The investigators hypothesize that large tropical river plumes with low nitrogen:phosphorus ratios provide an ideal niche for organisms that can fix nitrogen, and that this is responsible for significant carbon export in the Amazon River Plume. The PIs have identified a potentially significant but poorly understood, ecosystem-controlled, climate-sensitive carbon sequestration pathway that seems to violate the expectation of an inefficient open-ocean biological pump. Primary production fueled by external sources of nitrogen such as nitrogen fixation can drive a net transfer of carbon from the atmosphere to the ocean. Thus, this may represent a significant, yet previously overlooked biological pump mechanism.