Our Research

What sustains diatoms blooms in a nitrogen-poor tropical (10°N) river plume extending over a thousand miles offshore? Why are pCO2 concentrations in the plume more than 150 ppm below atmospheric concentrations? Why does the biological pump seem to be so efficient there? The answers to these questions lie in the unusual and poorly understood symbiotic relationship between diatoms and diazotrophs (N2-fixing microorganisms). In this project, we hypothesize that large tropical river plumes with low N:P ratios provide an ideal niche for diatom-diazotroph assemblages (DDAs).

We suggest that the ability of these organisms to fix N2 within the surface ocean is responsible for significant carbon export in the Amazon River plume. Our previous observations in the Amazon River plume during our NSF-funded PIRANA project helped reveal that blooms comprised of the endosymbiotic N2-fixing cyanobacterium Richelia and its diatom hosts (e.g. Hemiaulus) were a significant source of new production and carbon export.

Since our previous work focused largely on the sensitivity of DDAs to external forcing from dust and riverine inputs, the ecology of these organisms and the fate of their new production were largely unstudied. We now know that DDAs are responsible for a significant amount of CO2 drawdown in the Amazon River plume. Also, floating sediment traps at 200 m measured 4x higher mass fluxes beneath the plume than outside the plume. We hypothesize that this greater export is due either to aggregation and sinking of DDAs themselves or to grazing of DDAs by zooplankton.


In ANACONDAS, we propose a suite of field, satellite and modelling studies aimed at understanding the ecology and tracing the fate of C and N fixed by DDAs and other phytoplankton living in the plume. By examining C and silicate (Si) export from offshore surface waters, through the upper oceanic food web, the mesopelagic, and down to the deep sea floor, we will quantify the impact of the Amazon River on biological processes that control C sequestration and the implications of this regional processes on C, N and Si budgets. Our study will go beyond previous research because we will quantify 1) the distribution, nutrient demands, and activity of DDAs in the context of phytoplankton species succession, 2) the sensitivity of the CO2 drawdown to the mix of phytoplankton, 3) the grazing and aggregation processes contributing to the sinking flux, 4) the composition of this flux, and 5) the proportion of this material that reaches the seafloor. This effort truly represents a measure of C sequestration and pump efficiency. Our ecological model will be used to place observational results from field studies and satellites into the context of the larger Atlantic basin with tropical climate variability on interannual and longer time scales.


The ROCA project will develop enhanced predictive capabilities regarding the interplay between marine microbial communities, biogeochemical cycling and carbon sequestration in a major river plume environment and to understand the sensitivity of these interactions to environmental change.

Test Image

Above:  In this satellite image using “false color” to represent the amount of living plant material (chlorophyll) observed at the sea surface, fresh water flows from the Amazon River into the Atlantic Ocean to form a green plume that extends hundreds of miles off the coast of Brazil (Photo: SeaWiFS Project and GeoEye, Scientific Visualization Studio,Goddard Space Flight Center, NASA).

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