How do light and iron affect phytoplankton populations in the western Antarctic Peninsula?

Light and iron availability are the dominant factors controlling Southern Ocean primary production. Because some macromolecules needed for photosynthesis require iron, phytoplankton iron requirements are intertwined with light availability. The underwater light available to phytoplankton is particularly sensitive to climate-induced changes in sea ice cover, upper ocean circulation, and stratification. Similarly, regional iron inputs from glacial and sea ice melt, precipitation, and sediments are susceptible to changing temperatures and ocean circulation. In addition to primary productivity, changes in light and iron availability can affect phytoplankton community composition, which can significantly alter regional and global nutrient cycling, carbon drawdown, and food web structure. We conducted a series of bioassay experiments in the spring in the West Antarctic Peninsula. In these experiments, we manipulated light and iron availability to investigate the interactive impacts on phytoplankton growth, photosynthesis, photoacclimation strategy, and species composition. We found that early in the season, light is limiting while iron is not. Light impacted phytoplankton growth and photoprotective pigments, but not photosynthetic parameters. Diatom abundance increased while Phaeocystis antarctica decreased in most treatments and we saw some evidence that iron addition favors diatom growth. These findings can help predict species composition in the region based on future light and iron availability.