Spatial pattern of dissolved organic matter (DOM) along a stream drainage in a forested, Piedmont catchment

Understanding how dissolved organic matter (DOM) varies spatially in catchments and the processes and mechanisms that regulate this variation is critical for developing accurate and reliable models of DOM. We determined the concentrations and composition of DOM at multiple locations along a stream drainage network in a 79 ha forested, Piedmont, watershed in Maryland, USA. DOM concentrations and composition was compared for five stream locations during baseflow (drainage areas - 0.62, 3.5, 4.5, 12 and 79 ha) and three locations (3.5, 12, 79 ha) for storm flow. Sampling was conducted by manual grab samples and automated ISCO samplers. DOM composition was characterized using a suite of spectrofluorometric indices which included - HIX, a254, and FI. A site-specific PARAFAC model was also developed for DOM fluorescence to determine the humic-, fulvic-, and protein-like DOM constituents. Hydrologic flow paths during baseflow and stormflow were characterized for all stream locations using an end-member mixing model (EMMA). DOM varied notably across the sampled positions for baseflow and stormflow. During baseflow, mean DOC concentrations for the sampled locations ranged between 0.99-3.1 mg/L whereas for stormflow the range was 5.22-8.11 mg/L. Not surprisingly, DOM was more humic and aromatic during stormflow versus baseflow. The 3.5 ha stream drainage location that contained a large wetland yielded the highest DOC concentration as well as the most humic and aromatic DOM, during both, baseflow and stormflow. In contrast, a headwater stream location (0.62 ha) that received runoff from a groundwater seep registered the highest mean value for % protein-like DOM (30%) and the lowest index for aromaticity (mean a254 = 6.52) during baseflow. During stormflow, the mean % protein-like DOM was highest at the largest 79 ha drainage location (mean = 11.8%) and this site also registered the lowest mean value for a254 (46.3). Stream drainage locations that received a larger proportion of runoff along surficial flow paths produced a more aromatic and humic DOM with high DOC concentrations; whereas those with a greater proportion of groundwater contributions produced DOM with greater % of protein-like content. Overall, our observations suggest that occurrence of wetlands and the nature of hydrologic flow paths were the key determinants for the spatial pattern of DOM.