Underwater light climate, thermal and chemical characteristics of the tropical soda lake Chitu, Ethiopia: Spatio-temporal variations

Soda lakes are known for their extreme environmental conditions and unique assemblage of biota and provide great ecological and economic values. Although they are highly sensitive to environmental changes, soda lakes are among the least frequently studied inland water bodies. In this study, temporal and spatial (vertical) patterns of underwater light climate, thermal and chemical characteristics of a little known soda were studied. Parameters of underwater light climate showed light-limited conditions with more marked inter-monthly variations, associated with the accumulation of Arthrospira biomass in the shallow trophogenic zone. Water column conditions indicated superficial thermal stratification (0-3 m depths) with weak temperature gradients and weak mixing pattern. Dissolved oxygen (DO) varied considerably among months, ranging from subsurface supersaturation on certain occasions to persistent deoxygenation of most of the water column on other occasions, with the variations being attributable to the high productivity, high metabolic rates of microbes and weak vertical mixing. Alkalinity, conductivity and pH were generally high with moderate temporal and vertical variations, which were presumably associated with precipitation, evaporation and high algal biomass. In most cases, CO32−:HCO3− was high (>1), suggesting lower concentration of HCO3−. Most of the major algal nutrients showed considerable inter-monthly and vertical variations. NO3− and NH3 in the euphotic zone were often very low or undetectable while soluble reactive phosphate (SRP) and total phosphorus (TP) were considerably high throughout the study period. The observed dramatic increase in the levels of NH3 and SRP with depth is attributable to internal loading, which is enhanced by increased microbial activities and largely anoxic water column. The concentration of SiO2 was remarkably low, which was probably due to organic matter accumulation within the lake that tends to preclude internal loading. In general, the notable temporal and vertical variations in physicochemical parameters, associated chiefly with the lake's productivity, microbial activity, anoxic water column and meteorological conditions, probably suggest that Lake Chitu is sensitive to perturbations and that any environmental changes occurring in the lake are likely to affect the key planktonic alga (Arthrospira) and its ecosystem values.