Ultraviolet Radiation and Bio-optics in Crater Lake, Oregon, 2005
Optical Proxies and the Impact of Phytoplankton on UVR Transparency
As in Case 1 marine waters, phytoplankton in Crater Lake normally are the dominant optical attenuator of UVR. During summer the UV attenuation rises from a minimum near the surface to a peak at the deep chlorophyll maximum (DCM) near 120 m. Measurements of Kd,UV at different wavelengths parallel each other with increasing depth although only the longer UVR wavelengths can be detected deep in the water column. Optical proxies for UV attenuation include the scattering of light by phytoplankton in the upper part of the photic zone (cp660, measured with a red beam transmissometer), and fluorescence of phytoplankton (Fchl, 683 nm) in the deeper regions (75–150 m). Other optical proxies for the depth range 0–40 m include inverse Secchi depth and diffuse attenuation in the broad blue waveband (400–500 nm), corresponding to historical underwater measurements with blue filter photometers. Only during rare heavy rains in summer does the optical dominance of phytoplankton give way to an optical signal from light scattering by suspended mineral particles and a likely breakdown of the proxy relationships. Excluding occasional wet summer months, Kd,UV derived from Secchi depth was correlated with phytoplankton biomass as measured by chlorophyll-a concentration for the period 1984–2002.