Blue Lake, Jefferson County, Oregon, has high summer surface phosphorus concentrations (ca. 30 ug/l) yet is oligotrophic (summer Secchi depth is 11 to 16 meters). Nutrient enrichment experiments done with 1000 1 polyethylene enclosures indicate nitrate limitation of phytoplankton growth. Basin morphology may be an important factor in nutrient cycling in this lake. The lake has a maximum depth of 95.7 meters with an average depth of 42.7 meters. The lake basin has steep sides with only 4% of the lake bottom less than 3.3 meters deep. of recent volcanic origin. In contrast, Suttle Lake, which is immediately downstream from Blue Lake, is moderately eutrophic (Secchi depth 1.7 meters) and supports much larger populations of phytoplankton, including nitrogen fixing cyanophytes. Suttle Lake is shallower and more subject to wind mixing.