Observations over a 14-year period at Liberty Lake, Wash., have indicated a close relationship between the seasonal decline of aquatic macrophyte populations and the onset of planktonic Cyanobacteria blooms. Tracer methods, using radiophosphorus, have been employed in laboratory and inin situexperiments to investigate the ability ofElodea canadensis, an important component of the Liberty Lake macrophyte community, to translocate phosphorus from sediments to the open water. Results of these experiments showed good agreement between release rates determined in the laboratory andin situfor senescing macrophytes. Experiments with actively growingElodeaplants indicate some release or leakage of phosphorus from healthy plants. Nutrient budgets and a phosphorus model for Liberty Lake indicate that internal cycling of sediment phosphorus by aquatic macrophytes is an important source of phosphorus to planktonic primary production as well as direct sediment/water exchange. Indeed, in Liberty Lake, it is possible that macrophyte influence on the dynamic cycling of phosphorus in the lake may not only influence, but also control the pattern, timing, and community composition of planktonic production. A conceptual framework that can be applied by lake managers for determining the potential contribution of macrophyte phosphorus cycling in lakes is discussed.