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Rising Ocean Temps Threaten the 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.”

While phytoplankton are microscopic, they're critical to the foundation of the marine food chain like schools of fish.
Credit: Wiborg/flickr

That could be a very big deal. Phytoplankton are not only the very foundation of the marine 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.

A scanning electron microscope (SEM) image of phytoplankton.
Credit: Ulf Riebesell/GEOMAR

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.

“Food webs are very complicated,” said co-author Colin Kremer, also at Michigan State. “But certainly, if phytoplankton are affected, they’re connected to many other species that would likely be affected as well.”

At this point, nobody knows how likely any of these scenarios might be. “It’s even possible, that there will be no disruption,” Thomas said. “There’s lots of information we simply don’t have. There are all sorts of different ways it could play out. But I expect changes to happen.”

Nailing down precisely what those are will entail a lot more research. “One of challenges we face is we know so little about how phytoplankton are spread through the ocean,” Kremer said. “There are only handful of phytoplankton species whose distribution we understand even in a coarse sense.”

Kremer said the current study is an important step toward understanding the effects of ocean warming on these crucial organisms (to say nothing of another climate-related threat, ocean acidification). But there’s a great deal more scientists need to understand.

Related content: 
Scientists Eat Crow on Geoengineering Test. Me, Too
Ocean Acidification Threatens Food Security
Finding the Ocean’s Carbon


By Desertphle (Chama, New Mexico)
on October 26th, 2012

In year 1998 I was on a ship off the coast about 6 miles, deep inside the “super El Nino,” collecting sea samples for Costa Rica. We were all astonished at the water temperature: at times it was 96 degrees: something none of us had ever seen happen. The entire cost was void of macrobiology—- the little fish died, the kelp died, and the corral (such as inside the Sea of Cortez) was stressed. This was not a good thing.

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By Todd Tanner (Bigfork, MT 59911)
on October 26th, 2012

Phytoplankton also produce 50% to 85% of the planet’s oxygen.  Anything that impacts phytoplankton on a large scale could also potentially change our atmospheric oxygen levels.

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By Charles Hollahan (New Orleans, La 70122)
on October 27th, 2012

At this point I think we can say with absolute certainty that there will be major disruptions in primary production and phytoplankton. I spent thirty years on the West Coast and most of that time I was working underwater in Southern California and I’ve seen some changes in the last fifteen that are very disturbing.

Species which had been uncommon near Santa Barbara were becoming prevalent whereas they had been found previously in Baja. This was beginning to show up on the Oregon and Washington coasts as well where tuna was being caught and this was once a rare occurrence during most Summers.

Ocean acidification has been occurring off Washington and Oregon since 2005 and the oyster farmers first saw this but were unable to figure out what was happening. Its not just a lower pH but a rise in Vibrio spp. as well so the toxic soup that so often appeared near Los Angeles in the 1960s was showing up in the Columbia River estuary now as well.

We should be clear and strident when a disaster happens that we think is caused by Climate Change and say it out loud, and often. It was preventable.

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