Sediment and Aquatic Vegetation Effects On Phosphorus Concentrations in Farm Drainage Water.

Co-precipitation of phosphorus (P) with CaCO₃ is well documented in aquatic systems with high Ca and alkalinity levels. We hypothesized that increased photosynthetic activities in farm canal waters free of floating aquatic vegetation (FAV) can enhance high water pH and co-precipitation of P with carbonate. In a glasshouse study, 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), and at three sediment P levels (low, medium and high). Water samples taken at 0, 1, 2, 3, and 7 days after each weekly water exchange showed greater water pH in treatments free of FAV than in presence of FAV. Concentration of P in water samples decreased with time for all the treatments, and lower P was observed in the absence of FAV. Equilibrium water P concentrations were dominated by both particulate-P and dissolved organic-P fractions in the presence of FAV.  Low-P and medium-P sediment waters along with the presence of FAV had significantly (p<0.05) lower total-P concentrations.  The study indicates that drainage water free of FAV can enhance P precipitation and reduce drainage water P loads for the canal sediments with lower P loads. Maintaining drainage canal waters free of FAV can complement existing best management practices (BMP), such as canal cleaning, to reduce P loads and P losses in the Everglades Agricultural Area (EAA), South Florida.