Whale poo, and climate change, and microbes... oh my! (repost)

[This post was originally published at webeasties.wordpress.com]

A recent paper published online in the Proceedings of the Royal Society B (I have no idea what the B means, but maybe one of you can clarify with a comment) draws an enticing connection between whale excrement and climate change.  Because this study involved whale poop, it even made the mainstream news.  The folks at MSN were even nice enough to provide an areal photo of what it looks like when a whale goes to the bathroom.  I have been lucky enough to experience this first hand on a whale watch when a whale shared the contents of its bowels with those of us aboard the ship, so this study had a little bit of a personal connection for me!  Hat tip to Chrissy at Working Through the Blue for pointing me towards this article.

The paper (entitled "Iron defecation by sperm whales stimulates carbon export in the Southern Ocean") can be found here, and seems to be available for download without subscription.  You might be wondering whether this study draws a connection to the microbial world, or if I am just excited to write about whale poop... I assure you there is a connection.  In most environments certain nutrients are limiting, and therefore control the rate of primary production.  This means that no matter how much energy (food, sunlight, or chemical energy) is available, the microbes that form the base of the food pyramid can only convert that energy into biomass (storing carbon in the process) when that limiting nutrient is present.  In the case of the Southern Ocean, iron is the limiting nutrient.  The microscopic phytoplankton floating at the surface take CO₂ out of the atmosphere and convert it to biomass that eventually falls through the water column and acts as food for other organisms.  However, they are restricted in how fast they do this because there is not much iron dissolved in these surface waters.

This is where the Sperm Whales come in.  These giants descend into the cold dark depths to feed (mostly on squid), and come up to the surface to breathe and relieve themselves.  It turns out that their waste is iron-rich.  By depositing this lovely substance in the photic zone, they apparently stimulate primary productivity.  This means that the photosynthetic plankton absorb CO₂, grow, reproduce, are eaten, and so on, and eventually organic material sinks down and provides energy to life in the darker regions.  But wait you say (being the savvy reader that you are), don't the whales exhale a lot of CO₂ back into the atmosphere?  Yes, they do, but according to the modeling of these researchers, the plankton take up much more CO₂ as a result of the iron released by the whale than the whale releases.  "By enhancing new primary production, the populations of 12,000 sperm whales in the Southern Ocean act as a carbon sunk, remivong 2 x 10⁵ tonnes more carbon from the atmosphere than they add during respiration".  They go on to suggest that the reduction of whale populations due to whaling is likely to have diminished this carbon sink.

How did the scientists calculate all of this?  They first assume that these whales eat only cephalopods (mostly squid).  I do not study whales, so I don't really know how accurate of an assumption this is, but it seems to be true for this region (in other areas sperm whales eat more fish).  Then, they used published reports of the amount of iron in cephalopods, and then calculated the amount of prey consumed by the whales by averaging previous studies that had estimated how much these animals eat.  From what is known about iron retention in marine mammals (maybe as low as 10%, they used 15% as their estimate), they were then able to estimate how much iron must be released by the whales during defecation.  They then had to figure out how much of that iron stays in the photic zone long enough to initiate primary production.  They did this by assuming (based on observations) that undigested squid beaks sink almost immediately, but that the liquid material stays around for a long time (this is admittedly non-quantitative).  Finally, they know the form that iron is generally found in the gut (ferrous salts), and they know that that form  can be expected to dissolve quickly and therefore would hang around in the photic zone.  Their estimate was that 75% of the defecated iron would stay around in the photic zone.

I explain this because while headlines like "whale poop may help offset CO₂ emissions" are certainly catchy, to gauge the accuracy of this claim it is important to know how the numbers were crunched.  Flashy findings like this can trickle into the mainstream news, but people generally don't get an explanation of why they should believe this.  [SoapBoxAlert] I think this is part of why it can be so hard for non-scientists to determine which bits of "science" to "believe" and which to be critical of, especially when it comes to climate change [now back to the regularly scheduled science].

In this case the researchers have made some very interesting calculations (they also provide estimates of the losses in carbon sequestration that historical whaling may have caused).  I think it is important to know that their numbers are based on a model rather than actual measurements of the iron concentration of whale feces or what happens to that iron over time.  Models are only as good as the data that go into making them, and in this case it seems (to me, admittedly knowing very little about whales) that they have done a pretty good job.  Whales are very difficult animals to study, and something as simple as sampling their feces would take much more time, energy, and money than you might thing, so I am certainly not faulting the authors for this.  Similar calculations were done to estimate the rate of carbon withdrawn from the atmosphere, and then sunk, but since this is already getting long, I will spare you those details.

In summary, this study provided me the opportunity to write about whales, poo, microbes, the carbon cycle, and scientific methods at once... what better way to spend a Sunday morning!