Meereswissenschaftliche Berichte No 13 1996 - Marine Science Reports No 13 1996
http://doi.io-warnemuende.de/10.12754/msr-1996-0013
doi:10.12754/msr-1996-0013
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Ernährungsstrategie calanoider Copepoden in zwei unterschiedlich trophierten Seegebieten der Ostsee (Pommernbucht, Gotlandsee)

Meyer-Harms, Bettina

Abstract. The aim of the thesis was to analyze the feeding strategy of calanoid copepods in two areas of the Baltic Sea which were characterized by different levels of eutrophication in the summer and early autumn. The two research areas were the Pomeranian Bay and the Gotland Sea. Based on phytoplankton biomass, the former one is considered to be eutrophic to hypertrophic, whereas the Gotland Sea is an oligotrophic to mesotrophic basin. The goal of the Pomeranian Bay study was to determine alterations in the phytoplankton community caused by the mixing processes of water discharged from the Oder river with the water of the bay and the impact of these processes on the feeding activity of calanoid copepods. In contrast to the Pomeranian Bay, the Gotland Sea, in summer, is characterized, by a stable stratification of the water column, oligotrophic nutrient conditions in the mixed layer and a low phytoplankton biomass. When compared to the Pomeranian Bay, the phytoplankton community in the Gotland Sea consists of small phytoplankton species (≤ 10 μm) and diazotrophic cyanobacteria. The goal of the Gotland Sea study was to find out whether the copepods feed on nitrogen-fixing cyanobacteria and small phytoplankton species. In addition, the contribution of calanoid copepods to the carbon flux was to be estimated in both areas under investigation. The samples were analyzed using different methods: measurements of marker pigments (HPLC), cell counts using the Utermöhl method, zooplankton counts and determination of δ15N. In contrast to the routine methods, the zooplankton was not size-fractionated. The specimens were separated according to species and developmental stage in the analyses and grazing experiments. All experiments were carried out using natural phytoplankton concentrations and compositions. Acartia bifilosa was abundant in both areas under investigation and thus allowed direct comparisons of trophic pathways between the two areas. The composition of phytoplankton (the diet of the copepods) was determined using marker pigments and Utermöhl cell counts. The food spectrum of the copepods was determined by pigment analysis of gut content. Additional grazing experiments with A. bifilosa allowed a compact interpretation of the field data. The grazing experiments were evaluated in conjunction with marker pigment measurements and cell counts. The determination of ingestion of cyanobacteria by the copepods was complemented by the δ15N-analysis. This method allows the differentiation between ingested cyanobacteria and other phytoplankton organisms. In order to be able to compare the results of the cell counts and the pigment analyses, the data were converted into chl.a-equivalents. The conversions were made using chl.a/marker pigment ratios from the literature (laboratory cultures), as well as using chl.a/marker pigment ratios from pigment measurements in the two research areas (field and grazing experiments), estimated by multiple regression analysis. The main results of the thesis are summarized as follows: - The copepods in the Pomeranian Bay and in the Gotland Sea exhibit different feeding strategies. Copepods in the Pomeranian Bay, which has a mean phytoplankton biomass of 400 μg C * L^-1 feed selectively on crypto.- and dinophyceae. In the Gotland Sea, which has a mean biomass of 120 μg C * L^-1, feeding of copepods is very opportunistic, except when they prey upon the cyanobacteria, which means that the composition of the diet corresponds to the availability of the food. The ingestion rates determined by pigment measurements in the Pomeranian Bay were around 844 ng C * Ind.^-1 * day^-1. This corresponds to 71 % of body-C content. In the Gotland Sea, the maximum ingestion rate of A. bifilosa was 241 ng C * Ind.^-1 * day^-1 in 1993, in contrast to 1994, when this value was 914 ng C * Ind.^-1 * day^-1. These values correspond to 22 % (1993) and 51 % (1994) of the body-C content. The assimilation efficiencies show that the ingested food was used increasingly less economically at high ingestion rates. In 1994, the assimilation efficiencies were 58 % and 46 % in the Pomeranian Bay and in the Gotland Sea, respectively. In 1993, in the Gotland Sea, when the food supply was suboptimal, the assimilation efficiency was 89 %. - In the Pomeranian Bay, feeding upon the primarily colonies-forming cyanobacteria was low. In contrast, in the Gotland Sea, the ingestion of diazotrophic cyanobacteria probably depends on the physiological status of the bloom. It seems that at start of the bloom the cyanobacteria are hardly fed upon (2% of the cyanobacteria biomass, Gotland Sea 1993). However, during the stationary and the regressing phase of the bloom feeding on cyanobacteria increases considerably (20 % of the cyanobacteria, Gotland Sea 1994). - 1.1% and 3.3% of the primary production were ingested by calanoid copepods in the Pomeranian Bay and in the Gotland Sea, respectively. In the nutrient limited system of the Gotland Sea, 2.3 mg C m^-3 * day^-1 were excreted by the copepods into the mixed layer. According to the Redfield ratio, this corresponds to 0.3 mg N * m^-3 * day^-l. Consequently, at a mean depth of 0 - 15 m of the mixed layer, 321 μmol * m^-2 * day^-l were excreted by the copepods. 20 % of this amount of nitrogen results from the new production of diazotrophic cyanobacteria. - The comparison between the chl.a-equivalents determined by chl.a/marker pigment ratios from algal cultures, and those determined by chl.a/marker pigment ratios from pigment measurement in the Pommeranian Bay and the Gotland Sea, which were calculated by multiple regression analysis, showed that the estimation by multiple regression analysis is a good tool to consider the variation in the pigment composition caused by changing light conditions. The application of conversion factors gained from laboratory cultures is not recommended for the calculation of ch1.a-equivalents from marker pigment concentrations in estuarine systems which are characterized by variable light conditions due to horizontal and vertical mixing processes. - The agreement of the determinations of phytoplankton composition using cell counts and pigment measurements is good. However, the pigment composition of the isolated Dinophysis norvegica in the Gotland Sea, exhibiting alloxanthin (marker for cryptophytes) as marker pigment instead of peridinin (marker for dinoglagellates), demonstrates clearly that pigment analysis should be complemented by random samples of cell counts. - The evaluation of the grazing experiments using cell counts and pigment measurements showed in the Pommeranian Bay and the Gotland Sea, the pigment analysis is a useful method to determine taxon-spezific ingestion rates by calanoid copepods. Pigment analysis allows to obtain good reproductible results at a low time investment. Also, the variance is smaller when compared to that resulting from cellcounts according to the Utermöhl method.

Citation

Bettina Meyer-Harms: Ernährungsstrategie calanoider Copepoden in zwei unterschiedlich trophierten Seegebieten der Ostsee (Pommernbucht, Gotlandsee). Meereswiss. Ber., Warnemünde, 13 (1996), doi:10.12754/msr-1996-0013

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