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 were quantified and material budgets were developed over two years of contrasting 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 for nitrogen and phosphorus respectively. During floods the estuarine basin was completely flushed of brackish water and the majority of the nutrient loads passed directly through the estuary. The nutrient load retained in the estuary during floods was inversely proportional to flood magnitude. Annual budgets show that > 97% of the nutrient load entering the estuary was from diffuse catchment sources; precipitation, urban runoff, and sewage were negligible. Less than 2.5% of the nitrogen and < 5.4% of the phosphorus loads entering the estuary were retained in sediments. During dry seasons the estuary became a net sink for nitrogen input from the ocean and the estuarine sediments remained a net source of phosphorus to the water column and ocean. The process of flood scouring is likely to be the cleansing mechanism responsible for maintaining water quality both on an annual basis and over the last 50 years and may also be responsible for potential nitrogen limitation. The sub-tropical Richmond River estuary contrasts with the majority of temperate systems of North America and Europe which typically have lower inter- and intra-annual nutrient load variability, longer and less variable flushing times, and greater nutrient retention.