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Keywords:

Environmental variables, Species composition, Abundance, zooplankton, Ethiopian Rift Valley Lakes, Abaya & Chamo

Abstract:

The major Ethiopian Rift Valley Lakes Abaya and Chamo are some of the most remarkable aquatic ecosystems in the region. However the demands to meet the needs of the rapidly growing population and provision to accommodate large numbers of domestic animals in form of space, shelter, food, water, health service and the waste disposal, put very high pressure on the natural resources of the basin. The main reason for the current ineffective monitoring and management plan is lack of comprehensive scientific data on the ecology of the lakes. Thus comparative limnological studies and primary productivity experiment using carbon 13 stable isotope enrichment were conducted in Aug-Sept 2012 in both lakes. Sampling of abiotic and biotic parameters were conducted from 8 sampling stations along a transect from inflow to outflow of each lake and were tentatively analyzed by means of a PCA (Fig.1). Axes 1 and 2 explained 66.2% of the total variation in environmental variables. Axis 1, representing the major axis of variation (51.8%), was positively associated with total suspended solid, turbidity, organic carbon and available nutrients. This gradient was negatively associated with conductivity, pH, Chl ‘a’, salinity total phosphorus, dissolved oxygen and total dissolved solids. RDA results indicated that the set of environmental variables had significant variation in Abaya and Chamo Lakes (F: 4.38; P = 0.013). Copepods (cyclopoid and calanoid) were the dominant zooplankton in both Abaya and Chamo Lakes. Copepods, cladocerans and rotifer constituted 65%, 23% and 12% in Abaya Lake, respectively; in Chamo Lake this was 70%, 14%, and 16%, respectively. From all copepod individuals, cyclopoid were dominant in both lakes: 72% and 100 % in Abaya and Chamo Lakes, respectively. Calanoids were not present in Chamo Lake during those months. The cladoceran populations in Abaya and Chamo Lakes were mainly comprised of Daphnia magna, Daphnia barbata, Ceriodaphnia cornuta, Ceriodaphnia laticaudata, Moina mongolica and Diaphanosoma australiensis. Macrothrix spinosa was also rarely present in the samples.The cladoceran community composition did not differ significantly between Abaya and Chamo Lakes, except for C. cornuta which was present only in Chamo Lake while C. laticaudata was present only in Abaya. Cladocerans of Chamo Lake were characterized by a pronounced dominance of C. corunta (51.2 %), followed by D. barbata (22%) and M.mongolica (17.3%), where as the Abaya Lake cladoceran community was dominated by M.mongolica (47.3%), followed by D. magna (20.8%), C. laticaudata (14.3%) and D. australiensis (13.5%). The actual zooplankton density in Chamo Lake (420 individuals L-1) was significantly higher than in Abaya Lake (13 individuals L-1). Similarly, the results of the carbon 13 stable isotope enrichment experiment showed that primary productivity of phytoplankton in Chamo Lake (32,5 μmol C L-1 h-1) was significantly higher than in Abaya Lake (7 μmol C L-1 h-1). Lake Abaya appears to be unproductive as compared to Chamo which is highly associated with the high turbidity and total suspended solid. The absence of the large copepod (calanoid) in Chamo Lake might be associated with fish predation and low turbidity compared to Lake Abaya. Moreover the D. magna identified in Chamo Lake were smaller in size compared to Abaya Lake. The results of this research provide better knowledge on the differences in the ecology of the two Lakes. Furthermore, our data provide abundance and density estimates for zooplankton, which serve as a baseline for future studies on the ecology of Abaya and Chamo Lakes.