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a b s t r a c tSoda 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, CO 3 2− :HCO 3 − was high (>1), suggesting lower concentration of HCO 3 − . Most of the major algal nutrients showed considerable inter-monthly and vertical variations. NO 3 − and NH 3 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 NH 3 and SRP with depth is attributable to internal loading, which is enhanced by increased microbial activities and largely anoxic water column. The concentration of SiO 2 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.

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. NO 3 -and NH 3 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 SiO 2 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.

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The vital ecological functions of the East African soda lakes are much dependent on Arthrospira, which forms a natural nearly monoalgal populations and serves as the main diet for the huge flocks of flamingos in the present study lake Chitu (Ethiopia). This study investigated algal taxonomic composition and biomass, and the seasonal variability in the abundance of Arthrospira fusiformis in response to some environmental drivers (e.g. rainfall, salinity and nutrients) using samples collected over an annual cycle. The algal community was composed of a few taxa (15 species), with exclusive dominance of A. fusiformis. Chlorophyll-a (Chl-a) and dry weight, and abundance of A. fusiformis were notably high and exhibited seasonal variations, with significantly (P<0.05) higher levels of Chl-a and dry weight during the rainy season. The observed strong correlations of algal abundance and biomass with rainfall (positively) and alkalinity-salinity (negatively), probably suggest that hydrological control of the salinity is the major driving force for the seasonal variability of A. fusiformis in the lake. Further hydrological modifications that enhance salinisation may greatly affect A. fusiformis thereby causing instability of the flamingos with eventual impairment of the ecosystem values of the lake.