10 April 2004
Saturday, 10 April 2004
After a couple of pre-dawn CTD casts this morning, we slowed down to make measurements in the vicinity of the French CARIOCA buoy. We have finally left the patch that we’ve spent nearly 3 weeks chasing around this area of the South Pacific. We started to see increases in the number of phytoplankton cells toward the end of the patch experiment. It has been hypothesized that once the many cloudy days finally broke into sunshine, the phytoplankton were no longer light limited. But as always in science there are certain to be other contributing factors to the growth limitation of the phytoplankton. Possibly low levels of other trace metals besides iron contributed to the growth limitation.
We are now steaming to the first of 2 biophysical moorings that require servicing. Here we will also be taking water samples for the same suite of analyses that we have been performing on the patch water.
As for myself, I’ve been continuously monitoring the amount of total dissolved gas and oxygen in the surface ocean through the Tangaroa’s seawater supply. I’m also doing titrations for the analysis of oxygen, using the Winkler method, to keep the oxygen sensors on the CTD and in my underway system in calibration.
Coming into this experiment we expected to see significant increases in the amount of oxygen in the ocean produced by active phytoplankton. The grazing by zooplankton has seemingly kept pace with any increased phytoplankton biomass and as a result this expected increase in oxygen hasn’t been observed. The extreme weather we’ve had during the experiment that was detrimental for the patch in many ways was interesting for me however. It provides a unique opportunity to directly observe how high winds affect the concentration of dissolved gases in the open ocean. This increase happens as a result of breaking waves creating bubbles that are mixed throughout the upper part of the ocean and ultimately dissolve. Measurements of the dual tracers SF6 and He3 during the times of high wind speeds provide an accurate estimation of the gas transfer velocity, or piston velocity. Using the gas transfer velocity measurements and the observed gas concentrations, we should be able to test current hypotheses on how high wind speeds affect gas transfer and concentration in the oceans.
Dave Katz (University of Rhode Island, USA)