Bottoms up: how whale poop helps feed the ocean

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Bottomsup:howwhalepoophelpsfeedthe
ocean
ARTICLE·JANUARY2014
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3AUTHORS,INCLUDING:
LaveniaRatnarajah
UniversityofTasmania
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IndiHodgson-Johnston
UniversityofTasmania
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Availablefrom:IndiHodgson-Johnston
Retrievedon:27October2015

Lavenia Ratnarajah
Andrew Bowie
Indi Hodgson-Johnston
Bottoms up: how whale poop helps feed
the ocean
AUTHORS
PhD candidate in Marine
Biogeochemistry at
University of Tasmania
Senior Research Scientist
in Marine Biogeochemistry
at University of Tasmania
PhD candidate in Public
International Law at
University of Tasmania
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.
Whales make more than a splash in the southern ocean.
Micheline Jenner
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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.
What iron goes in must come out – in slurry or faeces from
the whales.
Lavenia Ratnarajah
Click to enlarge
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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.
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.
Whale faeces floating on seawater. The bright orange colour
is from the carapace of krill.
Micheline Jenner – Centre for
Whale Research
Click to enlarge
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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.
A Humpback whale diving down to feed on the abundant
krill.
Micheline Jenner – Centre for Whale Research
Click to enlarge
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