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

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 (I&) in the epilimnia of these lakes varied seasonally (39-8 1% decline from maximum Z&,&, with minimum Kduv values occurring near summer solstice. Declines in K,,, corresponded to reductions in UV absorbance by dissolved material (ad”“), caused by declines in both DOC concentration and UV absorptivity (a,: [DOC]). The seasonal decline in Kduv was also accompanied by substantial increases in an absorption coefficient ratio (Q~,,~,,, : ud36snm) 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: uduv (-35 to -52%), UV absorptivity (-3 1 to -48%), u~,~~“~,,, : udjgSnm (0 to +39%), spectral slope, S (0 to -27%), and DOC concentration (0 to -20%). Changes in uduv 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,,,. Results suggest that rates of photochemical degradation were sufficient to account for the summer reductions in Kduv observed in the three lakes. Discerning the role of UV radiation (UVR; 280-400 nm) in aquatic ecosystems is of considerable interest due to its potential for profound ecosystem effects (Karentz et al. 1994; Siebeck et al. 1994). This interest has been heightened by recent evidence of increasing incident UV-B radiation in response to stratospheric ozone depletion (Tevini 1993; Young et al. 1993). This phenomenon is well documented for Antarctic ecosystems but increases in incident UV-B radiation (lo-20% per decade) have also been reported for temperate latitudes (Kerr and McElroy 1993; Madronich 1992; Stolarski et al. 1992) that contain a large proportion of the world’s freshwater ecosystems. The influence of UVR on Antarctic ecosystems has been well documented (Neale et al. 1994; Vincent and Quesada 1994; Smith et al. 1992). In contrast, UV effects on freshwaters are not well understood and may not be easily inferred from marine models. Lakes may be extremely susceptible to the effects of UVR due to their morphology (global mean depth < 10 m; Wetzel 1990) and characteristic patterns of epilimnial circulation that limit the availability of UV refugia (Williamson 1995; Williamson et al. 1996). The heterogeneity of freshwater Acknowledgments We thank C. E. Williamson and Robert Moeller for their role in the conception and implementation of the sampling program that provided data on seasonal variations in attenuation coefficients in our study lakes. We also thank Neil Blough, Robert Moeller, and an anonymous reviewer for their insightful comments that improved this manuscript. Logistic support was provided by the Lacawac Sanctuary, Blooming Grove Hunting and Fishing Club, the Lake Waynewood Association, and in particular Sally Jones, Janice Poppith, Ken Ersbak, Bob Kobler, and Dave Westpfahl. Technical assistance was provided by Jim Agger and Joanne Davis. Financial support was provided by NSF (DEB 93-06978) and the Keck Foundation (921265). ecosystems (physical, chemical, and biological) also implies that UV effects may differ substantially among lakes. Transmission of UVR differs greatly among lakes and is largely regulated by the concentration and absorptivity of dissolved organic carbon (DOC) (Morris et al. 1995; Kirk 1994a; Scully and Lean 1994). These DOC compounds, which are primarily humic substances, contain chromophores that strongly absorb UVR. While these compounds can help to shield the water column from the direct effects of UVR, they are themselves subjected to considerable photochemical degradation (Kouassi et al. 1990; Gjessing 1980; Strome and Miller 1978). This degradation is often accompanied by changes in the optical properties of DOC (often referred to as photobleaching). Experimental evidence, mostly obtained with artificial light sources, suggests that photochemical degradation of DOC often reduces DOC absorptivity in the UV range (Lindell et al. 1995; DeHaan 1993; Mopper et al. 1991). Photochemical degradation may proceed to complete photomineralization (Miller and Zepp 1995; Allard et al. 1994; Salonen and Vahalalo 1994) or may result in production of low molecular weight DOC that in turn may be mineralized through microbial decomposition (Lindell et al. 1995; Amador et al. 1991). Both processes can result in a net increase in transmission of UVR through the water column. Because DOC concentration and absorptivity strongly influence water column transparency, photochemical degradation of DOC may result in substantial temporal variability in the UV transparency of lakes and other aquatic ecosystems. This variability could occur on a variety of time scales from seasonal (driven primarily by seasonal variations in incident solar radiation and depth of the mixed layer) to interannual according to changes in climatological factors that influence incident UVR (e.g. cloud cover, particulate