The role of photochemical degradation of dissolved organic carbon in regulating the UV transparency of three lakes on the Pocono Plateau

Abstract

The role of photochemical degradation of dissolved organic carbon (DOC) on UV transparency (280-400 nm) was investigated in three Pocono Plateau lakes. Diffuse attenuation coefficients (K(dUV) in the epilimnia of these lakes varied seasonally (39-81% decline from maximum K(d320nm)), with minimum K(dUV) values occurring near summer solstice. Declines in K(dUV) corresponded to reductions in UV absorbance by dissolved material (a(dUV)), caused by declines in both DOC concentration and UV absorptivity (a(d): [DOC]). The seasonal decline in K(dUV) was also accompanied by substantial increases in an absorption coefficient ratio (a(D250NM):a(d365nm)) and decreases in spectral slopes (S). Experimental studies of photochemical degradation were also performed using filtered lake water and natural sunlight. Exposure to solar radiation produced a number of changes in optical parameters and DOC that resembled those observed in the water column: a(dUV) (-35 to -52%), UV absorptivity (-31 to -48%), a(d250nm):a(d365nm) (0 to +39%), spectral slope, S (0 to -27%), and DOC concentration (0 to -20%). Changes in a(dUV) were correlated with absorbed UV dose. The derived rate constants were used in a model to estimate the contribution of photochemical degradation of DOC to water column declines in K(dUV). Results suggest that rates of photochemical degradation were sufficient to account for the summer reductions in K(dUV) observed in the three lakes.