Shift happens: Changes to the diversity of riverine aquatic macroinvertebrate communities in response to sewage effluent runoff

Rivers are amongst the most threatened ecosystems on the planet. They are particularly susceptible to common forms of anthropogenic influences including sewage effluent runoff and flow modification. We hypothesized that the diversity of aquatic macroinvertebrate communities from three rivers (Nyl, Mogalakwena and Limpopo) within a semi-arid region (Limpopo, South Africa) will decline in response to shifts in water quality resulting from sewage effluent runoff. We assessed the diversity of aquatic macroinvertebrate communities from ten sites using univariate and multivariate analyses, and community responses were determined based upon anthropogenic disturbances. Our results show distinct spatial and temporal variation between the sampling sites and their associated macroinvertebrate communities. Spatial separation is attributable to increased concentrations of nitrogenous-based pollutants (total nitrogen and nitrites) and manganese, both of which are associated with sewage effluent runoff released by the Modimolle Waste Water Treatment Facility. Temperature is the principle driver of temporal variation amongst the macroinvertebrate communities, with winter (June 2016) communities being more diverse than summer (February 2017) communities. Macroinvertebrates remain important bioindicator organisms for riverine ecosystems, specifically within semi-arid regions. Our study highlights the need for more comprehensive diversity assessments of riverine macroinvertebrates, specifically from southern Africa, a region of the world that remains significantly underrepresented in the scientific literature of aquatic ecology. These results further increase our understanding of aquatic biodiversity within semi-arid regions and provides a baseline for future monitoring efforts and management in South Africa.