Pelagic Sargassum in the Western Tropical North Atlantic: Does the Amazon Plume Drive Sargassum Growth?

Joseph Peter Montoya, Georgia Institute of Technology, School of Biological Sciences, Atlanta, United States, Erica Kelley Strope, Georgia Institute of Technology, School of Biological Sciences, Atlanta, GA, United States, Ajit Subramaniam, Columbia University, Lamont-Doherty Earth Observatory, Palisades, United States, Elana Jasmine Ames, Coastal Carolina University, Conway, SC, United States, Richard N Peterson, Coastal Carolina University, Coastal and Marine Systems Science, Conway, SC, United States, Mengqiu Wang, Wuhan University, School of Remote Sensing and Information Engineering, Wuhan, China and Chuanmin Hu, University of South Florida St. Petersburg, Optical Oceanography, St Petersburg, FL, United States
Sargassum abundance in the Western Tropical North Atlantic (WTNA) has increased tremendously over the last decade, leading to massive algal strandings on beaches and coasts in the Caribbean and Gulf of Mexico, with significant economic impacts in these regions. The factors supporting increased populations of Sargassum in these waters remain unclear, but the Amazon, like other tropical rivers, injects large quantities of fresh water, nutrients and terrigenous materials into coastal and offshore waters, supporting biological productivity in an extensive offshore region. We carried out focused studies of nutrient distributions, N2-fixation, and stable C and N isotope abundances in the WTNA in 2010-2012 and 2018-2019, finding that the supply of nutrients via the Amazon plume supports elevated biological production in waters tens to many hundreds of kilometers offshore. We encountered much higher abundances of Sargassum throughout our work area in 2018-2019 than during previous cruises to these waters, consistent with remote sensing data showing much higher populations of Sargassum in the Tropical Atlantic after 2010.

During our recent cruises, we collected samples of Sargassum throughout the WTNA. Our studies of the impact of the Amazon plume on nutrient cycling and productivity, and the diverse communities of N2-fixing organisms in this region provide a robust context for exploring the factors that may promote the recent increases in Sargassum abundance in these waters. We found strong variation in nutrient concentrations and ratios in and near the plume; these variations fostered the growth of different, spatially segregated, phytoplankton assemblages and supported strong regional variation in N2-fixation rates. Our Sargassum samples showed clear spatial differences in elemental (C:N ratio) and stable isotope composition (δ15N and δ13C) that reflect the influence of the Amazon plume in supplying nutrients to support macroalgal growth, and an increasing importance of N2-fixation and/or remineralized nitrogen in supporting growth in waters where nitrogen limitation is greatest. We will use our rich set of hydrographic, biogeochemical, and isotopic measurements to assess the role of the Amazon plume in supporting and promoting the growth of Sargassum in offshore waters of the Tropical Atlantic.