Transparency of Antarctic ice-covered lakes to solar UV radiation

Depth profiles of solar ultraviolet radiation (UVR), photosynthetically available radiation (PAR), and related variables were measured beneath the thick, permanent ice cover of four lakes in the McMurdo Dry Valleys (77’S, 162”E). These lakes span a range of phytoplankton c centrations (0.1-10 pg Chl a liter-l) butreceive little input of chromophoric dissolved organic matter (CDOM) from their barren, polar desert catchments. The diffuse atten-uation coefficients fordownwelling radiation (K,) in the upper water column of the lakes were at or below those for clear natural waters elsewhere, with minimum values in Lake Vanda of 0.080 (305 nm), 0.055 (320 nm), 0.036 (340 nm), 0.023 (380 nm) and 0.034 (PAR) m-l. The attenuation lengths (l/K,) for these lakes and for a set of high latitude lakes in the northern hemisphere (tundra nd boreal forest catchments) showed a close log-log rela-tionship with dissolved organic arbon (DOC) concentrations (r * L 0.90; n = 20); dry valley lakes were at the high transparency end of this polar-subpolar continuum. Phytoplankton exposure to UVR relative toPAR is known to rise steeply with decreasing DOC in the concentration range 2-4 g m-3; the addition of the dry valley lakes data shows the continuation of this upward, markedly nonlinear t end at lower DOC concentrations. Calculation of the biologically effective UVR dosage rate for the upper phytoplankton community of Lake Vanda indicated that sufficient UVR penetrates hrough the 3.5-m-thick la e ice to cause inhibition of algal growth. These results sho