Phosphorus Speciation and Concentration Attenuation in Waterways from Agricultural Fields to Indian River Lagoon.

Agricultural production and urban development has enhanced phosphorous (P) input to the Indian River Lagoon (IRL) in south Florida.  However, changes in the concentration and bioavailability of P along waterways from agricultural fields to the discharge point are not yet known. In this study, surface runoff water samples were collected along the waterways from agricultural fields to the discharge point at the IRL and analyzed for phosphorus speciation and related water quality properties. The concentrations of total P (TP), dissolved P (DP), particulate-P (PP), and orthophosphate (OP) in surface water samples were in the range of 0.04-3.33, 0.0-2.35, 0.001-0.97 and 0.01-2.02 mg L^-1, respectively, with corresponding mean values of 0.512, 0.419, 0.091, and 0.364 mg L^-1. Orthophosphate accounted for 71% of total P whereas DP was dominant in surface water (>80%), indicating that P in agricultural water is highly bioavailable. Total P was highest in runoff water from agricultural fields (~ 1.50 mg L^-1), significantly decreased in ditch and canal water (~ 0.25 mg L^-1), and relatively low in canal water near the discharge point (~ 0.10 mg L^-1), indicating that P flux was substantially attenuated during its transport from agricultural fields to the IRL and waterways may play an important role in reducing P input to the IRL. Total P in runoff water from agricultural fields was highest in July-September (~1.90 mg L^-1), followed by April-June (~1.50 mg L^-1), and relatively low in October-December and January-March ~ (0.95 mg L^-1). These results may be useful for evaluating the effectiveness of current best management practices in south Florida.

Key word: Agriculture, phosphorus flux and bioavailability, spatial and temporal variation, storm water