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We examined the effects of copepod feeding on estuarine microbial plankton, including bacteria, cyanobacteria, heterotrophic and autotrophic nanoflagellates, microzooplankton and larger phytoplankton cells, to elucidate any short-term control or release phenomena. As an example of such potential effects, it was hypothesized that through omnivory Acartia tonsa would reduce both competitors (large phytoplankton) and predators (microzooplankton) of small autotrophic flagellates, allowing the latter to proliferate. Copepod addition and removal treatments were conducted in 24 l containers in triplicate. As expected, the copepods significantly decreased both microzooplankton and large phytoplankton populations, and blooms of ciliates and diatoms (the dominant phytoplankton group) were apparent in the copepod removal treatment. Effects were qualitative (species composition changes) as well as quantitative. However, beyond this immediate impact, community level cascades in the microbial food web were not found, likely due to consumer recycling of resources and trophic level heterogeneity. Bacterioplankton responded positively to the addition of copepods, but negatively to the bloom of diatoms in the copepod removal treatment. These results suggest that the effects of copepods on the structure and function of estuarine microbial plankton communities may be more complex than simple linear effects would predict. KEY WORDS: Acartia tonsa · Microbial plankton · Grazing · Recycling · Trophic cascadeResale or republication not permitted without written consent of the publisher

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A three-week mesocosm experiment was conducted in order to study the effects of bottom sediment and nutrient enrichment on phytoplankton and zooplankton community structure in the Archipelago Sea, northern Baltic Sea. The transparent polyethylene enclosures included the whole water column and varied in volume from 30 to 40 m 3 . There were two types of enclosures: some with natural sediment as a bottom and others with a plastic bottom. The experiment was a 2 · 2 factorial design with presence of sediment and nutrient enrichment as treatment factors. Both the sediment presence and nutrient enrichment significantly increased water nutrient concentrations and the rate of primary production. However, external nutrient enrichment and the presence of sediment stimulated the growth of different phytoplankton groups, indicating that the effect of sediment was not related to nutrient fluxes alone, but involved more complex interactions. External nutrient enrichment was primarily channelled to picoplanktonic cyanobacteria, the biomass of which increased four-to fivefold due to enrichment. The presence of sediment increased the biomass of cryptophytes, chrysophytes and prasinophytes, but decreased the biomass of N 2 -fixing cyanobacteria. Zooplankton biomass increased during the experiment, but was not affected by the treatments. The study shows that sediment plays a significant role in phytoplankton dynamics, underlining the importance of including sediment in shallow-water mesocosm experiments.