THE ROLE OF PHOTOCHEMICAL DEGRADATION OF DISSOLVED ORGANIC-CARBON IN REGULATING THE UV TRANSPARENCY OF 3 LAKES ON THE POCONO PLATEAU

The role of photochemical degradation of dissolved organic carbon (DOG ) on UV transparency (280-300 nm) was investigated in three Pocono Pla teau lakes. Diffuse attenuation coefficients (K-dUV) in the epilimnia of these lakes varied seasonally (39-81% decline from maximum Kd(320nm )), with minimum K-dUV values occurring near summer solstice. Declines in K-dUV corresponded to reductions in UV absorbance by dissolved mat erial (a(dUV)), caused by declines in both DOC concentration and UV ab sorptivity (a(d):[DOC]). The seasonal decline in K-dUV was also accomp anied by substantial increases in an absorption coefficient ratio (a(d 250nm): a(d365nm)) and decreases in spectral slopes (S). Experimental studies of photochemical degradation were also performed using filtere d lake water and natural sunlight. Exposure to solar radiation produce d a number of changes in optical parameters and DOC that resembled tho se 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 corr elated with absorbed UV dose. The derived rate constants were used in a model to estimate the contribution of photochemical degradation of D OC to water column declines in K-dUV. Results suggest that rates of ph otochemical degradation were sufficient to account for the summer redu ctions in K-dUV. Results suggest that rates of photochemical degradati on were sufficient to account for the summer reductions in K-dUV obser ved in the three lakes.