Utilization of nitrogen and carbon by phytoplankton in Moreton Bay, Australia

Water samples were collected within river mouths, at river plume sites and at well flushed ocean-influenced sites within Moreton Bay, a shallow subtro pical embayment in south-eastern Queensland. Rates of inorganic nitrogen (N H4+ and NO3-) and carbon uptake were determined across temporal and spatial scales by use of N-15 and C-14 incorporation. Phytoplankton productivity, measured as CO2 uptake, was highest at the river mouths. Rates of NH4+ upta ke exceeded rates of NO3- uptake at all sites at all times. Relative prefer ence indices demonstrated a consistent preference by phytoplankton for NH4 uptake, and NH4+ uptake rates were higher at ocean-influenced sites than a t river-mouth sites. Inorganic nutrient and chlorophyll a concentrations we re highest at river mouths; however, the greatest NH4+ uptake occurred at t he ocean-influenced sites, reflecting a greater dependence on 'recycled' N than on 'new' N. Biomass-independent NH4+ uptake increased with increasing water temperature; however, NO3- uptake increased with decreasing water tem perature, reflecting the lower temperature optimum for nitrate reductase. T he range of NH4+ and NO3- uptake rates was greater than ranges reported for other coastal waters, reflecting the strong temporal and spatial gradients within Moreton Bay. This trend of strong gradients in C and N dynamics fro m oligotrophic to river-influenced waters with seasonal flows is likely to exist in many tropical and subtropical coastal waters of Australia.