Ocean depths contain regions where life struggles. These “dead zones” have extremely low oxygen levels, making them inhospitable for most marine organisms. The Arabian Sea hosts one of the most extensive and intense such phenomena on Earth.
What is an Ocean Dead Zone?
An ocean dead zone is a marine region where dissolved oxygen levels are severely depleted. Scientists refer to these conditions as hypoxia when oxygen is low, and anoxia when oxygen is completely absent. These environments arise when oxygen supply cannot meet biological demands, leading to a lack of oxygen for marine life. Most marine animals, including fish, crustaceans, and many invertebrates, require dissolved oxygen to survive. When oxygen concentrations drop below a certain threshold, these organisms either flee the area or perish. Dead zones are found at mid-water depths, between 650 to 2,600 feet (200 and 800 meters) below the surface.
The Arabian Sea’s Unique Dead Zone
The Arabian Sea hosts the world’s largest and thickest oxygen minimum zone (OMZ), often described as a “dead zone.” This expansive area, specifically within the Gulf of Oman, has been measured at approximately 63,700 square miles, an area comparable to the size of Florida or Scotland. Unlike some shallower coastal dead zones, the Arabian Sea’s OMZ extends through a significant portion of the water column, from about 330 feet (100 meters) down to 4,900 feet (1,500 meters).
This dead zone is a persistent feature of the Arabian Sea, known since the early 1960s. Recent research indicates an increase in its size, suggesting expansion. The area has shifted from low oxygen levels around its peripheries to minimal oxygen across a wider region.
Factors Contributing to its Formation
The formation and persistence of the Arabian Sea dead zone result from a combination of natural oceanographic processes and human-induced changes. Naturally occurring oxygen minimum zones are a feature of some ocean basins, caused by an imbalance between oxygen supply from the atmosphere and oxygen consumption by decaying organic matter. In the Arabian Sea, unique current patterns and strong monsoon winds contribute to this imbalance. These winds drive upwelling, bringing nutrient-rich deep waters to the surface, which fuels the growth of phytoplankton. The subsequent decay of this abundant organic matter as it sinks consumes large amounts of oxygen in the deeper waters.
Human activities further exacerbate these natural conditions. Nutrient runoff from agricultural and industrial activities along the coast introduces excess nutrients into the sea, intensifying algal blooms and subsequent oxygen depletion. Climate change also plays a significant role, as warmer waters hold less dissolved oxygen, and increased stratification due to warming makes it harder for oxygen from the surface to mix into deeper layers. Research indicates that warming of the Arabian Gulf could cause the intensification and expansion of the Arabian Sea dead zone, as warmer Gulf waters are less likely to sink and ventilate the OMZ.
Consequences for Marine Life and Humans
The expansion of the Arabian Sea dead zone impacts marine biodiversity. Fish, crustaceans, and other mobile organisms cannot survive in oxygen-depleted waters and are forced to migrate to shallower, oxygenated areas, confining them to a smaller habitat. Organisms unable to escape, such as slow-moving crabs and other shellfish, will suffocate. This reduction in habitable space and the direct mortality of marine life disrupt the entire food web.
These ecological shifts have direct consequences for human communities that depend on the ocean. Local fisheries face reduced catches and altered species distributions, impacting livelihoods and food security. Anoxic conditions can also alter nitrogen’s chemical cycling, leading to nitrous oxide production, a potent greenhouse gas approximately 300 times more powerful than carbon dioxide.
Investigating the Deep Waters
Studying the Arabian Sea dead zone presents unique challenges due to its vastness, depth, and the historical presence of piracy and geopolitical tensions in the region. Despite these difficulties, scientists employ advanced technologies to gather data. Specialized underwater robotic gliders, such as Seagliders, aid this research.
These torpedo-shaped autonomous vehicles can reach depths of up to 3,300 feet (1,000 meters) and operate for months at a time, collecting data on oxygen levels, temperature, and salinity in areas previously inaccessible to traditional research vessels. Scientists retrieve the collected information via satellite, allowing them to assess the extent of oxygen depletion and better understand the complex underwater mechanics that influence oxygen distribution. Combining this glider data with high-resolution computer simulations helps researchers forecast oxygen levels and understand seasonal shifts in the dead zone.