The Influence of Fluvial Discharge on Pelagic Production in the Gulf of Papua, Northern Coral Sea

The influence of river discharge on the behaviour of dissolved and particulate nutrients, and patterns of primary and bacterial production, were investigated in the Gulf of Papua, northern Coral Sea. Inshore, close to the mouths of the Fly and Purari Rivers, buoyant plumes of warmer river water, marked by high total suspended solid concentrations and floating plant debris, overlay cooler saline waters, and their movement with the tides results in marked short-term changes in salinity profiles. Mixing of silicate was conservative, but the behaviour of other nutrients (nitrate, DOC, phosphate) indicated biological uptake and/or production and release from particles. DIN:DIP ratios increased slightly into the gulf indicating possible phosphorus limitation to phytoplankton growth in mid-shelf and outer-shelf waters. At the mouth of the Fly Delta and in inshore waters, decreases in phosphate concentration corresponded to increases in standing stocks of chlorophylla. Primary production rates were <20 mgC m−2day−1in waters of salinity <4, but ranged from 133 to 942 mgC m−2day−1in river delta and inshore waters. Bacterial production was highly variable (range: 162–886 mgC m−2day−1), but correlated very significantly (r=0•97) with primary production. Rates of bacterioplankton production nearly equal or exceed primary production rates, indicating that pelagic bacteria in the gulf are utilizing other carbon sources (e.g. riverine DOC and POC) and that gulf waters are net heterotrophic. The pattern of primary production across the estuary—shelf salinity gradient off the southern Papua New Guinea coast is similar to that observed off the Amazon. However, the much lower volume of water exiting the Papuan rivers restricts maximum production to the inshore (<50 km from the coast) rim of the Gulf of Papua. This pattern is mirrored in benthic secondary production indicating close benthic–pelagic coupling.