Organic carbon and nutrient fluxes to the coastal zone from the Sepik River outflow

Abstract

The Sepik River is a major contributor of water, sediment and associated organic loads to the coastal waters of northern New Guinea and the Bismarck Sea. We compare dissolved and particulate organic carbon data from September 1997 during an extremely dry El Nino year with those from 1996, 1999 and 2000 during La Nina wet season discharges. Estimated Sepik River flux of DOC is 3.2×10 10 mol yr −1 and POC is 1.1×10 11 mol yr −1. The estimates for total river nutrient fluxes to the sea are 1.1×10 10 mol yr −1 for nitrogen and 4.6×10 8 mol yr −1 for phosphorus. The Sepik DOC flux is about equal to that combined from all four major rivers that enter the Gulf of Papua on the south coast of PNG. The Sepik inorganic PIC flux is low (1.4×10 8 mol yr −1) as the river does not drain carbonate soils. With a narrow continental shelf, and strong coastal currents, much of this exported material is available for long distance transport into the coastal Bismarck Sea and beyond. With data from the water column, a sediment trap study, and analysis of a series of sediment cores, we estimate total fluxes for carbon in this relatively remote coastal system. For the mass balance of organic carbon we calculated primary production in the area as 5.6×10 10 mol yr −1. These terrestrial and marine inputs gave a total of ∼2×10 11 mol OC yr −1 entering the near coastal system. Approximately 40% of the total OC load is degraded in the coastal water column or advected away from the immediate area in coastal currents. Approximately 13% of the annual river organic carbon load of exported sediments settles on the narrow coastal fringe and upper continental slope. About 2% of the total is lost through early diagenesis and 5% preserved in the shelf and upper slope sediments. The remaining ∼53% of the sediment organic carbon apparently flows with sediments along the coastal slope or into the deep sea. The carbon/nutrient regime leaving the Sepik River has a C/N molar ratio of 12, and a C/P ratio of 284 which is in between the Atkinson ratio of 550 and the Redfield ratio of 106. The particles settling through the water column in the nearby coastal waters retain a high C/N ratio; while the dissolved organic matter changes to a lower, more marine, Redfield ratio with distance offshore and with depth.