The influence of turbidity and water temperature on black fly species in the middle and lower Orange River, South Africa

Water clarity in the Orange River of South Africa alternates between highly turbid and clearer conditions. Multiple ecological effects result from this that affect aquatic community structure and species composition, including switching of dominant black fly species and growth of benthic algae. Fourteen sites in the middle and lower Orange River were sampled for black fly species and abundances, presence of benthic algae, and water quality and turbidity seasonally between November 2015 and December 2016. Logistic regression models, time series analyses, and multivariate ordinations showed that clearer water favours benthic algae and the minor pest black fly species Simulium adersi Pomeroy, 1922 and Simulium nigritarse Coquillett, 1902, while high turbidity favours the major pest black fly species Simulium chutteri Lewis, 1965 and Simulium damnosum Theobald, 1903. Moreover, these switches appear to be less frequent under post-impoundment conditions. We conclude that black fly species composition is dynamic and reflects changes in flow and turbidity levels. Flow regulation has resulted in a trade-off between improved water supply and ecological costs resulting from reduced ecosystem variability.