Who's nourishing the deep reef?

As we were steaming along yesterday, we encountered a mysterious yellowish slick along the surface.  Sometimes it formed into filaments stretched out like cobwebs on the surface, but in other areas it was thick enough to make the water surface totally opaque.   What could it be, so far off the coast?  fish spawn? coral spawn? algae? oil, maybe? So we slowed the ship and took a bucket sample from over the side (thanks Maurice!).  The culprit? Trichodesmium.  This blue-green alga is common in the nutrient poor waters of the tropics, and occasionally forms huge blooms like this.  How can it bloom when nutrients are so scarce?  The answer is that it makes its own nutrients; Trichodesmium is a “nitrogen fixer”.  This means it can take nitrogen from the air and incorporate it into its own molecules and tissues, a relatively rare feat (the best known example on land are legumes like peas and beans).

If Trichodesmium blooms like this, then the impact can ripple through the ecosystem because, once fixed, the nitrogen is available to the rest of the food chain.  This can make Trichodesmium a key species.

All of that brings us to Dr. Paulo Sumida from the University of Sao Paulo.  Paulo is on this expedition to study organic matter, like the products of all that Trichodesmium.  He’s especially interested in what’s happening on and just above the bottom, where the sub is visiting.  One of the biggest questions: is the organic matter in the sediment of the dark deep made by organisms on the bottom elsewhere and transported there, or is it made by plankton in the water column above (like Trichodesmium), and then rains down like nutrient snow?  One of their other hypotheses is that the southern part of Abrolhos is more productive than the north.  In other words, that more organic matter is produced there by greater numbers of organisms.  To work out the answer to these questions, Paulo looks for clues about how much organic matter there is, what “quality” it is and who made it. 

Dr. Paulo Sumida peers out of a porthole on the JSL sub

Measuring how much organic matter there is is relatively straightforward with an instrument called a CHN analyser (C = carbon, H = hydrogen, N = nitrogen, the key ingredients of organic matter).  To measure quality, Paulo looks at how much of the photosynthetic pigment chlorophyll is present.  If the organic matter is old, most of the chlorophyll will have broken down in a process scientists call diagenesis, leaving behind waste products called phaeopigments.  The relative amounts of chlorophyll and phaeopigments can be used as a measure of the quality of organic matter.  Perhaps the coolest part, however, is trying to work out who made the stuff.  To do that, Paulo uses a rare tool at the University of Sao Paulo, called a GC-IR-MS (gas chromatograph isotope ratio mass spectrometer, say that ten times fast).  This instrument can look for chemical signatures that tell you who made the organic matter.  For example, phytoplankton might produce organic matter with certain carbon isotopes in it, while benthic algae might produce a certain sterol compounds that, when Paulo sees them, he can say “Aha! Now I know that this organic matter was made by this group or that group”. It’s a great bit of detective work.

Taken together, all this information tells Paulo and the other scientists about how nutrients move from water to sediment and back again (properly called “flux”) and therefore how tightly life on the bottom is connected (“coupled”) to life in the water column.  It also speaks to how connected different parts of the bottom may be, especially if organic matter proves to be made somewhere else and then transported to the dark zones.    So what’s the answer? Is it produced on the bottom or in the water column?  By algae or by phytoplankton?  Unfortunately, we don’t know yet, because this is just the sample collection phase; his research is just beginning.  I hope in a future post I can tell you about the results of Paulo’s work.