Mechanistic models of nutrient dynamics in natural wetlands were developed and applied in a study of Kissimmee River (Florida) flood-plain marshes. The models describe hydrodynamics and transport diffusion in wetland basins and the ecological processes of nutrient uptake, conversion to organic forms, and release from the marsh to a receiving water body. Results of computer simulations suggested that more than 50 percent of dissolved phosphorus loadings could be permanently captured by the marsh ecosystem, at least up to a loading rate of 5 g/sqm/yr. In addition, 80 to 90 percent of the phosphorus exported from the marsh would be transformed from dissolved to detrital forms and would therefore be relatively unavailable to nuisance algae. The simulations suggested that consolidated peat would be laid down at a rate of approximately 0.5 cm/yr, giving a useful life of the marsh of perhaps 50 to 100 years.