Assessing the effects of multiple stressors on the Incomati River Catchment

Abstract

Freshwater ecosystems, such as rivers, lakes, and wetlands, are among the most diverse and productive ecosystems on the planet. They support a wide variety of species and provide important ecosystem services, including water for drinking, agriculture, and industrial activities. It also provides habitat for aquatic organisms. These freshwater resources are under immense pressure due to a growing population, urbanisation, industrialisation, agricultural expansion, and climate change. Anthropogenic activities such as agriculture, forestation, mining, as well as wastewater pollution contribute to the decline in the quality of these ecosystems. Although trace elements are naturally occurring, an increase in these anthropogenic activities close to river catchments has led to an increase in the concentrations of trace elements in water and sediment of freshwater systems. Some trace elements (zinc, copper, and iron) are essential for the metabolic processes of aquatic organisms, but at high concentrations, they pose a threat to the health of these organisms. Elements such as cadmium, mercury and lead, are non-essential and highly toxic even at low concentrations.The Limpopo River and Incomati River catchments are important rivers that provide water to millions of people that live in and around these catchments. Both of these rivers flow through the Kruger National Park (KNP) and where there are few anthropogenic activities that could affect the health of the ecosystem, whereas just outside of the KNP, both of these catchments are affected by many anthropogenic activities such as mining, agriculture, as well as industrial activities.

Therefore, the aims of the present study were to determine the water and sediment quality of both the Limpopo River and the Incomati River catchments, determine the accumulation of trace elements in Oreochromis mossambicus (Mozambique tilapia) from selected sites in the Limpopo River and Incomati River catchments, and to determine whether there is a relationship between environmental element exposure (as reflected in the bioaccumulation in O. mossambicus) and ecosystem health (as reflected by the water, sediment and fish index scores). Firstly, the physico-chemical variables of the water and sediment of these systems were assessed. Secondly, the bioaccumulation of trace elements in the muscle tissue of fish was assessed to determine the bioavailability of trace elements in the environment. Thirdly, the Aquatic Toxicity Index (ATI), Sediment Quality Index (SeQI) and Fish Response Index (FRAI) were assessed in relation to the accumulated trace element concentrations in the muscle tissue of fish.

From the results, it was evident that trace element concentrations in the water and sediment of the Limpopo River and Incomati River catchments are influenced by the unique anthropogenic activities in their respective tributaries. It was also determined that trace element concentrations in the water and sediment of the Limpopo River catchment were higher than the trace element concentrations in the water and sediment of the Incomati River catchment. Oreochromis mossambicus was an effective bioindicator for trace elements in both of these systems. The fish from the Limpopo River had higher trace element concentrations compared to fish from the Incomati River catchment. It was also evident that there was a relationship between environmental element exposure, indicated by element bioaccumulation in O. mossambicus, and the ecological status as indicated by water, and biotic (fish) indices.

The present study concluded that the Limpopo River and the Incomati River catchments are subjected to various stressors that contribute to increased levels of nutrients, as well as the trace element concentrations in these systems. The accumulation of trace elements in O. mossambicus from the Limpopo River and Incomati River catchments serves as a valuable indicator of environmental quality, pointing to significant pollution risks from industrial, agricultural, and mining activities. The stressors affecting these catchments need to be addressed, to help preserve both the aquatic organisms that inhabit these catchments, as well as the communities that are dependent on these water resources.