Nutrient enrichment (eutrophication) is a major theme in freshwater ecology. Some themes come and go, but the inevitable release of phosphorus and nitrogen that ac companies human presence seems to ensure that eutrophication will not soon become an outmoded subject of study. Eutrophication raises issues that range from the pressingly practical problems of phosphorus removal to the very fundamental ecological questions surrounding biological community regulation by resource supply. Although it is possible to take a reductionist approach to some aspects of eutrophication, the study of eutro phication is fundamentally a branch of ecosystem ecology. To understand eutrophication in a given setting, one is inevitably forced to consider physical, chemical, and biological phenomena together. Thus while eutrophication is the focus of our study of Lake Dillon, we have assumed that a broad base of lirnnological information is a prerequisite foundation. Eutrophication of a lake can be studied strictly from a lirnnological perspective. If so, the nutrient income of the lake is quantified but the sources are combined within a black box whose only important feature is total loading. It is also possible, however, to treat the watershed and lake as equally important components of a hybrid system. In this case the nutrient sources must be dissected and their variability and dependence on key factors such as runoff must be quantified.

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Introduction.- 2. Design of the Study.- Time Series Data Set.- Spatial Survey Series.- Special Studies.- 3. Methods.- Lake and Stream Sample Collection.- Filtration and Analysis of Particulates.- Phosphorus Analysis.- Soluble Nitrogen Analysis.- pH, Alkalinity, Oxygen, and Major Ions.- Chlorophyll a.- Phytoplankton and Zooplankton.- Primary Production.- Temperature, Conductance, and Transparency.- Discharge Measurements.- Precipitation Sampling.- Special Studies.- Precision and Accuracy.- 4. Physical Variables and Major Ion Chemistry of the Lake.- Geology.- Morphometry.- Hydrology.- Temperature.- Transparency.- Historical Changes in Transparency.- Composition of Major Ions.- Conductance.- pH and Alkalinity.- 5. Phosphorus and Nitrogen in Lake Water and Sediments.- Phosphorus in Lake Water.- Nitrogen in Lake Water.- Sediment Phosphorus.- 6. Particulates and Phytoplankton Biomass.- Total Particulates in the Water Column.- Chlorophyll a.- Phytoplankton Composition and Seasonality.- Elemental Composition of Phytoplankton Biomass.- 7. Zooplankton.- 8. Photosynthesis and Oxygen Consumption in the Water Column.- Photosynthesis.- Oxygen Concentrations and Oxygen Consumption.- 9. Nutrient Enrichment Studies.- 10. Horizontal Spatial Variation in the Lake.- The Five-Station Heterogeneity Study.- The 14-Station Heterogeneity Study.- 11. Overview of Limnology and Throphic Status.- The Annual Cycle.- Trophic Status of the Lake.- 12. Chemistry of Nutrient Sources as They Enter the Lake.- 24-Hour Variation in River Chemistry.- Chemistry of the Three Rivers as They Enter the Lake.- Chemistry of Miner's Creek and Soda Creek.- Frisco and Snake River Treatment Plant Effluents.- Atmospheric Deposition at the Main Station.- Precipitation Chemistry at Other Stations.- 13. Total NutrientLoading of the Lake.- Water Budget.- Phosphorus Loading.- Nitrogen Loading.- Overview of Phosphorus and Nitrogen Loading.- 14. Nutrient Export in Relation to Land Use.- Undisturbed Watersheds.- Residential Area on Sewer.- Urban Area on Sewer.- Residential Areas on Septic Systems.- Ski Slopes.- Interstate Highways.- Mining.- Point Sources.- Overview of Nutrient Yields.- 15. Separation of Nutrient Sources within the Watershed.- Snake River Drainage.- Blue River Drainage.- Tenmile Creek Drainage.- Relationship between Runoff and Nutrient Storage.- Comparison of Observed and Predicted Total P and N Loading.- Itemization of Lake Nutrient Sources for 1981 and 1982.- 16. Modelling.- The Land Use Component of the Model.- The Trophic-Status Component of the Model.- Effects Component of the Model.- Comparing Predictions and Observations for 1981 and 1982.- 17. Using the Model for Prediction.- Model Runs for Present Land-Use Patterns.- Scenarios 1A, 1B: Low Growth.- Scenarios 2A, 2B: Low Growth with Diversions.- Scenarios 3A, 3B: High Growth.- Scenarios 4A, 4B: High Growth with Perfect Nonpoint Controls.- Scenarios 5A, 5B: High Growth with Partially Effective Nonpoint Controls.- Overview of Scenario Studies.- 18. Summary.- References.