Zooplankton as ecosystem indicators and their effects on eutrophication in Lake Arekit (Ethiopia) – implication for freshwater habitat management

Abstract

This paper examines the zooplankton abundance and composition along with various environmental factors in Lake Arekit – a shallow freshwater habitat in Ethiopia’s rift valley system. Zooplankton samples and specific environmental data were collected by a seasonal campaign (rainy season: June to August) and dry season: March to May 2023) from three sites: inlet area (IA), pelagic area (PA), and macrophyte area (MA). In total, 11 zooplankton taxa from four different groups – rotifera (7 taxa), cladocera (2 taxa), copepod (1 taxa), and ostracoda (1 taxa) – were identified in Lake Arekit. Large-bodied copepoda and cladocera comprised 90% of the zooplankton density in the lake, but small-bodied rotifers dominated the zooplankton diversity. The most noticeable zooplankton in the lake were Mesocyclops aequatorialis, Daphnia barbata, and Cypridopsis vidua. Seasonal difference in the total zooplankton abundance was evident (p < 0.01): being high during the rainy season which was accompanied by an increase in the lake’s water temperature and nutrient levels. At MA, where D. barbata predominated, chlorophyll a was relatively low. Chlorophyll a had a strong negative correlation with the abundance of grazing zooplankton, especially with D. barbata (r = −0.563) and C. vidua (r = −0.518). The zooplankton composition and abundance of Lake Arekit reveal a low Shannon diversity index (H’) (1.03) and a predominance of a few species, indicating heavy contamination of the lake’s water. Moreover, Lake Arekit was determined to be hypertrophic (TSI > 70) using Carlson’s trophic index (TSI), which took into account the lake’s water transparency, total phosphorus, and chlorophyll a. Our results indicated that high densities of Daphnia should be the goal of biomanipulation since they can achieve filtration potentials high enough to enhance the water’s clarity in hypereutrophic lake. Even though high D. barbata concentrations cannot address the underlying cause of eutrophication, grazing can assist by lengthening the clear-water stages in a hypertrophic lake – Lake Arekit. The sources of pollution for the lake are the discharge of wastewater from nearby water bottling companies and possibly high nutrient levels from the lake’s geological history. Stopping the discharge of wastewater from the nearby commercial industries (the major source of contaminant) into heavily washed open areas and establishing a buffer zone may help manage the water quality of Lake Arekit.