Dissolved organic carbon excretion along environmental gradients in the Amazon River Plume

The Amazon River Plume (ARP) is a productive region of the Atlantic Ocean where riverine nutrients drive particulate productivity. A portion of the carbon produced by phytoplankton is released as dissolved organic carbon (DOC); this research investigated the effects of the ARP on the extracellular release (ER) of DOC. Particulate production (PP) and ER were measured during two cruises to the region, and were contrasted with salinity, dissolved nitrogen, and phytoplankton taxa and size. Chlorophyll normalized PP and ER were unaffected by size, and assemblages dominated by Trichodesmium sp. released a greater percentage of fixed carbon as DOC than sites mainly composed of diatoms. Salinity and total dissolved nitrogen (TDN) did not affect ER, but phytoplankton released a larger fraction of their total productivity at low TDN and high salinity. Particulate production correlated with total dissolved nitrogen, there was no effect of dissolved inorganic nitrogen on PP or ER, suggesting that the phytoplankton utilize the organic nitrogen pool in lieu of ammonium and nitrate.