Anthropogenic changes to a billabong in New South Wales. 1. Lagoon evolution and phosphorus dynamics

Anthropogenic changes detected in the sediment of a shallow (similar to 1.6 m) billabong (Horseshoe Lagoon) are the result of a Sewage Treatment Plant (STP) connection in 1958 and catchment urbanization around 1974. Nutrient- rich water caused the collapse of macrophyte populations, algal blooms beca me common, and urbanization increased sediment deposition (similar to 1 cm year(-1)). Changes in P retention were investigated by using dated cores, p ore-water element profiles, and water quality records. The lagoon is the la st of three STP holding ponds. Ponds 1 and 2 were sand-mining pits; they re ceive no sediment input and have negligible P adsorption. The Fe:P ratios i n sediments from three Australian rivers have a common slope and this relat ionship was used to examine Fe:P ratios in the anoxic bottom sediments of t he lagoon. A potential for effluent P adsorption developed in the lagoon th rough Fe mobilization and enrichment in the upper sediments and a strong cl ay-Fe-P association. The mean long-term P adsorption in the lagoon (35+/-18 %) was the result of maintaining clay input to an oxic waterbody. Bacterial sulfate reduction is the main process decreasing available Fe for effluent P adsorption. Appropriately designed systems could expect to maintain effl uent P adsorption efficiencies of around 70%.