High nutrient loading and climatic parameters influence the dominance and dissimilarity of toxigenic cyanobacteria in northern bays of Lake Victoria

Mark Olokotum,Marc Troussellier,Arthur Escalas,Sahima Hamlaoui,William Okello,Ronald Semyalo,Jean-François Humbert,Cécile Bernard

doi:10.1016/j.jglr.2021.04.021

Abstract

Eutrophication of Lake Victoria led to changes in its phytoplankton communities. However, different levels of eutrophication exist in the open lake and the bays, and between embayments. This study utilized spatial and temporal sampling of Napoleon Gulf and Murchison Bay, exhibiting different trophic conditions. Over one year, we investigated phytoplankton biomass, richness, diversity and dissimilarity, and related the dynamics of the dominant species to the limnological and climatic conditions. The results confirmed that Napoleon Gulf and Murchison Bay showed large differences in eutrophication status, with lower nutrient concentrations in Napoleon Gulf than in Murchison Bay, where a strong gradient was observed from inshore to offshore areas. These nutrient dynamics resulted in a 4 to 10 fold higher chlorophyll-a in Murchison Bay than in Napoleon Gulf. From the embayments, 135 phytoplankton taxa were recorded with no significant differences in alpha diversity. However, high dissimilarity in community structure was observed in beta diversity, mostly due to a turnover among the dominant toxigenic species. Thus, from a similar species pool, there was a shift in the dominant toxigenic cyanobacteria from Microcystis flos-aquae and M. aeruginosa in Murchison Bay, Dolichospermum circinale and Planktolyngbya circumcreta in Napoleon Gulf to D. circinale in the offshore stations. These cyanobacteria are toxigenic taxa with known health hazards. Using partial least square models, we showed that both climatic variables (e.g. wind, solar radiation) and levels of inorganic dissolved nutrients (e.g. SRP, NO3–, and NH4+) are the main drivers of differences and dominance in cyanobacteria communities in northern Lake Victoria.

Highlights

After years of anthropogenic pressures, Lake Victoria is considered to be persistently eutrophicated and polluted (Downing et al, 2012; Juma et al, 2014; Mbonde et al, 2015; Nassali et al, 2020; Olokotum et al, 2020)
This study assessed the spatio-temporal dynamics in phytoplanktonic communities in two embayments along a transect from inshore to open lake
Large differences in limnological characteristics were observed between the two bays, especially regarding the concentration of inorganic nutrients which were much higher in Murchison Bay than in Napoleon Gulf

Summary

Introduction

After years of anthropogenic pressures, Lake Victoria is considered to be persistently eutrophicated and polluted (Downing et al, 2012; Juma et al, 2014; Mbonde et al, 2015; Nassali et al, 2020; Olokotum et al, 2020). Beyond the ecosystem services identified for the whole lake (Sterner et al, 2020), these bays and gulfs provide crucial services for the riparian populations that are likely to be impaired by eutrophication. Most of these embayments are shallow and increasingly susceptible to point source pollution and, experience changes in phytoplankton diversity and increased biomass.

Methods

Results

Discussion

Conclusion