Seasonal and diurnal stratification in two small Zimbabwean reservoirs
Seasonal and diurnal stratification patterns were studied in two small Zimbabwe reservoirs. The water level of the upper reservoir fluctuated more than that of the lower one and theoretical water retention times were 9 days and 3.6 days, respectively. Rainfall is seasonal but water demand for irrigation and potable water persist throughout the year. There was a distinct cycle of seasonal thermal stratification with associated deoxygenation, but this was not as stable or pronounced as in some larger Zimbabwean reservoirs. Weaker thermal stratification was established during the winter. Seasonal trends were related to changes in incoming solar radiation and the corresponding changes in air temperatures. Diurnal cycles of thermal stratification and oxygen occurred in both reservoirs throughout the year, but were less established during the winter months. The frequent mixing recorded is a significant finding because, if there is mature sediment in such reservoirs, there may be a rapid exchange of nutrients between sediments and the water, leading to high overall productivity. The shallow nature of small reservoirs and the considerable fluctuations in their water levels makes them more vulnerable to external fluxes such as daily changes in incoming short-wave solar radiation and wind runs, and hence the prevalence of diel stratification regimes. Consequently, small reservoirs are characterised by low thermal stability as compared to larger, deeper reservoirs. Diurnal heat fluxes were observed in both reservoirs accounting for their weak thermal stability. Birgean energy budgets for these reservoirs were small (11.6 and 54.3 MJ m−2 yr−1 for the upper and lower reservoirs, respectively). Reasons for this are that their small size severely limits their capacity to capture and retain heat energy and, although not quantified in this study, high evaporation rates contribute to low energy budgets in tropical waterbodies as compared to temperate ones.