Oceans and Carbon Dioxide: Known Causes Creating Unexpected Effects

Scientists found two unexpected effects of climate changes in the world’s oceans this week: the Southern Ocean is absorbing less carbon dioxide than expected, while fish raised in water with higher carbon dioxide levels are growing larger ear bones. Scientists have long known that climate change would effect ocean ecosystems, but neither of these effects were on anybody’s list.

Photo Credit: David SifryOcean

Scientists are pointing the finger at the hole in the ozone layer to explain why the Southern Ocean is absorbing less carbon dioxide than they expected. In theory, as carbon dioxide levels in the atmosphere rise, oceans have been expected to absorb more CO2 as one of the major “carbon sinks” on the globe. Currently the Southern Ocean absorbs about 15% of CO2 emissions.

While this appears to be the case in most oceanic water, the Southern Ocean’s carbon absorption has leveled off, something none of the climate models saw coming. Studies have revealed that carbon absorption in the Southern Ocean declined by 2.5 billion tons between 1987 and 2004, a major reduction.

Scientists in France have created integrated ocean and atmospheric climate models that incorporate changes in the makeup of the ozone layer. By comparing models with and without ozone depletion factored in, scientist led by Andrew Lenton of the University of Pierre and Marie Curie in Paris determined that the hole in the ozone layer could be the cause of lower absorption rates in the Southern Ocean. They say that changing atmospheric conditions are causing the region to be windier, which may account for less CO2 absorption.

One possible related effect of higher winds may be greater ocean water circulation, bringing more CO2-laden water to the surface- and higher CO2 levels means more acidic waters, generally bad news for ocean life like coral and squid. “This result illustrates how complex the chain of cause and effect can be in the Earth system. No one would ever have predicted from first principles that increasing CFCs would have the effect of decreasing uptake of ocean carbon dioxide,” says Andrew Watson of the University of East Anglia, UK.

So What Do Higher CO2 Levels Mean?

Another set of findings this week show that these higher CO2 levels and more acidic ocean waters could have a direct effect on how fish develop- specifically how fish ear bones grow.

David Checkley from the Scripps Institution of Oceanography is conducting tests on fish development in water with higher CO2 levels. Scientists anticipated that acidic water would affect fish bone growth, but they had originally expected problems with bone formation. What they did not expect was that the higher CO2 levels would actually cause fish to grow LARGER ear bones, specifically otoliths, the bones fish use to help orient themselves in water.

The experiments measured the size of the otoliths when they were 7-8 days old and found that sea bass raised in water with more than six times the current amount of CO2 the fish grew otoliths 15-17% larger than typical. With results this surprising, researchers repeated the tests, finding the same results. Subsequent tests showed that sea bass raised in water with 3.5x the current carbon dioxide levels grew otoliths 7-9% larger than is currently typical. Scientists say that at current rates of CO2 absorption into the ocean, levels in 2100 could be at 3.5x the current CO2 levels.

Otoliths are often studied because they grow in layers that can yield information about a fish’s age, much like trunk rings in a tree. The study showed that white sea bass raised in seawater with high CO2 levels had far more extensive otolith growth than fish raised in water with typical CO2 levels. They have yet to find how larger otoliths affect the sea bass.

“If fish can do just fine or better with larger otoliths, then there’s no great concern. The assumption is that if you tweak them in a certain way it is going to change the dynamics of how the otolith helps the fish stay upright, navigate and survive,” said Dr. Checkley.

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