Assessment of water quality and reservoir water level in relation to phytoplankton abundance and diversity: A study in Magalla Reservoir in the North Western Province of Sri Lanka

This study examines the relationship between water quality and the abundance and diversity of phytoplankton in the Magalla reservoir in Sri Lanka. The study was conducted from May to December 2023 in four sampling sites. Site 1 was influenced by anthropogenic domestic waste, including biodegradable material, polythene, and plastics.   Site 2  a bathing area associated with inputs of soap, detergents, and paper waste. Site 3 received  urban waste, such as leachates from hotels, along with food, paper, and plastic waste. Site 4, serving as the reference site, was  minimally disturbed and was isolated from human activities. Standard methods were used for all in situ water quality parameters. Six replicates were taken for each parameter. The water quality parameters across sites 1, 2, and 3 significantly differed from the reference site (Kruskal-Wallis, Mann-Whitney U, P < 0.05). According to principal component analysis (PCA), the reference site clustered separately due to higher transparency, indicating minimal anthropogenic impact. pH and dissolved oxygen concentration were high in site 2. Water temperature, total dissolved solids, and electrical conductivity were high in sites 1 and 3. The highest abundance and diversity of phytoplankton were recorded in the reference site, and the lowest in site 3. Twenty-nine phytoplankton species, representing 7 distinct classes, were identified. One species from Trebouxiophyceae, and Euglenophyceae, two species from Chrysophyceae, and Zygnematophyceae, four species from Cyanophyceae, seven species from Bacillariophyceae, and twelve species from Chlorophyceae were observed during the study period. Melosira granulata constituted 69.4%, and Fragilaria capucina constituted 3.78% of the observed phytoplankton population, indicating their dominance in the reservoir. According to ANOVA, Tukey’s method, Shannon Wiener Diversity Index (SWDI) of phytoplankton in site 3 significantly differed (P < 0.05) due to different water quality conditions, and phytoplankton abundance and Pielou’s Evenness Index of phytoplankton at all three sites were not significantly different (P > 0.05) from the reference site. Abundance and SWDI of phytoplankton did not show a significant correlation (Pearson correlation analysis) with the reservoir water level.