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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Ocean deoxygenation | 3/4 | https://en.wikipedia.org/wiki/Ocean_deoxygenation | reference | science, encyclopedia | 2026-05-05T07:35:51.047511+00:00 | kb-cron |
=== Vertical extent of low oxygen conditions === The vertical extent of low oxygen conditions is also variable, and areas of persistent low oxygen have annual variation in the upper and lower limits of oxygen-poor waters. Typically, OMZs are expected to occur at depths of about 200 to 1,000 meters. The upper limit of OMZs is characterized by a strong and rapid gradient in oxygenation, called the oxycline. The depth of the oxycline varies between OMZs, and is mainly affected by physical processes such as air-sea fluxes and vertical movement in the thermocline depth. The lower limit of OMZs is associated with the reduction in biological oxygen consumption, as the majority of organic matter is consumed and respired in the top 1,000 m of the vertical water column. Shallower coastal systems may see oxygen-poor waters extend to bottom waters, leading to negative effects on benthic communities. Many persistent OMZs have increased in thickness over the last five decades. This happened because the upper limit of the OMZ became shallower and also because the OMZ expanded downward.
=== Variations in temporal duration === The temporal duration of oxygen-poor conditions can vary on seasonal, annual, or multi-decadal scales. Hypoxic conditions in coastal systems like the Gulf of Mexico are usually tied to discharges of rivers, thermohaline stratification of the water column, wind-driven forcing, and continental shelf circulation patterns. As such, there are seasonal and annual patterns in the initiation, persistence, and break down of intensely hypoxic conditions. Oxygen concentrations in open oceans and the margins between coastal areas and the open ocean may see variation in intensity, spatial extent, and temporal extent from multi-decadal oscillations in climatic conditions. Coastal regions have also seen expanded spatial extent and temporal duration due to increased anthropogenic nutrient input and changes in regional circulation. Areas that have not previously experienced low oxygen conditions, like the coastal shelf of Oregon on the West coast of the United States, have recently and abruptly developed seasonal hypoxia.
== Impacts ==
=== Ocean productivity === Ocean deoxygenation poses implications for ocean productivity, nutrient cycling, carbon cycling, and marine habitats. Studies have shown that oceans have already lost 1-2% of their oxygen since the middle of the 20th century, and model simulations predict a decline of up to 7% in the global ocean O2 content over the next hundred years. The decline of oxygen is projected to continue for a thousand years or more. Ocean deoxygenation results in the expansion of oxygen minimum zones in the oceans. Along with this ocean deoxygenation is caused by an imbalance of sources and sinks of oxygen in dissolved water. The change has been fairly rapid and poses a threat to fish and other types of marine life, as well as to people who depend on marine life for nutrition or livelihood. As low oxygen zones expand vertically nearer to the surface, they can affect coastal upwelling systems such as the California Current on the coast of Oregon (US). These upwelling systems are driven by seasonal winds that force the surface waters near the coast to move offshore, which pulls deeper water up along the continental shelf. As the depth of the deoxygenated deeper water becomes shallower, more of the deoxygenated water can reach the continental shelf, causing coastal hypoxia and fish kills. Impacts of massive fish kills on the aquaculture industry are projected to be profound.
=== Marine organisms and biodiversity === The viability of species is being disrupted throughout the ocean food web due to changes in ocean chemistry. As the ocean warms, mixing between water layers decreases, resulting in less oxygen and nutrients being available for marine life. Short term effects can be seen in acutely fatal circumstances, but other sublethal consequences can include impaired reproductive ability, reduced growth, and increase in diseased population. These can be attributed to the co-stressor effect. When an organism is already stressed, for example getting less oxygen than it would prefer, it does not do as well in other areas of its existence like reproduction, growth, and warding off disease. Additionally, warmer water not only holds less oxygen, but it also causes marine organisms to have higher metabolic rates, resulting in them using up available oxygen more quickly, lowering the oxygen concentration in the water even more and compounding the effects seen. Finally, for some organisms, habitat reduction will be a problem. Habitable zones in the water column are expected to compress and habitable seasons are expected to be shortened. If the water an organism's regular habitat sits in has oxygen concentrations lower than it can tolerate, it will not want to live there anymore. This leads to changed migration patterns as well as changed or reduced habitat area. Long term effects can be seen on a broader scale of changes in biodiversity and food web makeup. Due to habitat change of many organisms, predator-prey relationships will be altered. For example, when squeezed into a smaller well-oxygenated area, predator-prey encounter rates will increase, causing an increase in predation, potentially putting strain on the prey population. Additionally, diversity of ecosystems in general is expected to decrease due to decrease in oxygen concentrations.
=== Effects on fisheries ===