Deoxygenation turns the coastal Red Sea lagoons into sources of nitrous oxide

Abstract

Direct measurements of dissolved N2O concentrations, fluxes and saturation percentages undertaken for the first time in two coastal lagoons — Al-Shabab and Al-Arbaeen, along the east coast of the Red Sea, revealed the region as a significant source of N2O to the atmosphere. The exacerbated dissolved inorganic nitrogen (DIN) from various anthropogenic sources led to substantial oxygen depletion in both the lagoons, which turned to bottom anoxia at Al-Arbaeen lagoon during the spring season. We assume that the accumulation of N2O is caused by nitrifier-denitrification in the hypoxic/anoxic boundaries. In fact, the results indicated that oxygen-depleted bottom waters favoured denitrification when the oxygenated surface waters recorded nitrification signals. Overall, the N2O concentration ranged from 109.4 to 788.6 nM (40.6–325.6 nM) in spring and 58.7 to 209.8 nM (35.8–89.9 nM) in winter in the Al-Arbaeen (Al-Shabab) lagoon. The N2O flux ranged from 647.1 to 1763.2 μmol m−2 day−1 (85.9 to 160.2 μmol m−2 day−1) and 112.5 to 150.8 μmol m−2 day−1 (76.1 to 88.7 μmol m−2 day−1) in the spring and winter respectively, in the Al-Arbaeen (Al-Shabab) lagoons. The ongoing developmental activities may worsen the current situation of hypoxia and associated biogeochemical feedbacks; therefore, the present results underline the need for continuous monitoring of both lagoons to restrict more severe oxygen depletion in future.