Harmful algae in aquaculture systems in Ngerengere Catchment, Morogoro, Tanzania: Descriptive community structure and environmental concerns

Climate variability, anthropogenic activities, and hydrological shifts are fueling the nuisance of harmful algal blooms in water bodies. Unfortunately, cyanobacterial harmful algal blooms (CyanoHABs) dynamics have not received much attention in Tanzania. The study aimed to identify and characterize common species of cyanobacteria and examine their possible change in composition and succession in the Ngerengere catchment, Morogoro, Tanzania. Water samples from the selected reservoirs were collected quarterly between October 2017 and September 2018 for physico-chemical parameters in situ and in the laboratory. A benchtop FlowCAM was used for the identification of cyanobacteria and compared with the literature and available online databases. Principal component analysis (PCA) was used to examine the association between the physico-chemical variables and meteorological patterns. The study found common CyanoHABs such as Microcystis, Cylindrospermopsis, Anabaena (Dolichospermum), Lyngbya as well as other species such as diatoms and Euglena which are also considered nuisance. Virtually, a colonial cyanobacteria species dominated the fishponds, while Mindu Dam was more of filamentous species. The study suggest that Mindu dam, based on Carlson's Trophic State Index (TSI), falls under eutrophic while the fishponds were hypereutrophic. Associated physico-chemical conditions, heavy rainfall and prolonged dry conditions influenced cyanobacteria bloom dynamics. The hydrological connectivity between the fishponds and the Mindu Dam poses a threat to public health because a significant population in Morogoro depends on Mindu Dam for domestic water supply. There is a need for the development of a framework for mitigative and adaptive measures in the catchment, especially during pre-and post-occurrence of blooms.

Automated summary

This study focused on the dynamics of harmful algal blooms, specifically cyanobacterial blooms, in the Ngerengere catchment in Tanzania. The study identified common species of cyanobacteria and examined their composition and changes over time. The study found that physico-chemical conditions, rainfall, and dry conditions influenced the dynamics of cyanobacteria blooms. The hydrological connectivity between fishponds and the Mindu Dam posed a threat to public health. Mitigative and adaptive measures are needed in the catchment to address cyanobacteria blooms.