The high elevation (1800 m) and frequently clear summer skies combine with the UV transparent water and shallow upper mixed layer (usually < 10 m deep and persistent from summer through early fall, Larson et al., 1996a) to create an unusually broad and stable depth gradient of UVR exposure for aquatic organisms. Research on underwater UVR and its potential for biological impact has increased in response to concerns about depletion of stratospheric ozone and climate change (Schindler & Curtis 1997; Pienitz & Vincent, 2000). Several reports on the distribution of organisms in Crater Lake suggest a possible inhibitory role for UVR (McIntire et al., 1994; McIntire et al., 1996; Larson et al., 1996b), especially above 40 m. UVR measurements in Crater Lake resumed in 1996 when a submersible wavelength-scanning radiometer (300–800 nm) was added to the monitoring program and used to acquire incident and underwater solar spectra at a range of depths. Visiting scientists made additional optical measurements in UV and visible wavelengths starting in 1999. A recent report confirmed the unusual UV-B transparency of Crater Lake and derived estimates of UV-B attenuation by pure water and phytoplankton from measurements in its surface waters (Hargreaves, 2003).
Our approach here was first to characterize the spectral properties of near-surface and deeper water in Crater Lake, emphasizing UVR wavelengths, and to compare these with other clear lakes and ocean waters. We then used bio-optical signals and daily stratospheric ozone levels to examine factors controlling UVR attenuation with depth and over time, to develop proxies for estimating UVR attenuation, and to evaluate the impact of UVR on the Crater Lake ecosystem. Finally, we used the complete record of direct measurements and proxy estimates of UVR attenuation to look at long-term trends, and to speculate about the impact of future climate change.
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