"A Considerable Error" in Current Estimates of The Ocean Nitrification
21 Jun, 2007 11:14 am
Adrian Martin discusses his findings published in this week?s edition of Nature on the nitrification in the world?s oceans.
We need to separate the nitrogen cycle and the carbon cycle. People are interested in the rate at which carbon is pushed down to the depths of the ocean by biological processes. The main way this has happened in the oceans is through phytoplankton, these minute organisms that are the grass of the sea, that grow in the surface waters: they sink down to depths taking the carbon that they consumed down with them. Phytoplankton require nitrogen as well as carbon to grow. The thinking is to estimate the amount of nitrogen, which is brought up from depths in order for the phytoplankton to construct new phytoplankton to grow. Then you can indirectly estimate the amount of carbon going down. It works according to the principle of what goes down must come up. Youíre using the amount of nitrogen coming up from depths to estimate the amount of carbon going down to help you get a handle on how much carbon is sequestered at depths.
The way that nitrification becomes important is the way that people have traditionally estimated the amount of nitrogen coming up is by assuming that there are two types of nitrogen largely in the ocean. It takes the form, generally speaking, of nitrate and ammonia. People have generally regarded nitrate as purely being formed at depths below a few hundred meters in the ocean. If you can estimate the amount of nitrate being brought up to the surface, then the argument is that you can estimate the amount of carbon going down from the surface by equating the two. In order to do this you need to be very certain that nitrate is only generated at depths in the ocean. The reason nitrification is important is because it is a process by which dead phytoplankton is broken down by bacteria initially. Nitrification involves the conversion of ammonia, which is the other form of nitrogen in the ocean into nitrate. If there is any nitrification going on in the surface that means that nitrate is being produced locally. So, you cannot equate these fluxes of nitrate, the nitrate being taken up in the surface, with the amount of carbon going down. It essentially means that you canít use estimates of nitrate up take by the phytoplankton as a way of estimating the strength of the biological pump, the sink of phytoplankton. Hence, it makes it much more difficult to estimate the amount of carbon sinking down out of the surface.
In your study released in Thursdayís issue of Nature, you estimate the oceanís nitrification. What are your conclusions?
Essentially it suggests that people had thought because nitrification was confined in the dark waters that nitrification was relatively unimportant. In terms of changing this paradigm of being able to equate nitrate uptake with the amount of carbon being taken up, what we found by collating all the ocean elements we could find and use is that a large fraction in addition, about a quarter, to the nitrate that was being taken in surface waters was actually generated in surface waters through this process of nitrification. It means that if you use the uptake of nitrate to estimate how much carbon is sinking out of the ocean, then you actually end up with a very considerable error in your estimate.
What are the implications of your findings in terms of climate change?
In terms of climate change there arenít any direct consequences. Itís more about the way in which you go. If you wanted to draw conclusions about climate change itís more about the way in which you go about monitoring the earth system. In that if we want to understand the impact of climate change and we want to see if itís taking hold, then one of the things we may be interested in, in particular, is how much carbon is being sequestered in the deep ocean. Essentially, how efficiently the ocean is taking carbon dioxide out of the atmosphere, and putting it into the depths in the ocean. This export of carbon from the surface ocean to the deep ocean is clearly going to be a crucial one. That is effectively what we have done; to indirectly help this is to point out that currently there is a flaw in the way we actually calculate this flux from surface ocean to the deep ocean if we use this standard technique called the F-ratio Ė essentially using the nitrogen cycle to understand the carbon flux. Therefore, there are other ways of estimating the carbon flux that our work may be signaling, which maybe we should be paying more attention to other ways of estimating the flux of carbon to the deep ocean.
Nature 21 June. Ocean science: Nitrification in the oceans ó getting it right (pp 999-1002)
Interview by: Christopher Le Coq
Adrian Martin is a researcher at the National Oceanographic Centre in Southampton, in the UK.