There is a layer in the Eastern Tropical Pacific where the water’s oxygen content drops to the point where most fish cannot survive, generally between the surface and 700 meters below the surface. The layer has existed for many years; beginning in the middle of the 20th century, oceanographers identified and charted oxygen minimum zones, treating them as stable, rather unusual aspects of deep-ocean chemistry. They weren’t growing very quickly. They weren’t regarded as an urgent ecological emergency. They were a component of the ocean’s background state, found in some basins, significant to scientists researching ocean biogeochemistry, and mostly undetectable to the general public.
The description of “not growing” has become inaccurate during the last few decades. Since 1960, the amount of ocean water with oxygen levels too low to sustain the majority of marine life has increased by about 8% worldwide. This trend may seem insignificant when compared to the volume of ocean it actually represents. The expansion is not evenly distributed, with some areas suffering much greater losses than others, and it is not stopping. Deoxygenation is increasing rather than decreasing as a result of the physical and chemical processes driving it.
Warmth is the first step in the fundamental mechanism. Basic physical chemistry explains why a warm carbonated drink goes flat more quickly than a cold one: warm water contains less dissolved oxygen than cold water. Surface water’s oxygen content has dropped as ocean surface temperatures have increased due to ongoing greenhouse gas forcing. However, stratification is how the most important effect works. The density differential between the warm upper layer and the cold water below grows as the ocean’s surface heats because warm water is less dense than cold water. The process by which oxygenated surface water replenishes the oxygen in deeper levels is called vertical mixing, and it is lessened by this increased stratification. The oxygen minimum zone spreads outward and downward because the supply of fresh oxygen from above is being cut off, not because it is losing oxygen more quickly.
A second mechanism that intensifies the first is added by nutrient loading from agricultural runoff. Phytoplankton blooms that are greater than the ecology can effectively process are fueled by nitrogen and phosphorus washing into coastal and eventually open ocean settings. As those blooms die and sink, dissolved oxygen in the mid-water column is consumed by the organic material’s bacterial breakdown. The Gulf of Mexico dead zone is the most researched example in the United States, but similar dynamics can be found in the Arabian Sea and the Bay of Bengal. In areas where both agricultural runoff and thermal stratification are present, the two drivers compound each other, causing deoxygenation at rates that neither would produce on its own.
The ecological repercussions condense into a chain of cascades. Squid, crabs, and fish that need oxygen to breathe migrate away from growing minimum zones and congregate in the oxygenated surface layers where their physiology is still supported. Fish packed into narrow surface layers are far easier to locate and catch than fish dispersed throughout a whole water column, a result of habitat compression that the fishing industry has documented without always precisely linking to deoxygenation. When all of the fish are in the upper 50 meters, commercial fishing equipment made for the three-dimensional ocean becomes significantly more effective. In the short term, the resulting catch rates appear to be successful, but in the long run, they indicate unsustainable pressure.
Not only does the biological community shrink in the most severely deoxygenated zones, but it also transforms. Because they can metabolize without oxygen, anaerobic bacteria occupy the ecological space left by oxygen-dependent organisms. Hydrogen sulfide, one of its metabolic byproducts, is harmful to the majority of marine life at concentrations that anaerobic bacterial mats can generate in large quantities. These bacteria do not transport energy upward toward fish and marine animals as part of the marine food chain. They are a dead end because they eat organic matter and release substances that make it impossible for anything else to retake the area.
A feedback link between OMZ expansion and ocean productivity in general is added by the disruption of the nitrogen cycle. Denitrifiers, or bacteria that break down nitrate and extract fixed nitrogen from the water column, are among the bacteria that flourish in oxygen minimum zones. In most of the ocean, nitrogen is a vital ingredient that restricts the growth of phytoplankton. More denitrification takes place when OMZs grow, eliminating the nutrient that is essential to surface productivity. It is a slow-acting relationship that eventually reduces the productivity of the oxygen-rich surface ocean, which is the reverse of what a warming, crowded, fishing-stressed ocean need from its base of primary production.
