Several small wetlands have been constructed in the watershed of the Central Swiss Lakes in order to collect the drainage water from the grassland and to retain or eliminate phosphorus (P) and nitrogen (N). For evaluating and improving their retention efficiency, nutrient fluxes were studied during several years in two of these wetlands. Both wetlands retained an average of 1.1 g P m-2 yr-1 at a retention efficiency of 23% and 7%. N retention was 45 to 50 g m-2yr-1 corresponding to a retention efficiency of 28%. The minimum water residence time during high discharge events and the plankton concentration were the key factors limiting short-term nutrient retention. In the longterm, oxygen concentration in the water column and the input of P sorbing soil particles determined retention efficiency.