Diel variations in microbial activity in hypertrophic coastal waters

Heterotrophic bacteria depend for their carbon and energy supply on organic matter, which in aquatic systems is produced either autochthonously through primary production by phytoplankton and phytobenthos or is washed in from allochthonous sources. Studies were conducted in two hypertrophic coastal systems of the Baltic Sea, the upper Schlei fjord in Schleswig-Holstein, and the Oderhaff lagoon in Mecklenburg-Western Pomerania, to explore whether diel variations in bacterial activity can be found that correlate with the daily pattern of primary production. Both systems are highly eutrophied, with an annual primary production between 600 and 800 g C m super(-2). During the study period in early June (Schlei) and late August (Oderhaff), daily primary production ranged from 2.5 to 3.0 g C m super(-2); highest values for thymidine and leucine incorporation were 0.346 and 4.01 nmol l super(-1) h super(-1) respectively. The maximal turnover rate of glucose was 99% h super(-1) and that of acetate 67% h super(-1), both representing low-molecular organic compounds. None of these bacteriological parameters yielded statistically significant differences between the mean values for day and night. The only evidence of a possible diel variation was a sharp drop in the turnover rates of glucose in the early nocturnal phase in the Schlei. Nor could a diel rhythm be discerned in the measurements from a water sample obtained from the upper Schlei that was kept for several days under controlled laboratory conditions in a light incubator. It is assumed that the bacteria are abundantly supplied with organic matter from allochthonous sources and that processes independent of diel variations, such as zooplankton grazing or hydrolysis, prevail in these hypertrophic systems. Thus, despite its high values, primary production does not induce a diel rhythm in bacterial activity in highly eutrophied systems that receive a sub-stantial input of organic matter from allochthonous sources.Original Abstract: Heterotrophe Bakterien sind auf die Versorgung mit organischem Material zur Deckung ihres Kohlenstoff- und Energiebedarfs angewiesen. In Gewaessern kann dieses Material durch die Primaerproduktion des Phytoplanktons und Phytobenthos autochthon erzeugt werden oder allochthoner Herkunft sein. Auf drei Stationen, zwei in der inneren Schlei (Schleswig-Holstein) und eine im Oderhaff (Mecklenburg-Vorpommern), wurde durch Probenahmen in kurzen Abstaenden versucht, eine von der Primaerproduktion