No abstract

To explain summer declines in phytoplankton biomass in large rivers, we compared the effect of zooplankton grazing on the planktonic algae of two large European rivers, the Meuse and the Moselle. In situ grazing was measured during two years (1994 and 1995), using the Haney method. Total zooplankton community filtration rates recorded in the river Meuse ranged between 1 and 32% of the water volume filtered per day. A drastic algal decline was observed early July both years and may be explained by high densities of a rotifer-dominated zooplankton community (500-700 ind. l 1) with more than 75% of Brachionus calyciflorus. During the summer period in 1994, when grazing was over 20%, edible algal biomass was controlled by a diversified rotifer community (up to 2500 ind. l 1), while a non-edible algal assemblage developed. In contrast, phytoplankton biomass remained comparatively low in the Moselle throughout the low-flow period, as did zooplankton numbers during most of this time (fewer than 200 ind. l 1 during the summer period). The proportion of crustaceans in this zooplankton was rather higher than in the Meuse, and they dominated at times, in biomass as well as in numbers. Nevertheless, measured in situ grazing rates (1-15%) could not explain the low summer algal biomass, even if low filtration rates may at times represent a significant carbon loss for phytoplankton, when and where net algal production was low. As a conclusion, the role of phytoplankton-zooplankton interactions in controlling algal biomass in large rivers is discussed.

. W. 2005. The role of mixotrophy in plankton bloom dynamics, and the consequences for productivity. e ICES Journal of Marine Science, 62: 833e840.Mixotrophy (Zheterotrophy and photosynthesis by a single individual) is a common phenomenon in aquatic ecosystems, in particular under light-or nutrient-limitation. However, it is not usually considered in mathematical models of biological populations. This paper shows how different types of mixotrophy might be usefully incorporated into a general predatoreprey model, and explores the consequences for plankton bloom dynamics and productivity. It is demonstrated, analytically and numerically, that even small levels of type III mixotrophy (a small fraction of the zooplankton also being involved in primary production) have significant effects on a system's equilibrium structure, stability, and short-term dynamics. Type III mixotrophy has a stabilizing effect on the system by reducing its excitability, i.e. its propensity to exhibit blooms. Compared with the nonmixotrophic benchmark, for a phytoplankton bloom to be triggered in a system with type III mixotrophy, a much larger perturbation is necessary. Type II mixotrophy (a small fraction of algae engage in phagotrophy) and type I mixotrophy (equal phagotrophy and phototrophy) are briefly discussed. The potential consequences for productivity are also studied. Our results indicate that the phytoplanktonezooplankton system becomes more productive in the presence of type III mixotrophy.

Various freshwater, estuarine and coastal stations of the Southern Baltic Sea were comparatively studied to evaluate pelagic bacterial performance. Inner coastal waters (so-called bodden or lagoons) are highly productive systems and dominate the coast of the Southern Baltic Sea. Due to high nutrient loads up to the 1990s in combination with an enhanced primary production, increased amounts of particulate (POC) and dissolved organic carbon (DOC) accumulated in these waters. In the Darß-Zingst bodden chain, POC < 16 and DOC < 13 mg C l -1 , and C:N ratios of 9 to 11 in particulate matter were measured in winter and spring samples. Due to high POC concentrations, the average ratio of DOC:POC was 1.1:1, which is very low compared to other aquatic systems. Bacterial abundance and activities were rather high and reached 24 × 10 6 ml -1 and 18 µg C l -1 h -1 , respectively. Although 2 of the 3 investigated freshwater systems were classified as eutrophic, the highest measured POC concentrations, bacterial abundance and production were much lower (1.6 mg C l -1 , 11 × 10 6 bacteria ml ) than in the bodden. In contrast to that, the DOC load was as high as in these inner coastal waters (<12 mg C l -1 ). The coastal stations of the Baltic Sea, classified as mesotrophic, were not severely loaded with organic matter and bacteria (POC < 0.8, DOC < 5.5 mg C l -1 , bacteria < 3 × 10 6 ml -1). Bacterial production again was lower than at all other stations; however, levels did reach an exceptional 4.6 µg C l -1 h -1, which is comparable to values of the freshwater systems. Compared to the other investigated marine and freshwater systems, the bodden were heavily loaded with organic matter, especially particulate organic matter (POM). The origin of this material is assumed to be mainly autochthonous as it is known not to be transported by rivers into these estuaries. Although dissolved inorganic nitrogen (DIN) concentrations were high at least in winter, POM was of poor quality; this was reflected by high C:N ratios and a low contribution of phytoplankton carbon to POC. However, this is particularly surprising, because nitrogen should be readily available at all bodden sites by resuspension from the sediment caused by frequent winds in these very shallow systems of < 2 m depth. KEY WORDS: Organic matter · Bacterial production · Hydrolytic enzymes · Marine systems · Brackish systems · Freshwater systems Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 32: [121][122][123][124][125][126][127][128][129][130][131][132][133][134][135] 2003 organic matter input and decomposition, and subsequently reveal the most important regulating factors of bacterial activity.Besides nutrient availability, grazing as a top down control factor regulates bacterial biomass and species composition (e.g. Berninger et al. 1991). Protozoa, especially heterotrophic nanoflagellates, are an important, or often the dominating, loss factor for bacterial standing stock in pelagic systems (e.g. Jürgens et a...

No abstract