Spatial and Temporal Variability in the Amount and Source of Dissolved Organic Carbon: Implications for Ultraviolet Exposure in Amphibian Habitats

Brooks, Paul; O'Reilly, Catherine; Diamond, Stephen; Campbell, Don; Knapp, Roland; Bradford, David; Corn, Paul; Hossack, Blake; Tonnessen, Kathy

The amount, chemical composition, and source of dissolved organic carbon (DOC), together with in situ ultraviolet (UV-B) attenuation, were measured at one to two week intervals throughout the summers of 1999, 2000, and 2001 at four sites in Rocky Mountain National Park (Colorado). Eight additional sites, four in Sequoia and Kings Canyon National Park/ John Muir Wilderness (California) and four in Glacier National Park (Montana), were sampled during the summer of 2000. UV-B attenuation was significantly related to DOC concentrations over the three years in Rocky Mountain (R2 = 0.39, F=25.71, p<0.0001) and across all parks in 2000 (R2 = 0.44, F=38.25, p<0.0001). Fluorescence analysis of the fulvic acid DOC fraction indicated that the source of DOC significantly affected the attenuation of UV-B. Sites in Sequoia were characterized by DOC derived primarily from algal sources and exhibited much deeper UV-B penetration, while sites in Glacier and Rocky Mountain contained a mix of algal and terrestrial DOC dominated sites with more terrestrially dominated sites characterized by greater UV-B attenuation per unit DOC. Interannual variability in UV-B attenuation at the sites in Rocky Mountain was related to variability in precipitation and local hydrology that controlled the amount and residence time of terrestrial DOC. Attenuation was greater in a year with higher precipitation at sites that were hydrologically isolated from catchment drainages and where terrestrial carbon transported by precipitation along hydrological flowpaths accumulated in surface water. In contrast, UV-B attenuation was lower during the year with higher precipitation at sites located along drainages due to a more rapid flush of terrestrial DOC from the water body. Consequently, while DOC remains a strong predictor of potential UV exposure, variability in the sources of DOC, local hydrology, and annual precipitation introduce significant spatial and temporal variability in UV-B exposure.

Spatial and Temporal Variability in the Amount and Source of Dissolved Organic Carbon: Implications for Ultraviolet Exposure in Amphibian Habitats

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