Transport and retention of nitrogen and phosphorus in the sub-tropical Richmond River estuary, Australia - a budget approach

Abstract

Nitrogen and phosphorus loads in the sub-tropicalRichmond River estuary were quantified and materialbudgets were developed over two years of contrastingfreshwater discharge. During both years >74% of thenitrogen and >84% of the phosphorus load enteredthe estuary during one month when flooding occurred inthe catchment. Due to larger flood magnitude, loadsduring the 1995/96 year were 3.3 and 2.5 times greaterthan during the 1994/95 year for nitrogen andphosphorus respectively. During floods the estuarinebasin was completely flushed of brackish water and themajority of the nutrient loads passed directly throughthe estuary. The nutrient load retained in the estuaryduring floods was inversely proportional to floodmagnitude. Annual budgets show that >97% of thenutrient load entering the estuary was from diffusecatchment sources; precipitation, urban runoff, andsewage were negligible. Less than 2.5% of thenitrogen and <5.4% of the phosphorus loads enteringthe estuary were retained in sediments. During dryseasons the estuary became a net sink for nitrogeninput from the ocean and the estuarine sedimentsremained a net source of phosphorus to the watercolumn and ocean. The process of flood scouring islikely to be the cleansing mechanism responsible formaintaining water quality both on an annual basis andover the last 50 years and may also be responsible forpotential nitrogen limitation. The sub-tropicalRichmond River estuary contrasts with the majority oftemperate systems of North America and Europe whichtypically have lower inter- and intra-annual nutrientload variability, longer and less variable flushingtimes, and greater nutrient retention.