Assessment of the impact of anthropogenic activities on water quality, biodiversity and livelihood in Lake Tana, Northwestern Ethiopia

Abstract

Lake Tana is a biodiversity and natural reservoir for fresh water supply contributing significantly to the economy of Ethiopia and downstream recipient countries, namely: Sudan and Egypt. The Lake Tana Ecosystem provides a variety of goods and services such as: provisioning, regulating, amenity and supporting services. These services are affected by high human activities which threaten the water quality and biodiversity of the lake. Hence, this study aims to assess the impact of human activities on water quality, biodiversity and livelihood of Lake Tana and its shore sides.To assess the impact of anthropogenic activities of Lake Tana; physicochemical parameters, macroinvertebrates, macrophytes and livelihood of the Lake side communities were collected in the year 2014/2015 during dry and wet seasons for 11 sampling sites as indicated in Figure 1.1 and Table 1.1. The variations of physicochemical, metals and bacterial parameters were investigated. The overall water quality parameters (mean analytical results) of Lake Tana were found to be: Temp (Temperature)23.0OC, pH 7.5, EC (Electrical conductivity)180.1 μS/cm, BOD5 (Biological oxygen demand in 5 days)37.3 mg/l, COD (Chemical oxygen demand)316.5 mg/l, TSS 0.3 mg/l, TDS 93.1 mg/l, SO4 2- 11.0 mg/l, PO4 3- 42.4 mg/l, Cr (0.08 mg/l), (Mn (0.01 mg/l), E. Coli (13.4 Cell/ml), F. Coliform (82.5 Cell/ml), T. Coliform (113.0 Cell/ml), etc. These parameters did not show significant variation among the sites but were significantly different between wet and dry seasons (P<0.05). The highest concentration values were recorded during the wet season. However, most of the parameters under investigation were within the Ethiopian EPA (Environmental protection agency) permissible range except PO4 3-, S2-, E. Coli (Cell/ml), F. Coliform (Cell/ml) and T. Coliform (Cell/ml). Based on the analysis of the water quality index (WQI), Lake Tana water was unfit for drinking purpose and needs treatment. For theMacroinvertebrates analysis, a total number of 629 macroinvertebrate individuals are belonging to 9 orders and 38 families were found. In the study year impacted areas number of identified macroinvertebrates were 478 (76%); of this, 233 (37%) were in the wet season and 245 (39%) in the dry season and the total number of individuals identified in the reference area was 151 (24% of the total) in the wet season 61 (9.7%) and in the dry season 90 (14.3%) individuals. The diversity was more in the dry season. The dominant orders were Odonata (156 individuals), Coleoptera (153 individuals) and Hemiptera (141 individuals). The literature indicated that the presence of more Odonata, Coleptera and Hemipteran larvae is an indication of water quality deterioration due to pollution. From the collected samples, the total number of tolerant individuals was 303 (48.2%) and facultative individuals were 243 (38.7%) while intolerant individuals were 80 (12.7%). Most of the taxa (48.2%) had tolerance scores ranging from 7 to 10. The analysis of different forms of indices showed poor water quality. The water quality of Lake Tana was also determined by developing the LTMI (Lake Tana Metric Index). The index indicated the impairment levels of the study sites. Seven of the sites were in the category of poor (disturbed) and the other three were very poor (highly disturbed). The study on macrophytes recorded 43 species and 18 families during the two seasons (wet and dry), throughout the study year. 2687 individual macrophytes were collected; 1756 in the wet season and 931 in the dry season. Poaceae (15 species) with abundance mean 215.40 ± 421.7 was the most dominant family, followed by Cyperaceae (5 species) 35.40 ± 68.3. Sacciolepis africana was the dominant macrophyte species in Lake Tana. But in the study area Ambobahir, the dominant species was Cyperus papyrus while the Megech study area was invaded by the nuisance exotic weed Eichhornia crassipes. In the present study, the low macrophyte diversity values of Shannon Wiener index (2.90), Simpson Diversity Index (1-D) (0.90), Simpson Dominance Index (D) (0.10), Margalef’s index (M’) richness index (5.32) and Evenness Index (E) (0.77) throughout the study year indicate moderate water quality status while the presence of certain bio-indicator species like Eichhornia, Potamogeton and Cyperus in the lake also confirm pollution. Because of the effect of human activities on water quality and biodiversity, the livelihood of the riparian community is affected indirectly. Hence, to recommend mitigation and remediation actions, this study also focused on the assessment of the change of livelihoods of people living in the study area using qualitative research methods (key informant interview, focus group discussion (FGD), observations, published and unpublished materials and photographes). Lake Tana is a home to different flora and fauna including endemic species. The flora such as macrophytes and forest resources are used mainly for traditional medicine, fuel wood, rope, pole, habitat for birds, animal feed, etc. and the fauna includes fish, hippos, crocodiles, invertebrates, etc. Further, the Lake Tana area is a good habitat for indigenous cattle breeds (Fogera breed) and field crops gene center. The major resources around Lake Tana are land (the major source of livelihood), vegetation resources (macrophytes and forest resources), wildlife resources (fish, the other important source of livelihood) and cultural landscapes (churches and monasteries). Lake Tana is exposed to a set of interrelated environmental problems induced by human influence such as deforestation, erosion, sedimentation, water level reduction, erratic rainfall, flood, and competition for water resources, pollution and introduction of alien species. The causes to these problems were overgrazing, farmland expansion, cultivation of marginal lands (shorelines), encroachment of communal land, pollution and vegetation removal to meet demand for food and fuel wood. It is observed that alteration of Lake Tana and its fringe wetlands has affected the whole dynamics of the Lake’s ecosystem and the livelihood of the surrounding community. Ecosystem components are interlinked; hence correlation analysis was done between physicochemical parameters and macroinvertebrates of Lake Tana. Thus, correlations among many of the physicochemical parameters and macroinvertebrates families have been observed. To mention some of the correlations, the changes in the physical, chemical and biological characteristic of the lake affected the aquatic life forms and significantly affect economic activities that the lake supports. The RDA(Canonical redundency analysis) ordination of the species-environmental variable association indicated that pH, Cd, Pb and SO42- and Velidae, Chironomidae, Physidae, Gerridae, Corixidae, Dytiscidae, Caenidae, Coenogrionidae Simuliidae and Psephenidae were negatively correlated while Mussidae positively correlated with these environmental variables. This study concludes that the main threat to aquatic ecosystems in Lake Tana arises from agricultural activities, urbanization and industrialization that deteriorated water quality and biodiversity. Thus, it is recommended that proper management of Lake Tana should be put in place to prevent further deterioration of water quality and biodiversity of the lake for its sustainable development.