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Phosphorus (P) export from agriculture is a major cause of eutrophication in many lake ecosystems. Human activity, hydrology, and physicochemical and biological processes that store, transform, and transport P, define P export patterns over time and space. We suggest that anecosystem paradigm is useful to holistically view P dynamics within complex watersheds. An ecosystem model of a dairy agricultural systemwas created within a hierarchical compartment-flux structure of a conceptual watershed ecosystem. Mass balance calculations with our Agriculture Ecosystem model (AEP model) describe P dynamics for the farm system, which are driven by the amount of P stored in agricultural soils and system management practices. Long-term P dynamics respond predominately to human interventions in watersheds and define conditionsfor future generations. Model simulations suggest that long-term environmental protection programs should incorporate the notions of P sustainability into management decisions. Dynamic simulation modeling is a valuable paradigm for understanding how complex watersheds process P and for developing management perspectives and public policy to achieve goals of environmental quality as well as economic and resource sustainability. [ABSTRACT FROM AUTHOR]