Amazon River waters flow into the Western Tropical North Atlantic (WTNA) at a rate of almost 200,000 m3 s-1, creating a low salinity surface plume that can be detected over 1000 km from the river mouth. In the process, the River adds incredible amounts of nutrients to the oligotrophic water of the WTNA. The river nutrients have N:P nutrient ratios significantly less than those needed by most phytoplankton. As a consequence, nitrate is depleted prior to phosphorus and silica, creating an ideal niche space for diatom-diazotroph assemblages (DDA).

 

DDA are a symbiosis between nitrogen-fixing cyanobacteria and a few specific taxa of diatoms. These DDA are believed to play a unique biogeochemical role within the Amaron River Plume, due to their ability to fix dissolved dinitrogen gas and the presumed high sinking rates that would arise from their silicate frustules. Additionally, they are the only nitrogen fixers that are likely to be preyed upon efficiently by mesozooplankton.

 

The Amazon iNfluence on the Atlantic: CarbOn export from Nitrogen fixation by DiAtom Symbioses (ANACONDAS) project is a joint field sampling and modeling program aimed at determining the mechanisms controlling DDA growth as well as their global biogeochemical role. Working with Raleigh Hood and Victoria Coles, I developed a 5P,2Z plankton model and ran it in a three-dimensional model of the WTNA. The focus of the model is to determine the role of top-down, bottom-up, and physical forcing in controlling DDA blooms. Our model code can be found here.

 

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To obtain phytoplankton growth and loss rates, I also went to sea on the 2011 ANACONDAS cruise. On that cruise, I used the microzooplankton dilution technique to measure phytoplankton growth and protozoan grazing rates across the Amazon River Plume. These rate measurements, which indicated that protozoans consume ~70% of phytoplankton throughout the region, will help us constrain our parameterization of the model.