Benthic processes and sedimentary structures of intertidal mudbanks within the Fly Delta, Papua New Guinea, were examined to assess their role in the storage, utilization and export of nutrients from the river delta to the adjacent Gulf of Papua. Three types of mudbanks were recognized based on interpretation of x-radiographs of sediment cores: a physically dominated, high sedimentation regime (one station), a biologically dominated, low-to-moderate sedimentation regime (one station) and a "transitional" regime (two stations). Meiofaunal (29-340 ind.10 cm-2) and macroinfaunal (174-6185 ind.m-2) densities varied widely, being lowest at the physically controlled site and dominated mainly by small deposit- and suspension-feeding polychaetes, amphipods, oligochaetes and nematodes. Infaunal biomass was low (0.17-1.38 g AFDW.m-2) across stations. Bacterial densities (0.14-1.5 x 10(10) cells.g-1 DW) were highest at the physically and biologically controlled sites. Bacterial C production was estimated from rates of protein (H-3-leucine incorporation) and DNA (H-3-thymidine incorporation) synthesis. The methods did not agree well, with rates of DNA synthesis (187-413 mg C.m-2.d-1) several times higher than production estimates derived from H-3-leucine incorporation (18-75 mg C.m-2.d-1). However, specific growth rates (mu) derived using both methods correlated strongly (r = +0.90), with high rates across 20-cm depth horizons at all four stations. Levels of particulate organic C, total N, total P in the sediments and dissolved organic (DOC, DON, DOP) and inorganic (NH4+, NO2- + NO3-, Si(OH)4+, PO4(3-)) nutrients in the porewaters were low compared to other benthic habitats and generally varied little with sediment depth. The average stoichiometric ratio of solid-phase nutrients was low (atomic C:N:P = 42:6:1), suggesting that the material was in very early stages of diagenesis. Fluxes of dissolved inorganic and organic nutrients across the sediment-water interface were mostly undetectable and rates of O2 consumption (145-206 mg C.m-2.d-1) and gross primary production (159-240 mg C.m-2.d-1) were low, as were Chl a (0.5-0.8-mu-g.g-1 DW) and phaeopigments (2.5-4.2-mu-g.g-1 DW). The two transitional stations had large quantities (482-1075 g DW.m-2) of deposited macroparticulate plant litter. It is proposed that the rapidly growing, bacterial communities in these sediments sequester and mineralize most of the labile organic matter deposited onto these mudbanks, leading to the observed lack of nutrient flux across the sediment-water interface and the general absence of vertical solid-phase and porewater nutrient profiles. Intertidal mudbanks of the Fly Delta therefore act as nutrient sinks rather than as net exporters of nutrients which may support coastal food chains in the adjacent Gulf of Papua.
