Land Use/Land Cover Change in the Catchment of Northern Bay of Lake Tana, Ethiopia: Ramification for Water Quality and Plankton Community Structure
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Date
2025-05
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Addis Ababa University
Abstract
Rapid population growth-induced industrialization, urbanization, deforestation, settlement expansion and habitat destruction has emerged as critical drivers of land use/land cover (LU/LC) change on a global scale. The uncontrolled LU/LC change and anthropognic activities, in developing countries like Ethiopia, poses significant environmental threats to water quality and nutrient enrichment. The excessive enrichment with these nutrients emanating from intensive anthropogenic activities has substantially impacted water quality and biodiversity of lakes. The developmet of strategies of restoration or protection of lakes from further degradation necessitates the availability of adequate information on water quality, its environmental drivers and impact on plankton communities. There is, however, paucity of information available on the LU/LC changes, water quality, trophic status, and plankton community structure of the northern Gulf of Lake Tana, Ethiopia. Therefore, this study aimed to assess spatio-temporal changes in water quality, trophic status and plankton community structure in relation to the LU/LC dynamics in the northern Gulf of Lake Tana catchment. ArcGIS software and Landsat imagery were employed to produce maps for LU/LC changes. Samples for the assessment of water quality and plankton community structure were collected from May 2023 to April 2024 from five sampling sites, which were selected based on their proximity to human interference. Physico-chemical parameters were measured in situ or determined in collected water samples using standard methods. LU/LC changes were categorized into six classes with an overall accuracy and Kappa coefficient of 92.2% and 0.86%, respectively. The various LU/LC classes were highly correlated with particular water quality parameters. Settlement and cropland expansion were positively correlated with nutrients (NH3- + NH4+-N), NO3-N, soluble reactive phosphate-phosphorus (SRP), and total phosphorus (TP). Forestation was positively correlated with indicator of good water quality such as dissolved oxygen (DO) and pH, while it was negatively
correlated with temperature, electrical conductivity (EC), SRP, and NO3-N demonstrating a strong linkage between LU/LC and water quality. There were significant (p<0.05) variations in various water quality parameters such as temperature, DO, EC and turbidity between the dry and rainy seasons. The mean concentrations of nutrients such as NO3-N (0.48 mg/l), SRP (1.2 mg/l), total nitrogen (TN, 1.27 mg/l), and TP (2.1 mg/l) showed significant spatio-temporal variations, with higher concentrations occuring at the Megech and Dablo sites during the rainy season. The concentrations of SRP, TP, and TN, were slightly higher than the USEPA’s threshold concentrations for causing eutrophication. A total of 113 phytoplankton species belonging to six taxonomic groups and 11 Reynolds functional groups (RFGs) were identified. The predominant RFGs, which collectively accounted for 97% of the total phytoplankton abundance, were M, B, N, D, Lo, P and S2. Other RFGs, such as J and X1-X3, constituted 2.4% and 0.8% of the total abundance, respectively. RFGs exhibited significant association with EC, pH, turbidity, and some algal growth limiting nutrients. Forty-five zooplankton species belonging to three groups (17 Rotifera, 14 Copepoda, and 13 Cladocerans) were identified. Rotifers were the most species-rich group, with Brachionus angularis, B. caudatus, B. falcatus, B. havanaensis, and Keratella tropica being the most abundant. Significant (p<0.05) spatial variations in zooplankton diversity, evenness, and dominance were recorded. Shannon index values ranged from H’=2.98 of the GOR site to H’ =3.49 of the MEG site. Evenness values varied between 0.52 and 0.74, indicating less uniform distribution. In summary, the findings indicated that LU/LC changes and spatio-temporal water quality variations significantly affcte phytoplankton and zooplankton community structure and functions, subsequently impacting the overall health of the ecosystem. Thus, ongoing monitoring and intervention are imperative to avert future ecological imbalances and facilitate the restoration of Lake Tana.
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Keywords
Anthropogenic Activity, LU/LC Change, Nutrient Enrichment, Plankton Community Structure, Trophic State, Water Quality