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

Abstract

Anthropogenic changes detected in the sediment of a shallow (~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 became common, and urbanization increased sediment deposition (~1 cm year ⁻¹ ). Changes in P retention were investigated by using dated cores, pore-water element profiles, and water quality records. The lagoon is the last of three STP holding ponds. Ponds 1 and 2 were sand-mining pits; they receive no sediment input and have negligible P adsorption. The Fe:P ratios in sediments from three Australian rivers have a common slope and this relationship was used to examine Fe:P ratios in the anoxic bottom sediments of the lagoon. A potential for effluent P adsorption developed in the lagoon through Fe mobilization and enrichment in the upper sediments and a strong clay–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 effluent P adsorption efficiencies of around 70%.