| Summary: | Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2014 The nitrogen fixation and abundance of Trichodesmium colonies and their connections with physical processes were investigated through Video Plankton Recorder (VPR) and other observations collected in fall 2010 and spring 2011 in the western subtropical–tropical North Atlantic. A data processing procedure for estimating rare taxon abundance was devised to leverage the accuracy of manual classification and the effort savings of automatic classification. In fall 2010, local maxima in colony abundance were observed in a series of cyclones. We hypothesized Ekman transport convergence/divergence in cyclones/anticyclones as a driving mechanism and investigated the process using idealized three-dimensional models. Elevated abundances in anticyclones in spring 2011 were correlated with anomalously fresh water connected to river outflow. A bio-optical model based on carbon-normalized nitrogen fixation rates measured in fall 2010 and spring 2011 was used to estimate nitrogen fixation over the VPR transects. Mean VPR-based estimates of abundance and volume-specific nitrogen fixation rates at depth in the tropical North Atlantic were not inconsistent with estimates derived from conventional sampling methods compiled in a database by Luo et al. (2012). These findings did not reveal the systematic underestimation of deep colony populations and nitrogen fixation hypothesized by Davis and McGillicuddy (2006). This work was supported through a NASA Earth and Space Science Fellowship (NASA NNX11AL59H Understanding the role of the nitrogen- xing cyanobacterium Trichodesmium in the oceanic nitrogen and carbon cycles: in situ measurement, satellite observation, and biogeochemical modeling) as well as fellowship support from the Ocean Life Institute and Academic Programs O ce at WHOI. Additional grant support was provided by NSF OCE-0925284 ...
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