First results on bathymetry, stratification and physicochemical limnology of a small tropical African reservoir (Malilangwe, Zimbabwe)

Abstract

The study provides a 9-month record of Malilangwe Reservoir water chemistry periodicity, for the period between February and October 2011. Malilangwe Reservoir is a small (211 ha), shallow (mean depth 4.54 m) reservoir situated in the south-eastern lowveld of Zimbabwe. The reservoir has not spilled in nearly 11 years, which makes it a unique system as most reservoirs of comparable size spill annually. This is the first bathymetric and limnological study of the reservoir where the morphology and physicochemical quality of the water body were examined. The reservoir was not strongly stratified during the hot-wet and hot-dry season with oxygen depletion of < 2 mg·ℓ^-1 DO being observed in the bottom layers (<6 m depth). Nutrient concentrations varied throughout the seasons. The reservoir exhibited marked seasonal fluctuations in water level, which decreased by over 149 cm between February and October. The N:P ratio rose to as high as 10.9 and generally reflected high levels of phosphorus in the reservoir. There were significant differences (p<0.05) in Secchi depth transparency between the study sites. Differences observed in water quality were due to water level fluctuations, with poor water quality conditions being experienced during the hot-dry season and the cool-dry season when water levels were low. The reservoir was classified as being mesotrophic. Therefore, there is a risk of eutrophication, especially since the reservoir is currently merely a sink for nutrients.

Keywords: Bathymetry, limnology, water level fluctuation, stratification, Malilangwe