Could phosphorus concentrations increase with the development of cage culture in Lake Kariba, Zimbabwe?

Abstract

A comparison between historical and current data was done to determine whether the concentrations of total phosphorus, orthophosphate and chlorophyll a, and Secchi disc transparency, have changed in the Sanyati Basin of Lake Kariba following the commencement of cage culture of fish in 1998. Total phosphorus concentration is markedly elevated in comparison to historical offshore data, while offshore orthophosphate concentration is lower than when the lake was eutrophic. Chlorophyll a concentrations were similar to the mean value of 4 µg l−1 reported from open water, but lower than the mean of 22 µg l−1 reported from inshore waters. Current values of Secchi disc transparency are lower than the historical records. To date, cage aquaculture seems not to have changed the trophic status of Sanyati Basin, and any nutrient enrichment it caused may have been mitigated by nutrient losses, through both outflow at the dam and fish harvesting.