Decreasing Dissolved Oxygen in the Persian Gulf and the Gulf of Oman: Impacts and Strategies for Mitigation – A Comprehensive Review

The Persian Gulf hosts a variety of marine ecosystems, including mangrove forests, seagrass beds, and coral reefs, which have adapted to the challenging environmental conditions of the region to some extent. These ecosystems are increasingly influenced by rapid population growth, economic development in the countries surrounding the Persian Gulf, as well as, the impact of climate change in the region. They are facing mounting pressure from human activities. The influx of nutrients, coupled with the warming of the Persian Gulf, can result in a reduction in the dissolved oxygen concentration in the water. Consequently, the simultaneous occurrence of warming, reduced oxygen, and acidification in the Persian Gulf has become a significant challenge for its valuable ecosystems. According to studies, hypoxic areas in the Persian Gulf in mid-autumn can extend to more than 50,000 square kilometers. The occurrence of hypoxia in the Persian Gulf during late summer is confined to two primary areas in the western and central regions. In the eastern Strait of Hormuz, hypoxic conditions are observed in the near-bottom layer from early summer to mid-winter in areas distant from the Gulf's outflow. The reconstruction of dissolved oxygen concentrations in recent decades also indicates a declining trend accompanied by an increase in the extent and duration of hypoxia in the Persian Gulf. The degradation of water quality in the Persian Gulf, in addition to its adverse effects on its valuable ecosystems and consequently on fisheries and aquaculture in the Gulf, can significantly impact the Gulf of Oman and even the Arabian Sea in various ways, exacerbating the conditions of Oxygen Minimum Zones (OMZ) in these expansive seas. The Gulf of Oman and the Arabian Sea naturally and permanently host the widest and thickest OMZ in the world ocean. Depths exceeding 400 meters in the Gulf of Oman exhibit suboxic conditions (oxygen concentration less than 6 µmol/kg) during both summer and winter. The OMZ in the Arabian Sea and the Gulf of Oman has intensified over the past few decades. Oxygen levels recorded in 1960 in the core of the OMZ in the Arabian Sea and the Gulf of Oman ranged from 6 to 12 µmol/kg but have now dropped to levels below 6 µmol/kg (suboxic). This intensification of the OMZ in the Arabian Sea and the Gulf of Oman has adverse effects on the ecosystems of this region and the livelihoods of dependent populations. Marine organisms residing in the near-bottom layer or surface sediments are most affected by the expansion and intensification of hypoxic and suboxic conditions. Oxygen deficiency leads to a decline in the biodiversity of benthic communities, promoting resilient species such as polychaeta in surface sediments. The reduction in oxygen levels in the Arabian Sea has significantly influenced the composition of phytoplankton communities, the vertical migration of zooplankton, and myctophidae in the water column. The expansion and intensification of the OMZ, with its adverse impacts on fishing and aquaculture, also carry socio-economic implications for residents of the coastal areas of the Indian Ocean, the Gulf of Oman, and the Arabian Sea. Considering the prevalence of seasonal hypoxic conditions in the Persian Gulf and permanent suboxic conditions in the Gulf of Oman, addressing this trend and mitigating its environmental and economic impacts in the region is crucial. Management and policy strategies to halt or reduce the effects of oxygen depletion in the Persian Gulf and the Gulf of Oman can be classified into three main categories: a) ecosystem-based mitigation measures for environmental restoration and protection, b) adaptation-based measures to restore and protect marine organisms and fisheries, c) implementation and maintenance of monitoring programs and analysis of the information obtained. In essence, efforts to integrate research, management, and policy actions in oceans and seas across all biological, geochemical, and physical disciplines, addressing all climate change-related issues such as warming, acidification, and oxygen reduction, and involving all academic, industrial, governmental, and regulatory sectors, constitute the most effective roadmap recommended for combating the impacts of climate change on marine resources.