Primary production and other limnological aspects of Cleveland Dam, Harare, Zimbabwe

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.