Study Shows Toxicity from Increasing Temperatures May Put Fisheries at Risk

It happened in the past, and it could happen again. That is the warning from ocean researchers at the University of Toronto and the University of California, Santa Cruz in a study reported recently in Science that reveals how an increase in CO₂ in Earth's atmosphere over 50 million years ago, radically altered the chemistry of the planet's oceans.

The scientists state if contemporary global carbon emissions continue to increase, the future of a number of fish species in the oceans could be in jeopardy.

Our study shows that global warming is not only about extreme weather events, or hotter summers, but it has the potential to alter the ocean structure with unknown consequences for fisheries

Professor Uli Wortmann in the Department of Earth Sciences in the Faculty of Arts & Science at U of T, & co-author of the study

Also, this is not the first time this has occurred.

"We show that the last time large amounts of CO₂ were injected into the atmosphere, not only did the planet get hot - which is known as the so called Paleocene-Eocene Thermal Maximum, about 55 million year ago - but it also changed the chemistry of the ocean quite markedly," said Wortmann. He collaborated with U of T PhD student Weiqi Yao, lead author of the research, and Adina Paytan of the Institute of Marine Science at the University of California, Santa Cruz.

It is broadly accepted that an increase in CO₂ results in warming, which then leads to less oxygen in Earth’s oceans. Less oxygen allows sulfate-eating bacteria to flourish, which creates hydrogen sulfide—a wide-spectrum toxin which is fatal in small concentrations.

This will affect fish species that live or dive deep in the ocean. Most notably it would impact high level predators like tuna and whales, which in turn would have a ripple effect on fish species living in the commercially more relevant shallow waters - up to 200 metres below sea level - and those living in the middle depth approximately 1,000 metres below the surface of the ocean.

The middle and deep ocean are the largest and least explored part of our planet, home to giant squid and weird deep sea fish but also some commercially important fish like the Patagonian Toothfish, otherwise known as Chilean Sea Bass.

Professor Uli Wortmann in the Department of Earth Sciences in the Faculty of Arts & Science at U of T, & co-author of the study

The Paleocene-Eocene Thermal Maximum (PETM) obtains its name from the boundary between two periods in Earth's past. The quantity of CO₂ input into the atmosphere during the period was like the predicted trajectories of CO₂ in the atmosphere towards the year 2100 adopted by the United Nations' Intergovernmental Panel on Climate Change, if emissions continue to increase across the 21^st century. It is seen as a good model for the fossil fuel burning activity happening presently and is passionately explored by researchers in the context of climate change.

In the course of investigating how ocean chemistry evolved over the last 60 million years, Yao came across some rare data - hinted at in earlier low-resolution work by Paytan - that Wortmann suggests conventional wisdom would consider defective. Upon observing that it overlapped with the PETM interval, the researchers searched harder for more data and were able to reveal that it was no measurement artifact, but a real signal.

"Increased atmospheric CO₂ concentrations go hand in hand with oxygen loss in the ocean, and this is the first demonstration that the CO₂ release from human activity could be large enough to turn parts of the ocean into a toxic brew," said Wortmann.

While the scientists cannot pinpoint how long it would take for the impact of increased CO₂ levels to become obvious, they say the change would be rapid.

"Our study is another piece in the puzzle," said Yao. "It highlights an often overlooked aspect of the global climate change debate: what will happen to marine fisheries in a warming world?"

The findings are illustrated in the research paper "Large-scale ocean deoxygenation during the Paleocene-Eocene Thermal Maximum", reported this month in Science. Support for the study was provided by the Natural Sciences and Engineering Research Council of Canada and the National Science Foundation.