5. Nitrogen, Phosphorus and Organic Carbon in the Saudi Arabian Red Sea Coastal Waters: Behaviour and Human Impact

The Red Sea is an oligotrophic marginal sea where the main source of nitrogen and phosphorus is the Indian Ocean water flowing through the Bab al Mandeb entrance in the south. Therefore, the nitrogen and phosphorus concentrations are expected to decrease northward. External sources resulting from urban activities enhance the level of phosphorus, nitrogen and carbon in coastal waters around major cities along the Red Sea. We utilized the results of dissolved inorganic phosphorus (reactive phosphate), dissolved inorganic nitrogen (nitrate, nitrite and ammonium) and organic carbon (dissolved organic carbon (DOC) and particulate organic carbon (POC)) along the coast of Jeddah in the eastern Red Sea, in order to understand the distribution, sources, and biogeochemical processes that control their levels in sea water. Moreover, the results were used to calculate the anthropogenic flux and contribution to the total budget of the Red Sea. The spatial distribution patterns showed very high concentrations of nitrogen, phosphorus and organic carbon in the water at the southern coast of Jeddah in comparison to the northern coast. Most of the wastewater (>300,000 m³ per day) of the city is discharged at this part of the coast. The quantity is beyond the nominal treatment capacity of the existing wastewater treatment plants, resulting in poor treatment efficiency. Further evidence of the importance of sewage discharges was obtained using salinity variations. The salinity was remarkably low at these locations and the projections of salinity against nitrogen, phosphorus and organic carbon revealed significant negative correlations. The seasonal distribution of DOC and POC reflected the seasonality of the primary productivity, showing higher values in late spring. POC showed a substantial proportion, accounting for up to 29% of total organic carbon. Ammonium was the major component in autumn, representing about 60% of the total inorganic nitrogen (TIN), while in spring nitrate became the principal component, constituting approximately 62% of the TIN. The application of various techniques revealed that nitrogen was the potential limiting element. Direct measurements and calculations indicated that the daily production of total nitrogen and phosphorus associated with sewage discharges into Jeddah coastal waters is about 21261 and 3360 kg, respectively. The inorganic forms of nitrogen and phosphorus represent 43 and 45% of the total nitrogen and phosphorus introduced into the area. Anthropogenic nitrogen and phosphorus sources at this part of the Red Sea coast represent about 0.9 and 9.9% of the deficit of the two elements through the Red Sea/Indian Ocean water exchange process at the Strait of Bab al Mandab.