Another key variable is the ratio of N-to-P or ES (ecological stoichiometry) ratio in the dissolved and particulate pools; i.e., the C:N:P composition that we have just been talking about. Finally, there is the remineralization ratio, termed the r-ratio. Both the rate of remineralization and the change in C:N:P stoichiometry with time (i.e., depth) are important components of the r-ratio. All of these parameters are important to carbon sequestration and to the overall efficiency of the biological carbon pump. For sequestration to be most efficient, one would need a high e-ratio and a low r-ratio, so that a large percentage of recently produced organic matter would be exported deep into the sea instead of being completely remineralized in the euphotic zone or near the top of the thermocline.

imageAn important control on e-ratio is community structure, as depicted in the model originally presented by Dr. Louis Legendre. It presents the dichotomy between two fundamentally different groups of phytoplankton in the sea, namely the diatoms on the left and picoplankton, as represented here by Prochlorococcus, on the right. The important thing to remember from this diagram is that when diatoms grow there is a high probability, but not a certainty, that a large percentage of organic matter will be exported from those habitats (i.e., a high e-ratio). This is predicted because diatoms have the characteristic that they are eaten by large zooplankton who produce rapidly sinking fecal pellets, and under certain conditions they can aggregate and directly sink out of the euphotic zone without being grazed at all. This latter process leads to a very large e-ratio relative to a Prochlorococcus-dominated ocean; the very tiny cells do not sink and are generally grazed and fully remineralized within the euphotic zone.

So community structure matters. You can't just say, well, there's so much or so little primary production in the sea. You must also know which organisms are producing the bulk of the carbon in order to make fundamental predictions about whether or not that habitat is likely to be a net exporter of organic matter.


imageNow I'd like to draw from some field data collected in the subtropical North Pacific Ocean, Earth's largest habitat. Since October 1988 we have been conducting experiments at a location that we call Station ALOHA using research vessels and autonomous moorings and in-ocean experiments. We have a sister station in Bermuda where these measurements and experiments are being done in parallel.

The subtropical North Pacific Ocean represents an oligotrophic (low nutrient and low biomass) portion of the ocean. This is not a present target area for Fe fertilization, but it might be in the future. Subtropical areas like these represent nearly half of the surface of the Earth, and the Pacific, by itself, nearly 25%. So these are nontrivial oceanic habitats that are poorly understood.

imageFirst, I'd like to show a few of the groups of microorganisms that are present in these habitats. This graph shows the time course of the euphotic zone inventory for zeaxanthin, a biomarker pigment for a group of microorganisms called cyanobacteria, some of which fix N2. As you can see, over the period of our field observations, lasting approximately ten years or so, we can see that the euphotic zone depth-integrated inventory of this biomarker, especially over the last five years, has nearly doubled. This suggests that this important, perhaps, key group of microorganisms has recently been selected for and that their population dynamics are changing on time scales of years to decades.

One of the more remarkable things about this set of observations is that it is all happening in a portion of the world ocean that had previously been thought to be homogeneous and constant in time (a so-called "climax" community), and here we're seeing changes of a factor of two in a group of microorganisms that we believe may be key components of the oceanic carbon cycle and perhaps crucial to the efficient functioning of the ocean's biological carbon pump.

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