Award: OCE-0934073

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 worldÆs 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 diatomÆs 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. Last Modified: 01/29/2014 Submitted by: Douglas G Capone