The drivers of benthic macroinvertebrates communities along a subtropical river system: Sediment chemistry or water quality?

Abstract

Despite a critical need for ecological risk assessment to preserve and manage aquatic ecosystems in rivers, the impacts of metal pollution on benthic macroinvertebrate species in sub-tropical river system have not been investigated. The main aim of this study was to examine benthic macroinvertebrate community composition in relation to metal concentrations sediments and other physico-chemical variables in the Mutshundudi River system. Benthic macroinvertebrate sampling and community composition analysis, sediment collection, processing, metal analysis and assessment of other variables in the river system were done across two seasons at 12 sampling sites, which were categorized into three segments: Upstream (sampling point 1-5), midstream (sampling point 6-9) and downstream (sampling point 10-12). The results from geo-accumulation (Igeo) values showed that sediments were loaded with Na, Zn, and B in all river segments. Water quality ranged from good at upstream sites due to low anthropogenic activities to very poor in downstream sites due to sewage and urban pollution. Sediments from the Mutshundudi river showed significant differences on high concentrations of metals (i.e., Mg, K, Na and Cu) and Seasonal variations (ANOVA, p < 0.01). Water and sediment chemistry were considered the driving factors of benthic macroinvertebrates, since species densities and composition reduced with a decline in water and sediment quality during both cool-dry and hot-wet seasons. Thus, measures of taxa richness, diversity and densities of macroinvertebrates may provide information on the quality of a river system. Heavy metals, such as Mg, K, Na and Cu in river sediments may pose adverse impact on macroinvertebrate community structure.