Poo pump: whales as ecosystem engineers
The brown cloud bursts forth among the pod of sperm whales, dispersing a wealth of nitrogen and iron into the surface waters over the deep ocean. The whale-borne windfall is eagerly received by phytoplankton, the microorganisms at the foundation of the ocean food chain, which quickly capitalize on the surge of fuel. Poop packs a powerful energetic punch.
And an adult sperm whale packs a lot of poo.
Enough to dump 50 metric tons of iron into the ocean every year, according to Trish Lavery and colleagues at Flinders University in Adelaide. By concentrating and moving nutrients in space and time, the researchers say, these giants of the deep engineer key attributes of ocean ecosystems.
Sperm whale poop is particularly rich in iron thanks to the whales preferred diet of squid and fish. Iron is both scarce and essential. It’s availability limits the growth of phytoplankton, the photosynthesizing primary producers of the sea. (The other major source of iron in the ocean is wind-borne desert dust.)
Sperm whales dive over 2000 meters to hunt squid, but defecate at the surface, transporting nutrients between ocean realms. Blue whales also act as “whale pumps,” diving to 150 meters to capture krill (small, shrimp-like crustaceans, which in turn feed on phytoplankton).
The turbulence of diving, surfacing and exhaling also stirs the water column.
In their migrations, the baleen whales transport nutrients latitudinally, from the food rich summer feeding grounds of the polar oceans to safer, but food scarce, tropical waters. Mother Humpbacks fast through birth and lactation in the tropics, burning through their summer fat stores, and leaving behind the waste products of their metabolism in the warm coastal waters.
Great whales exert powerful effects on the environment as predators, prey, and corpses sinking to their final rest in the deep dark, sometimes boosting the productivity of ecosystems in non-obvious ways. But humans have reduced whale populations to a a tenth of their historical numbers. The mass disappearance of the great whales in the last few centuries has likely changed the functioning of ocean itself.
A group of marine ecology heavyweights led by Joe Roman review the science on the influence of whale on ocean ecosystems for Frontiers in Ecology and the Environment in an article published online today.
Read more from the University of Vermont news site.
Joe Roman, James A Estes, Lyne Morissette, Craig Smith, Daniel Costa, James McCarthy, JB Nation, Stephen Nicol, Andrew Pershing, and Victor Smetacek (2014). Whales as marine ecosystem engineers. Frontiers in Ecology and the Environment (e-View 3 July; scheduled for August print edition) http://dx.doi.org/10.1890/130220