The Effects of Artificial Floating Wetland Islands on Water Quality in a Eutrophic Lake

Alexandra S. Jangrell-Bratli

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Environmental Science and Policy, Department of Environmental Science and Policy, College of Arts and Sciences, University of South Florida St. Petersburg

Abstract

Artificial floating wetland islands are alternatives to traditional treatment wetlands in areas with limited shoreline. Artificial floating wetland island vegetation enhances water quality due to direct uptake of nutrients from the increased surface area available for microbial growth. Three artificial islands and two Pontederia tussocks were enclosed in an urban eutrophic lake in St. Petersburg, FL to determine the effects of artificial floating wetland islands on water quality for two months at the end of the growing season. Excess nutrients from anthropogenic input have decreased water quality in the lake which might affect Coffee Pot Bayou and Tampa Bay into which Crescent Lake water drains. The islands were fertilized with nitrogen and phosphorous to investigate the system response. Water samples were analyzed for dissolved oxygen, oxidized nitrogen, nitrite, ammonium, phosphate, and silica. Biomass was analyzed for nitrogen, carbon, and phosphorous. Aquatic enclosure volumes were measured along with estimates of total percent surface cover of biomass. Presence of islands had an effect of water quality. Dissolved oxygen concentrations were lower in enclosures with islands (0.033 to 3.432 mg/l) than without (5.54 to 26.9 mg/l). The nutrients added to the systems were removed within two days after fertilization for all enclosures. Significant differences in concentrations of oxidized nitrogen, ammonium, and phosphate were only observed for October 4, 2010 (p< 0.05). Nitrate concentrations were higher at the end of the study in enclosures (0.006 mg/l) compared to the open lake concentration (0.003 mg/l). Percent cover did have a significant relationship on removal rates for oxidized nitrogen as removal increased with increasing biomass (p= 0.014). Removal of phosphorous decreased with increasing xii biomass cover likely due to the presence of algae accumulating phosphorous quicker than plants (g/m2*d P = 0.6828818 - 0.0081878*% Total Cover (Oct4)). Accumulation rates of nitrogen (57±2.66) and phosphorous (3.22±1.07 g/m2*yr) were higher for the plants growing on the fertilized islands compared to the unfertilized islands (6.2±2.39 and 0.31±0.12 g/m2*yr respectively). The floating islands did have a short-term effect on nutrients. On a time scale exceeding that of this study, it is expected that a greater effect and an improvement would be observed on the water quality underlying the artificial floating wetland islands.