Spatio-temporal variations in underwater light climate, thermal and chemical characteristics of the tropical soda lake, Lake Shala, Ethiopia

Abstract

Although soda lakes are highly sensitive to environmental changes and suited for the study of vital ecological processes owing to their unique biological and physicochemical properties and closed nature, there are limited studies on and knowledge of their basic ecology. In this study, temporal and vertical fluctuations of underwater light climate, thermal and chemical characteristics of the soda lake Lake Shala were studied using data collected on 11 occasions over an annual cycle. Measured parameters of underwater light climate depicted light-limited conditions with slight temporal variations, which are attributable to non-biological turbidity and high water column mixing depth. Its water column exhibited superficial thermal stratification with turbulently mixing and well-oxygenated conditions. Alkalinity, pH and conductivity were high but exhibited small temporal and vertical variations. These parameters increased slightly during the rainy season, possibly due to saline inflows associated with precipitation. Most of the major algal nutrients showed small temporal and vertical variations. NO3 − and NH3 in the euphotic zone were often very low or undetectable, while soluble reactive phosphate and total phosphorus were very high throughout the study period. The concentration of SiO2 was remarkably low, which was probably associated with biological uptake and some chemical processes within the lake. In general, physicochemical parameters of Lake Shala exhibited small temporal and vertical variations despite the current threat of environmental degradation, possibly because of the high resilience of the lake associated with its low productivity and voluminous nature.