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After they were nearly hunted to extinction in the 20th century, whales are now making a strong, general recovery and reappearing in places they haven’t been seen in decades and even centuries. This is good news, and not just for nature lovers and whale watchers; it turns out that whales play an essential role in the Earth’s oceans.

The growth of Earth’s ecosystems is often limited by a sufficient quantity of certain nutrients, usually nitrogen or phosphorus. When a farmer fertilizes a field of corn, they create a surplus of those nutrients and allow the ecosystem, in this case the cornfield, to grow more productive until it reaches the next limit, be it another nutrient or some other constraint like light or water availability.

In the ocean, the limiting nutrient is often iron. Phytoplankton, the microscopic algae that form the base of the aquatic food chain, need iron to photosynthesize. The problem is that most of the iron in the ocean is inside of other organisms and thus is not available for phytoplankton. Whales complete the cycle by eating massive quantities of tiny creatures like krill and expelling iron in their feces. This iron acts as an oceanic fertilizer, creating algae blooms that feed the krill and countless other species.

Besides propping up marine food chains, phytoplankton also capture an enormous amount of carbon dioxide—the equivalent of around four Amazon rainforests. The climate journalist Sam Matey recently summarized just how important whales are to this process:

It looks like whale poop is the key factor determining the productivity of Earth’s oceans…

Preliminary calculations estimate that returning the whole Southern Ocean to historical whale numbers, with the concomitant phytoplankton boom, would be the equivalent of adding an entire continent’s worth of new forests.

Around a third of the world’s oceans are iron-limited, which means marine ecosystems have the potential to become much more productive and much more effective at managing the Earth’s climate. Unfortunately, whales reproduce slowly and take many years to reach sexual maturity, so it will be a while before whale populations return to historical levels.

Could humans help jump-start the process? Iron is, after all, one of the most abundant elements on Earth. Scientists have considered iron fertilization as a possible climate mitigation strategy for decades, but experiments have proven to be controversial and politically fraught. Perhaps whales will provide the evidence needed to pursue the idea further.

Malcolm Cochran, Digital Communications Manager

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