Crater Lake Limnological Studies Final Report
The National Park Service began a study of Crater Lake in 1982 because of indications that lake clarity might be declining. Later the same year, Congress passed Public Law 97-250, which authorized and directed the Secretary of the Interior to promptly initiate a 10-year program to assess the status of the water quality of the lake. Little was known about the ecology of the lake in 1982. Consequently, the National Park Service adopted the following major goals for the study of this unique lake: (1) develop a data base to compare present and future conditions of the lake, (2) develop an understanding of lake components and processes, (3) develop a long-term program for monitoring changes, (4) determine if the lake had experienced recent changes, and if so, (5) identify causes and recommend mitigation procedures if the changes were related to human activity.
Looking at the data in its entirety, researchers concluded at the end of the study that Crater Lake was a complex and dynamic system with considerable seasonal and annual variability. Although fish, which were introduced into the lake between 1888 and 1941, affected the lake’s food web, no other changes caused by human activities could be specifically identified or separated from those caused by natural phenomena. Although the possibility of long-term changes in the lake could not be dismissed, researchers regarded such changes to be too subtle for detection over a time scale represented by the available data.
Original concerns about changes in lake clarity were prompted by measurements of clarity with a Secchi disk. Measurements of lake clarity with a 20- cm (8 in) Secchi disk with black and white quadrants were included in the 10-year study in order to gain data for comparison with historical data. Analysis of the Secchi disk data revealed that clarity was now generally greater than 25 m (82 ft) but less than 35 m (115 ft). The shallowest reading during this study (21.9 m; 72 ft) was recorded in August of 1982, and the deepest reading (39.2 m; 129 ft) was recorded in June of 1988. This deepest reading was 0.8 m (3 ft) short of the maximum Secchi reading on record for the lake using the 20-cm disk. August Secchi disk readings in the range of 39-40 m (128-131 ft), a range that encompassed the maximum Secchi depths recorded in August of 1937 and 1969, were not observed between 1982 and 1992. In addition to measurements with a Secchi disk, changes in the composition and depth of penetration of surface light were measured with a photometer, and changes in the spatial distribution of particles in the water column were measured with a transmissometer. Natural variabilities were apparent using all three measurements of clarity, but as a whole, the data did not support the hypothesis that clarity of Crater Lake had undergone long-term change.