Nutrient biogeochemistry in the tropical Moresby River estuary system North Queensland, Australia

Dissolved, particulate and sediment-bound nutrients were studied in the Moresby River estuary and catchment, to gain a better understanding of the link between landuse practices and enhanced nutrient concentrations in the coastal waterways of the Great Barrier Reef region. During the wet season the estuary controls elevated fluvial concentrations of nitrate by dilution of dissolved nitrate with low nitrate seawater, biological removal of dissolved nitrate and denitrification of particulate nitrate. In the catchment, there is little leaching of phosphate from the soils and phosphate is dominantly transported bound to particulate material. In the estuary, it is suggested that low dissolved phosphate concentrations (0·02 ìmol l-1) are maintained by low pH which regulates the isoelectric point of the surface charge of particulate material (colloidal oxyhydroxides) and favours the adsorption of dissolved phosphate to particulate material). However, other parameters may also be important in controlling low phosphate concentrations such as: biological processes, low suspended sediment concentrations and the short residence-time of suspended material in the estuary. The Moresby Estuary acts to modify and equilibrate nutrient inputs and their transport mediums on the journey from the catchment to the coastal waters. Therefore, it is critical that complete estuary-catchment systems be included in studies designed to evaluate the cause-effect links between landuse practices and the possible impact of these anthropogenically elevated nutrient concentrations on the Great Barrier Reef.