Using data from southeast Zimbabwe intensive agricultural zone we investigated diurnal changes in the Runde River to discover the variations that can occur in the control and test sites of agricultural development. This should predict how the aquatic ecosystem responds when ’stressed' by human activities, such as runoff from the land. Diurnal changes were studied at a resolution of 2 h intervals between 0500 and 0600 h in the control sites and test sites of an area with agricultural development in July 2005 and 2006. The observations gave same results. The diurnal fluctuations are characterized by sharp rises and gradual declines each day. The maximal and minimal points in diurnal changes can be useful biogeochemical signatures for describing the fluvial conditions of a river in a river basin. Significant associations (R2 = 0.64, y = -1.30 x +24.93, P = 0.03) between measured dissolved oxygen concentration (mg/l) and temperature (°C) were demonstrated during the daytime only in the site above the effluent outfall. Insignificant associations (R2 = 0.04, y = -032 x +13.94, P = 0.75) between measured dissolved oxygen concentration (mg/l) and temperature (°C) were demonstrated during the nighttime in the site above effluent outfall. This suggests that there was more dissolved oxygen concentration added during the day than at night at the site above the outfall. The coefficients of determination between dissolved oxygen concentration (mg/l) and temperature (°C) for the daytime and nighttime were R2 = 0.35 and R2 = 0.29, respectively, for site below the outfall. There were almost identical regression lines demonstrated during the daytime and nighttime below the outfall, an indication that oxygen demanding wastes deplete dissolved oxygen concentration. No significant associations were found between dissolved oxygen and temperature during the daytime and nighttime below the outfall. This study indicates that fluvial conditions characterized by fluctuations in diurnal changes may be a strong signature to catchment activities but need to be looked at in conjunction with other fluvial measures such as flow rate, depth and channel discharge that may also be naturally challenging to aquatic organisms.
Key words: Zimbabwe, savanna semi-arid lowland river, diurnal changes, aquatic organisms.
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