Role of dissolved organic carbon in the attenuation of photosynthetically active and ultraviolet radiation in Adirondack lakes

1. We surveyed eighty-five lakes located in the Adirondack Mountain Region of New York State, U.S.A., to characterize the attenuation of photosyntheti cally active (PAR) and ultraviolet radiation (UVR) in relation to dissolved organic carbon (DOC) concentrations and pH. Attenuation of PAR was quantif ied in situ. Attenuation was also inferred by measuring the light absorptio n of filtered lake water samples at wavelengths (300, 340 and 440 nm) repre senting UV-B, UV-A and PAR. 2. Substantial variation in transparency was observed among lakes in this r egion. Attenuation depths (z(1%)) for PAR ranged from 0.5 to greater than 2 0 m, while inferred values for UV-B and W-A ranged from a few centimetres t o > 5 m. Median values of UV-A penetration (0.75 m) and UV-B penetration (0 .45 m) corresponded to 11% (UV-A) and 6% (UV-B) of lake maximum depth. 3. Much of the variation in PAR and UVR attenuation was explained by differ ences in lake DOC. Univariate power models based solely on DOC accounted fo r 85% (PAR), 90% (UV-A) and 91% (UV-B) of the variation in absorption. 4. Attenuation and absorption coefficients were generally lower for recentl y acidified lakes compared to acidic and circumneutral lakes which have not undergone recent acidification. However, differences among these three gro ups of lakes were not statistically significant. Our results suggest that t he effects of acidification on the optical properties of a regional populat ion of lakes, even in an area experiencing widespread acidification, are re latively subtle in comparison with other factors contributing to inter-lake variability. 5. The presence of near-shore wetlands is probably a key factor influencing regional variability in DOC and light climate among Adirondack lakes. Temp oral variability in climatic factors influencing wetland DOC production and export may mask more subtle influences on lake DOC associated with anthrop ogenic acidification.