The effects of climate change on freshwater fauna in the lower Olifants River, South Africa

Abstract

Freshwater rivers are under threat worldwide from anthropogenic disturbances including climate change. Climate change will increase air temperature and, consequently, water temperature in rivers. The survival of aquatic fauna is closely tethered to water temperature, therefore, the increase in water temperature will influence freshwater fauna at both population and community levels. The ecologically and economically important lower Olifants River, in the Kruger National Park (KNP), South Africa was investigated as it is already negatively affected by anthropogenic disturbances, with the aim of validating a statistical model to predict future water temperatures. The study validated a water temperature model for the Olifants River, with Nash Sutcliffe efficiency values of >0.9 for monthly and >0.75 for daily timesteps, indicating the robustness of this model. The model was further validated using the upper Klaserie River. This statistical model was then used to predict average water temperatures (WTavg) from projected air temperatures to predict monthly and daily WTavg under the Representative Concentration Pathway (RCP) 8.5 scenario using 16 General Circulation Models (GCMs) to 2100 CE. The results showed that monthly WTavg will increase by 3.7° C, and maximum WTavg will reach 33.6° C by 2100 CE. The daily results showed a similar increase of 3.9° C, with some extreme days reaching 42-44° C, and maximum WTavg will increase to 41.1° C by 2100 CE. These predictions were compared to the thermal tolerances of fauna present in the Olifants River to extrapolate which taxa may be vulnerable to the effects of climate change. The investigation used WTavg of 33.6° C and 41.1° C as the thresholds for chronic and acute heat stress, respectively. It was found that 12 fish species and 11 macroinvertebrate families will be susceptible to acute heat stress while five fish species and five macroinvertebrate families will be vulnerable to chronic heat stress. Although statistical modelling approaches are instrumental in modelling climate change as a driver of water flow and temperature over time, they do not extend beyond these parameters to address climate change impact-related questions on ecological functioning, such as trophic ecology. This study used stable isotope analysis (SIA) to extrapolate what the loss of these fish and macroinvertebrates could have on the trophic dynamics under the RCP 8.5 scenario. The results showed that the apex predator is vulnerable, along with other predators, algivores and insectivores. This will ultimately lead to top-down effects and over-proliferation of prey, plants, and algae. While this study is based on a single river, it represents a novel approach that combines: 1) a mechanistic approach of statistical modelling future water temperature; 2) the evaluation of thermal tolerances; and 3) the assessment of trophic cascades using SIA and applying them to evaluate faunal changes at both population and community levels in a river system. This study has implications at a global level on how climate change may affect not only freshwater water temperatures, fish, and macroinvertebrates, but also with ecological, economic, and human health implications in similarly data-deficient river systems globally.