The Clean Air Act and Dissolved Organic Matter (DOM): The effects of acidification and recovery on DOM quality and source in temperate forested watersheds

Background/Question/Methods

Over the last two decades concentrations of DOM, expressed as dissolved organic carbon (DOC) have increased significantly throughout much of the northern hemisphere.  DOM is ubiquitous within natural waters, comprises a large portion of the global carbon (C) budget, and is important in many processes critical to ecosystem function. The implications for changing DOM flux in temperate forests, a large C sink, are widespread and ecologically significant. While much attention has been paid to patterns and drivers of changing DOC concentrations (i.e. declining sulfate deposition), little is known about associated changes in DOM source and quality with acidification or recovery from acidification.  We used a combination of archived samples from Maine lakes (1993-2008) and intensive stream sampling at The Bear Brook Watershed in Maine (BBWM), a long-term, paired, whole-watershed acidification experiment with ammonium sulfate to elucidate the effects of acidification and recovery on DOM source and quality in northern forests.  All solutions were analyzed for DOC concentration, UV absorbance, excitation-emission fluorescence spectroscopy (EEMs), fluorescence index (FI), and fit to the Cory McKnight PARAFAC Model.  Fluorescence and absorbance spectroscopy provide information regarding the composition, source, and quality of DOM, and have been suggested as necessary measurements in long-term ecological research. 

Results/Conclusions

This work demonstrates that changes in acid deposition loading (i.e. sulfate or ammonium sulfate) not only change DOM concentrations in these waters, but also influence the source and quality of DOM in these ecosystems.  Results from archived lake samples reveal that increasing DOC concentrations were correlated with decreasing FI.  The FI provides a measure of the source (i.e. terrestrial versus microbial) and lability of DOM.  The decrease in FI over time suggests an increasingly aromatic and terrestrial source (i.e. soils) for fulvic acids in these lakes with recovery.  Samples from BBWM confirm these findings with fulvic acids from the reference watershed having consistently lower FI than those from the acidified watershed. Samples from the acidified watershed also exhibit greater fluorescence in the amino acid region (Ex_max/Em_max= 270-280/310-360 nm) suggesting subtle differences in microbial activity and DOM quality, likely as a result of added nitrogen and changing litter C:N ratios.