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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]

Dynamic simulation technology is integrated with mass balance concepts and compartment-flux diagramming to create computer models that estimate contaminant export from watersheds over long and short-term futures under alternative simulated policies of watershed management. The Watershed Ecosystem Nutrient Dynamics (WEND) model, applied to developed watersheds with a mix of urban, agricultural, and forest land-uses, predicted phosphorus (P) export from three watersheds; a 275,000 ha dairy/urban watershed, a 77,000 ha poultry/urban watershed, and a 23,000 ha swine dominated watershed. Urban, agricultural, and forestry activities contribute to P export in different proportions. In all cases the P imports to the watershed exceed total export and P accumulates in watershed soils. Long-term future P export patterns are compared for several watershed management strategies that range from encouragement of rapid urban growth to aggressive environmental protection. The specific response of each watershed to imposed management is unique, but management strategies designed to reduce export of P over the long-term need to consider options that promote P input/output balance. Using this same approach, the Watershed Ecosystem Bacterial Dynamics (WEBD) model assesses the dynamics of bacterial populations in a small case-study watershed over an annual cycle as influenced by dairy farm management actions. WEND and WEBD models integrate the diversity of activities and stakeholders interested in the watershed and promote development of a more holistic understanding of watershed function. Model outputs are designed to assist watershed policy-makers, managers, and planners to explore potential future impacts of management/policy decisions.