ill.; Caption title; Includes bibliographical references;
Abstract Maximum Secchi disk depth readings of 40 m and 39 m in Crater Lake were recorded in 1937 and 1969, respectively. Disk readings from 1978 to 1987 indicated an apparent decline in lake clarity, although some readings were in the high 30 m range. In this paper, Secchi disk readings of the lake were evaluated relative to seasonal and annual variations and how variations of chlorophyll, primary production, and phytoplankton effected the readings. Considerable temporal variation was found both within and among years. The evidence suggested that Secchi disk depth readings made during the month of August have declined about 25 percent since 1969. The mean values for July and August from 1968 to 1987 showed a decreasing trend through 1982; thereafter, the readings have steadily increased. A significant positive linear relationship was found between Secchi disk depth and the depth of 1 percent incident light. Chlorophyll and phytoplankton (density and biovolume) were poorly correlated to Secchi disk depth. A weak negative relationship was found between areal primary production (0 to 40 m) and the disk readings. It can be argued that Secchi disk clarity in clear lakes like Crater Lake are sensitive to very small changes in the density and optical characteristics of biotic and abiotic particles. As such, variations in the Secchi disk clarity of the lake are dynamic and not readily explained by small changes of lake productivity. Future optical studies of Crater Lake need to include an examination of the characteristics and optical properties of the total particle community. 2
Abstract Maximum Secchi disk depth readings of 40 m and 39 m in Crater Lake were recorded in 1937 and 1969, respectively. Disk readings from 1978 to 1987 indicated an apparent decline in lake clarity, although some readings were in the high 30 m range. In this paper, Secchi disk readings of the lake were evaluated relative to seasonal and annual variations and how variations of chlorophyll, primary production, and phytoplankton effected the readings. Considerable temporal variation was found both within and among years. The evidence suggested that Secchi disk depth readings made during the month of August have declined about 25 percent since 1969. The mean values for July and August from 1968 to 1987 showed a decreasing trend through 1982; thereafter, the readings have steadily increased. A significant positive linear relationship was found between Secchi disk depth and the depth of 1 percent incident light. Chlorophyll and phytoplankton (density and biovolume) were poorly correlated to Secchi disk depth. A weak negative relationship was found between areal primary production (0 to 40 m) and the disk readings. It can be argued that Secchi disk clarity in clear lakes like Crater Lake are sensitive to very small changes in the density and optical characteristics of biotic and abiotic particles. As such, variations in the Secchi disk clarity of the lake are dynamic and not readily explained by small changes of lake productivity. Future optical studies of Crater Lake need to include an examination of the characteristics and optical properties of the total particle community. 2