High-frequency dissolved organic matter quality variations during “Super Storm” Sandy

Abstract:

Rapid changes in aqueous dissolved organic matter (DOM) concentration and composition during hydrologic events are difficult to capture by conventional sampling. In order to better understand high frequency changes in DOM characteristics during hydrologic events, we co-deployed a traditional DOM peak C fluorometer with a custom, multi-wavelength deep ultra-violet (DUV) fluorometer at the forested W-9 watershed at Sleepers River, Vermont during “Super Storm” Sandy in late October, 2012. The custom sensor was equipped with three wavelength pairs centered at the commonly identified excitation-emission peaks, A, T, and M. Field sensor data agreed well with these peaks measured on discrete lab samples. We observed synchronous changes in the fluorescence intensity of all measured excitation-emission pairs with fluorescence intensity increasing rapidly on the rising limb before falling to pre-event levels within several days. We also observed differential changes in peak intensity as a function of discharge; the ratio of peak T (protein-like) to peaks C, A and M (humic- and fulvic-like) fluorescence peaked just following peak discharge. The increase in the ratio of protein-like to humic-like fluorescence likely represents the flushing of fresh organic matter along shallow soil flow paths activated by the storm. After battering the eastern U.S. coast, Sandy had weakened and dropped only 38 mm rain at Sleepers River. Sandy is put into context through analysis of T, C, A, and M peaks for several hundred discrete samples collected during hydrologic events of varying sizes from 2008 to 2013 and we use these results to extrapolate how DOM quality will be expected to shift during extreme storm events even larger than Sandy.