Award: OPP-1340886

Photosynthesis by phytoplankton (tiny plants floating in the surface ocean) is responsible for roughly half of the earthÆs biological carbon dioxide utilization, and hence is a major sink for fossil fuel-produced CO2. However, most of the CO2 taken up by phytoplankton will be respired by bacteria or zooplankton grazers and hence re-released into the atmosphere. For this carbon to be removed from the atmosphere for time periods of longer than a season, it must be transported from the surface ocean into the deep ocean. One of the primary mechanisms for export of carbon into the ocean interior is sinking of biological particles including large phytoplankton, zooplankton fecal pellets, and aggregates. Despite the importance of this process in the global carbon cycle, we still lack a mechanistic understanding capable of predicting how carbon export rates will respond to a changing ocean. The Southern Ocean is believed to take up a significant fraction of atmospheric CO2, so it is especially critical to understand these processes around Antarctica. One of the basic theories governing our understanding of carbon export is the balance between "new production" and export. In most regions, the surface ocean is limited by the availability of inorganic nitrogen. Thus the input of "new" nitrogen (primarily introduced when deep, nutrient rich waters are brought to the surface) to the upper ocean will stimulate phytoplankton productivity. Over long time periods and large spatial scales, the surface ocean retains a relatively constant nitrogen concentration. Thus, it is expected that the export of nitrogen to the deep ocean on biological particles must be balanced by the input of new nitrogen and its utilization by phytoplankton. Basically, you can envision the surface ocean as a cornfield. If corn is removed from the farm (export) but no fertilizer is added (new nitrogen) the farm will become depleted and nothing will grow. The addition of fertilizer is equivalent to the new production that happens when currents bring nutrients in deep water to the surface. While the simplicity of this theory is attractive, it is difficult to test because new production and export typically do not occur on the same scales. Often new production happens early in the phytoplankton growing season, leading to an increase in phytoplankton populations, which are exported at the end of the growing season. Furthermore, the constantly swirling ocean currents often transport communities long distances, leading to a spatial decoupling of new production and export. In order to test the equivalence of new production and export, we conducted a program to measure new production and export at a site near Palmer Station Antarctica, during the full phytoplankton growth season (Antarctic summer). We used three independent methods to measure new production and net community production, which agreed well and suggested fairly high new production as would be expected for summer in the nutrient-rich Southern Ocean. We also used two independent methods to measure export and found that while these two methods agreed with each other, they showed much lower export than new production. Our results thus suggest that, contrary to prior expectations, new production is not balanced by the sinking of particles out of the surface ocean in the region near Palmer Station. In fact, the biological pump was substantially weaker than expected. Nevertheless, this new production, must somehow leave the ecosystem (or biomass would continually accumulate). By analyzing nitrate and thorium data, along with physical profiles of the water column, we determined that persistent physically-driven vertical mixing (which may be much higher in the Southern Ocean than in other regions due to the cold surface temperatures) of particles to depth may be responsible for much of the imbalance. This has profound implications for the mechanisms ...