Physical and biological contributions to the western tropical North Atlantic Ocean carbon sink formed by the Amazon River plume

Dissolved inorganic carbon (DIC) and total alkalinity (TA) were measured in the upper 1000 m of the western tropical North Atlantic Ocean (WTNA; study area 3–15°N, 40–59°W) in January–February and July–August 2001. Concentrations of DIC and TA in surface samples (0–10 m) influenced by the Amazon River plume were up to 400 μmol C kg−1 (∼20%) lower than oceanic surface samples. In this region, physical dilution by river water dominates DIC and TA inventories, driving CO2 partial pressure (pCO2) well below atmospheric levels. Nevertheless, DIC concentrations at most plume‐influenced stations were 10–90 μmol C kg−1 below levels expected from conservative mixing of seawater with low‐salinity, low‐CO2 Amazon River water. In this otherwise oligotrophic region, the diazotrophs Trichodesmium spp. and Richelia intracellularis were often abundant, supporting a link between increased carbon drawdown and nitrogen fixation in the outer plume. Net community production in the plume must surpass the fluxes of inorganic carbon from below and air‐sea CO2 replacement to leave biologically mediated DIC deficits, which is possible under observed conditions. Biological activity lowers plume pCO2 30–120 μatm below the conservative mixing line, and contributes to a CO2 deficit in the northern WTNA that outlasts the plume's physical structure.