On the sub-seasonal processes controlling the natural phytoplankton abundance and biological pump in the Drake Passage

Veron, FabriceOliver, Matthew J.The biological pump is an important aspect of the global carbon cycle. Phytoplankton blooms in the surface ocean draw down atmospheric carbon dioxide and package it as organic particulates that sink into the deep ocean, where carbon can be sequestered on millennial timescales. Phytoplankton blooms are controlled by nutrient availability, changes in light exposure, respiration, and grazing. In the high nitrate, low chlorophyll surface waters of the open Southern Ocean (SO), blooms are further limited by insufficient iron concentrations. Hence, the SO draws considerable attention as a potential site for geoengineered atmospheric carbon concentrations via iron fertilization. However, there are no prolonged, in-situ observations of the mechanisms controlling the formation of naturally occurring phytoplankton blooms in the SO. To better understand the bloom formation mechanisms in the SO, an APEX biofloat was deployed in the energetic Drake Passage. The biofloat profiled the water column every two days and resolved the development of a natural phytoplankton bloom, along with a subsequent organic carbon export event. Satellite observations suggest that the phytoplankton abundance in the Drake Passage is low when the surface mesoscale kinetic energy (KE) is high. This observation is supported by biofloat observations that revealed, on average, high levels of KE deepen the mixed layer and may therefore control phytoplankton abundance through light limitation. Satellite observations further suggest that the relationship between high KE and phytoplankton abundance could be extended to the entire SO. Furthermore, our analysis indicates that when the high mesoscale kinetic energy control on phytoplankton abundance is alleviated, the mechanisms important for natural bloom formation include sub-mesoscale grazing relaxation and foreign iron deposition. Following the bloom, we infer that high levels of mesoscale kinetic energy contributed to organic carbon export.University of Delaware, Department of School of Marine Science and PolicyM.S