The sea that loses its oxygen from the top

The Reading

It is a shallow sea. Average depth is around 35 meters, under the height of the cranes on its container terminals. Evaporation runs at roughly 2 meters a year against 0.2 to 0.5 meters of rain and river water arriving, so it is concentrating salt continuously, and large parts of it carry salinity above 42 psu. Surface temperature swings between 12 and 36 degrees Celsius over a year.

Water comes in from the Indian Ocean along the northern side of the Strait of Hormuz, at the surface and fresher. It leaves along the bottom, dense and hypersaline, on the southern side. That is a reverse-estuarine circulation, the opposite of what a river mouth does, and it turns the whole volume over in something between under a year and two and a half years.

A sea like that should be well oxygenated, and the literature says so plainly: short residence time and shallow depth are the reasons to expect it. Two thousand twenty-five work in Ocean Science adds the other half of the setting. It is the sea with the highest average summer surface temperatures in the global ocean, and the basin-wide mean summer figure rose at approximately 0.31 degrees Celsius per decade between 1980 and 2018.

Sources. Lachkar and others, Frontiers in Marine Science: https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2022.891378/full — Lachkar and others, Ocean Science: https://os.copernicus.org/articles/21/3241/2025/os-21-3241-2025.html

A sea like that should be well oxygenated. It is not.

What the Water Does

The oxygen is going out of the top rather than being consumed at the bottom.

Warm water holds less gas. Warm water sitting on cooler water also stops mixing, so what little arrives at the surface does not reach the bed. The Frontiers work reconstructs dissolved oxygen for the basin from 1982 to 2010 and finds the oxygen content dropping by nearly 1 percent per decade, with near-bottom concentrations falling by between 10 and 30 millimoles per cubic meter between the early 1980s and the late 2000s. The paper counts water below 60 millimoles of oxygen per cubic meter as hypoxic, which is about 1.9 milligrams per liter, just under the 2 milligrams per liter NOAA uses for coastal hypoxia. The hypoxia-prone seafloor area grows over the same span from under 20,000 square kilometers to around 30,000, and the hypoxic season starts one to two months earlier than it did.

The paper attributes that to two things: reduced oxygen solubility near the surface, and enhanced respiration near the bottom. The respiration half needs feeding, and increasingly the feed is municipal. A 2025 review in Frontiers in Water puts the eight surrounding nations at an estimated 8.6 billion cubic meters of municipal wastewater in 2020, more than six times the annual discharge of the Tigris-Euphrates into the same sea. Sixty-nine percent of the treatment plants sit within 10 kilometers of the shoreline and 28 percent within 1 kilometer.

Source. Gastoldi, Al Gergawi and Burt, Frontiers in Water: https://www.frontiersin.org/journals/water/articles/10.3389/frwa.2025.1702212/full

The oxygen is going out of the top rather than being consumed at the bottom.

Who Pays for It

The World Bank puts the states around this sea at nearly half of global desalinated water output, produced for under 1 percent of the world's population. Their drinking water comes through intakes in the water described above.

What that costs shows up as an operating incident rather than as an environmental one. In July 2023 Nama Water Services said that the red tide phenomenon had significantly disrupted Barka's main desalination plant, activated reserve wells, interconnected the Sohar, Barka and Ghubra plants, and asked customers to limit consumption until seawater quality improved. That is a newspaper's report of a utility statement rather than a regulator's finding, and it is graded accordingly. It is also the most concrete thing in the accessible record connecting a bloom to a tap on this coast.

Sources. World Bank, through the Internet Archive capture of 11 August 2026: https://web.archive.org/web/20260811223857/https://www.worldbank.org/en/news/opinion/2024/03/26/from-scarcity-to-sustainability-the-gcc-s-journey-towards-water-security — Nama Water Services via Muscat Daily: https://www.muscatdaily.com/2023/07/24/red-tide-outbreak-impacts-water-desalination-in-barka/

Caveats

Most of the figures above come from a model hindcast, which is how the authors worked with the observations available to them. They describe physical and biogeochemical observations of the region as scarce, and understanding of how its hypoxia behaves as, in their words, in its infancy.

We weren't able to find a solid, current, comprehensive observational record. The first record of oxygen falling near that seafloor came from a research vessel named Mukhtabar Al-Bihar, in September 1985 and September 1986, within a few meters of the bed at depths beyond 40 meters. The lowest concentration recorded there, 26 millimoles per cubic meter, comes from cruises by the research vessel Khalij Fars between September 2018 and November 2019, which also put the near-bottom hypoxic area at potentially up to 50,000 square kilometers in autumn. That is one survey, on one side of one sea.

We weren't able to find a published multi-year, multi-station dissolved-oxygen baseline for a named basin on this coast. Until one is available, a change measured by a single instrument has little to be compared against.

A Question

From a reader who runs an intake: if the oxygen is leaving from the surface, does anything upstream help?

Less than the usual argument assumes, and the published record is what says so. The solubility half of this is set by temperature and salt, and no nutrient program touches it. The respiration half is the half that responds to what arrives, and the review above is fairly clear that what arrives here is municipal rather than agricultural, which is a different set of assets and a different ministry.

The more useful thing to take away is the shape of the year. The extremes fall in July to September, six of the nine hottest summers in the record coincided with La Nina, and the hypoxic season now opens one to two months earlier than it did in the 1980s. An intake's difficult weeks are becoming more of the year, and that is a planning horizon rather than a forecast.

What Now

A shallow sea that would be expected to be well oxygenated, losing oxygen from the top for four decades according to monitoring data that may not be comprehensive. One utility statement, one archived Bank page, and a thin baseline.

For an intake operator or a coastal authority on water like this, the next step is the baseline itself: a calibration phase that measures oxygen through the water column inside an agreed zone, against parameters written down beforehand. On SeaBreather, Alarivean's marine hull class, the oxidation stage runs as a contained process: affected water is drawn into a reactor on the vessel, treated through it, and returned oxygen-enriched with residuals inside the specification set for that discharge. Alarivean has not yet published open-water data for its advanced nano-oxidation systems. A client sees that data on their own water in a paid pilot, which carries no obligation to subscribe, or in the calibration phase of an Aquatic Prosperity Program (APP), where payment waits until Alarivean has delivered the water-quality outcomes it committed to.

A situation report starts it: https://alarivean.com/company/connect/