Nutrient characterisation of river inflow into the estuaries of the Gouritz Water Management Area, South Africa

Abstract

This study provides an overview of the nutrient status of river inflow into the estuaries within the Gouritz Water Management Area (WMA) of South Africa. Riverine inputs are a major source of macronutrients to estuaries and the adjacent coastal environments. Long-term water quality monitoring data (dissolved inorganic nitrogen, i.e. DIN; and dissolved inorganic phosphorus, i.e. DIP), collected by the Department of Water Affairs (DWA), were used to assess historical trends of river nutrient inflow within the Gouritz WMA. The results indicate that DIP concentrations exceeded the eutrophic limits for aquatic ecosystems (DWA) in 50% of the catchments assessed. Anthropogenic activities such as agriculture, wastewater discharge, urbanisation, and afforestation were significant factors influencing nutrient levels within these rivers. For the majority of the river systems (approx. 80%) there was no significant correlation (P > 0.05) between inorganic nutrient levels and freshwater inflow from the catchments. Wastewater treatment plant (WWTP) data (DWA) were assessed to explore the reasons for this 'disconnect' between freshwater inflow and inorganic nutrient levels. The Gwaing and Hartenbos estuaries would be most vulnerable to increased nutrient loading because of their small size and prolonged periods of mouth closure. The study highlights the importance of water quality monitoring of river inflows into coastal ecosystems, as it is needed to assess pollution trends and identify management priorities.

Nutrient enrichment as a threat to the ecological resilience and health of South African microtidal estuaries

“…Wastewater volumes and nutrient loads (N and P) were obtained from available data sources (e.g. Lemley et al 2014;Van Niekerk et al 2018;DWS 2019). Using the collected data, estuaries affected by different pollution sources (wastewater treatment works (WWTW), stormwater run-off and agricultural return flow) were compared between biogeographic zones (cool temperate, warm temperate, subtropical, tropical) and volumes and nutrient loads from WWTW discharges to the nine different estuary types (Van Niekerk et al 2020) were summarised for the country.…”

Dilemma of nitrogen management for future food security in sub-Saharan Africa – a review

“…Very little of the remainder (29%) benefited crop production as it was also deposited on several non-agricultural land use types such as settlements, roads, and marginal lands. Based on an assessment conducted in South Africa, Lemley et al (2014) reported that when there are no other limiting factors, concentrations of 400 and 30 mg L -1 of total dissolved N and P respectively, and an N : P ratio of 7-8 on a weight basis are enough for eutrophication to occur. Preventing eutrophication requires control of both N and P loadings into water bodies (Howarth and Marino 2006).…”

Deterioration in the water quality of an urbanised estuary with recommendations for improvement

“…5) and DIP ( Fig. 6) loads discharged from WWTWs, calculated according to Lemley et al (2014), between 2009 and 2013 were examined and related to concentrations measured in the river and estuary. DIN concentrations increased steadily from the mouth of the estuary to the tidal limit.…”