Whales are considered the ecosystem engineers of the oceans. They stimulate the growth of phytoplankton, which in turn supports the growth of their primary food source, krill. Trace metals such as iron play a central role in this process.
At the beginning of the 20th century, the Southern Ocean was teeming with whales—and with krill, their primary food source, which was also found in vast quantities. However, in less than 70 years, commercial whaling wiped out around 2 million large baleen whales, including blue whales, fin whales, sei whales, humpback whales, and minke whales.
At first glance, one might assume this was good news for krill, as the near-extinction of their predators should have allowed them to thrive undisturbed. However, the opposite proved to be true—the krill population also declined dramatically.
Only the “iron hypothesis” proposed by the US oceanographer John Martin (✝) could explain this apparent paradox. In regions where nutrients are abundant but phytoplankton—tiny single-celled algae—barely grow, the missing factor is iron. The Southern Ocean is one such “high nutrient, low chlorophyll” region. No matter how plentiful other nutrients like nitrate, phosphate, and silicic acid may be, significant growth cannot occur without iron.
And if phytoplankton can’t grow, krill is left without a food source. But where did all the iron come from that once sustained the vast krill populations before commercial whaling began? The concentration of iron in seawater is naturally very low—far too low to support hundreds of millions of tons of krill.
How whales cultivate their own food
This is where the large marine mammals come into play. In 2010, oceanographer Victor Smetacek, now professor emeritus at the German Alfred Wegener Institute, introduced a theory that became known as the “whale poop hypothesis”. He proposed that whales concentrate the iron they ingest with krill during digestion and release it back into the water in large quantities through their feces.
In the same year, krill scientist Stephen Nicol validated the hypothesis with concrete data. His research showed that the iron concentration in whale excrement is approximately 10 million times higher than in Antarctic seawater.
However, the whales do not excrete their feces anywhere, but strategically
at the sunlit water surface, where phytoplankton can utilize the nutrients for photosynthesis. The iron concentrate in whale poop acts as a booster for the tiny algae, triggering a plankton bloom that in turn supports the growth and reproduction of krill.
Whales themselves play a crucial role in keeping iron in circulation, ensuring a steady food supply. Deep-diving species like sperm whales further contribute by transporting nutrients from the depths—where they feed on squid—back to the surface, a process known as the “whale pump.”
Metals in bioavailable form
In a recent study published in Nature Communications Earth & Environment in January,
a research team from the University of Washington (UW) examined the role of whales in recycling trace metals. For their research, they collected fecal samples from humpback whales in the Southern Ocean and blue whales off the coast of California.
“Our focus was on the iron chemistry and constraining the availability of recycled metals to better understand the ecosystem services provided by whales,” writes Patrick Monreal, PhD student at the UW and lead author of the study, in an email to Polar Journal AG.
In their study, the researchers not only analyzed iron but also examined copper, as both are limiting factors in the ocean. However, they were surprised to find that the copper concentrations in the feces were unexpectedly high. This metal poses a risk to many marine organisms, as excessive amounts can lead to toxic effects.
“Our analysis suggests that the decimation of baleen whale populations from historical whaling could have had larger biogeochemical implications for the Southern Ocean, an area crucially important to global carbon cycling,” Monreal said in a university press release.
After further analysis, however, the team discovered that both iron and copper atoms are bound to organic molecules known as ligands. This binding process enhances the bioavailability of these essential micronutrients for marine organisms. At the same time, it neutralizes the toxic effects of copper.
The researchers have not yet determined the exact origin of the ligands, but they suspect that these organic molecules could be metabolic byproducts of bacteria in the whales’ digestive system.
“I think animals play a larger role in chemical cycles than many experts give them credit for, especially when thinking at the ecosystem scale,” Monreal said in the press release. “When I talk about animals, I actually mean their gut microbiome. From what we see, the bacteria in the guts of whales could be important.”
In the email to Polar Journal AG, he added: “I think on an ecosystem and regional-scale, that recycled iron was quite important to the health of the Southern Ocean food chain, especially because whales feed (and poop) near phytoplankton blooms.”
This well-established nutrient cycle, whose details are still not fully understood and which may owe its effectiveness to microorganisms in the whales’ intestines, once again highlights the intricate interplay of natural processes that underpin the success of entire ecosystems.
And for whales, the “gardeners” of the oceans who sustain the continuous regrowth of their own food, we can only hope that their populations will keep recovering from the relentless slaughter of the past—so they can once again bring more life to the oceans.
Julia Hager, Polar Journal AG
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