Relevance of peat draining rivers in central Sumatra for the riverine input of dissolved organic carbon into the ocean

Sources and discharges of dissolved organic carbon (DOC) from the central Sumatran river Siak were studied. DOC concentrations in the Siak ranged between 560 and 2594 mmol l 1 and peak out after its confluence with the river Mandau. The Mandau drains part of the central Sumatran peatlands and can be characterized as a typical blackwater river due to its high DOC concentration, its dark brown-coloured, acidic water (pH 4.4e4.7) and its low concentration of total suspended matter (12e41 mg l 1). The Mandau supplies about half of the DOC that enters the Siak Estuary where it mixes conservatively with ocean water. The DOC input from the Siak into the ocean was estimated to be w0.3 Tg C yr 1. Extrapolated to entire Indonesia the data suggest a total Indonesian DOC export of w21 Tg yr 1 representing w10% of the global riverine DOC input into the ocean.