Ultraviolet Radiation and Bio-optics in Crater Lake, Oregon, 2005
UVR Impact on the Crater Lake Ecosystem
In the absence of UVR experiments the impact of UVR on lake biota can be inferred only from correlations. Since 1984 the summer average stratospheric ozone over Crater Lake was correlated with chlorophyll-a concentration in the photic zone (for depth ranges 0–30 m and 40–140 m), suggesting a direct or indirect impact of UV-B radiation on Crater Lake phytoplankton. Particulate absorption at a wavelength typical of UV-B protective pigments in phytoplankton varied with depth in proportion to UV-B exposure. Few planktonic organisms appear in the upper water column where daily exposure to UV-B irradiance is stronger than 5–10% of surface irradiance. Phytoplankton at depths where irradiance levels were stronger than 1% of incident UV-B and 15% of incident PAR developed maximal photoacclimation to strong sunlight. Visible light penetration at noon should become limiting at about 150 m for fish visual predation upon large zooplankton. Diurnal migration of small fish and large zooplankton sufficient to avoid visual predators would also prevent their exposure to strong UVR.