OVERVIEW OF THE LIMNOLOGY OF CRATER LAKE

Crater Lake occupies the collapsed caldera of volcanic Mount Mazama in Crater Lake National Park, Oregon. It is the deepest lake (589 m) in the United States and the 7th deepest lake in the world. The water col umn mixes to a depth of about 200 m in winter and spring from wind ene rgy and cooling. The deep lake is mixed in winter and early spring eac h year when relatively cold water near the surface sinks and exchanges positions with water in the deep basins of the lake. The lake becomes thermally stratified in summer and early fall. The metalimnion extend s to a depth of about 100 m; thus most of the water column is a cold h ypolimnion. Secchi disk clarity measurements typically are in the uppe r-20-m range to the low-30-m range in summer and early fall. Concentra tions of nutrients are low, although conductivity is relatively high o wing to the inflow of hydrothermal fluids. Total chlorophyll is low in concentration, but typically maximal at a depth of 120 m during perio ds of thermal stratification. Primary production also is low, with the maximum levels occurring between the depth of 40 and 80 m. Phytoplank ton taxa are spatially segregated from each other within the water col umn to a depth of 200 m in summer and early fall. The same generalizat ion applies to the zooplankton taxa. Water level, clarity, concentrati ons of total chlorophyll, primary production, and abundances of zoopla nkton and introduced kokanee salmon exhibit long-term fluctuations. Ba sed primarily on a recent 10-year study of the lake, the lake is consi dered to be pristine, except for the consequences of fish introduction s.