TRANSPARENCY OF ANTARCTIC ICE-COVERED LAKES TO SOLAR UV-RADIATION

Depth profiles of solar ultraviolet radiation (UVR), photosyntheticall y available radiation (PAR), and related variables were measured benea th the thick, permanent ice cover of four lakes in the McMurdo Dry Val leys (77 degrees S, 162 degrees E). These lakes span a range of phytop lankton concentrations (0.1-10 mu g Chl a liter(-1)) but receive littl e input of chromophoric dissolved organic matter (CDOM) from their bar ren, polar desert catchments. The diffuse attenuation coefficients for downwelling radiation (K-d) in the upper water column of the lakes we re at or below those for clear natural waters elsewhere, with minimum values in Lake Vanda of 0.080 (305 nn), 0.055 (320 nm), 0.036 (340 nm) , 0.023 (380 nm) and 0.034 (PAR) m(-1), The attenuation lengths (1/K-d ) for these lakes and for a set of high latitude lakes in the northern hemisphere (tundra and boreal forest catchments) showed a close log-l og relationship with dissolved organic carbon (DOC) concentrations (r( 2) greater than or equal to 0.90; n = 20); dry valley lakes were at th e high transparency end of this polar-subpolar continuum. Phytoplankto n exposure to UVR relative to PAR is known to rise steeply with decrea sing DOC in the concentration range 2-4 g m(-3); the addition of the d ry valley lakes data shows the continuation of this upward, markedly n onlinear trend at lower DOC concentrations. Calculation of the biologi cally effective UVR dosage rate for the upper phytoplankton community of Lake Vanda indicated that sufficient UVR penetrates through the 3.5 -m-thick lake ice to cause inhibition of algal growth. These results s how that polar desert lakes are optical extremes in terms of their wat er-column transparency to UVR, and that their dilute, mostly autochtho nous CDOM offers little protection against the ultraviolet-B radiation flux that is continuing to increase over the polar regions.