The nutrient status of South African rivers: Concentrations, trends and fluxes from the 1970s to 2005
Eutrophication of river systems, resulting from nutrient enrichment, is globally considered to be one of the most serious threats to freshwater ecosystem services such as water quality and biodiversity. This study provides a comprehensive overview of the nutrient status of the 20 largest river catchments in South Africa, based on dissolved inorganic nitrogen (NO 3- + NO2-) and phosphorus (PO 43-) long-term water quality monitoring data collected by the Department of Water Affairs and Forestry. Nutrient levels exceeding recommended water quality guidelines for plant life are observed in all of the rivers, except one. Additionally, dissolved-phosphorus levels exceeding recommended concentrations for aquatic animal life prevail episodically in all but 6 of the catchments. Alarmingly, statistically significant (P < 0.05) upward trends in dissolved PO43- levels are found in almost 60% of the rivers evaluated. The most likely cause of increasing nutrient enrichment is effluent from dysfunctional sewage works and unsewered human settlements. This poses a serious and costly threat to water quality and biodiversity. Nutrient fluxes associated with agricultural runoff, representing loss of soil fertility, translate into fertilizer-equivalent costs exceeding several hundred million rands annually.