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. Results indicate that the Gwaing (267.73 kg·d-1 DIN; 77.46 kg·d-1 DIP), Goukou (49.71 kg·d-1 DIN; 17.38 kg·d-1 DIP), Knysna (41.77 kg·d-1 DIN; 13.92 kg·d-1 DIP) and Hartenbos (37.73 kg·d-1 DIN; 21.39 kg·d-1 DIP) systems received the highest daily loads from WWTPs. 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.Keywords: Water quality, eutrophication, inorganic nutrients, wastewater discharges
Highlights
Estuaries form the interface between the marine and freshwater environments and as a result are complex, dynamic and productive ecosystems
This study highlighted the benefit of long-term water quality monitoring datasets to estimate inputs to coastal ecosystems, as these are needed to assess trends, identify management priorities, as well as identify shortcomings associated with such assessments
The river systems in the Gouritz Water Management Area (WMA) were generally shown to be in fairly good condition; the risk of introducing nutrients to receiving coastal ecosystems is everpresent in the majority of the catchments
Summary
Estuaries form the interface between the marine and freshwater environments and as a result are complex, dynamic and productive ecosystems. Estuaries provide numerous ecosystem services, such as regulating (erosion control), provisioning (food and water), supporting (nursery areas) and cultural services (recreation and tourism) (Costanza et al, 1997; Van Niekerk and Turpie, 2012). They are one of the most heavily used and threatened ecosystems worldwide, due to their high socio-economic importance. Land-use changes in drainage basins/ catchments have increased significantly and have led to a considerable transformation from a natural to heavily developed landscape (Fohrer and Chicharo, 2012) This transformation is largely due to the rapid development and intensification of agricultural activities, afforestation, urbanisation, water abstraction, and industrial activities that have had a marked impact on the delicate balance between riverine and coastal ecosystems (Cloern, 2001; Fohrer and Chicharo, 2012). The National Biodiversity Assessment of 2011 showed that 42% of estuaries in South Africa are classified as being in fair to poor condition (Van Niekerk and Turpie, 2012)
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