Particle deposition, resuspension and phosphorus accumulation in small constructed wetlands.

To improve understanding of phosphorus (P) retention processes in small constructed wetlands (CWs), we analysed variations in sediment deposition and accumulation in four CWs on clay soils in east-central Sweden. Sediment deposition (in traps) generally exceeded the total suspended solids (TSS) load suggesting that resuspension and wetland base erosion were important. This was confirmed by quantification of particle accumulation (on plates) (1-23 kg m ^-2  year ^-1 ), which amounted to only 13-23% of trap deposition. Spatial mean P concentrations in accumulated sediment on plates (0.09-0.15%) were generally similar to temporal mean P concentrations of particles in water (0.11-0.15%). Deposition/accumulation was minor in one wetland with high hydraulic load (400 m year ^-1 ), suggesting that such small wetlands are not efficient as particle sinks. Economic support for CWs are given, but design and landscape position are here demonstrated to be important for effective P retention.