Photochemical changes in the dissolved organic matter of temperate lakes: Implications for organic carbon cycling and lake transparency

This dissertation studied the relationship between changes in the optics and chemistry of dissolved organic matter (DOM) exposed to natural solar radiation in lake ecosystems through experimental manipulations and field observations. I developed a spectral weighting function (SWF) for the photobleaching of CDOM, which is the fraction of DOM that absorbs ultraviolet radiation (UVR) and undergoes photochemical reaction. Results indicated that photons in the UV-B region of the solar spectrum (280--320 nm) are the most effective per unit energy absorbed in causing loss of absorbance in CDOM of lakes. In contrast, the overall photobleaching effect was attributed mainly to the UV-A region (320--400 nm). This result suggests that increased UV-B radiation from stratospheric ozone depletion may not greatly increase the loss of dissolved absorbance (ad) in the surface waters of lakes.; An average SWF was calculated and used to model changes in ad for both a clear and a humic lake in Pennsylvania. The model compared favorably with the measured changes in ad at times when, apparently, no input of CDOM occurred. This result underscores the importance of photobleaching in driving the changes in optical properties observed seasonally in the surface waters of lakes.; Chemical measurements of carbon stable isotopes and 13C nuclear magnetic resonance (NMR) showed changes in DOM chemistry that correspond to changes in its optical properties. Exposures of humic water collected from a bog to sunlight for 7 days showed a 15% loss of DOC, an increase in delta 13C of 1.2‰, and a loss of aromatic functionality and molecular weight---all indicating photochemical oxidation of the original DOM. However, ad decreased by 30%, suggesting that absorbance loss by CDOM occurred despite DOM photooxidation. Seasonal observations of the epilimnia of both the clear and the humic lake exhibited similar changes in DOM chemistry and optics during summer UVR exposure, and both chemical and optical signals were reset to initial spring values upon fall lake mixing. Hypolimnion measurements revealed heavy delta13C values that suggested microbial production of photochemically labile DOM; this production may contribute to the seasonal "re-setting" of the optical and chemical signals in these lakes.