Sperm whales as a carbon sink

Sperm whales in the Southern Ocean deserve credit for their fine work pumping iron for climate change, researchers say.

These whales have been falsely accused of breathing out enough carbon dioxide to contribute to the greenhouse gas buildup causing climate change, says Trish J. Lavery of Flinders University in Adelaide, Australia.

Of course the whales breathe, but earlier calculations overlooked the potential for whales to offset their emissions by introducing extra iron into the upper zone of water, Lavery said October 13 at the Biennial Conference on the Biology of Marine Mammals in Quebec City, Canada. The extra iron that whales bring up from their deep feeding encourages plankton growth. That growth traps carbon, much as human-run iron-enrichment experiments in the ocean might, Lavery and her colleagues contend.

According to the team’s calculations, sperm whales in the Southern Ocean should rank as carbon neutral at least. The animals may even be capturing a net 5 million metric tons of carbon from the atmosphere per year, Lavery says.

Some 210,000 of the world’s sperm whales swim around the Southern Ocean during a year, Lavery says. Whale numbers inspire a lot of debate, so she averaged results of several estimates.

The first analysis of whales’ effect on greenhouse gases determined that warm-blooded residents — with whales as the dominant force — might be respiring 25 percent of the carbon fixed in the Southern Ocean, she says. Later estimates have revised their share downward, and the most recent calculation puts their contribution at 0.3 percent. That’s not huge compared to global output, but it’s still 17 million tons of carbon a year.

Sperm whales, however, feed by diving for squid in the cold depths of the Southern Ocean. This zone normally acts as deep storage for nutrients, Lavery says. So anything the whales bring up effectively introduces something new to the upper waters.

Skimpy levels of iron in the Southern Ocean limit growth of the floating meadows of plankton there, Lavery says. This limitation has inspired human experiments in adding iron to trigger a big plankton bloom. A burst of iron-fed organisms would draw in carbon dioxide and then trap some of it as a portion of the bloom dies and sinks into deep cold storage.

Using numbers from studies of feeding and nutrition, Lavery and her colleagues calculate that each whale brings up about 10 grams of iron a day from the depths and then defecates it at the surface. The beauty of this sperm whale output is that it takes the form of drifting liquid plumes that can feed life in the upper ocean, Lavery says. She notes that experiments with iron have struggled with iron fertilizers that clump and sink before upper-water plankton can eat all of the goodies. Yet, she says, those experiments document measurable carbon trapping with even less iron fertilizer than sperm whales contribute.

Lavery points out that her calculations can show sperm whales as either a net carbon sink or as carbon-neutral depending on which numbers go into the model. Finding the exact values will take more research, she says, but she wants to call attention to an overlooked mechanism.

Asked if he’s ready to believe sperm whales could be carbon neutral, Ari Friedlaender of the Duke University Marine Lab in Beaufort, N.C. says, “Of course!”

He adds that is he is now thinking about the flip side of whales and climate change: what impact climate change will have on them.