Effects of pollution on phytoplankton and zooplankton communities in Murchison Bay, Lake Victoria

Abstract

Anthropogenic activities significantly threaten freshwater ecosystems worldwide by degrading water quality and harming aquatic biodiversity. Lake Victoria, the world's second-largest freshwater lake, is especially vulnerable, with Murchison Bay experiencing human pressures from urban and industrial discharges. This study evaluated how these activities affect water quality, zooplankton and phytoplankton communities in Murchison Bay. The objectives included: (i) assessing phytoplankton abundance and diversity, (ii) examining zooplankton abundance and diversity, and (iii) analyzing how water physico-chemical variables influence species composition of both groups. The research was carried out in Murchison Bay, located on the northern shores of Lake Victoria near Kampala, Uganda. Three sampling sites were chosen to represent different levels of human impact: Site A near the heavily polluted Nakivubo channel outlet with high organic and nutrient loads; Site B near Gaba landing site with nearby human activities; and Site C, less impacted, near Gaba 1 waterworks. In-situ measurements of physico-chemical parameters were taken with a multimeter probe. Zooplankton and phytoplankton samples were collected using a Van Dorn sampler, concentrated, fixed, and identified to the lowest possible taxa, using inverted microscopy and standard taxonomic keys. Zooplankton counts were performed with a Sedgewick-Rafter chamber, while phytoplankton were counted using the Utermöhl method. Data analysis was conducted with Excel and R software. Results showed that zooplankton in Murchison Bay included rotifers, copepods, and cladocerans, totalling 26 species. Rotifers were most diverse (13 species), followed by copepods (9) and cladocerans (4). Site A had the highest overall zooplankton diversity, although no cladocerans were found at Site C. Copepods dominated in abundance, mainly due to nauplii stages, with rotifers and cladocerans following. Despite spatial differences in group abundance, ANOVA indicated no significant variation in total zooplankton numbers across sites (p > 0.05). Zooplankton diversity was highest at Site A, then Site C, and lowest at Site B. The phytoplankton community consisted of 18 species from five groups, with green algae being the most common (7 species). Cyanobacteria, mainly Microcystis spp. and Anabaena spp., were dominant. Phytoplankton density peaked at Site A and was lowest at Site C, but no significant difference in total phytoplankton abundance was found across sites (p > 0.05). Unlike zooplankton, phytoplankton diversity was lowest at Site A and increased toward Sites B and C. These results highlight how human activities impact plankton communities in Murchison Bay differently.