Abstract： 

Tropical rivers inject nutrients and other terrigenous materials into coastal and offshore waters, supporting biological productivity in and around the river mouth and in extended regions affected by the offshore river plumes. We have carried out focused studies of nutrient distributions, nitrogen fixation, and stable C and N isotope abundances in the Amazon plume in the Western Tropical North Atlantic (2010-2012, 2018).  The supply of nutrients via the Amazon Plume supports elevated biological production in waters many hundreds of kilometers offshore.  We are exploring the impact of the Amazon and other river plumes on nutrient cycling and productivity, with a focus on the activity of the diverse communities of diazotrophic (nitrogen-fixing) organisms that regularly appear in and near these river plumes.  We have found strong variation in nutrient concentrations and nutrient ratios that foster the growth of different, spatially segregated, phytoplankton assemblages.  Our rate measurements show strong regional variation in nitrogen fixation, with measurable rates at almost all stations.  The highest volumetric rates we’ve found have been associated with Diatom-Diazotroph Assemblages (DDAs) in mesohaline regions of the plume, and with Trichodesmium in oceanic waters just outside the plume, though we found measurable rates at almost all stations. Stable isotope (delta-15N and delta-13C) abundances in particles and zooplankton provide a complementary, integrative measure of the movement of diazotroph N into the foodweb and of its critical role in supporting new production in and around the river plume.  Here, I will discuss the biogeochemical impact of the Amazon on the nitrogen cycle and the planktonic food web of the Western Tropical North Atlantic and the environmental and biological factors that appear to control nitrogen fixation in this region.

Bio：

Joseph Montoya is a Professor in the School of Biological Sciences at the Georgia Institute of Technology.  He is a biological oceanographer with research interests at the interface of biology and geochemistry.  His lab specializes in studies of the marine nitrogen cycle and the role of N2-fixation (diazotrophy) in structuring the flow of nitrogen and energy through planktonic ecosystems using a combination of direct experimental rate measurements and stable isotope natural abundance approaches. His work is very field-oriented, with recent and ongoing projects focused on the Gulf of Mexico,  the Western Tropical North Atlantic, and the South China Sea.