Limnological Studies on Lake Tinishu Abaya, Ethiopia
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Date
2019-06-02
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Addis Ababa University
Abstract
Lake Tinishu Abaya, hereafter referred to as LTA, is a small-sized inland water body in the rift valley system of Ethiopia. LTA was stocked with Tilapia fish (Oreochromis niloticus) in 1997 to enhance fishery and provide animal protein source to the local community. This study designed to investigate the limnological features (physical, chemical, and biological) of the lake and determine the ecological productivity, water quality, and its potential service for users in the neighborhood and the nation at large. Moreover, the baseline information from the study can be used by policymakers to design strategies for sustainable use of LTA.
All the data for this study were collected and analyzed using standardized sampling techniques and methods. The water samples for the analyses of physicochemical parameters, phytoplankton, and zooplankton were collected monthly between January and December 2016 from two selected sampling sites (open water and shore area). Benthic macroinvertebrates were collected from March to August 2017 from five sampling sites. For analysis of fish gut content and related studies, O. niloticus fish samples were collected from March to May and July to September in 2017.
The results of the various physicochemical parameters suggested that LTA was well oxygenated, slightly warm, and alkaline. From the values of conductivity (as a proxy to salinity) the lake can be classified as a freshwater body compared some other rift valley lakes of Ethiopia and TDS values. TDS of LTA was always less than 3000 mgL-1 indicating freshwater system. The water of LTA was turbid, low water clarity, shallow euphotic area, and hypertrophic state with nutrient enrichment.
In LTA, 37 phytoplankton taxa belonging to six groups were found. Bacillariophyceae (40%) were the most diversified phytoplankton followed by Chlorophyceae (30%), Cyanobacteria (19%), Euglenophyceae (5%), Dinophyceae (3%), and Cryptophyceae (3%). In this study, 24 zooplankton taxa comprising of rotifers, cladocerans, and copepods were recorded. The abundance of zooplankton was dominated by copepods (54%) followed by rotifers (40%). The overall mean total standing biomass of zooplankton was 230.87 in open water and 164.2 μg L-1 at shore area, and it was broadly dominated (94%) by the microcrustaceans (copepods & cladocerans).
The photosynthetic productivity of phytoplankton of LTA was determined in the open water, and the results showed that the rate of photosynthesis (Amax) ranged from 0.62 g C m-3 h-1 to 2.02 g C m-3 h-1. The hourly integral photosynthetic production (ΣA) and pattern of variation for the daily integral rates of photosynthesis (ΣΣA) ranged from 0.247 g C m-2 h-1 to 1.022 g C m-2 h-1 and 5.43 g C m-2 d-1 to 9.194 g C m-2 d-1, respectively. Because of the number of phytoplankton and zooplankton taxa, their abundance, and
biomass, and considering marked values of photosynthetic productivity it was concluded that LTA is biologically productive aquatic resource to support fish and other aquatic organisms.
The ecological condition of LTA was assessed using benthic macroinvertebrates as well, and a total of 5735 benthos specimens comprising of 23 taxa were collected from all the study sites. There was a spatial effect on the distribution of benthic individuals (ANOVA; p <0.05). It was high at Dacha riverside (n=2089) followed by Bobodo riverside (n=1145), Reference site (n= 963), Badober riverside (n= 859), and Main fish landing site (n=679). The Hemiptera family were the predominant macroinvertebrates that contributed the largest number (n=2546) of the total samples followed by Diptera (n=878), Coleoptera (n=835, 14.56%) and Gastropods (n=631). The majority (about 70%) of the benthic communities of LTA comprised pollution tolerant species as compared to pollution sensitive ones. This indicated the existence of organic pollution, and thus LTA is undergoing environmental stresses.
The diet and other related aspects of the stocked O. niloticus were also assessed. The relationship between the total length and standard length of O. nilotucus in LTA was linear and significant (R² =0.962, TL=0.1456SL+1.8088). The relationship between total length and the total weight of O. niloticus, on the other hand, was curvilinear with a strong relationship (R²= 0.9848, TW =0.0194 TL2.9876). The slope of the regression (b) was 2.9876, closer to the isometric growth value (b=3) of fish. The Fulton´s Condition Factor (FCF) of O. niloticus ranged from 0.96 to 3.51 (mean= 1.88). The well being of O. niloticus fish showed that the majority of the population in LTA were in good condition (FCF >1). Phytoplankton, detritus, zooplankton, and macrophytes were the most important food items while insects, nematodes, ostracods, and fish scales made up minor portions of the diet of O. niloticus in LTA. The importance of phytoplankton, macrophytes, and detritus increased with increasing fish-size and the significance of zooplankton, insects, and other animal origin food items declined with increasing fish size.
In a nutshell, LTA is a productive lake and an important aquatic resource for the surrounding area. But the lake and its watershed are facing many threats from intensive human activity. The major problems affecting LTA include excessive water abstraction, extensive shoreline modification, sedimentation, eutrophication, water pollution due to organic waste from agricultural area and overfishing. At the same time, efforts to ensure the health and normal functioning of this productive ecosystem are negligible. Thus, management solutions should be developed to avert the current and continuing degradation of LTA and its environs. Some of the actions that should be taken, but not limited to, are: demarcating a buffer zone, restoration of riparian habitats, regulating the excessive water abstraction, controlling mass fish catching, and empowering the local community to protect and conserve this aquatic resource.
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Limnological, Studies on Lake Tinishu Abaya, Ethiopia