Water Quality and phytoplankton community structure in the Southern Gulf of Lake Tana, Ethiopia

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Addis Ababa University


Water quality and phytoplankton community structure at one offshore (Zege) and three near-shore (Hospital, Resort and K. George) sites in the Southern Gulf of Lake Tana were investigated from Dec. 2015 to Mar. 2016. Surface water temperatures of all sampling sites were within the range of variation reported for most tropical water bodies (20-30ºC). Considering the shallowness (mean depth < 10 m) of the sampling sites and their exposure to southerly winds, frequent and complete mixing seems highly likely in the Southern Gulf of Lake Tana. Mean Secchi depth(cm), which was largely a function of total suspended solids, was significantly (at p <0.05) smaller at the relatively deep open water site (Zege, ≈ 83) than at the near-shore sites (mean: 130). The mean levels of pH (<7.3), TDS (< 110 mg L-1), K25 (< 160 μS cm-1) and salinity (<0.1 g L-1) showed that the Southern Gulf was a very dilute freshwater with near-neutral pH. Surface water DO (mg L-1), which was primarily a function of temperature and phytoplankton biomass, varied from 4.39 of the K. Georgis site to 6.82 of the Zege site. Mean levels of nitrate varied between 0.875 and 1.257 mg L-1, while those of ammonia ranged from 0.06 of the Zege site to 0.216 mg L-1 of the Hospital site. Mean SRP (in mg L-1), which was slightly smaller than TP, ranged from 0.237 of the Hospital site to 0.41 of the Zege site. Although it declined to below or slightly above the level regarded as limiting to diatom growth (<0.3 mg L-1) in the second half of February, silica (mg L-1) was generally at concentrations typical of tropical freshwaters (>10 mg L-1). The phytoplankton communities of all sampling sites were dominated, both in terms of species richness and abundance, by three alga groups, Cyanobacteria (blue-green algae), Bacillariophyceae (diatoms) and Chlorophyceae (green algae). Chlorophyceae was the most species-rich taxonomic group, followed by Bacillariophyceae. Microcystis flos-aquae, Aulacoseira granulata, and Oedogonium sp. were quantitatively the most important constituent species of blue-green algae, diatoms and green algae, respectively. The dominance by cyanobacteria constituted primarily by species of the genus Microcystis, which may be attributed to turbidity and nutrient availability, is regarded as a sign of eutrophic conditions and poor ecological status of water bodies. Mean Chlorophyll-a biomass (μg L-1) was very low (2.90), with individual observations varying from 0.31 of the K. George site to 8.40 of the Zege site. Cyanobacterial dominance represents a threat to public health, aquatic and terrestrial life. A continuous monitoring program is, therefore, necessary to ensure the protection of public health, aquatic and terrestrial life. Keywords: Cyanobacteria, nutrient availability, phytoplankton abundance, phytoplankton biomass, turbidity, water quality



Cyanobacteria, Nutrient Availability, Phytoplankton Abundance, Phytoplankton Biomass, Turbidity, Water Quality