Food web dynamics in a temperate temporarily open/closed estuary (South Africa)

Size fractionated chlorophyll-a (chl-a), primary production and zooplankton (mesozooplankton (200e2000 mm) and microheterotrophs (!200 mm)) grazing studies were conducted on nine occasions in the temperate, temporarily open/closed Kasouga estuary situated on the eastern seaboard of southern Africa over the period 2001e2002. Total chl-a concentration and primary production during the study ranged between 0.20 and 3.43 mg chl-a m 3 and between 12.94 and 53.17 mg C m 3 d 1, respectively. The highest biomass and production values were consistently recorded following periods of freshwater inflow into the estuary. The inflow of freshwater into the estuary also coincided with a shift in the size structure of the phytoplankton community with large cells (O5 mm) dominating total chl-a and production. During periods of reduced freshwater inflow into the estuary, total chl-a and production were dominated by small (!5 mm) phytoplankton cells, which constituted up to 77% of biomass and up to 74% of total daily production. Daily grazing impact of the microheterotrophs ranged between 0.7 and 15.2 mg C m 3 d 1 or between 7.7 and 37.4% of the daily phytoplankton production. The mesozooplankton grazing impact ranged between 0.2 and 24.2 mg C m 3 d 1. These rates are equivalent to a loss of up to 46% (range 0.2e45.6%) of the daily phytoplankton production. Although there was clear evidence of temporal variability in the size structure of the phytoplankton community, there was no associated pattern in the partitioning of carbon between the mesozooplankton and microheterotrophs. The absence of any clear pattern appears to be related to the high degree of overlap in the preferred food particle size between the microheterotrophs and mesozooplankton.