Limnocalanus macrurus, and
  • Some larger species, which were occasionally collected, like ephyrea of Aurelia aurita, and juvenile Mnemiopsis leidyi.

Situation in the Eastern Gotland Basin after the last salt water influx in 2003:
The occurrence of the marine species Oithona similis below the halocline in the central Gotland basin is an indication for higher saline water in combination with oxygenated conditions. There were still abundances up to 40 ind./m³ in 2007, despite of missing larger salt water inflows. In longer stagnation periods, like in the late nineteen eighties / beginning nineteen nineties, the species was totally missed.

Smaller maxima of mesozooplankton – abundance since the 1990s:
We observed a negative tendency in the maximum values of the total abundance by >50 % (in 2007) and by 70 % (2005) in comparison to the period of the early 1990s. A special decrease was recorded in the maximum abundance of rotifers - a taxon which stands for eutrophication - by nearly two orders of magnitude. Concentrations of Bosmina spp. - constituent of the food spectrum of small pelagic fishes: The maximum Bosmina spp. abundance of 32,000 ind./m³ remained below the concentration of the previous year by one order of magnitude due to the low summer temperature of less than 15 °C in the central Baltic Sea in 2007. It was the second lowest value since 1991.

Mnemiopsis leidyi - the invasive ctenophore in the Baltic Sea:
This ctenophora over wintered in Kiel Bight and in Mecklenburg Bight with up to 5 ind./m³ , and below the halocline in the Baltic proper with less than 1 ind./m³. In January 2007 reproduction was observed. Again, offsprings were recorded in the western Baltic Sea from June/ July. The spreading up to the Gulfs of Finland and Bothnia was recorded by Finish scientists in 2007. Further, there was a clear gradient from west to east, with higher concentrations in Kiel Bight. The abundance was smaller by two orders of magnitude east of Darss sill. Maximum abundances of 500 ind./m³ in Kiel Bight in June, reported by IfM-GEOMAR , were in the range of those in the native areas of North America as well as those of the Black Sea in the nineteen eighties. However, Mnemiopsis leidyi in the Baltic Sea remained smaller by a factor of three in comparison to the Black Sea, which is positively correlated with the reproduction potential according to the literature. We still recommend monitoring of the situation especially in respect of adaptation capacity and a match and mismatch in the food web in Northern European waters.

Macrozoobenthos

Macozoobenthos analyses comprised species composition, their abundance and their biomass. Whereas 7 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 with high salinity (18 psu) and showed with 77 species the highest taxonomical diversity. The other represents the central Pomeranian Bay with its low salinity (8 psu) and was colonised by 23 species. The 108 species for 7 "old" stations (133 at all 9 stations respectively) found in the macrozoobenthos in 2007 mark the second highest diversity during the last 17 years at the monitoring stations sampled by the IOW (Figure 13).
After the quick recovery following the oxygen depletions in 2002 and 2005 in the western Baltic (Station 010/Fehmarnbelt, 012/central Mecklenburg Bight and 018/Mecklenburg Bight south) the benthic communities reached highest species numbers in 2006 and 2007. Favourable salinity and oxygen conditions in 2006 allowed a rapid recolonisation of the western Baltic during 1 year. Probably due to a decrease of salinity in 2007 some marine species like the bivalve Abra alba and the brittle star Ophiura albida, dominant in the previous year at the westernmost stations, decreased as well.
With the whelk Buccinum undatum and the common pelican foot Aporrhais pespelecani two very rare species of the Baltic Sea were recorded. Furthermore the bivalve Macoma calcarea, threatened with extinction in German waters, was found in the Kiel Bight in low abundance.
At the Darss Sill (Station 030) the species number was comparable with the years before. Otherwise, a strong negative trend in abundance and biomass is to be observed since the beginning of the 1990ies. The main causes are the disappearance or decline of Hydrobia ulvae and Pygospio elegans. No reasons for this observations could be stressed. During the last years the community had a little bit changed to more species which prefer organic rich sediments (e.g. Lagis koreni). Only within the last 3 years a slight increase both in total abundance and biomass could be observed.
In the central Arkona Sea the values (species number, abundance and biomass) decreased in comparison with the previous years. Low oxygen values in bottom near waters during the year 2007 will expected. But in respect to historical data both the species number and the abundance/biomass are increased.
The northern Pomeranian Bay (Station 152) shows since several years relatively stable conditions in respect to colonisation by macrozoobenthos. Slight increase of biomass and abundance could be observed. The Bornholm Sea (Station 213) was characterised by no oxygen and a complete defaunation in 2007.

(http://falster.hh.bsh.de/bsh_edit/de/Meeresdaten/Beobachtungen/MURSYS-Umweltreportsystem/PDF/IOW_Biol_Monit_Ber_2007.pdf)

Table 1: The 10 most abundant phytoplankton species (percentage of total phytoplankton biomass) in the different sea areas (upper 10 m) in spring, summer and autumn 2007. The mean phytoplankton biomass (in µg/L) is given on the top of each block.

The complete report is available in German on the internet at :
(http://www.io-warnemuende.de/publikationen.html)

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 © 2016 Bundesamt für Seeschifffahrt und Hydrographie Last Update: 19.04.2013 17:59:56  
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