The Individual and Interactive Effects of Climate, Land Use and Nutrient Enrichment on Macroinvertebrate Dynamics in South African Rivers.

Freshwater ecosystems are increasingly degraded by multiple stressors arising from climate change and local anthropogenic pressures. However, the potentially important interactive effects of stressors are not well understood. While there is growing evidence of multiple stressor effects from experimental studies, there is a need for more field-based studies to provide complementary insights where there is high biological variability and context dependencies. We conducted an extensive survey of benthic macroinvertebrate communities across 30 streams in South Africa to examine responses to both the single and interactive effects of climate, human land cover and nutrient enrichment. We sampled macroinvertebrates and physicochemical variables at each site during the cold-dry and hot-wet seasons. Land use was classified using the 2022 South African National Landcover (SANCL) data. We used temperature and rainfall data from the South African Weather Service (SAWS) as climate proxies. Ammonium was the most pervasive stressor, acting both independently and in interaction with climate and human land cover to influence macroinvertebrate community dynamics, including independent effects on Ephemeroptera, Plecoptera and Trichoptera (EPT) richness and mean biomass (mg) and in interaction with human land cover to negatively affect overall and EPT abundance. The interaction of human land cover and rainfall, as well as ammonium, rainfall and season interaction showed positive effects on mean body size, indicating that rainfall is an important stressor in modulating the effects. Overall, our study suggests that both the individual and interactive effects of climate and anthropogenic stressors shape freshwater macroinvertebrate community dynamics. Our study highlights that reducing nutrient inputs and effective land use management would go a long way towards conserving freshwater invertebrates.