In partial fulfillment of the requirements for the degree of

Master of Science in Biology

in the

School of Biological Sciences

Elijah Harrison

Will defend his  thesis

“Pelagic Sargassum in the Western Tropical North Atlantic”

April 13th 2026

9:00 AM

https://us05web.zoom.us/j/2211521813?pwd=72tXmkQCSzs1VwZr5w4xbYxqasJgaC.1&omn=82170664459

Meeting ID: 221 152 1813 (Passcode will be shared the day of)

Thesis Advisor:

Dr. Joseph Montoya

School of Biological Sciences

Georgia Institute of Technology

Committee Members:

Dr. Mark Hay

School of Biological Sciences

Georgia Institute of Technology

 Dr. Martial Taillefert

School of Earth and Atmospheric Sciences

Georgia Institute of Technology

Abstract: 

Pelagic Sargassum forms extensive mats across the North Atlantic in what is called the Great Atlantic Sargassum Belt and causes large scale ecosystem and economic damage through recurring coastal inundations. While the Amazon River Plume (ARP) has been implicated as a major nutrient source supporting Sargassum proliferation, its effects on elemental content and stoichiometry in different Sargassum morphotypes, habitats, and at different times remain poorly understood. In this study, we quantify carbon (C), nitrogen (N), and phosphorus (P) content and their molar ratios in Sargassum natans I, Sargassum natans VIII and Sargassum fluitans III collected across spatial and temporal gradients within and outside of the ARP during three summer research cruises (EN614, EN640, M174). 

            Elemental analyses were conducted using isotope ratio mass spectrometry for C and N and flow injection analysis following persulfate digestion for total phosphorus. Nitrogen and phosphorus content showed high variation among phytoplanktonic habitats, cruises and morphotypes. In contrast, carbon content showed comparatively low variability across space, time, and morphotype. Comparison with previously published datasets indicates stronger phosphorus limitation of Sargassum is more phosphorus-limited than previously thought, both within the ARP and in oceanic waters. C:P and N:P ratios well above Redfield proportions reflect phosphorus limitation while high C:N ratios indicate that nitrogen may act as a sequentially limiting nutrient. Morphotypic comparisons suggest that S. natans VIII and S. fluitans III generally exhibited higher nitrogen and phosphorus content than S. natans I, while samples collected from subtropical North Atlantic waters suggested greater nutrient efficiency in S. natans I under increasingly oligotrophic conditions.