When one says "Amazon," most people imagine rainforest, tropical birds, and jaguars; some may conjure up piranhas or pink dolphins. Few will evoke cyanobacteria, marine diatoms, or salps. In the course of extending the river ecosystem to include its oceanic component, we aim to change that perception. We suggest that the popular idea of the Amazon as "the lungs of the earth" must include the breath of the river plume far offshore. In this program, we investigate how the Amazon continuum and its associated carbon cycle extend hundreds of miles from the river to the open ocean and suggest that making such a connection is vital to understand regional and global human impacts and potential feedbacks to climate change.
Our international team of multidisciplinary aquatic and marine scientists aims to connect Amazon carbon cycle research along the continuum from the last river station at Obidos (800 km upriver from the mouth), though the river plume, to the open ocean. We will investigate key components of our central working hypothesis that climate and land-use driven modifications to river discharge and upriver carbon and nutrient concentrations and ratios will significantly impact carbon fluxes across the entire Amazon continuum, including the tropical open ocean.
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 completed 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.
Chong, L. S., W. M. Berelson, J. McManus, D. E. Hammond, N. E. Rollins, & P. L. Yager. (2014). Carbon and biogenic silica export influenced by the Amazon River Plume: Patterns of remineralization in deep-sea sediments. Deep Sea Research Part I: Oceanographic Research Papers, 85, 124-137. doi: 10.1016/j.dsr.2013.12.007
Coles, Victoria J., Maureen T. Brooks, Julia Hopkins, Michael R. Stukel, Patricia L. Yager, & Raleigh R. Hood. (2013). The pathways and properties of the Amazon River Plume in the tropical North Atlantic Ocean. J. Geophys. Res. Oceans, 118(12), 6894-6913. doi: 10.1002/2013jc008981
Goes, Joaquim I., Helga do Rosario Gomes, Alexander M. Chekalyuk, Edward J. Carpenter, Joseph P. Montoya, Victoria J. Coles, Patricia L. Yager, William M. Berelson, Douglas G. Capone, Rachel A. Foster, Deborah K. Steinberg, Ajit Subramaniam, & Mark A. Hafez. (2014). Influence of the Amazon River discharge on the biogeography of phytoplankton communities in the western tropical north Atlantic. Prog. Oceanogr., 120, 29-40. doi: 10.1016/j.pocean.2013.07.010
Moran, M. A., B. Satinsky, S. M. Gifford, H. Luo, A. Rivers, L. K. Chan, J. Meng, B. P. Durham, C. Shen, V. A. Varaljay, C. B. Smith, P. L. Yager, & B. M. Hopkinson. (2013). Sizing up metatranscriptomics. ISME J, 7(2), 237-243. doi: 10.1038/ismej.2012.94
Ward, N.D., R.G. Keil, P.M. Medeiros, D.C. Brito, A.C. Cunha, T. Dittmar, P.L. Yager, A.V. Krusche, & J.E. Richey. (2013). Degradation of terrestrially derived macromolecules in the Amazon River. Nature Geosci., 6(7), 530-533. doi: 10.1038/ngeo1817
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