Award: OCE-1736947

The Amazon River is the largest river in the world in terms of water volume discharge over a year. It carries nutrients and sediments into the Western Tropical North Atlantic through a vast, buoyant, freshwater plume that travels hundreds to thousands of kilometers from the river mouth. The extreme amounts of nutrients delivered to the ocean strongly impact microbial production and resulting fisheries near the coast. The factors that control the transport and delivery of nutrients offshore are complicated and interconnected, but these nutrients are known to influence phytoplankton community structures across the entire plume ecosystem. To better understand the behaviors of this massive river plume, we used naturally occurring radium isotopes (223Ra, 224Ra, 226Ra, and 228Ra) to examine transport and mixing behaviors through two research cruises to the Amazon River plume in 2018 and 2019 during periods of high discharge. The four natural isotopes of radium have a large range of radioactive half-lives (from 3.5 days to 1600 years). Radium is an ideal chemical tracer for this task because it occurs in elevated activities at the low end of a salinity gradient, and decreases offshore as a function of (1) dilution, for all isotopes, and (2) radioactive decay for the short-lived isotopes (223Ra and 224Ra). The radioactive nature of these naturally-occurring isotopes allows us to calculate apparent radium ‘ages’, which can help us assess dissolved material transport scales and rates, including those of dissolved nutrients that support phytoplankton communities. Our findings are relative intuitive, but offer the first quantitative measures of water transport behaviors in the plume: near the river mouth, waters are relatively young and a mixed rather vigorously with ambient ocean waters. Our results suggest that low salinity plume waters travel at a rate of 77 - 136 cm/s and that this rate is seemingly determined by the amount of river discharge. Transport rates across the plume boundary (i.e., between the plume and ambient waters) were significantly lower, ranging between 13 and 44 cm/s. Mixing in this region is dominated by advective, rather than diffusive, processes. Eddy diffusion coefficients estimated for the core of the plume mixing vertically with ambient waters averaged 3.9 x 10 -4 ± 1.5 x 10-4 m2/s, varying significantly with the relative strength of salinity profiles. Determining the temporal scale of dissolved material (e.g., nutrients) movement across this region is a valuable first step in examining the foundation of planktonic food web dynamics of the Amazon River plume. This project afforded a number of undergraduate and graduate students at Coastal Carolina University to receive hands-on laboratory and at-sea research experiences. They learned sample collection, preparation, and analysis techniques as well as data organization, interpretation, and communication skills. Each of these students has received their degrees and begun their careers in science fields. Last Modified: 01/21/2021 Submitted by: Richard N Peterson