Wintertime Carbon and Nitrogen Fluxes on Australias Northwest Shelf

Hydrographic characteristics, water column productivity, nutrient cycling, particle and nutrient sedimentation fluxes were measured in shallow waters of the southern Northwest Shelf, Australia, near an offshore oil and gas production platform during September 1994 and 1995. Shelf waters were found to be highly productive (mean primary and bacterial production RATES= 0·75 and 0·31 g C m-2day-1, respectively) despite very low ambient nutrient concentrations. Measured daily primary production rates ranged between 0·45 and 2·5 g C m-2. Uptake rates of15NH4+indicate ambient DIN is cycled on time scales of <3 h. Water column stocks of particulate matter appear to be turned over one to two times daily by sedimentation and resuspension. Despite elevated concentrations in produced formation water (PFW), daily discharge fluxes of DOC and PO42-from the production platform were negligible compared to local water column stocks and fluxes of these materials. In contrast, discharges of NH4+in PFW were equivalent to water column stocks within a 22 km2area. This NH4+, however, was taken up within a short distance of the platform. All evidence points to a highly productive regional ecosystem in which nutrients and organic matter are rapidly recycled.