Saturday, October 27, 2012

Rising Ocean Temperatures Threaten Ocean Food Chain

Of all the plants and animals facing a potentially dire future because of climate change, a study released Thursday in Science paints a potentially grim picture for one of the most important and underappreciated groups of living things on Earth.

The study reports that phytoplankton — water-dwelling, single-celled micro-organisms including algae and other species — may have trouble adjusting to rising ocean temperatures.

“Phytoplankton have evolved to do really well at current temperatures,” said lead author Mrudil Thomas, of Michigan State University, “but if they don’t evolve further, the warming this century is going to lead them to move their ranges, and their diversity in tropical oceans may drop considerably.”

That could be a very big deal. Phytoplankton are not only the very foundation of themarine food chain, but they also consume about half of the carbon dioxide that enters the atmosphere, and take it to the bottom of the sea with them when they die.

Significant disruptions to the world’s phytoplankton could therefore have major repercussions for the world’s food supply, and at the same time allow more CO2 to remain in the air to trap heat, accelerating climate change.

All of this is highly speculative at the moment. What Thomas and his co-authors have actually shown is that phytoplankton — or at least, the more than 130 species they studied — are highly sensitive to water temperature. Their growth rate slows considerably when the temperature changes, and especially when it goes up. It’s possible that the tiny organisms will be able to evolve and adapt to rising ocean temperatures, but as Thomas said, “we don’t know how fast they can do that.”

They might also respond by shifting their range: tropical phytoplankton could, in principle, colonize more temperate waters as the tropics heat up. But they don’t just need comfortable temperatures: they also need a certain amount of sunlight to carry out photosynthesis, for example, and the sun is most powerful at the Equator.

They need a mix of nutrients as well, which might easily be less abundant in a new location. And even if the migration were largely successful, the tropics — currently home to the most diverse and productive phytoplankton communities on Earth — would be left significantly depopulated, removing a major food source for animals that feed on them. More