Urban rivers in developing countries, including South Africa, are increasingly being impacted by human activities. The Bloukrans and Palmiet Rivers, where the current study was undertaken, are no exception. The Bloukrans River, the main object of this study, was impacted by wastewater effluent discharges and run-off from informal settlements and agricultural activities. The present study was aimed at evaluating the ecological health of the Bloukrans River using macroinvetebrate-based biomonitoring approaches, including the biotic index (i.e. the South African Scoring System version 5), multimetric and multivariate analyses of the assemblage structure. 18 metrics were selected and their potential for discriminating between the control and impaired sites evaluated. Macroinvertebrates were sampled bi-monthly (April – August 2013) at one control site (i.e. Site 1) in the Palmiet River and four impaired sites (i.e. Sites 2, 3, 4 and 5) in the Bloukrans River. The Scoring index indicated that the river health condition at the control site was good, whereas apart from Site 5, conditions at Sites 2, 3 and 4 were either poor or critically modified. Twelve metrics discriminated between Site 1 and the four impaired sites. Canonical correspondence analysis indicated that dissolved oxygen, electrical conductivity, nutrients and turbidity exerted the greatest influenced on the assemblage structure. Overall, the study indicated that pollution in the Bloukrans River had significant impact on the river health, influencing metrics in the diversity, composition and richness categories. The 12 metrics that proved sensitive to deteriorating water quality, enabling the discrimination of the control site from the impacted sites, could be used with the Scoring index to assess river health in the Bloukrans River. However, although using metrics in addition to the Scoring index provided greater understanding, where analytical resources are limited the summary biotic Scoring index can provide an acceptable rapid indication of river health.
Currently, the species sensitivity distribution (SSD) approach is considered a useful tool in aquatic ecotoxicology for predictive effect assessments of toxicants. This approach is based on the variation in species sensitivities expressed by a statistical distribution, and the data comes from many ecotoxicological studies. In recent times, biomarkers have been suggested to be included in ecotoxicological risk assessment. This study, therefore, sought to answer the question: Could biomarker sensitivity distribution (BSD) of a single species be used to derive water quality guideline values? To answer this question, different biological system scales of Caridina nilotica were exposed to environmentally relevant concentrations of Roundup® herbicide in a series of short-term (< 4 d) and long-term (> 4 d) experiments. Short-term biomarkers assessment included different life stages, lipid peroxidation (LPx) and acetylcholinesterase (AChE), while adult life stage, AChE, LPx, growth and reproductive measures were assessed for long-term biomarkers. Biomarker sensitivity distributions (BSDs) were constructed and used short-term and long-term water quality guideline (WQG) values were estimated. These were compared to SSD-derived water quality guideline values for Roundup®. Observations from the comparison suggested that the different biological system scales of C. nilotica are more sensitive to long-term Roundup® exposure than SSD results. This study has shown that an in-depth study with one species using the BSD approach can provide reasonable ecotoxicological data that can be used to derive site-specific guidelines to protect sensitive species or taxonomic groups. As a novelty, this study has demonstrated the possibility of using the BSD approach in deriving WQG values.
Although a plethora of models exist to describe the characteristics and risk assessment of chemical mixtures in ecotoxicology, there is no specific procedure to decide on the mixing ratios (i.e. proportions of the individual chemical substances that form the mixture) at any desired level of concentration in an ecotoxicological mixture experiment. In this study, an attempt was made to develop a procedure for determining the mixing ratios in ecotoxicological experiments. In brief, from a single salt exposure test, the relative toxic fractions, which represent the toxic effect exerted by the individual salts, are determined. Thereafter, the proportions of each individual salt at any level of concentration in the mixture are estimated by multiplying the desired concentration with the relative toxic fraction of that particular salt. The procedure was applied to ecotoxicological experiments involving four binary salt mixtures (MgCl + MgSO, NaCl + NaSO, MgCl + NaSO and NaCl + MgSO) and Caridina nilotica, an indigenous South African freshwater shrimp. It is hoped that the application of this developed procedure will ensure administering the correct proportions of individual chemical substances in chemical mixtures in order to obtain the desired levels of concentration in aquatic ecotoxicological mixture experiments.
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