Investigating Effects of Lemna Removal on Oxygen and Phosphorus Recovery in Urban Ponds
Traditional ecological models often treat aquatic communities as having a linear relationship with water nutrient levels. However, certain aquatic systems more closely resemble an alternative stable state model in which the system tends to gravitate towards one of two sets of biotic conditions based on a critical nutrient threshold. Lemna, an aquatic plant taxa, has long been recognized as an instigator of low-oxygen, high phosphorus (eutrophic) conditions in freshwater ecosystems due to their ability to block sunlight and restrict photosynthesis in the water column. As such, this study aims to determine if the presence of Lemna acts as a driver for the establishment and maintenance of an alternate steady state regime in small pond ecosystems through modulation of oxygen and phosphorus levels. The oxygen and phosphorus levels of four, small, Lemna infested urban ponds were monitored over the course of one summer, and Lemna was manually removed from one of these ponds partway through the monitoring period. Following removal, the Lemna-free pond experienced rapid reoxygenation and decrease in phosphorus levels to that of a non-eutrophic ecosystem. These results indicate that the presence of Lemna can constitute an alternative stable state regime, and that methods to remove or control Lemna growth can effectively reverse eutrophic conditions.