Size structure and energetics in a plankton food web

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Studies were conducted to quantify how the size distribution of plankton affects ecological transfer efficiency in the pelagic food web. Simultaneous measurements of phytoplankton and zooplankton size (equivalent spherical diameter, ESD), biomass, and productivity were determined from bimonthly samples collected over a 13-month period at four pelagic sites in a large subtropical lake. Plankton ESD values were compared statistically with ratios of zooplankton:phytoplankton biomass and productivity, which served as two indices of ecological transfer efficiency. Communities dominated by large phytoplankton (generally filaments or colonies of cyanobacteria) and/or small-bodied zooplankton (rotifers and nauplii) displayed lower ratios of zooplankton:phytoplankton biomass than communities with smaller phytoplankton(nanoflagellates) and/or larger zooplankton (copepods and cladocerans). This pattern was robust, and was apparent both on inspection of data from individual sampling sites, and when all data were combined. Productivity ratios displayed the same general relationships with phyto- and zooplankton ESD, although those patterns were evident only when data were examined on a site-by-site basis. Overall, the results supported the hypothesis that when the pelagic community is dominated by large phytoplankton and small zooplankton, only a small fraction of the carbon fixed by photosynthesis is passed directly to the herbivore trophic level. At such times, alternative routes for carbon flow may predominate, including bacterial-based pathways with protozoa as intermediate consumers, or direct links from phytoplankton to fish.

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