Stratification, diel and seasonal energy transfers in M alilangwe R eservoir in the south‐eastern lowveld of Z imbabwe

Abstract

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.