Nutrients in the sediments of a tropical north-eastern Australian estuary, catchment and nearshore coastal zone

Abstract

The distribution, transport, accumulation, modification and seasonal variability of sediment nutrients was studied in the Moresby River Estuary, catchment and nearshore zone. Phosphorus was examined by using a quick and analytically simple three-stage sequential extraction scheme that allows for natural environmental processes and differences in bio- availability. Elevated sediment phosphorus concentrations in the catchment during the wet season reflect the application of phosphate fertilizers to the agricultural lands that dominate the catchment. It is suggested that the dispersion and seasonal variability in concentration and distribution of phosphorus are controlled by sedimentary processes (e.g. erosion and transport of sediment) that are intimately linked to hydrological conditions. The physical processes that operate on a time-scale of hours (e.g, tidal currents) dominate sediment phosphorus concentration in the estuary and preclude chemical and biological equilibrium from becoming established in the sediment phosphorus along the length of the estuary. Elevated sediment total Kjeldahl nitrogen concentrations in the catchment reflect the application of nitrogenous fertilizers (mainly urea). The nitrogen distribution patterns differ from those of phosphorus, suggesting that different processes operate to control sediment nitrogen and sediment phosphorus. Low phosphorus concentrations in the sediments suggest that agricultural practices in the catchment and associated anthropogenic inputs are having little, if any, impact on the catchment, river and estuary.