Response of tropical marine phytoplankton communities to manipulations of nutrient concentration and metazoan grazing pressure
Manipulations were conducted to test the effects of variations in nutrients and grazing on the structure of marine phytoplankton communities, from oligotrophic to eutrophic tropical waters surrounding Jamaica, West Indies. In 16 sets of experiments, conducted using 21 L transparent carboys, the metazoan zooplankton community was enhanced, reduced or eliminated by using a range of screens with mesh size from 45–600 μm. Phytoplankton response, measured as size-fractionated chlorophyll, was followed over five days. In all environments, there was a pronounced increase of chlorophyll in response to the addition of nutrients, response being greatest in oligotrophic waters, and in the netplankton size-fraction. The lower responses by picoplankton and nanoplankton size-fractions appear to be due in part to grazing control by protists, whose populations exploded upon removal of the metazoa. Microcosms not receiving nutrients showed a slow decline in chlorophyll, the pattern of which varied with size fraction and grazing pressure. Sedimentation was most significant in the netplankton, and insignificant in the picoplankton. Despite large differences in the intensity of metazoan zooplankton grazing pressure created by the different manipulations, their grazing effects were less pronounced than nutrient effects. We conclude that phytoplankton biomass in larger size fractions is more strongly affected by nutrient supply (bottom-up) than by metazoan zooplankton grazing in areas of lower productivity, but can become more strongly affected by their grazing (top-down) in areas of higher productivity.
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Document Type: Research Article
Publication date: 2003-09-01
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