Whale poop contains iron that may have helped fertilize past oceans

As whales were decimated, some thought the krill would proliferate in predator-free waters. But that's not what happened. Krill populations dropped, too, and neither population has yet recovered.

A recent theory proposes that whales weren't just predators in the ocean environment. Nutrients that whales excreted may have provided a key fertilizer to these marine ecosystems.

Research led by University of Washington oceanographers supports that theory. It finds that whale excrement contains significant amounts of iron, a vital element that is often scarce in ocean ecosystems, and nontoxic forms of copper, another essential nutrient that in some forms can harm life.

The open-access study, the first to look at the forms of these trace metals in what's commonly known as whale poop, was published in January in Communications Earth & Environment.

"We made novel measurements of whale feces to assess how important whales are to recycling important nutrients for phytoplankton," said first author Patrick Monreal, a UW doctoral student in oceanography. "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."

The Southern Ocean encircling Antarctica harbors little human life but is thought to play an important role in the global climate. Strong circumpolar currents bring deep ocean water up to the surface. Huge blooms of plant-like organisms known as phytoplankton support populations of krill, which are still harvested in unprotected waters today for aquaculture and pet food.

To investigate what role whale poop may have played in this ecosystem, the study analyzed five stool samples. Two samples were from humpback whales in the Southern Ocean and three were from blue whales off the central Californian coast. The samples were collected when researchers out studying whale populations saw an opportunity.

"The nice thing, I guess, is that whale excrement floats," said senior author Randie Bundy, an assistant professor of oceanography at the UW. Researchers collect it using a net attached to a jar to collect the substance typically found as a slushy or slurry material.

"The hypothesis is that the whales were actually adding nutrients to the ecosystem that these phytoplankton were able to use, so they would bloom more and then the krill could eat them," Bundy said.

Previous research had found significant amounts of major nutrients, like nitrogen and carbon, in whale poop samples. The new paper instead looked for metals that are in short supply far from land and are often a limiting factor for the growth of ocean ecosystems.

"In the Southern Ocean, iron is considered to be one of the most scarce, or limiting, nutrients that phytoplankton need to survive," Bundy said

Results showed iron was present in all the samples. The researchers also found another metal, copper.

"We were really shocked by how much copper was in the whale poop. We initially thought, 'oh, no, is the whale poop actually toxic?'" Bundy said.

Further analysis showed that organic molecules known as ligands attached to the copper atoms transformed them into a form that is safe for marine life. Other ligands helped make the iron accessible to living organisms. The researchers don't yet know the source of the ligands but suspect they may come from bacteria in the whales' stomachs.

Bundy's research focuses on trace metals in the ocean environment. This project began as Monreal's introductory research project as a graduate student but it grew into a larger endeavor as the results came in.

"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. "When I say animals, I really mean their gut microbiome. Based on what we see, it seems like bacteria in the whales' guts could be important."