Water temperature, water chemistry, and optical properties of Crater Lake were studied from 1983 to 1991. In winter and spring, wind energy and convection mixed the water column to a depth of 200 to 250 m. The lake was thermally stratified in summer and early fall; however, the epilimnion was only 5 to 20 m thick, and most of the 589 m deep water column was a cold hypolimnion. The lake was slightly basic, with moderate alkalinity and conductivity. The water column was oxygenated, although slight decreases in dissolved oxygen concentration were noticed near the lake bottom in late summer and early fall. Phosphorus and nitrogen concentrations were low. Orthophosphorus-P concentrations increased slightly with increased lake depth, whereas nitrate-N was below detection limits in the upper 200 m of the water column and then increased with increased lake depth. Secchi disk clarity typically varied from the high-20-m to low-30-m range. The depth of 1 % surface incident light (425–655 nm) in July and August typically ranged between 80 and 100 m. The results also indicated that water temperature, water chemistry, and optical properties of the lake between 1983 and 1991 were consistent with those observed between 1896 and 1982.
