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

Nitrogen and phosphorus loads in the sub-tropical Richmond River estuary we re quantified and material budgets were developed over two years of contras ting freshwater discharge. During both years > 74% of the nitrogen and > 84 % of the phosphorus load entered the estuary during one month when flooding occurred in the catchment. Due to larger flood magnitude, loads during the 1995/96 year were 3.3 and 2.5 times greater than during the 1994/95 year f or nitrogen and phosphorus respectively. During floods the estuarine basin was completely flushed of brackish water and the majority of the nutrient l oads passed directly through the estuary. The nutrient load retained in the estuary during floods was inversely proportional to flood magnitude. Annua l budgets show that > 97% of the nutrient load entering the estuary was fro m diffuse catchment sources; precipitation, urban runoff, and sewage were n egligible. Less than 2.5% of the nitrogen and < 5.4% of the phosphorus load s entering the estuary were retained in sediments. During dry seasons the e stuary became a net sink for nitrogen input from the ocean and the estuarin e sediments remained a net source of phosphorus to the water column and oce an. The process of flood scouring is likely to be the cleansing mechanism r esponsible for maintaining water quality both on an annual basis and over t he last 50 years and may also be responsible for potential nitrogen limitat ion. The sub-tropical Richmond River estuary contrasts with the majority of temperate systems of North America and Europe which typically have lower i nter- and intra-annual nutrient load variability, longer and less variable flushing times, and greater nutrient retention.