Zooplankton biomass to chlorophyll ratios in relation to trophic status within and between ten South African reservoirs: causal inferences, and implications for biomanipulation

Abstract

Rising eutrophication in South African reservoirs is of major concern, leading to the consideration of top-down biomanipu lation as a management option – reducing zooplankton-eating fish to sustain zooplankton grazing pressure and thus restrict autotrophic plankton that proliferate with nutrient increases. The biomass ratio of zooplankton to phytoplankton (ZB/PB) is used as an index of the likely value of biomanipulation to achieve this outcome, but values have not been explored for South African systems. Using chlorophyll (Chl) as a surrogate for PB, available ZB/Chl data are assembled for the first time for ten reservoirs of three types (minerally-turbid systems, oligo/mesotrophic clear water systems, and eutrophic/hypertrophic systems), and the results are discussed in relation to a generalised conceptual model proposed. With the exception of one minerally-turbid system, ZB/Chl values decline quasi-exponentially with rising chlorophyll within individual reservoirs.

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