Payette Lake was studied during water years 1995-96 to determine the 20.5-square-kilometer lake's assimilative capacity for nutrients and, thus, its eutrophication potential. The study included quantification of hydrologic and nutrient budgets, characterization of water quality in the limnetic and littoral zones, development of an empirical nutrient load/lake response model, and estimation of the limnological effects of a large-scale forest fire in the lake's 373-square-kilometer watershed during the autumn of 1994.

Streamflow from the North Fork Payette River, the lake's primary tributary, delivered about 73 percent of the lake's inflow over the 2 years. Outflow from the lake, measured since 1908, was 128 and 148 percent of the long-term average in 1995 and 1996, respectively. The larger volumes of outflow reduced the long-term average water- residence time of 2.35 years to 1.84 and 1.42 years for 1995 and 1996, respectively. The lake retained 54 percent of its 1995-96 influent load of nitrogen and 79 percent of its influent load of phosphorus. The North Fork Payette River contributed an average of 69.4 percent of the lake's nitrogen load and 28.2 percent of its phosphorus load. The 1994 forest fires substantially increased the loads of nitrogen and phosphorus delivered to the lake; however, only nitrogen concentrations were noticeably increased in the lake.

Payette Lake was classified as oligotrophic on the basis of annual geometric mean concentrations, in micrograms per liter, of total phosphorus (4.7), total nitrogen (225), and chlorophyll-a (1.3) during 1995-96. Secchi-disc transparencies ranged from 2.3 to 8.0 meters, indicative of mesotrophic conditions. Median ratios of dissolved inorganic nitrogen to dissolved orthophosphorus ranged from 38 to 254, thereby indicating phosphorus limitation of phytoplankton growth. Phytoplankton populations were taxonomically dominated by diatoms; blue-green algae were rare. One diatom, Tabellaria fenestrata, contributed 52 percent of the biovolume. Within the littoral zone, median periphyton production, normalized to photosynthetically active radiation input, ranged from 0.0007 to 0.02 milligrams of chlorophyll-a per square meter per Einstein, a difference of 28.6 times. Multiple linear regression analysis failed to detect any significant relation between periphyton production and various indices of nearshore development. Nine genera of aquatic macrophytes were identified, including Eurasian milfoil (Myriophyllum spicatum var. spicatum), which is considered a nuisance aquatic plant.

Despite its oligotrophy, Payette Lake had substantial dissolved-oxygen deficits in 1995-96, which led to 4-month periods of anoxia in the near-bottom waters of its southwest basin. The hypolimnetic dissolved-oxygen deficit was much larger than that predicted by the nutrient load/lake response model. The southwest basin's propensity for developing anoxia was related to the lengthy water-residence time and incomplete water-column circulation and reaeration during the spring and autumn, coupled with a long-term accumulation of oxygen- demanding organic matter produced within the lake or delivered by its watershed.