Warmer oceans are acting like a catalyst for one of the world's most abundant species of plankton, triggering earlier blooms of blue-green algae in the waters of the North Atlantic. Because of plankton's fundamental role in the marine ecosystem, researchers expect this shift to have far-reaching impacts throughout the world's oceans.
The study, published in the journal Science, focused on Synechococcus, a type of blue-green algae that is one of the most abundant phytoplankton in the ocean. The authors drew on 13 years worth of data to measure the spring blooms that cover the North Atlantic in a carpet of green each year.
For every degree increase in water temperature, they found, the plankton bloomed four to five days earlier. From 2003 to 2012, the warmest years in their study, the bloom shifted by 20 days. The change will likely continue as the region warms in coming years.
"To the extent that they're shifting, we do expect there to be impacts throughout the ecosystem," said Heidi M. Sosik, senior scientist at the Woods Hole Oceanographic Institution.
Phytoplankton are mostly microscopic organisms that use photosynthesis to convert sunlight into energy, so they are critical to nearly all marine life. Because they perform about half of the world's photosynthesis, they also play a tremendous role in pulling carbon dioxide out of the atmosphere.
Some migratory whales and other marine creatures have synchronized their own lifecycles to the blooms. As a result, scientists have theorized that changes in the timing of these events could lead to a "mismatch" between predator and prey, with predators essentially arriving late to the party.
"There's this worry that if those two predators and prey don't track each other, we're going to see big effects," said Sara Rivero-Calle, a postdoctoral scholar in oceanography at the University of Southern California who was not involved with this study.
Still, Sosik said, the shift in timing may be reshaping the larger ecosystem. Earlier blooms could lead the plankton or the creatures that consume it to out-compete other organisms, for example, or could lead to a mismatch in timing higher up the food chain. The researchers did not attempt to look at these broader implications.
Even if the effects remain unknown, Sosik said that studying plankton such as Synechococcus can play an important role in trying to understand broad environmental changes. They reproduce and die quickly, so they respond to changes almost immediately. "If you tried to go out and say how is environmental change affecting birds or marine mammals, you're looking at impacts that have occurred across years or decades," Sosik said. "They provide us with a sentinel of being able to pinpoint responses that are already occurring in marine ecosystems."
To isolate the cause of the shift, researchers used a device installed about a mile off the coast of Martha's Vineyard that pumped seawater through a chamber. Inside, a laser beam measured the abundance of the individual species at specific intervals every day for 13 years. "We're able to measure thousands and thousands of cells every hour," Sosik said.
Now they want to expand their work, and are hoping to use similar devices installed at locations around the world. By amassing such detailed and focused bits of data, scientists can begin fitting them together like puzzle pieces to better understand the larger ecosystems.
"This is a really solid piece of work," said Michael Behrenfeld, an oceanographer at Oregon State University who studies marine algae. "How you take these specific conclusions and relate them to mixed populations, and how mixed populations behave over the annual cycle—that's going to be the next challenge."