Ocean acidification and iron: bad for phytoplankton

Acidifying Ocean May Stifle Phytoplankton - Science News

Some people argue that increased CO2 will result in more phytoplankton blooms, which will help sink more CO2 to the bottom of the sea.

One study that appeared a couple of weeks ago in Science suggests that this may not happen due to the lower water pH that the increased CO2 is definitely already causing:

Research by oceanographer Dalin Shi and his colleagues at Princeton University hints that rising CO₂, instead of providing extra nutrients for phytoplankton, may actually curb the growth of these organisms, which form the base of the ocean’s food chain. The team reports these findings online January 14 and in an upcoming Science.

In their tests, the researchers studied how acidification, a decline in ocean pH, affects the ability of phytoplankton to take up dissolved iron, another nutrient required for growth. The scientists measured growth rates of four species of the marine microorganisms — including two that Shi described as “the lab rats of phytoplankton” — in ocean water with pH values that ranged from 8.8 to 7.7. On average, the pH of ocean surface waters today is about 8.08, says Shi.

Across large swaths of the ocean, phytoplankton are already starved for iron, Shi says. And the team’s research suggests that acidification will make things worse: If ocean pH drops by about 0.3 units over the next century — the acidification expected if CO₂ emission trends continue — iron uptake by phytoplankton could drop by between 10 and 20 percent, the data suggest. Ironically, even though more-acidic waters are able to hold increased amounts of dissolved iron, a larger percentage of that nutrient would be chemically bound to organic matter dissolved in the water and therefore unavailable to nourish phytoplankton, Shi says.