Abstract

HARDING, WR  and  HART, RC. Food-web structure in the hypertrophic Rietvlei Dam based on stable isotope analysis: Specific and general implications for reservoir biomanipulation. Water SA [online]. 2013, vol.39, n.5, pp.611-614. ISSN 1816-7950.

Fish predation on zooplankton is the basic foundation for top-down biomanipulation of lacustrine ecosystems. To test this premise, we determined stable isotope (SI) values (δ¹³C and δ¹⁵N) of representative samples of major planktonic (phy-toplankton, zooplankton), benthic (submerged macrophytes and associated epiphytes, benthic macro-invertebrates) and nektonic (fish) food-web components, collected from 3 to 7 shallow inshore locations (with additional plankton samples at 1 or 2 deep offshore sites) in Rietvlei Dam over a period of 30 months. The resulting δ¹³C values did not indicate significant consumption of zooplankton by fish, while the δ¹⁵N values for fish confirmed their wide trophic separation from zooplankton. Instead, SI values indicated that fish relied mostly on food resources of benthic origin (through direct consumption or piscivory). The SI signatures of individual fish species were consistent with their known feeding habits. The lack of trophic couplings between zooplankton and fish accords with previous gut content analyses of fish and analyses of zooplankton abundance and size structure in hypertrophic reservoirs. Marginal utilisation of zooplankton by indigenous reservoir fish is attributable to their native origin as riverine species unaccustomed to feeding on zooplankton. These findings indicate that top-down biomanipulation is unlikely to be effective as a management tool in eutrophic South African reservoirs. Primary producer components exhibited surprisingly wide and unsystematic temporal fluctuations in both δ¹³C and δ¹⁵N values; some potential contributory factors are considered. Changes in phytoplankton δ¹³C values were broadly tracked by zoo-plankton - their nominal consumers. Some questions arising from the study, and some apparently anomalous findings are identified and discussed.

Keywords : δ¹³C; δ¹⁵N; trophic structure; plankton; benthos; fish; warm-water ecosystems; South Africa.

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