A quantitative study of the trophic relationships within the fish community of the Mhlanga estuary, South Africa

Abstract

The importance of benthic floc, zoobenthos, zooplankton and epiphytic flora and fauna as energy sources for the fishes of the Mhlanga estuary was studied between January and December 1978. Benthic floc (detritus and associated microorganisms) was the most important food resource with an annual standing crop of 161 kJ m⁻² (9·46 g m⁻²). More than 90% of the fish biomass utilized this energy source compared to 1·8% feeding on zoobenthos (26·4 kJ m⁻², 1·5 g m⁻²), 1·7% on zooplankton (1·7 kJ m⁻³, 0·08 g m⁻³) and 3·1% on epiphytic flora and fauna (4·0 kJ m⁻², 0·27 g m⁻²). Since much of the detritus entered the Mhlanga estuary from the fringing Phragmites swamp, the biological productivity of the estuary was closely linked to that of the reed beds. During the closed phase of the estuary the standing crops of the fish food resources increased due to the relative stability of the physical environment. The highest biomasses of zooplankton, zoobenthos, epiflora and epifauna were recorded after extended periods of closure and decreased significantly when the estuary opened. This was due to two main factors: firstly prolonged exposure of vast areas of the lagoon and reed swamp, and secondly large scale substrate movement caused by floodwaters leaving the estuary.