Sinking organic particles in the ocean constitute a major part of the biological pump (Figure 1) and are commonly assigned two fates: mineralization in the water column and accumulation on the seafloor. Mineralization is often assessed by the decline in vertical flux of the particles with increasing depth. Yet there is another fate of export hidden within the vertical decline of particle flux, and that is the transformation of sinking matter to fine suspended and/or dissolved organic fractions. This process has often been suggested in the literature, but has rarely been observed or quantified. We know that deep microbes respond to the sinking flux of organic matter, so we have presumed that the solubilized fraction is largely mineralized over a short time scale, whereas the insoluble fraction can accumulate in the deep ocean. The nature of this accumulation is difficult to discern because of complicating factors such as i) vertical advective export of dissolved organic carbon (DOC) into these systems and ii) very deep water columns over which the accumulation is spread/diluted.
Figure 1. The Biological Pump. As particulate or dissolved organic carbon (DOC) sinks, it is exported out of the mixed layer into the deep ocean where it is mineralized, or converted into CO2. This CO2 is then brought back into the mixed layer via upwelling, where algae can grow in the presence sunlight. These algae photosynthesize at the surface ocean, converting the upwelled CO2 into DOC and the cycle repeats itself. Arrows denote water movement on seasonal to millennial timescales. Click figure to enlarge.
The goal of our research is to study the transformation from sinking-to‐solubilized phases of the export flux in the Ross Sea, a model system for this process because the basin experiences i) very high export production as particles, ii) very low export as DOC with overturning circulation, and iii) a predictable succession of production and export events. The Ross Sea is also a shallow basin (<1000 m) when compared to the average ocean depth (~4000 m), so the water we sample will be relatively concentrated with signatures of export production. In the Ross Sea, we will utilize both well-established and novel biochemical, biogeochemical and optical measurements of export production and its fate during the austral summer of 2013. Our goals are to capture the senescence of the colonial phytoplankton species, Phaeocystis antarctica (Figure 2a), during their blooms in the central Ross Sea, and their early fate in the water column, which we will look for at the end of February. In March, we aim to capture export by silicifying diatoms (Figure 2b) in the western Ross Sea, where export continues later in the season than in the central Ross Sea. Our research and field campagne will move the scientific community towards closing the carbon budget in the Ross Sea, a long held goal.
Figure 2. (a) A colony of Phaeocystis antarctica, and each P. antarctica is approximately 5 µm in diameter and (b) a diatom, which is approximately 10 µm in diameter. The diatom image was provided by the Monterey Bay Aquarium. Click figure to enlarge.