Distribution and sources of particulate organic matter in the water column and sediments of the Fly River Delta, Gulf of Papua (Papua New Guinea)

Abstract

Suspended particles from surface and bottom waters and surficial sediments from the seabed were collected throughout the Fly River subaqueous delta region during the monsoon season in January 2003. Because of the unusually low river discharge associated with a strong El Niño, water-column salinities were relatively high (10 to 32) throughout most of the delta, with brackish salinities (<10) only measured within the main distributary channel of the Fly River. The concentration and composition of particulate organic matter (POM) in these samples showed distinct spatial differences and marked contrasts between water column and seabed samples. Overall, relatively low concentrations of total suspended solids (TSS), particulate organic carbon (POC) and particulate nitrogen (PN) were measured in surface (27 ± 50, 0.83 ± 1.2, and 0.05 ± 0.05 mg/L, respectively) and bottom (400 ± 743, 7.1 ± 7.3, and 1.7 ± 7.2 mg/L, respectively) waters throughout the delta. Particles in both surface and bottom waters displayed elevated organic carbon contents (%OC > 4 wt.%), relatively high organic carbon:nitrogen molar ratios (OC:N > 20 mol:mol) and quite depleted stable isotopic compositions of organic carbon (δ 13C OC < −27‰). In contrast, surface sediments in the seabed displayed spatially uniform compositions that were characterized by markedly lower %OC contents (1.1 ± 0.8 wt.%), lower OC:N ratios (17 ± 9 mol:mol) and relatively enriched δ 13C OC compositions (−25.5 ± 1.1‰). The radioisotopic compositions of OC from a selected set of seabed samples (Δ 14C OC of −408 ± 82‰) indicate OM in surface sediments is old ( 14C ages of 2800 to over 6000 years before present). The ratios of organic carbon to mineral surface area exhibited by these sediments are within the typical (mono-layer equivalent) ranges characteristic of shelf sediments and do not reflect the preferential removal of terrigenous OM. Overall, these compositions indicate that while water-column POM appears to be derived mainly from terrigenous vascular plant debris and riverine/estuarine phytoplankton, the source of most of the sedimentary POM is aged, soil OC ultimately derived from C3 vegetation. We speculate that the concentrations and sources of suspended POM in the water column of the Fly River delta region reflect the conditions of low river discharge, low wave energy and neap tides encountered at the time of sampling. In contrast, POM compositions in surface sediments are consistent with the transport and deposition of old, mineral-bound OC most likely eroded from the upland regions of the Fly River watershed, which is characterized by steep slopes, high precipitation and C3 tropical forests and grasslands.