Abstract
•Nutrient loads and dispersion from Jeddah into the Red Sea were quantified.•Dissolved inorganic forms made up the bulk of the nutrient loads.•Impacts restricted to the inner bay and lagoons exhibiting hypertrophic conditions.•Organic and particulate nutrients dominated the coastal oligotrophic waters.•Nitrogen deficiency may limit primary production in the coastal waters.
Large amounts of waste water are discharged from the Jeddah Metropolitan Area into the Red Sea. Daily loads of total nitrogen (TN) and phosphorus (TP) amount to 6564kg and 2241kg, respectively, comprising 83% of dissolved inorganic nitrogen and 33% of dissolved phosphate. Steep gradients prevail nearshore, ranging from 2000μMTN and 250μMTP in the hypertrophic city lagoons to 6μMTN and 0.4μMTP in the adjacent oligotrophic water. Sewage inputs from Al Khumra, Jeddah’s main outfall, cause a widespread but moderate increase in surface nutrient concentrations due to the submerged diffuser. The nutrient pool in the oligotrophic water is dominated by dissolved organic and particulate forms, with nitrate frequently below the detection limit, indicating rapid transformation of inorganic nutrients. N:P ratios, as well as half-saturation constants for phytoplankton growth, suggest that nitrogen is the limiting factor restricting primary production in the area.