Monday, June 27, 2011

Bloom hunting

- Tom Bell [UC Irvine]

Now that you’ve been introduced to the science party, it seems logical to introduce what we are trying to achieve on this cruise (or, as the French call it, Research Mission, which is far cooler). The primary focus of these few weeks at sea is to make measurements of the movement of gas between the ocean and atmosphere and the various factors that affect this process. The gases we are focusing on are carbon dioxide (CO2) and DMS (a sulphur compound that is thought to play some role in the reflectance of incoming solar radiation away from the earth surface). Many measurements of concentrations of CO2 and DMS have previously been made in the atmosphere and ocean and some estimation can be made of the sea-air flux from these. However, a number of assumptions have to be made in order to do this.
Measuring the flux directly - our current experimental method - avoids these assumptions, but requires that we make very rapid (10 times a second) measurements of the concentration in the atmosphere while recording the vertical dynamics of the wind, water vapour and temperature.
Almost all of our sampling takes place from a mast on the bow at the front of the ship. I’m currently reading Moby Dick, which contains a pertinent quote as to why we have to measure from there:
“The Commodore on the quarter-deck gets his atmosphere at second hand from the sailors on the forecastle. He thinks he breathes it first, but not so.”

When measuring the flux of any gas, we have to ‘breathe’ it first, before the gusts of wind are physically and chemically disturbed and the signal lost.
Currently we are steaming North and East away from the North American seaboard - our ultimate destination is the waters close to Iceland. Why go so far away from land to make our measurements? Every year, the waters in the North Atlantic light up with the green colour of chlorophyll.

The oceanic phytoplankton (algae) grow quickly as they make the most of available light and nutrients in the summer months. In the waters to the South and West of Iceland, the annual bloom is consistently dominated by a group of phytoplankton called coccolithophores. These are known to produce large amounts of DMS and, during a previous research cruise, strong flux signals were observed. We hope to try to find similar conditions this time round.

Further information:
Marandino C.A., W.J. DeBruyn, S.D. Miller, and E.S. Saltzman (2008): DMS air/sea flux and gas transfer coefficients from the North Atlantic summertime coccolithophore bloom. Geophys. Res. Lett. 35, L23812

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