Microbes of the genus Prochlorococcus, a photosynthetic plankton, are the base of the food chain in the world's oceans and also absorb a large amount of carbon dioxide from the atmosphere. Pictured: plankton bloom off the coast of South America. Image: Flickr/NASA Goddard Photo and Video
For a while, Adam Martiny and some of his fellow scientists had suspected something was not right in how researchers understand the oceans. The object of their suspicion was something called the Redfield ratio, a principle stating that, when nutrients are not limiting, ocean microorganisms always have the same ratio of three elements: carbon, nitrogen and phosphorus.
This matters now because the Redfield ratio is used to help modelers and biogeochemists understand how important elements like nitrogen and carbon cycle in the oceans. If the Redfield ratio does not hold true, climate researchers might have to adjust how that process is represented in their climate models.
So Martiny, an associate professor of Earth system science at the University of California, Irvine, and a few of his colleagues set out to sample the ocean and test the ratios. What they learned, detailed in a paper published Sunday in Nature Geoscience, was that the ratios of carbon to nitrogen to phosphorus varied in different parts of the ocean. They also discovered the patterns of variation corresponded to different latitudes.
"How much carbon is attached to each molecule of nitrogen or phosphorus just used to be [considered] a constant," said Francois Primeau, a co-author on the paper and an associate professor of Earth system science at UC Irvine.
But that's not the case. For example, in warm zones near the equator that are low on nutrients, the ratio of carbon to nitrogen to phosphorus measured was 195:28:1; in cold, high-latitude regions with plenty of nutrients, the ratio changed to 78:13:1. Redfield's ratio is 106:16:1 oceanwide.
Many models have predicted that a warming ocean will take up less carbon because higher temperatures lead to smaller phytoplankton, which take less carbon to the bottom of the ocean when they die. The amount of carbon these plankton take with them is typically calculated based on the Redfield ratio.
What happens in science when a 'constant' isn't?
This change from a constant ratio to one that varies depending on latitude will likely shake up climate models, because it demonstrates regional variability, said Jasper Vrugt, another co-author.
"Likely we will see regional differences in the ocean. That will also have an effect on the climate change patterns that you simulate," he said.
Because the ratios vary by latitude, the plankton may actually take more or less carbon with them as they sink down to the ocean floor, depending on where they are.
The way the data played out with the variations correlated with latitudes is actually good news for modelers, even though they will need to do some revisions, Martiny said.
"I think most modelers, they get sort of a tired look when they hear the biologists come out with yet another mechanism, saying this is real important, you have to incorporate that," he said, laughing a little.
"But when I can also tell the modelers, here are what the ratios are in different ocean regions, and we can describe why it is different," he added, "I think that way it can pretty easily now be incorporated into a model."
Mick Follows, a senior research scientist at the Massachusetts Institute of Technology and a modeler who works on ocean circulation and biogeochemical cycles, said that while he was not surprised the Redfield ratios were "flexible," it was useful that the study mapped out latitudinal patterns in the variations.