Land-derived inorganic nutrient loading to coastal waters and potential implications for nearshore plankton dynamics

Abstract

Algoa Bay represents a complex suite of aquatic ecosystems that support high biodiversity and provide significant socio-economic benefits. Since land-ocean interactions are central to shaping the structure and functioning of these ecosystems, the aim of this study was to quantify land-derived inorganic nutrient loads entering the coastal waters of Algoa Bay. Dissolved inorganic nitrogen (DIN) and phosphorus (DIP) data together with flow discharge volumes were sourced from long-term monitoring programmes and existing literature to determine annual and seasonal nutrient loads. The annual loads of DIN and DIP entering the coastal waters of Algoa Bay were estimated at 8.7 × 105 and 1.4 × 105 kg, respectively. Seasonal peaks were observed in winter for DIN and in spring for DIP. Wastewater discharges were the dominant source of inorganic nutrients to the nearshore environment, representing 71.3% and 62.0% of total DIN and DIP loads, respectively. The DIN and DIP inputs entering estuarine waters of Algoa Bay accounted for 21.9% and 38.0% of total loads, respectively. Inshore zooplankton biomass in Algoa Bay has increased significantly (p < 0.01) since 2013 concomitant with elevated DIN inputs from WWTPs. Additionally, anthropogenic nutrient loading to the estuarine and coastal waters of Algoa Bay has facilitated, in part, the increased observations of eutrophic symptoms, including harmful algal blooms (e.g. Heterosigma akashiwo and Lingulodinium polyedra) and hypoxia (<2 mg l−1). Effective ecosystem-based management is key to preventing the exacerbation of these undesirable disturbances, and thus maintaining ecosystem functionality.