Biogeochemistry of carbon in the Amazon River

Abstract

Depth‐integrated, discharge‐weighted water samples were collected over 1,800 km of the Amazon River on eight cruises at different stages of the hydrograph, 1982–1984. Fine (FPOC, <63 µ m) and coarse (CPOC, >63 µ m) particulate organic carbon as weight percentage of suspended sediment varied between 0.9–1.5% for FPOC and 0.5–3.4% for CPOC. Concentrations of FPOC ranged from 5 mg liter−1 upriver to 2 mg liter−1 downriver in the mainstem and from 6 mg liter−1 in the Rio Madeira to <1 in the Rio Negro. CPOC had similar distribution patterns, but with concentrations 15–30% those of FPOC. Dissolved organic carbon (DOC) averaged 4–6 mg liter−1 in the mainstem and up to 12 mg liter−1 in the Rio Negro. Upriver dissolved inorganic carbon (DIC) concentrations of about 1,200 µ M were diluted by tributaries and floodplain drainage to 600 µM at the most downriver site. Evasion of CO 2, invasion of O 2, and in situ oxidation were of comparable magnitude, 3–8 µ mol m−2 s−1. The average export of total organic carbon (TOC) was 36.1 Tg yr−1 (8.5 g m−2 yr−1), of which 62% was DOC, 34% was FPOC, and 4% was CPOC. TOC inputs were insufficient to support in situ oxidation by a factor of at least two. A relatively small, rapidly cycling pool of labile organic matter may coexist with a much larger pool of more refractory material.