Deoxygenation and acidification of the Persian Gulf in a changing climate

Dissolved oxygen (DO) concentrations in coastal areas have been decreasing in many regions over the past decades mainly due to land and river-based anthropogenic nutrient inputs, with the evolution of massive agriculture and the use of fertilizers [۱-۳]. The seafloor in the Persian Gulf, as a shallow basin, is typified by ecologically and economically valuable benthic species. Low concentration of dissolved oxygen could lead to significant losses in biomass and diversity. Here, dissolved oxygen results of several research cruises to the Persian Gulf and the Strait of Hormuz during ۲۰۱۸-۲۰۱۹ on the research vessel Kavoshgar Khalij Fars were discussed. The results showed that summer to autumn hypoxia (O۲ ≤ ۶۱ μmol/kg) occurs mainly at depths ≥ ۵۰ m to the bottom in the Persian Gulf. This seasonal hypoxia starts in late summer (when the strong stratification prevents ventilation of the water column) in the near-bottom layer reaching its greatest severity in mid-autumn with an unexpected maximum area of more than ۵۰,۰۰۰ km۲. The minimum oxygen measured at the near-bottom layer of the western basin in autumn (۲۵.۸ μmol/kg or ۰.۸۵ mg/l) was lower than any previous measurement in the open waters of the Persian Gulf. In the Strait of Hormuz, the seasonal hypoxia appears in early summer in the near bottom of the most eastern part at the Iranian side, which is less affected by the outflowing Persian Gulf Water. Although the highest hypoxic area in the Persian Gulf was observed in autumn, the greatest area and thickness of low-oxygen waters (O۲ ≤ ۹۲ μmol/kg) in the Persian Gulf occurred in late summer. Deepening vertical mixing in late autumn and destratification and full vertical mixing of the water column due to the surface cooling in winter are suggested as the foremost influential force of hypoxic dissipation. In the hypoxic and low-oxygen areas, high phosphate and nitrate concentrations and low pH values were found. pH values recorded in hypoxic waters were as low as whatis predicted for surface ocean under ocean acidification in ۲۱۰۰ (assuming the worst-case CO۲ emissions scenario). Considering the significant hypoxic and more acidic waters of the bottom layer in the late summer and autumn, we strongly suggest evaluating the effects of hypoxia and acidification on the Persian Gulf ecosystems [۴].

کلیدواژه ها:

Deoxygenation ، acidification ، Persian Gulf ، Persian Gulf Explorer