Preliminary study on the food web structure of two contrasting estuaries along the Eastern Cape coast, South Africa

Abstract

Water column profiles, surface macronutrient concentrations, size fractionated primary production and grazing by protozooplankton were investigated at 8 stations in the Kariega and Great Fish River estuaries in autumn (May) 1999. As a result of the low freshwater influx, the Kariega estuary was characterised by a well-mixed water column, low macronutrient concentrations and low turbidity (NTU <35). In contrast, the water column in the Great Fish River estuary was stratified largely as a result of sustained freshwater influx. Associated with the high freshwater input, its macronutrient concentrations and turbidity values (NTU >240) were elevated. Size fractionated chlorophyll-a and primary production in the Kariega estuary ranged from 1.50 to 3.39mg.Chl-a.m^-2 and between 171.10 and 234.45mg.C.m^-2d^-1 respectively. Throughout the survey picophytoplankton (<2.0µm) dominated chlorophyll-a biomass and production, which can be related to low macronutrient concentrations. In the Great Fish River estuary total integrated biomass and production were higher, varying between 3.42 and 4.79mg.Chl-a.m^-2 and between 222.87 and 284.63mg.C.m^-2d^-1. The elevated production recorded in the Great Fish River estuary appears to be related to increased macronutrient concentrations associated with the freshwater input. Despite high nutrient availability, total integrated biomass and production were dominated by nanophytoplankton (2–20µm). The predominance of small cells appears to be related to the unfavorable light environment conferred by the high turbidity. Protozooplankton densities in the Kariega estuary ranged from 2 570 to 3 150 cells.l^-1 and between 7 250 and 7 830 cells.l^-1 in the Great Fish River estuary. Protozooplankton grazing on phytoplankton in the two systems was similar, corresponding to <10% of the initial standing stock or <55% of the daily phytoplankton production. The high grazing impact in the contrasting estuaries can be related to size structure of the phytoplankton assemblages.


Keywords: estuaries; primary production; protozooplankton; grazing


(Afr J Aqua Sci: 2000 25: 13-22)