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.
Normal 0 false false false EN-US X-NONE X-NONE The Amazon River is the largest river in the world and has a flow volume five times that of the world’s second largest river, the Congo. The river flows into the Atlantic Ocean at the Equator and is deflected northward by the Brazil Current. Since freshwater is lighter than saltwater, as it mixes with seawater it basically produces a plume on the surface of the ocean which covers about 1.3 million square kilometers. The nutrients, dissolved nitrogen, phosphorus, silicon and iron, in the Amazon River stimulate the growth of marine phytoplankton in the plume. Initially the growth of microscopic diatoms is stimulated, however, after a period of days to weeks, much of the nitrogen sediments to the seafloor, leaving an excess of phosphorus, silicon and iron. This stimulates the growth of a diatom/cyanobacterial symbiotic association in which the cyanobacterial symbiont is able to convert dissolved gaseous nitrogen into ammonia, and eventually amino acids and proteins. These symbioses flourish in five different species of diatoms and basically act to fertilize hundreds of thousands of square kilometers in the plume. As microscopic zooplankton graze on the diatom symbioses particulate matter is transferred to deep water as fecal pellets and exoskeleton molts. Furthermore, as these plankton photosynthesize they take CO2 up and this causes a deficit in CO2 in the plume water relative to atmospheric CO2. This in turn leads to the plume water drawing CO2 from the air and into the seawater. We calculate that the carbon which is transferred to deep water amounts to 1.3 Tmols of carbon annually. In lay terms, this is roughly three billion pounds of carbon annually, and using this calculation, we see that the North Atlantic Ocean is actually a net sink for carbon rather than a source of carbon to the atmosphere. This has important implications for the global carbon cycle as well as climate change. The accompanying pictures are of several species of diatoms which have a filamentous cyanobacterium externally on the cell surface (A), or Richelia intracellularis living symbiotically within the diatom’s silica frustule (B, C). These symbioses are extremely abundant in the Amazon River plume and through nitrogen fixation are responsible for the high rates of photosynthesis and carbon-draw down from the atmosphere.
