Importance of Biologically Mediated Removal of Fine Sediments from the Fly River Plume, Papua New Guinea

A freshwater plume from the Fly River, Papua New Guinea, stretched to the south-west over a distance of more than 150 km from the estuary towards Torres Strait. The highly turbid freshwater was mixed with the relatively clear water of the Gulf of Papua, producing a sharp boundary around 23 salinity. Sediment concentrations were typically more than 50 mg l−1inside the turbid water mass, and were rapidly reduced to a few milligrams per litre in the area where salinity was still as low as 26. The area had relatively high levels of phytoplankton (>1 μg chl l−1), resulting in rapid depletion of nitrate and phosphate. Biological as well as physico-chemical (flocculation) processes seemed responsible for the rapid removal of sediments from the plume. Settling particles collected near the estuary consisted largely of fine sediments bound loosely by flocculant organic matter, and contained very few plankton and their remains. Further away from the estuary, however, settling particles were dominated by colonies of phytoplankton, fecal pellets and macroscopic aggregates of apparently biological origin. Fine sediments were found firmly attached to or embedded in these settling particles. The area of high biological activities in the plume probably functions as a barrier, limiting a further offshore transport of sediments.