Water quality and plankton dynamics in two small dams in Zimbabwe

Abstract

Two small dams on the Munwahuku River was investigated in relation to the influence of hydrology on their water chemistry over the period January 2000–August 2001. The water level of the upper dam fluctuated more than that of the lower dam and their theoretical water retention times were nine days and 3.6 days, respectively. Seasonal fluctuations in conductivity, TDS, TSS and transparency were due to effects of the first rains, while increases in BOD and COD suggested increased organic matter at low water levels. There were no dramatic seasonal variations in pH, alkalinity or nutrients, although total nitrogen was higher in the upper dam. The N:P ratios ranged from 0.57–3.75 in the upper dam and 0.92–14 in the lower dam and generally reflect the high levels of phosphorus in the dams. This could make them susceptible to colonisation by Cyanophyta. There were significant differences (p < 0.05) between the dams in total phosphorus, reactive phosphorus, nitrites and chlorophyll a. Small differences observed in the water quality of the two dams could be due to short water residence times. Extremes in water quality conditions were experienced during the dry season, when water levels were low, and during the rainy season when levels were higher. Peaks in phytoplankton and zooplankton abundance occurred during summer in both dams, and differences observed between the plankton community of the period January–October 2000 and that of November 2000–August 2001 were unexpected. The changes in the plankton communities, especially in the lower dam, were remarkable. There were no significant differences in plankton abundances between the dams. Phytoplankton dynamics were weakly associated with environmental factors in both dams and, apart from predation by fish and grazing by zooplankton, washouts during the rainy season and unstable stratification regimes were considered to be contributory factors. Canonical Correspondence Analysis showed that temperature and water transparency (also linked with suspended solids) were important variables in zooplankton dynamics in both dams. Hydrological factors, such as water retention time, floods and water levels, are key determinants of plankton communities in small dams.African Journal of Aquatic Science 2007, 32(2): 139–151