Mzime Regina Ndebele-Murisa

Abstract: This paper provides a synthesis of current knowledge on phytoplankton production, seasonality, and stratification in tropical African lakes and considers the effects of nutrient enrichment and the potential impacts of climate warming on phytoplankton production and composition. Tropical African lakes are especially sensitive to climate warming as they experience wide fluctuations in the thermocline over a narrow range of high water temperatures. Recent climate warming has reduced phytoplankton biomass and production in the lakes. A decline in the production of palatable chlorophytes and an increase in cyanobacteria has led to reduced zooplankton production and a consequent decline in fish stocks, all of which can be associated with the elevated water temperatures. This indicates that even moderate climate warming may destabilise phytoplankton dynamics in tropical African lakes, thereby reducing water quality and food resources for planktivorous fish, with consequent negative impacts on human livelihoods.

Abstract: Primary production, plankton community and some limnological aspects of Cleveland dam were investigated for 15 weeks to ascertain the health of the reservoir’s ecological system. Sampling was conducted on one site near the dam wall on a weekly basis, at a 1 m interval depth profile. The sampling period (from September 2005 to January 2006) reflected the results of a hot, dry season (from September to November) and a hot, wet season (from December to January). On average, the pH, temperature, conductivity, turbidity, dissolved oxygen concentration, light penetration and photosynthetically active radiation decreased with depth. In contrast, the primary production, phytoplankton biomass, phytoplankton density, and the total nitrogen and total phosphorus concentrations did not conform to the same pattern, instead tending to increase at a depth of 4–5 m below the water surface, suggesting it was the optimum plankton habitat. There were no significant differences between depths for all parameters, suggesting that the reservoir, although exhibiting signs of stratification, was nevertheless too shallow to exhibit the varied differences down a vertical profile typical of tropical reservoirs, at least during the sampling period of this study. Although the present limnology of Cleveland Dam confirms its oligotrophic nature, the conditions prevailing in its catchment could ultimately lead to adverse effects in the reservoir. The ecological health of Cleveland Dam is discussed in regard to land use patterns in the catchment that pose threats as potential pollution sources. Cleveland Dam is possibly in danger of being polluted. There is a need to address the legal instruments currently in place for water management in order to curb such environmental degradation as they do not adequately address the problems faced.

Abstract: Lake Chivero is a eutrophic reservoir, initially constructed to supply drinking water to the City of Harare, Zimbabwe. Blooms of blue-green algae have been a problem in the lake for many years and concern has been expressed about the toxins produced by Microcystis spp. The concentrations of the toxin, microcystin-LR, produced in cultures of Microcystis aeruginosa from Lake Chivero, were investigated from March–April 2003. Microcystin-LR was found in algal cells cultured from the lake water in concentrations ranging between 18.02 and 22.48 μg L−1, with a mean concentration of 19.86 μg L−1. These concentrations are the highest recorded to date for the lake, raising concerns about the possible effects of the toxin on the health of people who are drinking the water. Based on these study results, there is a need to control eutrophication, reducing algal blooms in order to prevent their potentially detrimental effects from blue-green algal toxins produced under such conditions.