A little while ago I wrote a post on the actions of Russ George, a businessman who has been trying to sell ocean iron fertilisation as a viable method for reducing carbon dioxide in the atmosphere. His arguments about the success of such schemes are way out in front of the science required to support them. He released 100 tons of iron sulfate into the north-eastern Pacific in what he calls an experiment, but what nearly all other pundits have been calling an irresponsible and reckless action.
His supporters have rallied behind him though and have had a presence in nearly every comment thread of prominent science news sites that have covered the issue. So, I thought I would take some time to put my views on the potential dangers of large-scale fertilisation of the ocean with iron.
Phytoplankton is not only reliant on iron to survive. There are several other important nutrients that limit phytoplankton numbers in areas where iron is abundant, such as phosphorous. Large blooms of phytoplankton that are produced by the addition of iron could rapidly deplete other limiting nutrients. Once the bloom has consumed the added iron and collapsed the population may not recover back to what it was prior to the bloom because it is limited by more than iron.
When the bloom dies and decomposes, it could reduce a molecule that is highly important to a great many marine organisms, oxygen. Areas of the ocean that become so low in oxygen that they are no longer able to support life and know as dead zones. The appearance and expansion of dead zones is often caused in areas where human inputs of important nutrients, such as phosphorus and nitrogen, increases the abundance of phytoplankton. The bacteria that consume the dead phytoplankton also consume the dissolved oxygen, depleting it to levels dangerously low for many other organisms.
The rain of dead phytoplankton could also have serious effects on the seafloor community beneath. Indeed, one of my very first posts on this blog was about how the energy balance in the deep sea was critical to maintaining a high diversity of species in an energy poor, seemingly homogenous environment. A huge input of nutrients from the detritus of a plankton bloom that reaches the seafloor is likely to upset the ecology of the communities found there.
While I think that these are all legitimate concerns, it is important to note that, to my knowledge, none of these effects have been observed in iron fertilisation experiments. Oxygen depletion is a known and well documented effect of phytoplankton blooms. But, iron fertlisation on the scale of Russ George's venture may not be large enough or persist for long enough to have this effect. And natural phytoplankton blooms on the scale of the one observed in the area that George dumped his iron are not unknown.
In fact, some researchers are claiming that the observed phytoplankton bloom was already underway before George released the iron sulfate. Many reports I have read say that that the iron fertilisation occurred in July at the time the plankton bloom was getting started. But, it seems the ship which released the iron sulfate didn't leave port until the 8th of August and probably couldn't have started fertilising until a week later. Natural blooms are known to occur regularly in the area, particularly during summer when offshore currents carry iron-rich water hundreds of kilometers out to sea. Unfortunately, the design of the experiment is so poor that it's hard to tell whether the iron sparked the bloom or how much it contributed to it.