Influence of Mineral Precipitation and Aquatic Vegetation on Phosphorus Removal in Canal Water from the Everglades Agricultural Area of Southern Florida.

Phosphorus enrichment has been identified as a contributor to the diminished water quality and negative ecological changes occurring in the Everglades National Park (ENP) in southern Florida. Discharges from the Everglades Agricultural Area (EAA), a former sawgrass marsh drained for agricultural use in the early 1900s, have been identified as a source of elevated P levels in the Everglades. Discharges from the EAA contain approximately 50% particulate P, predominantly from plant detrital material, with the remaining P in the more reactive dissolved form. Calcium has been assumed to be the predominant element that to precipitate with phosphorus and remove P from the mobile phase, however, direct comparative experiments of the potential for other metals (Mg, Fe, Al, Mn) to precipitate P under EAA conditions have not been tested. Additionally, floating a vegetation, which can alter pH and oxidation-reduction potential, has not been examined for its potential impact on different mineral precipitations with P. To quantify the different contributions of vegetation and minerals to phosphorus removal, we performed a greenhouse level study comparing the removal of P from canal water in the presence of Ca, Mg, Fe, Al, and Mn minerals in 125 L drums. Three replicate of each mineral were seeded with floating aquatic vegetation, while another three replicate were grown without floating aquatic vegetation. New canal water was introduced to the drums every two weeks, with weekly or bi-weekly collection of phosphorus species, pH, oxidation-reduction potential, mass of accumulated detrital matter, mass loss of mineral, and analysis of water Ca, Mg, Fe, Al, and Mn concentrations. In the absence of floating aquatic vegetation, our results indicated that Mg and Ca were the most effective minerals at removing P, with 48% and 45%, respectively, likely due to the elevated pH levels found in the canal water. Iron removed 23% of P, while Al and Mn had the lowest removals at 17% and 15%, respectively. Floating aquatic vegetation had a negative impact on phosphorus removal, with 12-31% higher concentrations of P in treatments with vegetation to those without. Our results indicate that Mg may play an important role in P removal in the EAA along with Ca, and warrants further research.