The ecological health and biodiversity of the Mukuvisi River system were investigated during this brief study to assess the functioning of the natural river system, utilizing biological monitoring. The study covered a period of 5 weeks from 15 September to 12 October 2005, during which a weekly sampling routine was conducted, and the effects of pollution on the river were investigated. The pH, dissolved oxygen concentration, temperature and electrical conductivity, as well as the two major nutrients in the water (phosphorus; nitrogen) were measured at four sites along the Mukuvisi River. Macroinvertebrate species also were identified and enumerated at each site, using the South African Scoring System version 4 (SASS4) to assess the biological diversity and to evaluate pollution levels at each site. Site 2 was the most polluted, while site 4 was the cleanest site. It was concluded that, although the river receives pollutants along its course, it also has a self-purification capacity that can be enhanced by remedial measures against pollution. Possible remedial actions are discussed as recommendations in this study.

The present study examined spatial, seasonal and depth variations in phytoplankton biomass and primary production (PP), compared with those reported for other tropical African lakes, to determine whether or not measured phytoplankton changes might be linked to climate warming. The biomass of three major phytoplankton classes (Cyanophyceae; Chlorophyceae; Bacillariophyceae) and net PP were measured during the midwinter and midsummer at six different depths at 35 sampling sites distributed across the lake’s five basins. A more rigorous sampling regime was used in the fifth basin, with phytoplankton biomass and PP rates measured every second month over a 24 month period at six different depths at ten sampling sites located in riverine and lacustrine waters. Cyanophyceae, which displayed a gradient of decreasing biomass from Basins 2 to 5, contributed 69% of the total phytoplankton biomass in the lake’s five basins during summer. This percentage was approximately four times greater than that contributed by the Bacillariophyceae and about ten times greater than that contributed by the Chlorophyceae. During winter, Bacillariophyceae biomass was equivalent to that of the Cyanophyceae, but displayed an opposing gradient of increasing biomass from Basins 1 to 3, with a subsequent biomass decline from Basins 3 to 5. Chlorophyceae exhibited no distinct biomass gradient across the five lake basins, being undetectable during winter. The biomass of all three phytoplankton classes and the net PP varied in magnitude and direction monthly between the lacustrine and riverine waters, with increasing water depth and with no distinct seasonal patterns being evident. The monthly variations in biomass were related to the thermal stratification cycle, hydrological gradients and the extent of water mixing, being similar to those reported for other tropical African lakes. It is noteworthy that total phytoplankton biomass and PP in Lake Kariba have declined by about 95% and 50%, respectively, since the 1980s. These declines correspond to an upward shift in the depth of the thermocline, associated with an average temperature increase of 1.9 °C and a 50% reduction in the depth of the euphotic zone, since the 1960s.