Whale carcasses sinking to the ocean floor bring a buffet of nutrients to the deep sea. But whales don’t have to be dead to be big movers of nutrients. Migrating baleen whales transport more than 3,700 tons of nitrogen and more than 46,000 tons of biomass each year from high-latitude feeding areas to warm, shallow breeding waters near the tropics, according to a recent study published in Nature Communications.
“In places like Hawaii, or the Caribbean, or the coastal waters of Western Australia, where nitrogen is often a limiting nutrient, migrating whales can have a big impact on the local biogeochemistry,” said Joe Roman, lead author of the new study and a conservation biologist at the University of Vermont.
“It’s a bit like adding fertilizer to a garden in New York City,” he said. “On the scale of the entire city, any change is probably undetectable, but the garden is profoundly affected.”
Roman and his colleagues found that in some breeding areas, the transport of whale-borne nutrients like nitrogen can be as significant as that from nonbiological processes, such as nutrient-rich upwellings. In the Hawaiian Islands Humpback Whale National Marine Sanctuary, nitrogen brought in each day by migrating humpback whales can be 125%–175% that of nitrogen from abiotic processes during the breeding season.
Though whales move only a small portion of the total nutrients swirling through the oceans, they still have a significant effect on the ecosystems in the breeding area, according to Matt Savoca, a marine ecologist at the Stanford University Hopkins Marine Station, who wasn’t involved with the study. “It’s a bit like adding fertilizer to a garden in New York City,” he said. “On the scale of the entire city, any change is probably undetectable, but the garden is profoundly affected.”
Nitrogen in Whale Pee
Roman and his colleagues used publicly available databases and whale sightings from ships and aerial surveys to estimate populations in feeding and breeding areas. They focused on gray, humpback, and right whales. (They avoided other baleen whales such as blue, fin, and minke because less is known about the migration patterns of these species.)
To calculate how much nitrogen migrating whales transport to breeding areas, the researchers turned to a perhaps unexpected animal: the northern elephant seal. “What makes northern elephant seals and baleen whales similar is that they are both capital breeders,” Roman said. Capital breeders bulk up for part of the year while in their feeding grounds. Then, after traveling to their breeding areas, they gestate, give birth, and lactate, all while fasting. This behavior contrasts with that of income breeders, such as seabirds, which feed throughout the year.
The elephant seal is the only capital breeding marine mammal for which data on nitrogen levels in urine for feeding and lactating animals exist. The researchers used information on elephant seal urine and supplemented the calculations with limited existing measurements of urine from whales in feeding and breeding areas to estimate how much nitrogen whales transport. (They didn’t include data on whale feces because adult baleen whales that are not feeding while in the breeding areas rarely defecate.)
Each year, whale species in the study may be adding more than 3,700 tons of nitrogen and more than 46,000 tons of biomass—which includes placentas released during births as well as carcasses of newborn and adult whales that die—to breeding areas. More conservatively, mothers and calves alone may transport more than 2,300 tons of nitrogen and 12,000 tons of biomass per year.
The Great Whale Conveyor Belt
Scientists still don’t fully agree on why whales migrate, usually from cold, nutrient-rich waters in high latitudes to warmer, nutrient-poor tropical waters. Some baleen whales make tremendously long journeys—gray whales can travel more than 11,000 kilometers from the waters around Sakhalin Island, Russia, to Baja California, for example.
“Other mammals and birds also migrate long distances, but what makes baleen whales different is their size and the fact that they are capital breeders,” Roman said. That means “most of the waste generated by the whales in the breeding areas, from placentas to urine, introduces external nutrients into the ecosystems.”
And baleen whales urinate a lot; a 2003 study estimated that one fin whale can produce almost a thousand liters of urine each day. Even whales that are fasting while in the breeding areas urinate copiously because they are breaking down stored fats and proteins to make milk for calves. Whale urine contains many elements, including phosphorus, magnesium, potassium, and nitrogen. The researchers were interested in nitrogen because it is often a limiting nutrient in many marine ecosystems.
“This study helps us realize that whales are not only charismatic species, but they also provide vital ecosystem services by connecting environments separated by thousands of miles.”
The researchers estimate that the nitrogen that whales bring in could increase the amount of food available in breeding waters. Whale urine contains nitrogen mainly in the form of urea, which organisms such as phytoplankton can readily use to convert carbon dioxide into thousands of tons of biological carbon per year through photosynthesis.
Some uncertainty is unavoidable when researching large marine mammals that travel huge distances, Savoca said. “But the study provides data-driven estimates that are as good as it gets at this time.”
“We are at the early stage of understanding how the nutrients, like nitrogen, that the whales bring in move through the ecosystems and the food chain,” Roman said. This understanding is especially important as whale populations face various threats, including pollution and climate change. “This study helps us realize that whales are not only charismatic species, but they also provide vital ecosystem services by connecting environments separated by thousands of miles,” Savoca said.
—Adityarup Chakravorty, Science Writer
