Reports that the world’s oceans showed a two percent decline in oxygen content over the past 50 years gathered a lot of press attention a couple of weeks ago. This was not the first time that oceanographers have reported human impacts on the marine environment, and it is not likely to be the last.
Oxygen accumulates in seawater as a result of photosynthesis by marine algae, also known as phytoplankton. If the rate of photosynthesis goes down, so will the oxygen content. We have no good idea that the rate of photosynthesis in the world’s oceans has declined, but our measurements of increasing seawater acidity and mercury content suggest that human impacts on the sea can extend globally. About half of the world’s photosynthesis, known as net primary production, occurs in the sea. Increasing seawater acidity is likely to affect coccolithophores—a major phytoplankton group with carbonate shells that dissolve in acid.
Oxygen might also decline if the circulation of waters between the surface ocean and the deep sea has declined. Oceanographers call this process “overturn,” which renews or “ventilates” the oxygen content in the deep sea, which is not in contact with the atmosphere. Several oceanographic studies have indicated that the overturn of waters in the North Atlantic Ocean has slowed in recent years, perhaps as a result of warmer surface waters. Warmer surface waters also hold less oxygen, which is more soluble in colder waters.
Oxygen is consumed in the deep ocean by the decomposition of sinking dead materials.It is possible that the downward movement of these materials has increased during the past few decades, increasing the rate of oxygen loss.
Of course, it is possible that a two percent decline in oxygen, recorded over 50 years, stems from changes in the accuracy of measurements and from the difficulty of synthesizing such a large dataset. Recording such a small change, over a long period, in such a large volume of water is not easy.
The oxygen content of Earth’s atmosphere has declined very slightly as a result of the burning of fossil fuels. If the analysis of the oceans holds up, it will be another alarming harbinger of ongoing human impact on our planet.
When I was growing up, we took family vacations on the shore of Cape Cod. Looking out into the Atlantic, my father would say that the ocean could dilute an infinite amount of wastes. Now I am not so sure.
References
Behrenfeld, M.J. and 9 others. 2006. Climate-driven trends in contemporary ocean productivity. Nature 444: 752-755.
Keeling, R.F. and S.R. Shertz. 1992. Seasonal and interannual variations in atmospheric oxygen and implications for the global carbon cycle. Nature 358: 723-727.
Kelly, K.A., K. Drushka, L. Thompson, D. Le Bars, and E.L. McDonagh. 2016. Impact of slowdown of Atlantic overturning circulation on heat and freshwater transports. Geophysical Research Letters 43: 7625-7631
Lozier, M.S. 2012. Overturning in the North Atlantic. Annual Review of Marine Science 4: 291-315.
Schmidtko, S., L. Stramma, and M. Visbeck. 2017. Decline in global oceanic oxygen content during the past five decades. Nature 542: 335-339.