Author: Maren  Voss, Ph.D. Professor Marine Biogeochemistry (Leibniz Institute Baltic Sea Research)

Designation: 

Description:
The Amazon River delivers 20% of the global riverine freshwater input to the oceans and enhances primary production in the Western Tropical North Atlantic. Nitrogen fixation or species composition were prime foci of previous work in the river plume while nitrate uptake has rarely been studied. Here we present nitrate uptake and nitrogen fixation rates from two cruises to the region on RV Endeavor in May 2018 and June 2019. Highest nitrate uptake rates of up to 24 nmol L⁻¹ h⁻¹ were observed in 2018 in the waters off French Guyana and Suriname and a maximum of 3.6 nmol L⁻¹h⁻¹ off Barbados in 2019. Unexpectedly, nitrate uptake was rather low at most mesohaline stations dominated by coastal phytoplankton like diatoms or <em>Synechococcus</em>. The percentage of primary production supported by nitrate varied considerably between years and was 11±29 in 2018 and only 0.25±0.24 in 2019. Volumetric nitrogen fixation rates were between 0.005 to 0.156 nmol L^-1 h⁻¹ in 2018 and appear to be lower than nitrate uptake rates in some plume influenced areas. The natural abundances of the stable isotopes of nitrogen and oxygen in nitrate collected near the river mouth during a cruise on R/V Meteor in 2018 suggest that nitrogen fixation and nitrification both contribute to the nitrate pool in surface waters. Overall the data reveal nitrate uptake as an important process where nitrate had concentrations were measurable in surface waters, and is of minor importance in other parts of the river plume.

Category: Scientific Program Abstract > Special Sessions > SS39 The present and future of nitrogen fixation in aquatic systems