Intra- and inter-annual variability in the different forms of diffuse nitrogen and phosphorus delivered to seven sub-tropical east Australian estuaries

Intra- and inter-annual variability in the different forms of diffuse nitrogen and phosphorus delivered to seven sub-tropical east Australian estuaries was examined using river concentration data collected in 1996 and historical flow records. Rainfall during the study period was above average due to high rainfall in January, May and November (southern catchments only) which resulted in early January and May, and late November (southern catchments only), floods. A total load of 12,321 t of nitrogen and 1594 t of phosphorus was exported from the seven catchments during 1996. Percentage forest cover was a good predictor of total phosphorus (TP) exports (kg/km²/mm) and a weak predictor of total nitrogen (TN) exports (kg/km²/mm), except for the Bellinger and Manning catchments where high exports from non-forested land uses masked the percentage forest cover/export relationship. Intra-annual variability was large due to the three floods transporting up to 76% of the annual TN load and 73% of the annual TP load from individual catchments. As such, 75% of the annual TN and TP loads in all seven catchments were transported in less than 20% of the time. This contrasts with typical temperate systems where it takes 50% or more of the time to deliver 75% of the annual load. Inter-annual variability was also larger than typical temperate systems with up to a 29 times difference in TN and TP loads between years over the period of historical flow records (approximately 50 years). The large intra- and inter-annual variability reflects Australia's highly variable hydrology. Nitrogen exports were dominated by organic forms with an average of about 80% of the TN load consisting of particulate nitrogen (PN) and dissolved organic nitrogen (DON) reflecting the high percentage forest cover in the catchment. Phosphorus loads were more evenly distributed between the three forms, dissolved inorganic phosphorus (DIP), particulate phosphorus (PP) and dissolved organic phosphorus (DOP).