Warming Oceans Starved of Oxygen

Global warming is depleting oxygen from the ocean waters and threatening the survival of fisheries and the entire marine ecosystem. Dr. Mae-Wan Ho

Large volumes of ocean waters at intermediate depths are starved of oxygen. The latest studies confirm that this oxygen-poor zone has been expanding vertically for the past fifty years [1], and will continue into the foreseeable future [2].

Tropical oceans deprived of oxygen

Oxygen has a very low solubility in seawater to the extent that 99 percent of it is in the atmosphere at any one time. Yet this dissolved oxygen not only supports the entire ocean biosphere, but also has major impacts on the global carbon and nitrogen cycles. These concentrations are very sensitive to changes in air-sea fluxes and interior ocean dynamics. Important organisms such as fish are stressed or die under ‘hypoxic' conditions, which can be anywhere between ~ 60 to 120 m mol/kg depending on the species; the fully oxygenated pre-industrial level is ~170 m mol/kg [2]. Regions with oxygen concentrations <10 m mol/kg are ‘suboxic', whereas ‘anoxic' regions have no dissolved oxygen. At present, the intermediate-depth low oxygen layers, called the oxygen-minimum zone (OMZ), are suboxic in the eastern tropical Pacific Ocean and the northern tropical Indian Ocean, and hypoxic in the tropical Atlantic Ocean.

Large fluctuations in the ocean's oxygen have occurred in the past history of the earth. The anoxic ocean at the end of the Permian 251 million years ago was associated with elevated atmospheric CO₂ and massive extinctions on land and sea.

Disturbances in oxygen levels everywhere in the world's oceans

Repeated surveys have indicated that the upper 3 km of the oceans have warmed and intermediate waters of high-latitude have freshened over the past decades [3]. Studies with models have confirmed the suspicion that these are the consequences of global warming from burning fossil-fuels and other human activities. The surveys have also detected a decrease in dissolved O₂ in intermediate waters in all the oceans with small increases in deeper waters of the North Pacific and South Indian Oceans. Strong fluctuations in oxygen have been observed in the upper 100 m from year to year and over decades; and long-term changes are reported in the subpolar and subtropical regions [1]. For example, the subarctic Pacific Ocean Station Papa (50 ^o N, 145 ^o W) has recorded oxygen concentrations dropping from depths of 100 to 400 m between 1956 and 2006.

A global reduction in dissolved O₂ is predicted in ocean general circulation models driven by increasing greenhouse gases, warming of the ocean surface and enhanced stratification (formation of static layers) of ocean water [3]. Stratification has two opposing effects on oxygen concentrations below the surface of oceans. First, it reduces the upwelling of nutrients from deeper waters to the surface, thus decreasing photosynthesis and the associated flux of organic detritus into the ocean interior, which is often referred to as the ‘biological pump'. Reducing the rate of this pump increases subsurface O₂ by reducing its rate of utilization. Second, stratification limits the downward transport of O₂ from the well-oxygenated surface waters into the ocean interior (from upwelling and overturning), thereby reducing subsurface O 2 concentration. In the modelling studies, the effect of stratification on O₂ transport was found to exceed that on subsurface utilization, leading to a net decrease in O₂ , as observed.

In the study projecting future O₂ depletion of the oceans [2], a slow-down of just 15 percent in overturning from stratification decreases ocean exchange by up to 77 percent, and mean ocean O₂ by up to 54 percent; compared with the maximum 23 percent decrease in O₂ with no slow-down in overturning.

Oxygen in the oceans can decrease much faster due to positive feedbacks acting at different spatial and temporal extents.

Reconstructing ocean history for the past 50 years

The international research team led by Lothar Stramma at Kiel University in Germany reconstructed and analyzed oxygen levels in select areas of the tropical oceans where there are quality-controlled historical data collected since 1960.

They found reductions in minimum oxygen concentration and vertical expansion of the OMZ since 1960 in three areas of the tropical Atlantic Ocean. The most striking is in the oxygen-poor region of the tropical North Atlantic (10 ^o to 14 ^o N, 20 ^o to 30 ^o W), where core oxygen values in the OMZ has declined and the OMZ expanded vertically with time. The vertical extent of the layer with oxygen concentration <90 m mol/kg increased 85 percent from a thickness of 370 m in 1960 to 690 m in 2006. Similar, less striking changes occurred in the OMZ of the central equatorial Atlantic (3 ^o S to 3 ^o N, 18 ^o to 28 ^o W), and at the tropical South Atlantic (14 ^o to 8 ^o S, 4 ^o to 12 ^o E).

The OMZ in the tropical North and South Pacific Oceans reaches suboxic (and in the most oxygen-deprived regions, nearly anoxic) levels, so it is difficult to detect further changes, especially as the data are too sparse.

In the Indian Ocean, the lowest oxygen values in the OMZ are not located in the eastern tropics as in the Atlantic and Pacific, but to the north in the Arabian Sea and the Bay of Bengal. In addition, minimum oxygen concentrations within the Indian Ocean OMZ are generally deeper (near 800 m) than in the other two oceans. Oxygen values in the OMZ are suboxic and data too sparse to reveal any long-term change.

Oxygen in oceans linked to phytoplankton decline?

Oxygen was found to decrease from 0.09 to 0.34 m mol/kg/y at depths of 300 to 700 in the reconstruction study [1], somewhat less than those reported previously in the North Pacific at 100 to 400 m [4], which were 0.39 to 0.7 m mol/kg/y. These trends not only threaten the marine ecosystems and fisheries, but also impact on the carbon cycle and global warming.

There may be a more fundamental cause of oxygen decline in the oceans that has not been mentioned in the studies: the failure of phytoplankton in the oceans to regenerate oxygen by photosynthesis. Phytoplankton is responsible for most of the primary productivity in the oceans that supports the entire marine food web and account for approximately half of the planet's primary production from photosynthesis. Yet it represents only 0.2 percent of the global primary producer biomass [5]. That's because the phytoplankton grows more than a thousand times faster than green plants on land, turning over in 2 to 6 days compared to ~19 years on average for land plant. However, warming and acidification of the oceans have seriously damaged the phytoplankton [6] ( Shutting Down the Oceans Act III: Global Warming and Plankton; Snuffing Out the Green Fuse and other articles in the series, SiS 31). Respiration increases in the ocean ecosystem faster than photosynthesis, turning large parts of the oceans into carbon source instead of carbon sink [7] ( Oceans Carbon Sink or Source , SiS 31).

This is corroborated by the disturbing downward trend in the oceans' primary productivity. Studies based on ocean colour measurements have shown that the ocean's net primary productivity (NPP), a measure of total plant growth in terms of C, has declined by more than 6 percent globally since the 1980s [8]. N early 70 percent of the NPP global decline occurred in the high latitudes above 30 degrees. In the high latitudes, rates of plankton growth declined by 7 percent in the North Atlantic basin, 9 percent in the North Pacific basin, and 10 percent in the Antarctic. This trend has continued, and is especially strong in the tropics [9]. The underlying cause is the decrease in supply of nutrients to the upper mixed layer because of climate-related changes in ocean circulation and stratification reducing vertical nutrient fluxes.

The decrease in ocean NPP will surely impact on fisheries. A study on the marine ecosystem of the continental shelf of western North America published in 2005 revealed strong linkages between phytoplankton, zooplankton and resident fish, extending to regional areas as small a 10 000 square kilometres [10].

More importantly, as phytoplankton are the fastest growing photosynthetic organisms on earth, they are literally the live line of the entire biosphere, both land and sea, in terms of regenerating oxygen. The decline of phytoplankton may be causing a simultaneous decrease in atmospheric oxygen that's in large excess of the amount that could be accounted for from burning fossil fuels [11] (see O₂Dropping Faster than CO₂ , Implications for Climate Policies , SiS 44). When the phytoplankton goes, we can expect anoxic oceans and mass extinctions of air-breathing animals, as had happened at the end of the Permian 251 million years ago; only this time round, the human species will be included in the death toll,

Article first published 10/08/09

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References

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