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

Abstract

Water samples were collected within river mouths, at river plume sites and at well flushed ocean-influenced sites within Moreton Bay, a shallow subtropical embayment in south-eastern Queensland. Rates of inorganic nitrogen (NH₊₄ and NO_-3) and carbon uptake were determined across temporal and spatial scales by use of ¹⁵N and ¹⁴C incorporation. Phytoplankton productivity, measured as CO₂ uptake, was highest at the river mouths. Rates of NH₊₄ uptake exceeded rates of NO_-3 uptake at all sites at all times. Relative preference indices demonstrated a consistent preference by phytoplankton for NH₊₄ uptake, and NH₊₄ uptake rates were higher at ocean-influenced sites than at river-mouth sites. Inorganic nutrient and chlorophyll a concentrations were highest at river mouths; however, the greatest NH₊₄ uptake occurred at the ocean-influenced sites, reflecting a greater dependence on ‘recycled’ N than on ‘new’ N. Biomass-independent NH₊₄ uptake increased with increasing water temperature; however, NO_-3 uptake increased with decreasing water temperature, reflecting the lower temperature optimum for nitrate reductase. The range of NH₊₄ and NO_-3 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 from oligotrophic to river-influenced waters with seasonal flows is likely to exist in many tropical and subtropical coastal waters of Australia.