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Seasonal changes in selected physico-chemical and biological variables in the temporarily open/closed Kasouga estuary, Eastern Cape, South Africa


PW Froneman

Abstract

Seasonal changes in selected physico-chemical (temperature, salinity and turbidity) and biological (chlorophyll-a (chl-a), microphytobenthos and microheterotrophs) variables were measured at 79 day intervals for a period of 14 months in the temporarily open/closed Kasouga estuary. In addition, zooplankton biomass was determined monthly over the same period. During the course of the study, 13 overtopping and two breaching events were documented. Freshwater inflow into the estuary resulted in a decrease in salinity coupled with increases in total suspended solids (TSS), particulate organic matter (POM) and turbidity. Chlorophyll-a ranged between 0.29 and 8.01mg chl-a m-3 and microphytobenthic biomass between 3.87 and 209.09mg chl-a m-2. Generally, the highest chl-a concentrations were recorded during the wet season (spring to early summer). In contrast the microphytobenthic biomass attained highest levels during periods of little or no freshwater influx into the estuary. The influx of freshwater into the estuary resulted in a change in the size structure of the phytoplankton community from one dominated by nano- (220m) and picophytoplankton (<2.0m) to a community dominated by microphytoplankton (>20m). Densities of microheterotrophs (<200m) (range 1 760 to 13 750 cells l-1) demonstrated an inverse relationship to total phytoplankton with maximum densities (>5 000cells l-1) recorded during winter. Total zooplankton biomass ranged between 15.91mg Dwt m-3 and 106.93mg Dwt m-3 and demonstrated a similar pattern to chl-a with maximum biomass recorded during the wet season. These data indicate that riverine input into the temporarily open closed Kasouga estuary is important for estuarine structure and function.


Keywords: temporarily open/closed estuary; plankton; Kasouga; South Africa


(Afr J Aqua Sci: 2002 27(2): 117-123)

Journal Identifiers


eISSN: 1727-9364
print ISSN: 1608-5914