Drainage ditches have sufficient adsorption capacity but inadequate residence time for phosphorus retention in the Everglades

• Subtropical drainage ditch sediments possess P retention capacity.
• P uptake and release are more rapid in ditches with organic sediment.
• Residence time of 1–4 days can reduce P discharge significantly.
• Ditches can be a source of P after successive rainfall events.
• Drainage management can enhance residence time and P retention.

Although considered a sink of nutrients, limited information exists to verify phosphorus (P) and nitrogen (N) retention in drainage networks such as ditches and canals. Nutrient retention within two ditches on a cattle ranch in the greater Everglades region was measured using in-situ mesocosms. Phosphorus and N concentrations of water inside the mesocosms were increased or decreased to evaluate the effect of P and N concentration on P and N flux across the sediment-water interface. Ditch sediments possess high P retention capacity which was related to aluminum and iron contents. Soluble reactive P (SRP) retention over seven days varied between 13 and 55% of the starting water column concentration, though retention was significant after one day (p = 0.035). Significant P release, although observed only at one site, does indicate the potential for the ditch to be a source of P to downstream locations. The P uptake and release were greater and more rapid in a ditch with organic sediment and emergent macrophytes compared to a ditch with a mineral substrate. P retention over time was rate-limited by diffusion and sediment pore-water exchange. Nitrogen transformation processes were unsubstantial, possibly because in warm sub-tropical Florida most nitrate is denitrified in the presence of an energy source. Several empirical P models predicted (r2 > 0.90) P retention based on the known water column SRP concentrations and hydraulic residence time. The residence time of the site (0.46 and 0.11 days for the two ditches) is not long enough to significantly reduce P loads passing through the ditches under the current drainage regime. Delaying the drainage by increased water retention as a voluntary Best Management Practice (BMP) or Payment for Environmental Services (PES) program can help increase the residence time to reduce P leaving the ranch. Although these results are derived from a cattle ranch, similar benefits can be expected for drainage ditches in other agricultural systems such as agronomic (e.g. sugarcane) or horticultural (e.g. citrus, vegetable) crops, and the models presented in this paper may be useful for basin-scale water quality management.