Fertilizer restrictions are not sufficient to mitigate nutrient pollution and harmful algal blooms in the Indian River Lagoon, Florida

Abstract

In Florida's Indian River Lagoon (IRL), anthropogenic eutrophication has resulted in harmful algal blooms and catastrophic seagrass losses. Hoping to improve water quality, policy makers enacted fertilizer bans, assuming that this would reduce the nitrogen (N) load. To assess the effectiveness of these bans, seawater and macroalgal samples were collected at 20 sites “pre” and ~ five-years “post” bans and analyzed to determine concentrations of dissolved nutrients and stable nitrogen isotope values (δ15N). Higher concentrations of ammonium and nitrate were observed post-ban and macroalgal δ15N values increased. A comparison of nutrient concentrations and δ15N between brown tide (Aureoumbra lagunensis) blooms indicated that the post-ban bloom was more strongly N-enriched with higher δ15N values than the pre-ban bloom, which had depleted values in the range of fertilizers. These data indicate a primary role of human waste influence in the IRL, suggesting that current management actions have been insufficient at mitigating eutrophication.