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N. Thake

As world warms, ocean habitats shrink

Large swaths of the ocean could become inhospitable to many kinds of marine life as the world warms. That’s the conclusion of a new study, which finds that rising water temperatures will make perhaps 20% of the ocean less habitable for many fish and crabs by reducing available oxygen. Another study warns that corals fleeing the tropics for cooler waters may find themselves without enough light to photosynthesize adequately in the winter.

Most kinds of coral require water that is relatively warm, which generally limits them to the tropics. But if water gets too hot, corals may die. So the common wisdom is that over the next decades coral will expand north and south into water that used to be too nippy.

But marine biologist Paul Muir of the Museum of Tropical Queensland in Townsville, Australia, thinks they will face a limit. While studying corals in eastern Australia and Japan, he was startled to notice that these species—inhabiting some of the southernmost and northernmost ranges of staghorn coral—seemed restricted to shallow water. That was intriguing, because the shallows are risky places for them: Strong waves can break their delicate branches, and water can occasionally get too warm or salty for comfort.

Curious to see if there were any general patterns to the depth ranges of coral species, Muir surveyed museum specimens and graphed the depths at which 104 species of staghorn coral had been collected at various latitudes. When he saw the results, he says, “my heart definitely skipped a beat.” For every degree of latitude farther north or south of the equator, species of staghorn coral live 0.6 meters closer to the surface, his team reports online today in Science

The reason, he and colleagues say, is how sunlight varies with latitude. Corals require light for their symbiotic algae, called zooxanthellae, to photosynthesize; sunlight is most intense at the equator and weakens to the north and south, especially in the winter. That would explain why corals at higher latitudes move to shallower water, where light is stronger. Muir and his colleagues conclude that limited light could “severely constrain” how far north and south corals might be able to expand. As the tropics get too warm, this would mean their overall habitat will shrink, with uncertain ecological consequences.  

Fish may face a different constraint. Several years ago, Curtis Deutsch, an oceanographer at the University of Washington, Seattle, read a scientific article suggesting that global warming would harm fish in the North Sea by depriving them of oxygen. It’s a double punch: Warm water contains less oxygen, but it also increases the amount required by fish and other cold-blooded organisms, by speeding their metabolism. To figure out the extent of this risk more broadly, Deutsch and his colleagues needed a measure of how much oxygen marine animals need to thrive at various temperatures. They derived an index by comparing experimental data that others had gathered on a dozen species—fish, crustaceans, and a sea squirt.

For four species in the North Atlantic and North Sea—cod, sea bream, eelpout, and rock crab—they compared this requirement with future marine oxygen levels. The discouraging news is that they all will lose about 20% of their habitat by 2100 under current trends in ocean warming, the team reports online today in Science. Other species in midlatitude northern oceans could face greater losses of habitat, up to 50%. One benefit of the research, says marine ecologist Joanie Kleypas of the National Center for Atmospheric Research in Boulder, Colorado, is that understanding metabolic constraints will help in predicting how the habitats will change, providing useful information to fisheries managers, for example.

It’s possible that natural selection will help species respond to the stresses of warming. Oxygen stressed fish, for example, might eventually evolve more efficient gills. And corals have tricks for dim conditions, such as flattening branches to collect more light and relying less on photosynthesis and more on catching plankton with stinging tentacles. Given the pace of climate change, nature will need to be resourceful. “There are basic limits to how far you can stretch organisms,” Kleypas says. “That’s why there have been extinctions in the past.”