Impacts of land and water use on plankton diversity and water quality in small man-made reservoirs in the Limpopo basin, Zimbabwe: A preliminary investigation

This paper reports on a study carried out from February to April 2005 in the southern part of Zimbabwe in the Mzingwane catchment, Limpopo basin to investigate the impacts of land and water use on the water quality and ecosystem health of eight small man-made reservoirs. Four of the reservoirs of were located in communal lands while the remaining four were located in the National Park Estates, considered pristine. Plankton community structure was identified in terms of abundance and diversity as an indirect assessment of water quality and ecosystem health. In addition, phosphorus, nitrogen, pH, transparency, electric conductivity and hardness were analysed. The results obtained indicate that a significant difference in abundance of phytoplankton groups was found between the communal lands and the National Park Estates (P < 0.01). Though the highest phytoplankton abundance was observed in April, February showed the highest number of taxa (highest diversity). Chlorophytes was the major group in both periods with 29 genera in February and 20 in April followed by Diatoms with 17 genera in February and 12 in April. The zooplankton community was less diverse and less abundant and did not show any seasonality pattern. Phosphorus (0.022 ± 0.037 mg/l) and nitrogen (0.101 ± 0.027 mg/l) had similar trends in the study area during the study period. Transparency of water was very low (ca. 27 cm secchi depth) in 75% of the reservoirs with communal lands’ reservoirs having a whitish colour, likely reducing light penetration and therefore photosynthetic potential. Evidence from the study indicates that, at this time, activities in the communal lands are not significantly impacting the ecosystem health of reservoirs, as water quality characteristics and plankton diversity on communal lands were not significantly different from the pristine reservoirs in National Park. However, water managers are urged to continuously monitor the changes in land and water uses around these multipurpose reservoirs in order to prevent possible detrimental land and water uses that might occur in the future.