This study examined the extent to which zooplankton in the James River Estuary (Virginia, USA) exploit the available algal-food resources. Zooplankton abundance, community composition and grazing rates were assessed at three locations which included a site with high algal production (near the VCU Rice Center) and two sites where algal production was lower. Grazing rates were measured by determining the rate of phytoplankton decline (as chlorophyll a) during 48-hour incubation experiments in the

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... presence and absence of zooplankton. Significant differences in zooplankton abundance were observed among the three sites, with the greatest average zooplankton density (434 ± vii 69.04 ind/L) occurring at the phytoplankton maxima. There were no significant differences in zooplankton filtration or ingestion rates among the three sites. The amount of chlorophyll a consumed was low at all sites (< 5%), suggesting that only a small proportion of primary production was directly passed to higher trophic levels. Low grazing rates were attributed to poor food quality owing to the presence of non-algal particulate matter and a predominance of cyanobacteria comprising the phytoplankton community. These findings are consistent with prior studies showing that cyanobacteria are a poor quality food source for zooplankton and that non-algal particulates may interfere with grazing rates. Further, this study lends support to the hypothesis that food quality is an important determinant of trophic transfer efficiency in aquatic food webs.