Temperature alone does not explain where in the ocean an animal can live.
Climate change is expected to make large stretches of the oceans uninhabitable for marine animals and it won’t just be because they can’t handle higher temperatures, scientists say.
“Temperature alone does not explain where in the ocean an animal can live. You must consider oxygen: how much is present in the water, how well an organism can take up and utilize it, and how temperature affects these processes,” explains Curtis Deutsch, an associate professor of oceanography at the University of Washington in the United States.
A variety of factors determine where certain marine species can live in the oceans, including their biological characteristics such as metabolism, water temperatures and the levels of oxygen present in the water. Because there is less oxygen in warmer water, warming weather will make large areas of the planet’s oceans uninhabitable for numerous marine species, according to Deutsch, one of the authors of a new study published in the journal Nature.
Oxygen levels and temperatures vary widely across ocean waters from warmer, less oxygen-rich water in the tropics to cooler water near the poles. Deeper beneath the surface both oxygen and temperature are lower than near the surface. The metabolic rate of marine animals determines how much oxygen they need in seawater where they live, which limits their range to specific areas. Active animals are especially affected by drops in oxygen levels because of their high energy requirements.
Deutsch and his colleagues examined 72 species from five different groups of marine animals — including cold-blooded vertebrates like fish; crustaceans; mollusks; segmented worms; and jellyfish — to see how they are expected to fare in warmer water with less oxygen. They found that the current range of a species generally overlaps with the areas of the oceans that it can naturally inhabit.
Their computer model predicts that the northern shrimp can get enough oxygen in cool waters north of about 50 degrees north latitude, which is where the shrimp’s actual range is. Meanwhile, the small-spotted catshark can inhabit temperate and cool waters at various depths, but can survive only near the surface (above about 100m) around the tropics, which was also confirmed by their model.
Many species already inhabit ranges that are on the edge of breathability, which means that warming weather will reduce those ranges as a result of drops in oxygen levels, the scientists say.
“Organisms today are basically living right up to the warmest temperatures possible that will supply them with adequate oxygen for their activity level — so higher temperatures are going to immediately affect their ability to get enough oxygen,” Deutsch explains. “In response to warming, their activity level is going to be restricted or their habitat is going to start shrinking. It’s not like they’re going to be fine and just carry on,” he adds.