A constructed wetland (0.08 ha) receiving drainage water from a small agricultural catchment (22 ha) with clay soil, was investigated with respect to phosphorus dynamics and retention. The aim was to evaluate the function of the wetland with respect to phosphorus retention, and relate that to gross sedimentation as measured with sediment traps. Hydraulic load and phosphorus retention were estimated for 2003-2010 based on monitoring data. Furthermore, water quality dynamics was studied during three intensive sampling periods of 3-5 days during 2010. For each period, phosphorus retention was calculated and the relationship between flow and phosphorus concentrations analysed. Additionally, the gross sedimentation rate was estimated using sediment traps, and the phosphorus, carbon and nitrogen content analysed. The results suggested that there was no net retention of phosphorus during 2003-2010, except for 04/05. During the intensive sampling periods, release of phosphorus from the wetland mainly occurred during high flow. Sediment analyses showed that settling of inflow particles mostly occurred in the inlet pond, while the sediment found in a shallow vegetated area and outlet pond likely originated from internal processes rather than from the catchment. In fact, the gross sedimentation of phosphorus during April-July and July-August, respectively, exceeded the measured phosphorus inflow. The results showed that short periods with rapid flow increases were crucial for the wetlands function and thus high frequency sampling must be done during these periods. Furthermore, it seems that the particles lost from the catchment during high flows are too small to settle in the wetland.