Nutrient behaviour during post-flood recovery of the Richmond River estuary northern NSW, Australia

Dissolved and particulate inorganic and organic forms of phosphorus and nitrogen, suspended sediments, dissolved silica and physico-chemical parameters were examined in the Richmond River estuary following a small flood event. Under flood conditions, nutrient concentrations were elevated, estuarine processes were bypassed and freshwater, sediments and nutrients were discharged directly onto the continental shelf. The Estuary recovered by way of a salt wedge penetrating landward along the channel bottom, and progressed from a small highly stratified, through a moderately stratified, to a large vertically homogeneous system. Flushing times were the dominant control on the degree to which nutrients were internally processed, with most of the river-supplied nutrients transformed under normal conditions due to very long flushing times. Dissolved inorganic phosphorus is apparently removed at low salinities by adsorption to iron and aluminium colloidal oxyhydroxides which aggregate and undergo sedimentation. At higher salinities, dissolved inorganic phosphorus is apparently desorbed due to a sharp rise in pH. Dissolved nitrate and silica were probably depleted by phytoplankton, and some nitrate may also have been removed through denitrification. As the riverine supply of nutrients is potentially nitrogen limited (i.e. low dissolved inorganic nitrogen: dissolved inorganic phosphorus), primary production for much of the year appears to be supported by the benthic flux of ammonium produced by the mineralization of particulate organic nitrogen deposited during the recovery stage. When this benthic supply of ammonium is exhausted, riverine nitrate becomes an important source of nitrogen. There also appears to be continuous wind-driven resuspension and deposition of bottom materials under the shallow well-mixed conditions that prevail during normal conditions. A three-stage (flood, recovery, normal) conceptual model is presented, which may be more applicable to highly variable Australian estuaries than the typical North American and West European estuarine models found in the literature.