A comparative study of the limnology of two small dams in Capricorn District, Limpopo Province, South Africa

Abstract

The aim of this study was to provide baseline limnology data that can be used in the management of small dams, Molepo Dam and Hout River Dam. Physico-chemical parameters were assessed to determine the water quality and the effect of water level fluctuations on water quality. Water temperature, pH, dissolved oxygen, turbidity, electrical conductivity, nitrate, total phosphorus, total nitrogen and chlorophyll-a were assessed during the dry season (May-Oct) and wet season (Nov-Apr). Physico chemical parameters showed seasonal variations with the exception of pH. The mean total phosphorus (1.06 and 0.98 mg/L), total nitrogen (1.27 and 1.56 mg/L) and chlorophyll-a (2.97 and 3.32 µg/L) were higher in the dry season than wet season in Molepo Dam and Hout River Dam respectively. Water level fluctuations had a significant effect on total nitrogen, total phosphorus and turbidity in Molepo Dam and total nitrogen in Hout River Dam. Plankton were used as indicators of water quality. Phytoplankton and zooplankton were sampled using nets of 71 µm and 132 µm mesh respectively and identified to genus and species at both Molepo Dam and Hout River Dam. Individual species, number per litre for phytoplankton groups that contributed significantly in terms of abundance were Microcystis aeruginosa and Oscillatoria sp. in both Molepo Dam and Hout River Dam during the dry season months. The Shannon-Wiener diversity index was high during the wet season in these small dams. The abundance of Cyanophyta was greatest during the dry season followed by Desmids in Molepo Dam and Hout River Dam. During the wet season, Bacillariophyta were not recorded for Hout River Dam. The highest composition for Chlorophyta (15.4%) in Molepo Dam was recorded during the wet season months. Phytoplankton abundance was highest in Molepo Dam as compared to Hout River Dam throughout the sampling period. Zooplankton was dominated by Cladocerans (Bosmina longirostris) in both small dams. This study showed that plankton can be used as indicators of water quality. Zooplankton communities were dominated by Bosmina longirostris which was present throughout the sampling period possibly indicating clear water in Molepo Dam and Hout River Dam. Copepoda were sub-dominant phyla mainly represented by Cycloid sp. which are also indicators of clean water. The study determined the abundance and composition of fish species and growth parameters of the most abundant fish species (Oreochromis mossambicus) in the two small dams. Scales were successfully used for the age determination of O. mossambicus. The rings on the scales were validated as being true annuli by marginal increment analysis which indicated that the distance between the last annuli and the scale edge was shortest in the wet season months in Molepo Dam and Hout River Dam. This is the time in which annulus formed. Annulus formation of O. mossambicus at these small dams coincided with the onset of increased water temperature and rainfall (increased water levels and increased water inflow into the dams) in this region. Furthermore, the growth performance index (ϕ) of male O. mossambicus was higher than that of female O. mossambicus in both Molepo Dam and Hout River Dam. The study provides the first record of limnology baseline data of Molepo Dam and Hout River Dam