A study of the macroinvertebrate biodiversity and selected physico-chemical parameters of the Marico River

Abstract

The rapid growth in world population, coupled with mega droughts over the past few decades, is progressively responsible for a global decline in water quantity and quality. In South Africa alone, nearly 71% of the main rivers are already regarded as threatened. In order to support the sustainability of freshwater ecosystems, it is of great importance to manage and conserve rivers such as the Marico River, a National Freshwater Ecosystem Priority Area. Freshwater macroinvertebrates, a diverse group, are often used as indicators for determining the biotic integrity and ecological health of river systems, as well as to monitor environmental change. However, there is limited information on the macroinvertebrate diversity of Southern Africa, especially up to species level. The aim of this thesis was to determine the aquatic macroinvertebrate biodiversity and to establish whether selected physico-chemical stressors have an adverse effect on the biodiversity of the Marico River. The first objective of the study was to determine concentrations and values of selected physico-chemical parameters, potentially toxic to aquatic macroinvertebrates, in water and sediment. Water quality variables such as pH, electrical conductivity and temperature were mainly within the tolerance ranges of macroinvertebrates. Some metal concentrations in the water and nearly all metal concentrations in the sediment exceeded quality guideline values. However, results suggested that these metals originated mainly from natural sources such as geological weathering. Metal concentrations in the water were also considerably lower than in the sediment, which indicated that metals in the sediment were most probably not bioavailable. The second objective was to conduct comprehensive biodiversity surveys of aquatic macroinvertebrates in the Marico River and selected tributaries. A total of 187 taxa and more than 18 900 specimens were recorded, including both tolerant and highly sensitive species. This supported the general perception that the Marico River is mainly unimpacted by anthropogenic activities. Results showed that biotope and food availability, as well as the measured environmental variables play a significant role in the occurrence, abundance and distribution of species in this river system. Thirdly, the molluscan diversity in the Marico River was compared with a similar study in the highly impacted Crocodile River. Twenty species were recovered from the Marico River, while only nine species were recovered from the Crocodile River. The relatively high mollusc diversity, as well as the fact that juveniles were present throughout the study, demonstrated that current habitat and environmental conditions were suitable to promote recruitment and the sustainability of diverse mollusc populations in the Marico River and its tributaries. The fourth objective was to utilize a biomarker approach linking biological response to metal exposure using metallothionein (MT) levels in selected macroinvertebrate families. Results revealed that metals accumulated in the macroinvertebrates in various concentrations, depending on both their proximity to the sediment, as well as their feeding behaviour. Metallothionein levels in the organisms correlated positively with increased metal concentrations in the macroinvertebrates, suggesting that MTs probably act as metal detoxifying proteins and that it may be used as a biomarker for metal exposure. To conclude, the selected physico-chemical factors measured in the Marico River did not seem to have a detrimental impact on the macroinvertebrates. Both the high total biodiversity and high mollusc diversity compared favourably to similar studies on impacted rivers, supporting the general perception that the Marico River, in view of biodiversity and water quality, is still largely unimpacted by anthropogenic disturbances