Scientists ‘shocked’ by scope of changes
FRISCO — Global warming is likely to have a big effect on the abundance and diversity of ocean phytoplankton, with some species dying out and other flourishing, researchers said after completing a study that tries to anticipate the impacts of ocean acidification.
Since pre-industrial times, the pH of the oceans has dropped from an average of 8.2 to 8.1 today, and by end of the century, could drop to 7.8 — much lower than any levels seen in open ocean marine communities today.
The new study published in the journal Nature Climate Change assesses phytoplankton response to ocean acidification and other climate change impacts, including ocean temperatures and lower nutrient supplies.
Some modeling shows phytoplankton communities will migrate poleward as the planet warms. Based on global simulations, however, they found the most dramatic effects stemmed from ocean acidification.
“I’ve always been a total believer in climate change, and I try not to be an alarmist, because it’s not good for anyone,” said Stephanie Dutkiewicz, a principal research scientist in MIT’s Center for Global Change Science. “But I was actually quite shocked by the results,” Dutkiewicz said.
“The fact that there are so many different possible changes, that different phytoplankton respond differently, means there might be some quite traumatic changes in the communities over the course of the 21st century. A whole rearrangement of the communities means something to both the food web further up, but also for things like cycling of carbon,” she said.
The paper’s co-authors include Mick Follows, an associate professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences.
In all, the papers examined 154 experiments of phytoplankton, divided into different groups. A key part of the study was projecting growth rates under more acidic conditions.
They found a whole range of responses to increasing acidity, even within functional groups, with some “winners” that grew faster than normal, while other “losers” died out.
The experimental data largely reflected individual species’ response in a controlled laboratory environment. The researchers then worked the experimental data into a global ocean circulation model to see how multiple species, competing with each other, responded to rising acidity levels.
When they crunched all the data in a global computer model, they found that ocean acidification prompted some species to grow faster, and others slower, and also changed the natural competition between species.
“Normally, over evolutionary time, things come to a stable point where multiple species can live together,” Dutkiewicz said. “But if one of them gets a boost, even though the other might get a boost, but not as big, it might get outcompeted. So you might get whole species just disappearing because responses are slightly different.”
Dutkiewicz says shifting competition at the plankton level may have big ramifications further up in the food chain.
“Generally, a polar bear eats things that start feeding on a diatom, and is probably not fed by something that feeds on Prochlorococcus, for example,” Dutkiewicz says. “The whole food chain is going to be different.”
By 2100, the local composition of the oceans may also look very different due to warming water: The model predicts that many phytoplankton species will move toward the poles. That means that in New England, for instance, marine communities may look very different in the next century.
“If you went to Boston Harbor and pulled up a cup of water and looked under a microscope, you’d see very different species later on,” Dutkiewicz says. “By 2100, you’d see ones that were living maybe closer to North Carolina now, up near Boston.”
Dutkiewicz says the model gives a broad-brush picture of how ocean acidification may change the marine world. To get a more accurate picture, she says, more experiments are needed, involving multiple species to encourage competition in a natural environment.
“Bottom line is, we need to know how competition is important as oceans become more acidic,” she says.