Biogeochemistry of nitrogen and phosphorus in Australian catchments, rivers and estuaries: effects of land use and flow regulation and comparisons with global patterns

This paper reviews the factors influencing the nitrogen (N) and phosphorus (P) exports from Australian catchments. Pristine, forested catchments export little N and P and the predominant form of N is dissolved organic nitrogen (DON). As catchments are cleared, exports increase and the predominant form of N changes from DON to dissolved inorganic N (DIN). Soluble reactive P (SRP) represents a roughly constant fraction of total P in these systems. As catchments are cleared, DIN:SRP export ratios increase sharply and DIN comes to represent a larger and larger fraction of the total N. The ratios of total N:P and DIN:SRP in rivers reflect land use and the residence times of the water. In Australian lakes and reservoirs, DON and total Kjeldahl N (TKN)are consumed and DIN is exported downstream. Australian freshwater systems with long residence times show stoichiometric evidence of N limitation, and the frequent occurrence of N-fixing cyanobacterial blooms. Despite TN:TP loading ratios equalling or exceeding Redfield stoichiometry, many Australian estuaries and coastal lagoons also show extensive evidence of rapid denitrification and N limitation. Coastal lagoons also have long water residence times (up to 1 year) and a high proportion of the N load is denitrified.