The ocean is losing its breath

Photo : Shoot-n-design

Photo : Shoot-n-design

The international research group GO2NE, including its co-chair Marilaure Grégoire , a physicist civil engineer, FRS-FNRS Research Director in the FOCUS Research Unit and ocean modelling specialist, warns in an article published in Science: Ocean oxygen levels are declining in the global and coastal ocean since 1960. The international team of scientists says that to halt this decline, the world needs to limit both climate change and water pollution by nutrients.

T

he oceans, a planet's lung is in bad shape. A study by the GO2NE (Global Ocean Oxygen Network), a working group set up in 2016 by the United Nations’ Intergovernmental Oceanographic Commission, has just produced worrying results. In their review published in Science, scientists report that over the past 50 years, the amount of water in the open ocean with zero oxygen has gone up more than fourfold. In coastal water bodies, including estuaries and seas, low-oxygen sites have increased more than 10-fold since 1950. Scientists expect oxygen to continue dropping as Earth warms. "We expect that oxygen levels will continue to decline as our planet warms up", explains Marilaure Grégoire "large areas of the Pacific Ocean, already naturally poor in oxygen, see their oxygen content decreasing  to critical or lethal levels for living organisms."

Low-oxygen zones are spreading around the globe. Red dots mark places on the coast where oxygen has plummeted to 2 milligrams per liter or less, and blue areas mark zones with the same low-oxygen levels in the open ocean. (Credit: GO2NE working group. Data from World Ocean Atlas 2013 and provided by R. J. Diaz).

At the ocean level, surface waters are well oxygenated by the dissolution of atmospheric oxygen and photosynthesis. Deep down, oxygen is consumed by respiration and its renewal depends on the existence of mechanisms capable of bringing well oxygenated surface water to depth, this is called ocean ventilation. "Climate change is disturbing this balance. The warming of surface waters reduces the intensity of water ventilation and the solubility of oxygen," recovers Marilaure Grégoire "and in coastal waters, it is nutrient pollution from the land that creates algal blooms that consume a lot of oxygen when they die and decompose." "Today’s processes are reminiscent of those thought to have promoted the occurrence of oceanic anoxic events (OAEs) that occasionally occurred during the past several hundred million years and that led to major extinction events. While a full-scale OAE would take thousands of years to develop, the small oxygen inventory of the ocean (~0.6% of that of the atmosphere) makes it particularly sensitive to perturbations of its equilibrium oxidative state. The consequences of such perturbations on the biogeochemistry and ecosystem state are not well known."     

In  areas traditionally called "dead zones," like those in Chesapeake Bay and the Gulf of Mexico, oxygen plummets to levels so low that many animals suffocate and die. In the Black Sea, the depth of the oxygenated layer has been reduced from 140 m in 1960 to 90m in 2015. Below that depth, the absence of oxygen only allows the development of bacteria that produces hydrogen sulfide, a toxic component for most animal life As fish avoid these low-oxygen areas, their habitat is reduced and they are more vulnerable to predators and fishing. In their report, scientists also point out that the problem goes far beyond the phenomenon of "dead zones". "Even smaller decreases in oxygen can slow species growth, hinder their reproduction and lead to disease and death". The change in oxygen levels can also trigger the release of dangerous chemicals such as nitrous oxide, a greenhouse gas up to 300 times more powerful than carbon dioxide, and toxic hydrogen sulfide. While some animals can thrive in dead zones, overall biodiversity falls. "To give an example, continues Marilaure Grégoire, in the Philippines, fish mortality due to deoxygenation in the catchment areas of a single city costs more than $10 million. Coral reefs, which are a major tourist attraction for many countries, are also threatened by lack of oxygen."

Low oxygen caused the death of these corals and others in Bocas del Toro, Panama. The dead crabs pictured also succumbed to the loss of dissolved oxygen. (Credit: Arcadio Castillo/Smithsonian)

A Three-Pronged Approach

To keep low oxygen in check, the scientists said the world needs to take on the issue from three angles:

  • Address the causes: nutrient pollution and climate change, via dramatically reducing  argricultural fertilizer use and GHG emissions.While neither issue is simple or easy, the steps needed to win can benefit people as well as the environment. Better septic systems and sanitation can protect human health and keep pollution out of the water.
  • Protect vulnerable marine life and ocean resources. With some low oxygen unavoidable, it is crucial to protect at-risk fisheries from further stress. According to the GO2NE team, this could mean creating marine protected areas or no-catch zones in areas animals use to escape low oxygen, or switching to fish that are not as threatened by falling oxygen levels.
  • Improve low-oxygen tracking worldwide. Scientists have a decent grasp of how much oxygen the ocean could lose in the future, but they do not know exactly where those low-oxygen zones will be. Enhanced monitoring, especially in developing countries and in very low oxygen areas, and numerical models will help pinpoint which places are most at risk and determine the most effective solutions.

The findings presented in this article and the many related activities to this international endeavor will contribute to the United Nations Decade of Ocean Sciences for Sustainable Development. Deepening our knowledge of the threats posed by deoxygenation, global warming, acidification and a multitude of other human-induced stressors will be key to sustainable management of oceans and seas.

Scientific reference

Breitburg D., Levin L., Oschlies, A, Grégoire M., Chavez,P., Conley, D., Garçon, V, Gilbert, D., Gutiérrez, D., Isensee, K., Jacinto, G., Limburg, K., Montes, I., Naqvi, W., Pitcher, G., Rabalais, N. Roman, M., Rose, K., Seibel, B., Telszewski, M., Yasuhara, M. and Zhang, J, 2018. Declining oxygen in the global ocean and coastal waters. Vol. 359, eaam7240. aam7240