Increases in humic and bioavailable dissolved organic matter in a forested New England headwater stream with increasing discharge.

Understanding the processes controlling the transfer of organic matter from terrestrial to aquatic ecosystems is of fundamental importance for the aquatic sciences. Over the course of a full year dissolved organic matter (DOM) fluorescence, absorbance, and bioavailability were characterized in Bigelow Brook, a forested headwater stream in Massachusetts, USA. Parallel factor analysis (PARAFAC) identified a 4 component model to describe observed DOM fluorescence (C1 to C4). C2 exhibited the characteristics of a more humic-like fluorophore with a potentially more reduced redox state and increased with discharge while more fulvic-like (C1) and protein-like (C3, C4) fluorophores decreased. Under both dark and light exposed conditions percent bioavailable DOC (%BDOC) increased with discharge (r2= 0.37 and r2= = 0.56). C2 and specific absorptivity (SUVA) were reduced following BDOC incubations while C1, C3, and C4 increased. These changes to DOM characteristics with increasing discharge were observed under both baseflow and stormflow conditions indicating that with rising water table, loading from a large riparian or hyporheic pool of organic matter is likely occurring. Other headwater streams, where loading is controlled by hillslope processes, are likely to exhibit a similar pattern of increasing export of more humic and bioavailable DOM during hydrologic events.