Exploring the effects of ocean acidification on iron for phytoplankton in North Pacific

Orono, Maine — The effect of ocean acidification on iron availability to phytoplankton in the eastern North Pacific is the focus of a three-year, more than $954,000 National Science Foundation collaborative research grant to the University of Maine, University of Washington and University of South Florida.

UMaine School of Marine Sciences professor Mark Wells will lead the project, in collaboration with Charles Trick from the Western University and Kristen Buck from the University of South Florida. Joining them will be Shigenobu Takeda of the University of Nagasaki, and graduate and undergraduate students from the four universities.

The international collaboration also will feature educational outreach for the public, with Maine K-12 students and their teachers engaged in learning opportunities during and after the research cruise.

Ocean acidification is caused by increasing atmospheric carbon dioxide from fossil fuel burning. Carbon dioxide dissolves from the atmosphere into the surface ocean and reacts with seawater to form acid, causing lower seawater pH. This acidification already can be measured, but it will be greatly magnified by the end of the century.

One of the outcomes from ocean acidification will be changes in the availability of iron to marine phytoplankton, the grasses of the sea that support the marine food web and account for more than half the biomass of the oceans. Like humans, phytoplankton require iron to grow, but much of the iron dissolved in seawater is bound with organic molecules in ways that limit the ability of phytoplankton to access it.

Much of the biological production in the global ocean is limited by this iron availability, and it is uncertain whether ocean acidification will lead to decreases in ocean productivity.

The scientific teams will conduct a major research cruise in 2020, transecting the coastal waters off Washington state, the northern margin of the North Pacific subtropical gyre and the northeast subarctic Pacific. The researchers will collect samples of the surface waters, adjust the seawater pH to levels predicted for the end of the century, and measure how the phytoplankton respond at high and low light levels, a factor that changes the iron demand of phytoplankton.

The goal is to develop proxies for quantifying iron availability under present and future ocean acidification conditions, and learn more about how ocean acidification-induced changes in iron chemistry affect phytoplankton production and the composition of the phytoplankton community — critical factors that will affect food webs and fisheries productivity, according to the researchers.

Phytoplankton production also leads carbon transfer from the atmosphere into the deep ocean as cells grow, die and sink from the surface ocean, a process that has removed about one third of the human-released carbon dioxide from the atmosphere. But limited iron availability has restricted this removal in large regions of the oceans, including the subarctic Pacific. Changes in iron availability here will have important consequences to how rapidly carbon dioxide is removed from the atmosphere over the next several decades.

“Understanding the effect of ocean acidification on the iron cycle is a critical unknown in global biogeochemical models, and their projections of climate change effects on the ocean system over the next century,” note the researchers.

###