Management of Floating Aquatic Vegetation in Farm Canals to Reduce Phosphorus Loads.

Management of farm canal waters free of floating aquatic vegetation (FAV) can enhance co-precipitation of P with carbonates, reduce drainage water total P (TP) loads, and complement existing best management practices (BMP) in the Everglades Agricultural Area (EAA) in  South Florida. In a glasshouse study, we evaluated impacts of FAV control  on canal water P concentration at  sediment P concentrations encountered in EAA farm canals.  Canal waters of  P concentrations ranging from 0.08 – 0.54 mg L^-1 were incubated weekly for 8 weeks in 115-liter PVC drums with and without water lettuce (pistia stratiotes) as FAV, and at three sediment P levels. Water samples taken at 0, 1, 2, 3, and 7 days after each weekly water exchange showed reduced soluble reactive P (SRP) with time for all the treatments.  Higher P concentrations were found in FAV treatments (SRP = 0.011 – 0.074 mg L^-1; TP = 0.055 – 0.094 mg L^-1)compared to no FAV (SRP = 0.005 – 0.056 mg L^-1; TP = 0.032 – 0.087 mg L^-1). Greater P reduction was observed after 7^th day of each of eight water exchanges in treatments without FAV (71 - 97%) than in the presence of FAV (52 – 92%). The FAV treatments consistently show greater water pH (7.8 – 9.7) than treatments without FAV (pH = 7.3 – 8.9). Effects of sediment P loads on water soluble P was not significant. The study suggests that farm canals free of FAV enhance precipitation of P and reduce drainage water P loads.