Sack Lunch Seminar (SLS)

SLS - Elise Olson (MIT-WHOI) - Investigating Trichodesmium spp. abundance and nitrogen fixation in the southwestern North Atlantic through observations and models
Date Time Location
February 18th, 2014 12:10pm-1:00pm 54-915
Abstract:
Converting atmospheric dinitrogen to biologically available form, Trichodesmium represents an important source of new nitrogen to oligotrophic regions and contributes to new production. Trichodesmium colony abundance was sampled in the tropical to subtropical North Atlantic during fall 2010 and spring 2011 using the Video Plankton Recorder (VPR), a towed underwater microscope.

Trichodesmium colony abundance and distribution were related to physical properties, including the mesoscale eddy field. Abundance was elevated in cyclones on the fall cruise and in fresh anticyclones on the spring cruise. Both patterns contrast with a previously identified association with salty anticyclones. Physical aggregation of buoyant colonies through Ekman flux convergence is proposed as a mechanism contributing to elevated abundance in cyclones and supported through idealized simulations.

Additionally, the vertical distributions of abundance and nitrogen fixation were investigated in order to evaluate the hypothesis, based on VPR findings in the subtropical North Atlantic, that Trichodesmium nitrogen fixation may be underestimated by conventional sampling methods through underestimation of deep populations. To estimate nitrogen fixation rates from VPR abundance measurements, a bio-optical model was developed based on measured carbon- normalized nitrogen fixation rates and incubation irradiance. In the regions analyzed, VPR-based estimates of abundance and volume-specific nitrogen fixation rates at depth in the tropical North Atlantic were consistent with estimates derived from conventional sampling methods.