Associate Professor Paul I. Boon and Dr Paul C. Bailey
Environment Australia, 2000
There is some evidence that the aquatic vegetation of shallow lakes and wetlands can exist in a number of alternative metastable states, depending on the nutrient loading. We examined this concept experimentally using large (3000 L) mesocosms (experimental pools) in a well-vegetated shallow urban lake (Victoria Park Lake) in Shepparton, central Victoria. There was little or no effect of low nutrient loadings on the submerged macrophytes (mainly Vallisneria americana), but moderate or high loadings could cause the compete loss of plants. The loss was caused largely by extensive growth of the floating fern, Azolla, which resulted in marked decreases in the dissolved oxygen concentration and penetration of light into the water column. Nutrient loading also increased the abundance of phytoplankton, resulting in algal blooms. Epiphytic algae responded less to nutrient enrichment than phytoplankton, increasing only under high levels of enrichment. Most of the nutrients added to the mesocosms were bound to sediments, and did not appear as free forms in the water column. Harvesting the aquatic plants - a technique commonly used in managing shallow urban lakes - also had major effects on the macrophyte and algal communities. The results of the study have many implications for the management of wetlands and shallow lakes, including improved understanding of the factors controlling the biodiversity of aquatic environments, the effectiveness and risks associated with techniques routinely used to control aquatic plants, and the design of monitoring programs designed to give early warning of the effect of eutrophication on the vegetation of wetlands.