Tidal flushing, primary production and phytoplankton and zooplankton communities were investigated to establish the processes leading to persistently high levels of phytoplankton (in excess of 15 to 60 μg l−1 chlorophyll a) in a particular reach of Berowra Estuary, a relatively large drowned river valley estuary in New South Wales, Australia. Residence times at the bloom site were sufficiently long to provide an opportunity for exponential growth of algae and to result in large accumulations of cells and high rates of primary production. Phytoplankton taxa over the study period were dominated by various diatoms, Chaetoceros, Pseudonitzschia, Thalassiosira and Skeletonema. Zooplankton biomass (wet weight of taxa between 90 and 900 μ m equivalent circular diameter), estimated using image analysis technology was significantly greater in the bloom area (200 mg m−3), than upstream (140 mg m−3) and downstream (120 mg m−3) sites. Numbers of individuals in the smallest zooplankton size classes were significantly related to chlorophyll a concentrations. The relationship decreased with increases in the particle size classes, showing an uncoupling between chlorophyll a concentrations and abundance of larger zooplankton. Residence times in the bloom reach were sufficient for secondary production (somatic and egg production) to occur in the zooplankton taxa present. Growth of phytoplankton under the conditions of nutrients, light and temperature exceeded losses due to zooplankton grazing and estuarine flushing, maintaining chlorophyll concentrations of more than 10 μ g·l−1.

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