Abstract
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 (δC and δN) of representative samples of major planktonic (phytoplankton, 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 δC values did not indicate significant consumption of zooplankton by fish, while the δN 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 δC and δN values; some potential contributory factors are considered. Changes in phytoplankton δC values were broadly tracked by zooplankton – their nominal consumers. Some questions arising from the study, and some apparently anomalous findings are identified and discussed. Keywords : δC, δN, trophic structure, plankton, benthos, fish, warm-water ecosystems, South Africa
Highlights
Nutrient pollution or cultural eutrophication is recognised as posing the major threat to water quality of inland fresh and coastal saline waters globally (Smith and Schindler, 2009)
Wide fluctuations were evident in both isotopes (e.g. 12.5‰ and 17.5‰ for phytoplankton δ13C and δ15N)
The regularity and consistency of phytoplankton sampling at inshore and offshore sites permitted an examination of spatial variation in stable isotope values of this autotrophic component
Summary
Nutrient pollution or cultural eutrophication is recognised as posing the major threat to water quality of inland fresh and coastal saline waters globally (Smith and Schindler, 2009) This well-known, serious and intransigent problem is symptomatically manifest in the excessive growth of primary producers released from intrinsic natural nutrient limitation, and generates a cascade of qualitative and quantitative changes in food-web structure and ecosystem functioning (e.g. Carpenter, 2005). At least 35% of water stored in such reservoirs is already classed as eutrophic or hypertrophic, with an additional 30% bordering on the eutrophic status (Harding et al, 2009) Since this nutrient pollution is largely attributable to wastewater effluents (Harding, 2008), the problem can be constrained by enforcing appropriate wastewater treatment and effluent discharge restrictions. Such ‘preventative’ treatment has been largely ineffectual (e.g. DWA, 2009), leading to proposals to implement ‘curative’ measures – notably ‘biomanipulation’ (Shapiro et al, 1975) – to rehabilitate highly eutrophic systems such as Hartbeespoort Dam (Anon. 2004a, 2004b; Harding et al, 2004)
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