Investigating Effects of Lemna Removal on Oxygen and Phosphorus Recovery in Urban Ponds

Abstract

Traditional ecological models often assume a linear relationship between aquatic communities and 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 ecological conditions based on a critical nutrient threshold. Lemna, an aquatic plant genus, has the ability to instigate eutrophic conditions in lakes by blocking sunlight at the water surface. This restricts photosynthesis in the water column, resulting in a low oxygen, high phosphorus ecosystem. As such, this study aims to determine if the presence of Lemna acts as a driver for the establishment and maintenance of an alternative stable state system in small pond ecosystems through modulation of oxygen and phosphorus levels. The levels of these nutrients were measured in four small Lemna-infested urban ponds over the course of one summer. The Lemna was manually removed from one of these ponds partway through the monitoring period. Following removal, the Lemna-free pond experienced rapid reoxygenation to levels characteristic of a non-eutrophic ecosystem. One of the control ponds experienced a marginally significant change as well, but at a much lesser magnitude. Total phosphorus in the water column of the experimental pond decreased, but it was not significant. These results indicate that the presence of Lemna can create an alternative stable state system, and that methods to remove or control Lemna growth can effectively reverse eutrophic conditions.