Declining oxygen in oceans threatens ocean health

The phrase “more data are needed” is unfortunately a common refrain in the Earth sciences. But better data are exactly what is needed to understand declining oxygen levels in Earth’s oceans and what that will mean for marine life in the future, according to a report published on November 15, 2011 in EOS, Transactions of the American Geophysical Union.

Ocean oxygen levels are declining, and it’s not just limited to coastal areas where nutrient inputs often trigger oxygen depleting algae blooms. Since the 1950s, scientists have measured a steady decline of oxygen in open waters of the North Pacific Ocean on the order of about 0.3% per year. The steady decline in ocean oxygen levels was discovered amongst a roughly 20-year cycle of fluctuating oxygen that is driven by the effects of lunar precession (i.e., changes in the rotational axis of the moon) on the tides.

Scientists are currently trying to identify what is causing the current declines in ocean oxygen. According to the authors of the EOS report:

It is unclear whether the loss throughout the basins in the open ocean is a long-term, nonperiodic trend related to climate change, the result of natural cyclical processes, or a combination of both. If related to climate change, a number of important factors may be involved, including decreased solubility of oxygen as waters warm, decreased ventilation at high latitudes associated with increased ocean stratification, and changes in respiration in the ocean interior.

The EOS report was produced by 18 scientists from a variety of research organizations in the United States who participated in a 2010 workshop on ocean deoxygenation sponsored by the NASA Astrobiology Institute.

Lead author Paul Falkowski is a professor in the Departments of Geological Sciences and Marine and Coastal Sciences at Rutgers University, where he studies biological oceanography. Biological oceanography is a field of study that seeks to understand what controls the distribution and abundance of different types of marine life, and how living organisms influence and interact with processes in the oceans.

To help understand the causes and consequences of ocean deoxygenation, the scientists reviewed geological records from numerous low oxygen events that occurred thousands to millions of years ago in Earth’s past. From these records, the scientists were able to deduce that both the configuration of the continents and climate are critical factors that can influence oxygen depletion. Zones of low oxygen in the ocean were observed to be contracted during cold periods and greatly expanded during warm intervals.

According to authors of the report:

The overwhelming message emerging from the historical record is that the extent of oxygen-depleted waters is very sensitive to climate.

The geological record also demonstrates that low oxygen levels in the ocean can have devastating impacts on marine life. Notably, widespread and extremely low ocean oxygen levels during the Permian-Triassic extinction event approximately 252 million years ago were associated with the loss of about 90% of all marine animal taxa.

Future trends in ocean health are difficult to predict due to the scarcity of long-term data on ocean oxygen levels. A decline of 1 to 7% in global ocean oxygen over the next century is possible according to climate models, and the decline could persist for 1,000 years or more. Such a decline in ocean oxygen could lead to a variety of negative impacts including habitat loss, decreases in ocean biodiversity and productivity, alterations in biogeochemical cycling and a reduction in fisheries.

According the authors of the EOS report, the collection of better data will be critical for understanding what the future holds in store for the oceans. In the report, the authors state:

Data must be obtained from remote regions and sustained over long periods (multiple decades). The needed time series of ocean oxygen and physical, chemical, and biological phenomena requires a commitment to such long-term observing campaigns, without which our ability to quantify the long-term health of the oceans is put at risk, along with society’s potential ability to sustainably obtain resources from ocean margins in the coming centuries.

Bottom line: Eighteen scientists published a report on November 15, 2011 calling for the need for more data, to understand declining oxygen levels in Earth’s oceans.

[Excerpts reproduced by permission of American Geophysical Union Source from Falkowski P.G., et al. (2011) Ocean deoxygenation: past, present, and future. EOS, Transactions, American Geophysical Union. Volume 92(46):409-410.]

[Disclaimer: I am a member of the American Geophysical Union]

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