Storm event patterns of particulate organic carbon (POC) for large storms and differences with dissolved organic carbon (DOC)

This study compared the storm event patterns, sources, and flow paths for particulate (POC) and dissolved organic carbon (DOC < 0.45 mu m) with a special focus on responses during large storm events. The study was conducted in a 12 ha forested catchment in the mid-Atlantic, Piedmont region of USA. A total of 14 storm events were sampled over a 16-month period (September 2010 to December 2011) including large, intense storms (precipitation > 150 mm) associated with two hurricanes-Nicole (2010) and Irene (2011). Storm-event concentrations for suspended sediment (SS), POC and DOC varied between 10-7589, 0.05-252, and 0.7-18.3 mg L-1, respectively. Within-event POC concentrations continued to increase for the large hurricane storms whereas DOC displayed a dilution at peak streamflow discharge. Flow-weighted mean POC concentrations decreased for closely spaced, successive storm events whereas no such decrease was observed for DOC. These results suggest that there are important differences in the supply and transport (leaching rates and kinetics) for POC and DOC which occur at different temporal scales. The % POC content of SS was highest for the summer events. Summer events also registered a sharper increase in DOC with stream discharge and then a decline for peak flow, suggesting critical seasonal controls on storm-event POC and DOC responses. End-member mixing analysis revealed POC is transported with surface runoff while DOC is transported by saturation overland flow and rising groundwater into the soil horizons. A mixing model for sediment sources failed to identify key end-members but event mixing patterns revealed near-stream sources for small events and more distal, upland sediment sources for large and intense storms. This study highlights the need to better understand POC and DOC responses in headwater catchments especially for the large, intense, storm events that are predicted to increase in intensity with climate change.