The influence of shifting habitat heterogeneity and environmental stressors over space and time on macroinvertebrates communities in the Upper Olifants River, South Africa

Abstract

The selection of sampling sites was considered in relationship with sites from an earlier study conducted in 1968. The water quality analysis of the current study showed that the river receive acid mine drainage from upstream abundant coalmines. During spring 2011, the sampling site in the Olifants River had higher macroinvertebrate community diversity than the reference site with sensitive families (Perlidae, Heptageniidae, Chlorocyphidae and Psepheniidae) despite increasing sulphate and electric conductivity levels. The lower habitat heterogeneity, such as little aquatic macrophytes and less riparian vegetation may have account for the lower diversity and imbalance in macroinvertebrate functional feeding group composition at the reference site in comparison to the Olifants River site.The number of taxa from the same period in 1968 was comparatively lower and was dominated by more tolerant family taxa such as Chironomidae and Oligochaeta which were indicators of high nutrient and low oxygen concentrations in the water. Oligochaeta was particularly high during winter time (n > 400) in 1968, indicating there may have been a higher nutrient inflow during that specific sampling period. By comparing the water chemistry of the current study with the 1968 study, it was evident that there was a decrease in nitrate nitrogen and orthophosphate. However the concentration of sulphate at the Olifants River site increases by tenfold since the 1968 study.This phenomenon was possible associated to the increase of coal mining activities in the upper catchment. The pH levels at both sites remained relatively similar during both study periods. The outcome of the study showed that there was a significant shift of macroinvertebrate community from more tolerant groups in 1968 to more sensitive groups in the present study.This phenomenon can possibly be related to the development of reedbeds over a period of >40 years at the Olifants River site, which provides a better habitat for macroinvertebrates. It is known, that all stages of the reedbeds succession support important macroinvertebrate communities.The expansion and colonization of reedbeds dominated by the salt-tolerant aquatic plant Phragmites australis seen in the current study at site in the Olifants River (when compared to the 1968 study) could be due to the increases in water column electrical conductivity over time caused by mining activities.