The 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. Their previous observations in the Amazon River plume 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. The previous work focused largely on the sensitivity of DDAs to external forcing from dust and riverine inputs, so the ecology of these organisms and the fate of their new production were largely unstudied. It is now known that DDAs are responsible for a significant amount of CO2 drawdown in the Amazon River plume, and floating sediment traps at 200 m measured 4x higher mass fluxes beneath the plume than outside the plume. This led the researchers to hypothesize that this greater export is due either to aggregation and sinking of DDAs themselves or to grazing of DDAs by zooplankton.
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.
Intellectual Merit: The PIs have identified a potentially significant but poorly understood, ecosystem-controlled, climate-sensitive C sequestration pathway that seems to violate the expectation of an inefficient open-ocean biological pump. 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.
Broader Impacts: The Amazon River has captured the public's imagination more than any other river. This study aims to take advantage of such high profile earth science to promote science literacy among all our citizens. This project will support graduate and postdoctoral education, undergraduates through training cruises, and ocean science education of K-12 teachers and undergraduates through the COSEE-West, the Mid Atlantic COSEE and the COSEEOS programs. The results of this research will be made available to other scientists through peer reviewed publications, public databases, and an ANACONDAS website, as well as to the general public through the SFSU RTC-Bay Area Discovery Museum Program.
The ANACONDAS (Amazon iNfluence on the Atlantic: CarbOn export from Nitrogen fixation by DiAtom Symbioses) project was an interdisciplinary study of the the biogeochemistry of the Amazon plume, focusing on its impacts on phytoplankton diversity and production and the N and C cycles of the Tropical Atlantic. This was a field-oriented project with multiple cruises to the Western Tropical North Atlantic to capture the behavior of the Amazon plume and its interactions with surrounding waters at different times of year. Multiple institutions and investigators were involved in this project, providing an integrative study of the biogeochemical interactions between the Amazon plume and adjacent oceanic waters. Within the overall project, the specific responsibilities of the Montoya lab focused on measurements of nutrient concentrations, the elemental (C & N) and isotopic composition of particles and zooplankton, and the rates of activity of nitrogen fixing organisms in the plume and surrounding waters. The first two cruises of this project sampled the Amazon plume during the low- and high-flow periods of 2010 and 2011, respectively. The structure of the plume was quite different in the two years, with a much larger plume extending well to the east of the South American coastline in 2011. Although the Amazon River introduces substantial quantities of dissolved nutrients to the Atlantic, nutrient concentrations decreased rapidly within the plume as water moved away from the river mouth, and the ratio of nutrients in surface waters was consistent with strong nitrogen limitation of primary production. The isotopic composition of particles and zooplankton provided an index to the relative importance of different nutrients sources in supporting biological production, and we found that nitrogen fixation contributed a significant fraction of the total nitrogen needed by plankton in this region. Interestingly, the highest rates of nitrogen fixation occurred in fresher water than we originally expected, suggesting that the plume proper is a critical site of this important biogeochemical process. Measurements of the isotopic composition of zooplankton provided strong evidence for close coupling of secondary (animal) production to nitrogen fixation throughout the region. The third ANACONDAS cruise in 2012 was the first to sample within Brazilian waters, providing our first direct access to the high nutrient and high sediment-load source waters of the Plume. The major contributions of the Montoya group to the ANACONDAS project included extensive measurements of the concentrations of nutrients in the Western tropical Atlantic as well as the first assessments of the flow of nitrogen through the planktonic community from phytoplankton to zooplankton. The broader impacts of this project included extensive training of students at all levels, technicians, and postdoctoral scholars in oceanographic field research. One undergraduate student who received REU supplement support is pursuing a career in ocean science, and one of the two postdoctoral scholars involved in this project has moved to a faculty position in Germany. Our findings have been incorporated into course materials at Georgia Tech and have been shared with the general community through lectures and other public presentations.
