Ultrafine (<0.45 m) particulate SRP fluxes entering and leaving Everglades Stormwater Treatment Areas as a function of STA management and performance

Ultrafine particulate SRP may have an important role in the transfer of P from the Everglades Stormwater Treatment Areas (STAs), constructed wetlands designed to reduce phosphorus (P) in stormwater that is dis-charged to the Everglades Protection Area, to sensitive downstream water bodies due to increases in mobility associated with small particle size. P concentrations in discharges from the STAs are monitored using routine methods for particulate P (>0.45 & mu;m) and dissolved P (<0.45 & mu;m), an approach that does not consider ultrafine particulate P within the <0.45 & mu;m fraction. Ultrafine particulate SRP was determined by ultrafiltration and was operationally defined as SRP associated with particles between <0.45 & mu;m and <3 kDa. This study determined the magnitude and proportion of ultrafine particulate SRP within the <0.45 & mu;m fraction of inflow, outflow, and in some cases, midflow waters from a selection of well-performing (discharge flow weighted mean concentration (FWMC) of P & LE; 20 & mu;g L-1) and under-performing flow-ways (FWs) (FWMC P > 20 & mu;g L-1) containing emergent aquatic vegetation (EAV), submerged aquatic vegetation (SAV) or a mix of EAV/SAV during the wet and dry season. During the wet season, inflow <0.45 & mu;m SRP was predominately (60-100%) truly dissolved SRP (< 3 kDa) in all flow-ways. In the under-performing flow-ways, <0.45 & mu;m SRP remained largely as truly dissolved, highly available SRP from inflow to outflow, whereas in the well-performing flow-ways, the percentage of <0.45 & mu;m SRP present as ultrafine particulate SRP increased up to 100% as the distance from the inflow increased and the amount of truly dissolved SRP was very low. Two flow-ways having 100% particulate SRP at the outflow (STA-3/4 CFW and STA-5/6 FW1) contain an EAV cell followed by an EAV/SAV cell. However, in flow-ways containing more SAV (STA-2 FW3), ultrafine particulate SRP was a smaller proportion of outflow SRP (29%). During the dry season, ultrafine particulate SRP similarly increased from inflow to outflow at some well and under-performing sampling locations, but to a smaller proportion, and ultrafine particulate SRP never made up 100% of the <0.45 & mu;m SRP fraction. These findings reveal that ultrafine particulate SRP is largely present within the <0.45 & mu;m SRP fraction, at times making up >50% of TP, making it an important but poorly understood component of STA P budgets.