The Amazon River plume ecosystem, extending thousands of km from the river mouth, represents a potentially significant but poorly understood, climate-sensitive C sequestration pathway that seems to violate the expectation of an inefficient open-ocean biological pump. The ANACONDAS investigators hypothesize that large tropical river plumes with low N:P ratios provide an ideal niche for ... diatom-diazotroph assemblages (DDAs). They suggest that the ability of these organisms to fix N2 within the surface ocean is responsible for significant C export in the Amazon River plume. Since primary production fueled by allochthonous sources of N such as N2 fixation can drive a net, biologically mediated transfer of C from the atmosphere to the ocean, C sequestration by DDAs in the Amazon River plume is a regionally significant process. Because DDAs have been found in other tropical river systems, they may represent a globally significant, yet previously overlooked biological pump mechanism.
The ANACONDAS project aims to address specifically 1) how C cycling and sequestration in the tropical North Atlantic is influenced by the Amazon River through its impact on pelagic ecosystem dynamics and 2) the sensitivity of this ecosystem to anthropogenic climate change. Our focus is to identify the links between riverine micronutrient ratios, enhanced N2-fixation, phytoplankton community ... structure and succession, and the sequestration of excess C into the deep ocean via the biological pump. To predict the evolution of this regional C export as climate changes, we must understand these links and their sensitivity to changes in the Amazon itself and other climate impacts on the tropical Atlantic.
ANACONDAS was funded as part of the US National Science Foundation (NSF) Emerging Topics in Biogeochemical Cycles (ETBC) program (Directorate for Geosciences, NSF 07-049, September 2007) explicitly intended to support emerging areas of interdisciplinary research. In this study the researchers will undertake a suite of field, satellite and modeling 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, they will quantify the impact of the Amazon River on biological processes that control C sequestration and the implications of these regional processes on C, N and Si budgets. The study will go beyond previous research because they 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. Ecological modeling 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.
Following NSF funding of ANACONDAS, the Gordon and Betty Moore Foundation, Marine Microbiology Initiative (GBMF-MMI) funded a multi-disciplinary expansion project to further link the plume study to the hydrology and biogeochemistry of the lower reach of the Amazon River and expand our analysis of microbial communities along the River Ocean Continuum of the Amazon (ROCA). The combined project aims to improve predictive capabilities for climate change impacts on the marine biosphere with resulting climate feedbacks via the carbon cycle.