Response of the Plankton to Three Different Hydrological Phases of the Temporarily Open/closed Kasouga Estuary, South Africa

The responses of the plankton, including pelagic phytoplankton biomass and productivity, microphytobenthic algal concentration, zooplankton biomass and zooplankton grazing to three distinct hydrological phases of the temporarily open/closed Kasouga estuary were investigated in summer (September–November) 2000. The three hydrological phases were the closed phase, the over-wash phase and the flood phase. During the closed and over-wash phases, oligotrophic conditions prevailed throughout the estuary. In response to the low macro-nutrient concentrations, total biomass and productivity were generally low <0·40 mg chl a m 2 and <30 mg C m 2 d 1, respectively. Picophytoplankton (<2 m) dominated total chl a and production during the closed and over-wash phases. Microphytobenthic chl a concentrations ranged between 57·8 and 132·9 mg chl a m 2 during the closed phase and between 48·7 and 83·4 mg chl a m 2 during the over-wash phase. Total zooplankton biomass during the closed phase ranged between 35·7 and 41·3 mg Dwt m 3 and between 33·8 and 48·13 mg Dwt m 3 during the over-wash phase. Grazing impact of herbivorous zooplankton was equivalent to <15% of total daily phytoplankton production during the two phases. The inflow of freshwater into the estuary result in an increase in macro-nutrient concentrations, which promoted the growth of the nano-(2–20 m) and microphytoplankton (>20 m). Total pelagic chl a biomass and productivity during the flood phase ranged between 2·38 and 5·68 mg chl a m 2 and between 40·09 and 64·66 mg C m 2 d 1, respectively. Microphytobenthic chl a concentrations during the flood phase were always <35 mg chl a m 2. Zooplankton biomass and grazing impact varied between 68·7 and 88·3 mg Dwt m 3 and between 13·4 and 53·9% of the daily phytoplankton production. Results of the study indicate that the influx of freshwater into the temporarily open/closed Kasouga estuary has a dramatic impact on the plankton community size structure, production and grazing impact of zooplankton.