One of the most important changes occurring in the oceans is that oxygen levels have been decreasing since at least the middle of the 20th century. Oxygen is essential for biological productivity in the oceans. The paleoclimate record has revealed that low oxygen levels are associated with extinctions. About 450 million years ago at the end of the Ordovician (488.3 to 443.7 million years ago) there was a mass extinction called the Ordovician–Silurian extinction event in which 85% of species extant during the Ordovician were lost. Recent research has associated this event with lowered oxygen levels. The mass extinction at the end of the Permian about 250 million years ago, called the Permian–Triassic extinction event, annihilated both marine and terrestrial biological organisms. Data on rocks from Spitsbergen and Italy and Slovenia indicate that the world’s oceans became anoxic, totally devoid of oxygen, at both low and high latitudes in the Late Permian. It is believed that such conditions were responsible for the greatest known mass extinction event.
Declining oxygen in open oceans
The open ocean is estimated to have lost 2% of its oxygen over the past 50 years. Repeated hydrographic observations revealed oxygen declines at locations ranging from the northeast Pacific and northern Atlantic to tropical oceans. Oxygen-minimum zones or OMZs are ocean zones where oxygen concentrations fall from the normal range of 4–6 milligrams per liter (mg/l) to below 2 mg/l. Over the past 50 years the total area of oxygen-minimum zones has expanded by 4.5 million square km of the equivalent of half the area of Canada.
The total volume of water completely devoid of oxygen (anoxic) has increased more than four-fold over the same period. There are two main causes of falling oxygen content in the open ocean, and both are the result of ocean warming. As the ocean warms less oxygen dissolves in the water. It is estimated that reduced oxygen solubility accounts for 15% of current total global oxygen loss. The effect is much higher in surface waters where it is believed to account for more than 50% of the oxygen loss. The remaining 85% of reduced oxygen content in the oceans is ascribed to the increased stratification of the oceans resulting from warming. Enhanced stratification inhibits the transfer of oxygen from surface waters to deeper waters.
Declining oxygen in coastal waters
Coastal waters are also losing oxygen, but through a different mechanism. The number of hypoxic sites resulting from sewage discharges and agricultural fertilizer runoff has continued to increase. Since 1950, more than 500 sites in coastal waters have reported low oxygen concentration or hypoxia. Before 1950 fewer than 10% of these systems were known to have hypoxia. Human population has nearly tripled since 1950 and there has been a 10-fold increase in global fertilizer use over the same period. When nutrients such as nitrogen and biomass from human waste and agriculture stimulate the growth of algae, the enhanced biological production in surface waters increases the deposit of organic matter to bottom waters where it decomposes thereby consuming oxygen. Low oxygen levels are difficult to recover from because there are feedback mechanisms that tend to keep oxygen levels low. Research has found that nitrogen discharges from rivers to coastal waters increased by 43% in just 30 years from 1970 to 2000. The main culprit is agriculture – more than three times as much nitrogen is derived from agriculture as from human sewage.
Oxygen is fundamental to oceanic biological and biogeochemical processes and its decline can cause major changes in ocean productivity and biodiversity. Analyses of direct measurements at sites around the world indicate that oxygen-minimum zones in the open ocean have expanded by several million square kilometers and that hundreds of coastal sites now have oxygen concentrations low enough to limit the distribution and abundance of animal populations.