Zooplankton grazing dynamics: top-down control of phytoplankton and its relationship to an estuarine habitat

Top‐down control of algal blooms was investigated in the Swan–Canning Estuary, to determine whether the resident zooplankton community was able to substantially reduce phytoplankton biomass through grazing, or whether high phytoplankton biomass effectively inhibited grazing. In situ grazing by zooplankton at two depths in the water column was measured over a diel period on six occasions and examined in relation to the biotic and abiotic habitat conditions. Measured weight‐specific ingestion rates varied with phytoplankton and zooplankton species composition, but were significantly greater at the surface than at 5 m, close to the bottom (ranges of 1·59 to 252·41 and 0·51 to 28·99 µg phytoplankton C µg zooplankton C−1 day−1, respectively). There was no evidence of significant diel changes in ingestion rates of zooplankton. Phytoplankton biomass removed by in situ community grazing ranged between 0·9 and 45·0% of standing stock per hour. The maximum loss occurred when phytoplankton biomass ranged from 249·0 to 320·8 µg phytoplankton C l−1. Zooplankton grazing impact was negatively associated with high phytoplankton and low dissolved oxygen in bottom waters. The implications of, and mechanisms associated with, top‐down control of algal biomass in this estuary are discussed in relation to plankton succession and the physical environment. Copyright © 2001 John Wiley & Sons, Ltd.