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In a shallow lagoon a couple of clicks from the Hawaiian coast, a humpback whale (Megaptera novaeangliae) gives birth. The one-ton baby slips from her gigantic girth and is soon nudged up to the surface of the water to take a first breath. A few minutes later, the mother whale delivers the placenta that nourished her baby for nearly a year. Now it will nourish something else: the food chain that hums through these warm, clear waters.
The mother whale nestles her rostrum beneath her baby’s body, gently supporting him while he gets his bearings in this wide new world. The calf tail-slaps experimentally. Meanwhile the placenta sinks through the water, perhaps nibbled by a fish or two, until it comes to rest on the bottom, where the 200-pound organ will provide a resource bonanza for an array of local scavengers.
The humpback placenta is a small part of what scientists in a paper published Monday called “the great whale conveyor belt,” a phenomenon that may qualify as the largest long-distance transfer of nutrients on the planet. Their analysis estimates the magnitude of nutrient transfer via the placentas, carcasses of adults and calves, and especially the urine of whales.
Whale carcasses are already known to provide vital pulses of nutrients to both deep-sea and seashore ecosystems, and seed seafloor “whale-fall” ecosystems teeming with dozens of species that can persist for a century or more. At whale feeding grounds, their eating and defecating creates a biological “pump” that moves nutrients up and down the water column over the course of a day. The new work adds to this picture, showing how whales move nutrients across latitudes and longitudes over the span of a year.
The 200-pound placenta will provide a resource bonanza for an array of local scavengers.
Focusing on four species with relatively well characterized annual migrations—humpbacks, gray whales (Eschrichtius robustus), and North Atlantic (Eubalena glacialis) and southern (E. australis) right whales—the researchers calculated that these animals deposit an estimated collective 46,512 metric tons of biomass and 3,784 metric tons of nitrogen at their low-latitude marine wintering grounds each year. That biomass is equivalent to more than four times the weight of the Eiffel Tower and the nitrogen is equivalent to the amount that would be fixed by fields of chickpeas covering an area five times the size of Paris.
The new analysis represents the latest evidence that whales are major arteries in the circulatory system of the planet, says Joe Roman, a conservation biologist at the University of Vermont and one of the study’s authors. “They move nutrients vast distances.”
The four whale species the researchers studied migrate every year between high-latitude summer feeding grounds and winter calving and breeding grounds in tropical and subtropical areas, often over 3,000 miles away. Whales typically fast while migrating and while circulating their wintering grounds, so anything they excrete or leave behind during these times represents a resource transfer from nutrient-rich high-latitude waters to relatively nutrient-poor low-latitude marine ecosystems.
In subtropical waters, nitrogen is generally a limiting nutrient, so the nitrogen that whales add to the system is likely to act as fertilizer to increase the growth of photosynthetic phytoplankton, the base of marine food chains. The nitrogen contributed to low-latitude ecosystems by the four species of migrating whales could result in the fixation of 18,180 tons of carbon per year, equivalent to the amount fixed by 835,000 mature trees.
The idea of a whale conveyor belt isn’t entirely new. But Roman and his collaborators are the first to put some detailed hard numbers to it thanks to recent advances in the understanding of whale migrations; new publicly available databases of tagging information, whale sightings, and historical whaling industry information; and studies of marine mammal metabolism and physiology.
The analysis is “wonderfully meticulous,” says Yadvinder Malhi, a professor of ecosystem science at the University of Oxford, who was not involved in the work, “drawing across a range of evidence to calculate the size of the nutrient fluxes involved, and showing that they make a significant difference to the local environments.”
Today, no equivalent exists on land. The migrations of large herbivores, such as buffalo, may have in the past had a similar ecological impact. But these herds are now much diminished, and their migrations constrained by fences and habitat fragmentation so that we now only “see a few whispers of these once widespread movements,” as Malhi puts it.
The overall scale of nutrient transfer by whales is about on par with that of seabirds who move marine nutrients onto land in the island ecosystems where they nest. But the birds are feeding relatively locally, moving nutrients across tens to hundreds of kilometers, not thousands.
The impact of the whale conveyor belt may be even bigger than the current estimates. That’s because the migratory patterns of other species of whales that were not included in the researchers’ calculations, such as fin (Balaenoptera physalus) and blue (B. musculus) whales, are less well understood. If those two species feast, migrate, and fast in a manner that is similar to the four species included in the study, the total movement of nitrogen would be three to four times higher, the researchers estimate. Whale skin sloughed into the water or nibbled by fish and seabirds, calf poop, and milk spilled into the water represent additional sources of nutrient transfer that the researchers have not yet quantified.
But the resource transfer from whales is likely also much weaker than it once was. Prior to industrial whaling, the nutrient fluxes from all great whales would have been three times larger than they are now, they estimate.
The ecological effects of that loss are difficult to reckon. “These changes happened before anyone was looking,” Roman says. But recovering whale populations “gives us a chance to both observe but also restore these planetary systems” that the whales help to keep thrumming. 
Lead photo by Martin van Aswegen / NOAA
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