Today we did our first deep CTD cast of the cruise, to 3500 meters below the ocean surface (over 10,000 ft). In addition to sampling important water column properties, we entrusted the CTD with a precious task: transporting decorated Styrofoam cups! No, we didn’t release Styrofoam into the ocean to perpetuate the global marine debris problem; Styrofoam cups make great cruise mementos and oceanographic teaching props!
Since pressure increases with water depth (more water weight sits above something at 3500 meters than at 10 meters depth), and since Styrofoam is compressible, sending cups down into the ocean squeezes them to less than a quarter of their normal size at the surface. This makes them hardly useful for drinking out of, but great for demonstrating how much pressure a fish must adapt to in order to live several thousand meters below the surface.
Our scientific impetus for conducting deep CTD casts is to measure dissolved organic carbon (DOC) in the deep water column. DOC is released into surface waters by animals that produce or take up carbon and then extrude it in dissolved form – usually from phytoplankton that release DOC as they die, or from zooplankton regurgitating bits of carbon as they feed. The pool of DOC in the ocean is similar to the amount of carbon dioxide in the atmosphere, making DOC an important component of the ocean and global carbon cycle.
Brandon Stephens, a Ph.D. candidate in Lihini Aluwihare’s lab at Scripps, is interested in isolating a carbon component known as ‘refractory’ DOC, which is the unpalatable and therefore very old component of organic carbon in the ocean (nothing wants to eat it, so it just cycles around for thousands of years). Stephens is interested in aging DOC and determining the distinct composition of the pool in the California Current System. He also uses deep DOC as a baseline reference for the upper ocean DOC values that he measures during the cruise. His findings suggest, among other things, that deep DOC in the California Current is some of the most carbon-depleted in the world. The ocean in our backyard continues to surprise us!
Posted by: Laura Lilly, SIO