Measurements of spectral optical properties and their relation to biogeochemical variables and processes in Crater Lake, Crater Lake National Park, OR

Spectral inherent optical properties (IOPs) have been measured at Crater Lake, OR, an extremely clear sub-alpine lake. Indeed Pure water IOPs are major contributors to the total IOPs, and thus to the color of the lake. Variations in the spatial distribution of IOPs were observed in June and September 2001, and reflect biogeochemical processes in the lake. Absorption by colored dissolved organic material increases with depth and between June and September in the upper 300 m. This pattern is consistent with a net release of dissolved organic materials from primary and secondary production through the summer and its photo-oxidation near the surface. Waters fed by a tributary near the lake's rim exhibited low levels of absorption by dissolved organic materials. Scattering is mostly dominated by organic particulate material, though inorganic material is found to enter the lake from the rim following a rain storm. Several similarities to oceanic oligotrophic regions are observed: (a) The Beam attenuation correlates well with particulate organic material (POM) and the relationship is similar to that observed in the open ocean. (b) The specific absorption of colored dissolved organic material has a value similar to that of open ocean humic material. (c) The distribution of chlorophyll with depth does not follow the distribution of particulate organic material due to photo-acclimation resulting in a subsurface pigment maximum located about 50 m below the POM maximum.