Understanding the quantity and quality of dissolved organic matter (DOM) in potential watershed sources is critical for explaining and quantifying the exports of DOM in stream runoff. Here, we examined the concentration and quality of DOM for ten watershed sources in a 12 ha forested catchment over a two-year period. DOM composition was evaluated for: throughfall, litter leachate, soil water (zero and tension), shallow and deep groundwater, stream water, hyporheic zone, and groundwater seeps. DOM quality was measured using a suite of optical indices including UV-visible absorbance and PARAFAC modeling of fluorescence excitationemission matrices (EEMs). DOM concentrations and quality displayed a pronounced trend across watershed sources. Surficial watershed sources had higher DOM concentrations and more humic-like DOM with higher molecular weight whereas deeper groundwater sources were rich in % protein-like fluorescence. The greater % contribution of proteinlike fluorescence in groundwater suggested that a larger fraction of groundwater DOM may be bioavailable. DOM for wetland groundwater was more aromatic and humic-like than that at the well-drained riparian location. Principal component analyses (PCA) revealed that the differences in surficial watershed compartments were dictated by humic-like components while groundwater sources separated out by % protein-like fluorescence. Observations from optical indices did not provide any conclusive evidence for preferential association of dissolved organic carbon (DOC) or dissolved organic nitrogen (DON) with any particular DOM quality pools.

[1] The concentrations and quality of dissolved organic matter (DOM) and their sources were studied for multiple storm events collected over a three-year period in a forested headwater (12 ha) catchment in the mid-Atlantic Piedmont region of the USA. DOM constituents were characterized using a suite of indices derived from ultraviolet absorbance and PARAFAC modeling of fluorescence excitation emission matrices. Runoff sources and hydrologic flow paths were identified using an end-member mixing model, stable isotope data, and groundwater elevations from valley-bottom saturated areas. DOM constituents and their sources differed dramatically between base flow and storm-event conditions. The aromatic and humic DOM constituents in stream water increased significantly during storm events and were attributed to the contributions from surficial sources such as throughfall, litter leachate and soil water. Groundwater sources contributed a large fraction of the DOM constituents during base flow and were responsible for the high % protein-like fluorescence observed in base flow. Hydrologic flow paths and runoff sources were critical for explaining the differences in DOM among the storm events. This study underscored the value of studying multiple storm events across a range of hydrologic and seasonal conditions. Summer events produced the highest concentrations for humic and aromatic DOM while the corresponding response for winter events was muted. A large event following summer drought produced a complex DOM response which was not observed for the other events. These extreme events provided important insights into how DOM quality may change for future changes in climate and water quality implications for sensitive coastal ecosystems.

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[1] Runoff mixing patterns for base flow and 42 storm events were investigated for a 3 year period (2008)(2009)(2010) in a 12 ha forested catchment in the mid-Atlantic, Piedmont region of the USA. Eleven distinct runoff sources were sampled independently and included: precipitation, throughfall, stemflow, litter leachate, wetland soil water, tension soil water, shallow groundwater, groundwater seeps, hyporheic water, riparian groundwater, and deep groundwater. A rigorous end-member mixing analysis (EMMA) was implemented and all base flow, storm-flow, and end-member chemistries were evaluated in a two-dimensional mixing space. End-members enclosed stream water chemistry and displayed a systematic continuum in EMMA space. Base-flow chemistry of stream waters was similar to groundwater seeps. Storm-event runoff was attributed to contributions from surficial sources (precipitation, throughfall, stemflow, and litter leachate) on the rising limb of the discharge hydrograph that was followed by soil and shallow groundwater sources on the recession limb of the hydrograph. The shapes of the storm-event hysteresis loops (wide versus tight, linear patterns) varied with hydrologic conditions from wet, hydrologically well-connected conditions to a dry, disconnected state. Detailed temporal data on endmember chemistry allowed us to explain the changes in stream water hysteresis patterns and runoff mixing space to shifts in end-member chemistry that occurred as the catchment became hydrologically disconnected. These results highlight the need to recognize the temporal variation in end-member chemistry as a function of catchment wetness and the need to collect high-frequency data on both--stream water as well as potential runoff endmembers to better characterize catchment flow paths and mixing responses.

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Although lasting only a fraction of the year, large storms may represent a significant, but highly variable, control on watershed nitrogen (N) fluxes. We determined the exports of particulate N (PN) and total dissolved N including nitrate-N (NO 3 -N) and dissolved organic N (DON) in streamflow from a 12 ha temperate forested watershed. Sampling was performed for 15 storms over September 2010 to December 2012 and included four large tropical storms-Nicole (2010) , respectively. Nitrate-N concentrations displayed a dilution trajectory for peak stormflows suggesting supply limitation, a response that was not as strong for PN. These results underscore the importance of large storms for PN export which is significant given that climate-change predictions indicate an increasing intensity of large tropical storms for the northeast U.S.. Elevated PN exports could further exacerbate water quality and eutrophication problems in sensitive aquatic ecosystems already subjected to excess dissolved nitrogen loads.

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The composition of dissolved organic matter (DOM) in a mid-Atlantic forested watershed was evaluated using two fluorescence models—one based on previously validated model (Cory and McKnight, 2005) and the other developed specifically for our study site. DOM samples for the models were collected from multiple watershed sources over a two-year period. The previously validated parallel factor analysis (PARAFAC) model had 13 DOM components whereas our site-specific model yielded six distinct components including two terrestrial humic-like, two microbial-derived humic-like, and two protein-like components. The humic-like components were highest in surficial watershed sources and decreased from soil water to groundwater whereas the protein-like components were highest for groundwater sources. Discriminant analyses indicated that our site-specific model was more sensitive to subtle differences in DOM and the sum of the humic- and protein-like constituents yielded more pronounced differences among watershed sources as opposed to the prevalidated model. Dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) concentrations and selected DOM metrics were also more strongly correlated with the site-specific model components. These results suggest that while the pre-validated model may capture broader trends in DOM composition and allow comparisons with other study sites, a site-specific model will be more sensitive for characterizing within-site differences in DOM.