Ocean Acidification and Phytoplankton

OCEAN ACIDIFICATION, climate change’s menacing little brother, might just be the greatest threat to ocean life the planet is currently facing. How that threat plays out is likely to depend in large part on how the tiniest of ocean plants, phytoplankton, respond.

Ocean acidification—you’ve no doubt heard—is what happens when carbon dioxide from things such as fossil fuels dissolves in seawater, lowering the water’s pH. We tend to think of ocean acidification mainly as a problem for shell-making organisms; however ocean acidification could also affect the growth of various marine organisms regardless of whether they make shells. (See our story on juvenile salmon and acidification in the November 2015 CIRCulator.) One such group of creatures is phytoplankton, a microscopic, single-celled plant that forms the base of our oceans’ food web. The effect of ocean acidification on phytoplankton population could lead to cascading effects that travel up the food chain, affecting the entire marine ecosystem. But predicting how phytoplankton growth rates will respond as the ocean continues to absorb a sizable portion of atmospheric carbon emissions is challenging on multiple fronts. One reason is that there are several thousand species of phytoplankton, and each species could respond differently to ocean acidification.

Tracking these varied responses of different phytoplankton species to ocean acidification is the work of Stephanie Dutkiewicz and colleagues. The researchers recently published their findings in Nature Climate Change. Compiling data from 49 previously published studies observing the effect of elevated levels of dissolved carbon dioxide (pCO2) on various phytoplankton species, Dutkiewicz and colleagues tallied and compared the studies’ observations of how different phytoplankton species (43 in their analysis) may change under future, elevated levels of pCO2.

To figure this, the researchers calculated the growth rate response of each phytoplankton species as the ratio of growth rates under elevated pCO2 (representing conditions in 2100 for a high emissions scenario that assumes CO2 emissions will not be significantly cut in the future—similar to RCP 8.5) versus present-day, ambient pCO2 conditions. To simplify things somewhat, the researchers also grouped each of their 43 species into 1 of 6 representative types, which the researchers defined as depending on the species’ respective roles in the ocean ecosystem’s nutrient cycle.

Comparing the previous studies’ results revealed that among the three larger phytoplankton types analyzed (diatoms, dinoflagellates, and coccolithophores) that dominate regions of high nutrient supply—such as the California Current System of coastal upwelling zone along the U.S. West Coast—the median response was one of increased growth rates, although rates varied between types and among species within each type, with several experiencing decreased growth. If those results seem confusing that’s because they are. Here’s how the researchers parsed it out.

Dutkiewicz and colleagues conjecture the widely varying responses suggest that under elevated pCO2, the phytoplankton species within and between types may end up competing with one another. The competition, in turn, could result in new community makeups.

To test this, Dutkiewicz and colleagues fed the differing responses into a marine ecosystem computer model embedded in an earth system model simulation. Basically, they created a virtual phytoplankton community of 96 simulated species that were randomly assigned growth rates (from data drawn from the published studies they looked at) within each defined “type”. This helped the researchers simulate competition between phytoplankton on a region-by-region level. The projected future levels of pCO2 the researchers used in their simulations correlated to atmospheric levels of CO2

As one might expect, phytoplankton species with a reduced growth rate in response to elevated pCO2 were “losers,” meaning their populations decreased, ceding territory to the competition. However, even some species with increased growth rates were also losers, and in those cases the simulations showed the species were being outcompeted by other species. Globally by the end of the century about half of the original phytoplankton community remained at a given location. The researchers also noted a general tendency for phytoplankton species to shift their ranges poleward. What’s noteworthy is overall there was little change in primary production, suggesting, at least in some places, that new species replaced ones that left. However, there were regional differences. For example, primary production increased in the Southern Ocean and North Pacific but decreased in the North Atlantic.

However, the researchers note, changes in the marine ecosystems are not driven by ocean acidification alone, but also by warmer oceans and changing levels of light and nutrients. To tease out the effect of ocean acidification on phytoplankton from these other drivers, the researchers ran another model experiment in which pCO2 increased while other drivers remained unchanged. On its own, ocean acidification increased global primary production—mainly in nutrient-rich regions—while the other drivers tended to decrease primary production. The competition between phytoplankton species due to differing ocean acidification responses resulted in average range contraction, countering the poleward range expansion with increasing temperatures. Ocean acidification was also the strongest driver of changes in phytoplankton community within any given location.

This study’s results should give us pause. Different phytoplankton types perform different and essential functions in the marine ecosystem, so any changes in the community structure largely due to ocean acidification, as suggested by this study, are likely to impact the nutrient and food supply at all levels of the marine food web. Climate change’s menacing little brother could one day give his bigger sibling a run for his money.

Citation: Stephanie Dutkiewicz, J. Jeffrey Morris, Michael J. Follows, Jeffery Scott, Orly Levitan, Sonya T. Dyhrman, and Ilana Berman-Frank, Impact of ocean acidification on the structure of future phytoplankton communities, Nature Climate Change 2015 5 1002-1006; November 2015, doi: 10.1038/NCLIMATE2722.

Photo: Phytoplankton bloom in the Barents Sea. (Photo: NASA Goddard Space Flight Center, some rights reserved.)

At OCCRI since 2011, Meghan Dalton works as CIRC’s project manager. A trained climate researcher with a BA in Mathematics from Linfield College and an MS in Atmospheric Science from Oregon State University, Meghan has worked closely with several Northwest communities working on Community Adaptation, including the water provider Seattle Public Utilities on the PUMA project. Meghan has worked as the lead on several regional climate assessments, including “Climate Change in the Northwest: Implications for Our Landscapes, Waters, and Communities” and “The Third Oregon Climate Assessment Report.”