Spatio-temporal Dynamics of Phytoplankton and Microphytobenthos in a South African Temporarily-open Estuary

Of South Africa’s 250 estuaries, 182 (73% of the total) are classified as temporarily-open. There is, however, little information currently available on their physico-chemical-biological properties and processes. This study addresses some aspects of the primary biomass available in these systems, in order to provide basic information for their sustainable management. Six surveys were undertaken in the Mpenjati Estuary, on the Kwazulu-Natal south coast, during 1998/99. Results show that microphytobenthic chl a attains concentrations of up to 616 mg chl a m 2, i.e. among the highest ever recorded for any South African estuary. Phytoplankton biomass in the estuary, on the other hand, varied from 0·14 to 15·4 mg chl a m 3. These concentrations are lower than those recorded in the permanently open estuaries of southern Africa. Chl a concentrations in the sediment were one to three orders of magnitude higher than in the water-column, with an average ratio of water-column to sediment chl a of 0·054 0·11 SD. These results are analysed in the context of the alternation of closed and open phases of the estuary, and in relation to some key physico-chemical factors.