Centuries of over-exploitation of whales for their meat and blubber has seen populations of most species plummet. But with no small amount of irony, the tables have turned with research discovering that we need whales for a healthy marine ecosystem, or at least their poop.
Large areas of the Southern Ocean are known as high-nutrient, low-chlorophyll (HNLC) waters. This is where phytoplankton abundance is very low despite high concentrations of major nutrients such as nitrate, phosphate and silicate.
Phytoplankton is crucial in marine ecosystems as the main food source that supports all marine life. It also plays a key role in removing carbon dioxide from the atmosphere through photosynthesis.
Uneaten phytoplankton eventually die and sink from the euphotic zone - the top 200-300m where light can penetrate - transferring the carbon to the deep sea.
Iron's role in the Southern Ocean
One factor that limits the production of phytoplankton in HNLC waters has been the availability of iron. Iron is an important nutrient that acts as an electron carrier and a catalyst during photosynthesis.
When iron is in short supply, phytoplankton can't grow, leading to less carbon dioxide removed from the atmosphere.
Until recently, it was thought that the main new sources of iron in the ocean were from atmospheric dust, shelf sediments, underwater volcanoes and icebergs.
Research has shown that large animals, including whales, are part of a positive feedback loop that consumes and retains nutrients at the ocean surface, and consequently are a slow-release source of recycled iron to the upper ocean.
Diving mammals such as whales require iron for myoglobin, the oxygen-storage protein in muscles. Filter-feeding, or baleen, whales mostly eat Antarctic krill and are capable of converting the iron found in solid form in their prey into a liquid form that is released as a slurry into the euphotic zone.
Other types of whales, such as sperm whales, migrate to waters much deeper than the euphotic zone and they can scavenge iron from below and then return to the surface layer to defecate.
Unlike some animals, such as zooplankton, that can defecate at depth, whales only defecate near the surface. Their warm, fluid-like faeces rise to the surface before being dispersed, thereby releasing the nutrients exactly where the phytoplankton need them.
The richness of whale poop
During the short summer feeding season in the Southern Ocean, adult blue whales consume approximately two tonnes of krill per day. As they are accumulating blubber instead of building muscle to last them through the subsequent calving period, most of the iron consumed is excreted in their faeces.
Whale faeces floating on seawater. The bright orange colour is from the carapace of krill. Photo / Micheline Jenner - Centre for Whale Research
The concentration of iron in whale faeces was found to be more than 10 million times higher than seawater concentrations. So whale poop acts as a fertiliser that increases phytoplankton growth, leading to a more productive ecosystem and enhanced atmospheric carbon dioxide removal.
If whale populations had not been hunted to near-extinction, whales would have recycled more iron because of their abundant numbers.
In the Southern Ocean, iron defecation by the 12,000-strong population of sperm whales removes approximately 200,000 tonnes of carbon per year from the atmosphere.
This is equivalent to 70,000 vehicles that each travel 15,000 km per year. Blue and fin whales, being much larger than sperm whales, could recycle far more iron.
Conservation of whales in the Southern Ocean
With the discovery of the important role of whales in recycling iron and its link to ecosystem productivity and carbon removal, our thoughts turn to the greater context of conserving and restoring whale populations in the Southern Ocean.
In addition to the moratorium on whaling under the International Whaling Commission's Schedule to the International Convention for the Regulation of Whaling, there is also a specific ban on commercial whaling in the Southern Ocean Sanctuary. This reflects the conservation-based approach of most members of the International Whaling Commission.
The only permissible killing, taking or treating of whales from the Southern Ocean falls under scientific permit whaling.
The whale hunting's over ... for now
Japan is the only country that has issued such permits in the Southern Ocean, with the International Court of Justice recently ruling that Japan's whaling actions in the Antarctic were not "for the purposes of scientific research".
The court ruling also ordered that Japan revoke its current Southern Ocean scientific permits.
The decision did not ban Japan from returning to the Southern Ocean with a revised programme, so lethal scientific whaling may soon resume.
The International Whaling Commission's next meeting in September 2014, the first since the court's decision, may raise the scientific value of research into Southern Ocean whale populations and the methods by which they are studied.
Greater cooperation between member nations in researching these populations (the lack of which was raised in the court's decision) could lead to greater knowledge of whales, and the potential of the Southern Ocean to become a more productive ecosystem and hence a more effective carbon sink.
Lavenia Ratnarajah receives funding from the University of Tasmania, the Antarctic Climate and Ecosystems Cooperative Research Centre and the Holsworth Wildlife Research Endowment.
Andrew Bowie currently receives funding from the Australian Research Council. He has previously received funding from the Cooperative Research Centres program through the Antarctic Climate and Ecosystems CRC and funding for shiptime from Australia's Marine National Facility.
Indi Hodgson-Johnston is affiliated with, and receives funding from, the University of Tasmania and the Antarctic Climate & Ecosystems CRC.