It’s a sunny day, I’m on one of my favourite train routes (up the Northumberland coast, between Newcastle and Edinburgh), en route for St Andrews – home, indeed, of the Sea Mammal Research Unit – so it seems appropriate to forget a recent funding disappointment, and the brewing scientific feud I described last week, and to concentrate instead on cool stuff that whales do. Please don’t expect anything about communication, intelligence, or other cuddly things; this is a tale of death and defecation. Two recent papers highlight the vital roles that the great whales have played in marine ecosystems, and in the global climate system more generally. Given the couple of centuries of concerted effort humanity expended in transforming whales into lamp oil, scrimshaw and corsets, they provide another sobering case study of how we’ve managed to fritter away those very natural resources that keep the world a habitable place.
The first paper, by Pershing et al., concerns the role of the great whales in locking up carbon. As they say,
In terms of their size and potential to store carbon for years or decades, marine vertebrates [especially the large whales] are the only organisms in the ocean comparable to large trees.
They use simple metabolic scaling to show that large animals are more efficient at storing carbon than are smaller animals:
…the same amount of primary productivity can support a higher biomass of large individuals due to the increase in metabolic efficiency with increasing size.
So krill, the small crustaceans on which the Southern Ocean ecosystem is based, are pretty rubbish as carbon stores – too small, and with too rapid a turnover. Blue whales, on the other hand, are great. What’s more, if they aren’t eaten or otherwise removed from the sea, even when they die whales perform an important carbon storage function: sinking carcasses can export carbon from the shallow, sunny surface waters to the deep ocean, where it is likely to remain for 100s or 1000s of years.
Perhaps the neatest part of the paper is in using estimates of pre-industrial whaling biomass of the really big species, to calculate the ‘carbon footprint’ of whaling. First, they estimate that whaling removed 1.7 × 10^7^ tons of C from marine ecosystems. They estimate the upper-bound of the pre-whaling flux to be “1.9 × 10^6^ tons C yr^-1^, or 0.1% of the ocean’s net carbon sink”, and they estimate that whaling contributed a total of 2.35 × 10^7^ t of C to the atmosphere.
The logical consequence of this is that restoring whale stocks will benefit the climate. For instance,
rebuilding the southern hemisphere blue whale population would sequester 3.6×10^6^ tons C in living biomass…[these] new blue whales would be equivalent to preserving 43,000 hectares of temperate forest, an area comparable in size to the City of Los Angeles.
The authors note that such figures compare favourably to other climate remediation schemes, notable geoengineering plans to use iron to fertilise the oceans. Intriguingly, whales may perform this role too. The second cool paper is by Nicol et al., and the start of the abstract sets it up nicely:
Iron is the limiting micronutrient in the Southern Ocean and experiments have demonstrated that addition of soluble iron to surface waters results in phytoplankton blooms, particularly by large diatoms. Antarctic krill (Euphausia superba) eat diatoms and recycle iron in surface waters when feeding. Baleen whales eat krill, and, historically, defecation by baleen whales could have been a major mechanism for recycling iron, if whale faeces contain signiﬁcant quantities of iron.
They go on to show that whale poo is, indeed, rich in iron. More specifically, krill are rich in iron, and whales absorb this. But their most crucial role is in converting the iron locked up in krill into a form that is more ‘bioavailable’:
Baleen whale faeces are released as a slurry, which disperses rapidly in the surface layer. [Pre-exploitation]… whales consumed some 190 million tonnes of krill per year … thus they converted some 7600 tonnes of iron in krill into iron-rich faeces every year… Thus, whales would have been part of a positive feedback loop that maintained primary productivity by efficiently recycling iron in the surface waters. Larger populations of whales would have led to enhanced primary productivity and this larger food source would have potentially supported bigger krill populations.
Which is exactly what the geoengineers want to achieve with artificial iron fertilisation. Who’d’ve thunk the climate change generation would be recycling the rather retro rallying cry of the 70s environmental movement?
Save the Whale!