A study of the limnology and ecology of Mazvikadei Reservoir

Abstract

An assessment of the limnology of Mazvikadei reservoir was carried out from May to October 2015. The main objective of the study was to determine whether limnology of the reservoir has changed since the filling phase. The reservoir has matured into an oligotrophic status characterized by low algal biomass, low concentrations of P and N, high Secchi disk clarity and low chlorophyll a concentration. Mazvikadei is well oxygenated with dissolved oxygen concentration of up to 7.9 mgl-1 and a slightly alkaline pH (8.54). During the entire sampling period the reservoir was not strongly stratified except in July when there was stratification in the first 5 m. The average conductivity in the reservoir has increased to 263.2 μScm-1 and the transparency has increased to 4.6 m. The improvement in transparency is linked to change in the trophic status of the reservoir. The phytoplankton community in the reservoir comprised of six main groups namely Dinophyta, Euglenophyta, Chlorophyta, Bacillariophyta, Desmids and Cyanophyta. Bacillariophyta and Dinophyta dominated in the cool dry season and Chlorophyta in the hot dry season. Cyanophyta had less representation although it occurred in small numbers right through the sampling period with the highest numbers being observed in May. Euglenophyta and Desmids were most abundant in May. Fifty four species were recorded.Species richness was highest in May at the onset of the cool dry season in response to high nutrient concentrations. Evenness decreased during the cool dry season. Dominance with respect to species representation was Chlorophyta (21 species), Bacillariophyta (14 species) and Cyanophyta (7 species). Phytoplankton abundance and composition were significantly correlated with temperature, nitrates and total nitrogen. The major groups of zooplankton recorded were the Cladocera, Copepoda and Rotifera. Cladocera were the most dominant although rotifers and copepods were well represented. Cladocerans became most abundant during the cold dry season (June and July) when they assumed 75 % dominance. Rotifers and copepods dominated during the hot dry season. Ostracods were only observed in September in the hot dry season. Species richness has increased to 19, being represented by 6 rotifers, 6 cladocerans, 6 copepods and 1 ostracod. The zooplankton community was highly influenced by the amount of reactive phosphorus and phytoplankton abundance. The relationship between reflectance and ammonia, nitrates and reactive phosphorus was not stable over time. The best time to use remote sensing for water quality monitoring was in the cool dry season where processes in the reservoir are relatively stable. Ammonia and nitrates could best be approximated by the near infra red and red band respectively. No relationship was found between reactive phosphorus and reflectance. The study showed that Mazvikadei reservoir has matured and assumed the physico-chemical characteristics and plankton community typical of an oligotrophic lake. Due to its oligotrophic status, remote sensing will not be effective in water quality monitoring.