Influence of aquatic biodiversity on the self-purification of tropical rivers

Water resources face several challenges from human activities that increase their vulnerability and negatively impact on their resilience, particularly concerning the preservation of biodiversity and ecosystem services. This study hypothesized that aquatic biodiversity contributes to maintaining the ecosystem service of the improving water quality in the Almada and Cachoeira Rivers, in northeastern Brazil. To test the hypothesis, samples were taken upstream (clean water zone) and downstream (active decomposition zones) from three urban areas of each river basin, and the Streeter-Phelps self-purification model was applied. The landscape was predominantly characterized by pastures and forest formations. Principal Component Analysis indicated greater variability in the physical and chemical variables of the active decomposition zones. The headwater stretches were the most affected by organic pollution; the Almada River presented a more efficient self-purification process. Zooplankton richness and abundance showed significant differences between the clean water and the active decomposition zones. There was a correlation between attributes of both benthic and planktonic communities and certain physicochemical variables, such as dissolved oxygen (DO), pH, and biochemical oxygen demand (BOD). The correlation between community attributes, including tolerant groups, and DO and BOD concentrations along each stretch, generated by the model, demonstrated the positive effect of the composition and abundance of planktonic organisms on self-purification processes. Given the observed anthropogenic impacts, sewage treatment measures should be implemented to sustain ecosystem services provided by water.