Stratification regimes and thermodynamic modelling of a subtropical African reservoir

The stratification of a lentic water system is an important factor regulating its biotic activities and processes. The present study investigated changes in temperature and oxygen stratification regimes in Mazvikadei Reservoir 27 years after the first study was conducted during its filling phase. The FLake‐Global model was also tested as a predictive tool for the first time to analyse annual limnological trends in a subtropical reservoir. The lake‐modelling tool FLake‐Global enables an instantaneous estimation of the seasonal cycle of temperature and mixing conditions in shallow freshwater lakes around the world. The results of the present study illustrate that the reservoir was weakly stratified, with the overall thermal stratification patterns somewhat similar to those recorded during the filling phase (1992). The oxygen stratification patterns observed in the present study correspond well to those of thermal stratification. The depth‐integrated oxygen profiles for both the deep and shallow sampling sites exhibited the same pattern, with higher concentrations from July to September. The water column was well oxygenated, indicating the reservoir has matured since its filling phase in 1992, when there was significant hypolimnetic deoxygenation. The FLake‐Global model exhibited good results in predicting annual trends, with the results also indicating no differences in stratification levels in the reservoir for the current and Masundire (1992) filling phase studies. The model predicted lower surface and bottom temperatures, however, compared to those measured in the reservoir. There was lowering of the mixed layer depth from the hot‐wet (November–April) months through the cool‐dry (May–August) season. Latent heat fluxes agreed with the net longwave (LW) predictions for Mazvikadei Reservoir, which exhibited an overall net cooling effect. The stratification has not changed significantly since the previous study in 1988. The FLake‐Global model proved a useful predictive tool when tested in a subtropical system.     

Stratification,diel and seasonal energy transfers in Malilangwe Reservoir in the south‐eastern lowveld of Zimbabwe

This study provides a 9‐month record of Malilangwe Reservoir stratification and diel and seasonal energy transfers between February and October 2011. The reservoir was not strongly stratified during the hot‐wet season (February and March), exhibiting low dissolved oxygen concentrations of <2 mg DO L^?1 in the bottom layers (<6 m depth). During the cool‐dry season, when both the air temperature and solar radiation input decreased, the water temperatures in the epilimnion and across the thermocline decreased, and the water column was very weakly stratified. In this state, the reservoir could easily be mixed, even by light winds, resulting in overturning, thereby giving the bottom water an opportunity to become aerated, as observed during the cool‐dry season. The stability was greatest when the water level was high (during midday), while it was considerably lower during the predawn hours. The calculated annual Birgean heat budget for the reservoir was 110 MJ m^?2 year^?1 for February to October 2011. The diurnal Birgean heat budget was consistently associated with heat loss over a 24 hour period in all seasons, mostly between 0400 and 1000 h. Spearman's correlation suggests that the DO was significantly correlated with stability for the three study months (February r = 0.69, P < 0.05; June r = ?0.76, P < 0.001; October r = 0.96, P < 0.001). The results of this study suggests that warming temperatures (warmer months or seasons) will result in high Birgean heat budgets, which could potentially trigger some important changes in plankton population dynamics in small reservoir, with severe water quality implications. The nutrient concentrations varied throughout the seasons.