Phytoplankton across the Southern Ocean are increasingly starved of iron—a building block for their photosynthetic machinery—and there are signs their productivity might be declining.
Off the icy shores of Antarctica each spring, an explosion of life unfolds that is so large it’s visible from space. As iron-rich waters rise from below, the surface of the Southern Ocean swirls with psychedelic clouds of bright green phytoplankton—single-celled plants that suck up carbon from the atmosphere and form the base of the food chain by sustaining krill, which is in turn a major food source for fish, whales, and penguins..
Ocean iron levels, although known to be an important factor limiting phytoplankton productivity in the Southern Ocean, are notoriously difficult to study. Neither robotic sensors nor research ships routinely look for the nutrient. So scientists have recently begun to infer its levels by looking for signals that phytoplankton are coping with an iron shortage.
Even if the decline is real, it’s not certain that iron is playing a role. Philip Boyd, a biogeochemist at the University of Tasmania who has studied iron dynamics in the Southern Ocean for decades, points to other potential factors. For example, marine animals could be eating more phytoplankton. “It’s a long bow to directly connect iron stress with net primary production,” he says.
Teasing apart what’s happening will be important not only for understanding future ecosystem changes in the Southern Ocean, but also for predicting the fate of the global climate. The Southern Ocean is a significant carbon sink; half of all the carbon pollution that dissolves in the ocean does so there. Some of that dissolved carbon is taken up by phytoplankton and stored away as the plants—or the organisms that feed on them—die and sink to the bottom.
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