MS Thesis by Shannon Salter

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In partial fulfillment of the requirements for the degree of


Master of Science in Biology

in the

School of Biological Sciences


Shannon Salter


Will defend her thesis


“Trophic structure in the Western Tropical North Atlantic Ocean using Stable Isotope Abundances (δ15N and δ13C)”


Wednesday, 27, April , 2022

11 AM


Thesis Advisor:

Dr. Joseph Montoya

School of Biological Sciences

Georgia Institute of Technology


Committee Members:

Dr. Jennifer Glass

School of Earth and Atmospheric Sciences 

Georgia Institute of Technology


 Dr. Mark Hay

School of Biological Sciences

Georgia Institute of Technology


Dr. Natalie Loick-Wilde

Leibniz-Institute for Baltic Sea Research



We investigated the biomass concentration and natural abundance of nitrogen and carbon stable isotopes (δ15N and δ13C) of size-fractionated zooplankton collected on two separate cruises to the Amazon River Plume (ARP) region. Mean animal biomass concentrations ranged between 0.106 to 0.811 g/ m3 during the day and 0.242 to 1.90 g/m3 at night for Cruise EN614. The mean animal biomass on Cruise EN640 ranged from 1.541 to 6.874 g/ m3 during the day and 2.505 to 6.291 g/m3 at night. Sampling stations were classified into habitat types identified by Weber et al. (2019) and Pham et al. (in prep): the young plume core (YPC), old plume core (OPC), outer plume margin (OPM), western plume margin (WPM), and oceanic seawater (OSW). In general, zooplankton δ15N varied markedly and significantly among habitats for both cruises, with generally higher values in the YPC, OPC, WPM, and OSW habitats relative to the OPM habitat. Zooplankton δ15N values reflect the sources of nitrogen supporting biological production, and trophic processing within the food web. Suspended particles collected from these habitats showed lower d15N values than the zooplankton, but we did not find a consistent increase in animal δ15N with size. Vertical migration led to diel shifts in zooplankton d15N at the surface, with generally higher d15N values at night than during the day during both cruises. Zooplankton d13C values varied less than their δ15N, with no significant differences among habitats or with animal size.




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