‘Hypoxia’ is a well-used word among JEB authors. These past 10 years have seen it used in over 80 titles, 200 abstracts and 800 articles, which means great effort is being put towards understanding the physiological responses of animals to low oxygen. Good thing, because rising atmospheric temperatures are making the Earth's aquatic habitats more hypoxic. Sunke Schmidtko and his team at GEOMAR's Helmholtz Centre for Ocean Research in Kiel, Germany, were familiar with the biotic and abiotic theories underlying this trend, but decided to quantify just how much oxygen the World Ocean has lost in recent years.
The Kiel team isn't the first to inventory the World Ocean for oxygen, but they certainly went farther and deeper than anyone before. They started by locating five publicly accessible databases from the likes of NOAA and Pangaea and then used these to piece together an elaborate profile of oceanic oxygen at 78 depth levels over the past 50 years. This more than quadrupled the volume of water previously analysed for oxygen and allowed them to paint a detailed picture of how the past 50 years has affected the World Ocean's oxygen content. Perhaps unsurprisingly, they have not been kind.
From the broadest perspective, Schmidtko and his colleagues found that the World Ocean currently contains around 227 petamol (1015 mol) of oxygen. That's a lot of oxygen – over seven quadrillion kilograms of the stuff – but it's about 4.8 petamol less than it was in 1960, meaning 150 trillion kilograms of oxygen have disappeared from the World Ocean over the past 50 years. This begged two questions: how were these changes distributed throughout the ocean?; and where did all the oxygen go?
The team set about answering the first question by directing their analyses towards particular ocean basins. All 10 basins they examined lost oxygen, but some lost more than others. The largest fraction of oxygen (∼40%) was lost in the North and Equatorial Pacific, while the smallest fraction (∼1%) was lost in the North Atlantic. Interestingly, the team found that the rapidly changing Arctic Ocean swung well above its weight class in terms of its contribution to oxygen depletion: 7.6% of the total lost oxygen despite containing a meagre 1.2% of the world's seawater. Perhaps most unsettling, the World Ocean's anoxic regions have quadrupled in size since 1960.
As for where all the oxygen went, the team's prime suspect was reduced oxygen solubility; as rising atmospheric temperatures drag world oceanic temperatures up with them, less oxygen is able to dissolve into the seawater, owing to water's physicochemical properties. The team combed through their temperature data and found that reduced oxygen solubility did indeed play a role – particularly in the ocean's uppermost 1000 m – but only accounted for 15% of all oxygen lost. To account for the other 85%, the team suspected biological and chemical patterns of oxygen consumption in combination with complex ocean ventilation dynamics. These processes are each influenced by climate variability in ways that could theoretically explain the Kiel team's results, but future studies will be needed to address these hypotheses directly.
So, the world ocean is definitely becoming more hypoxic, impacting the marine ecosystem as a whole in ways that are complex and, in many cases, not good. It also means that there will be plenty of work for those researchers publishing on the hypoxic adaptations of animals, keeping them busy for years to come as we attempt to get to grips with this complex ecological calamity.