Evolution of particulate organic matter (POM) along a headwater drainage: role of sources, particle size class, and storm magnitude

Large storm events can not only increase the runoff mass exports of particulate organic matter (POM) from watersheds, but can also alter the sources, size distribution, and composition of POM. We investigated the quantity, particle size distribution, carbon (C) and nitrogen (N) content, and sources of POM for five locations longitudinally along a forested Piedmont stream. POM was sampled for multiple storm events of varying magnitude and intensity over a two-year period. POM was separated into coarse (CPOM), medium (MPOM), and fine (FPOM) size classes, and sources were estimated using stable isotopes of C-13 and N-15 with a Bayesian mixing model. CPOM largely resembled less-degraded vascular plant material characteristic of forest floor litter, which was estimated to contribute to similar to 40% of CPOM in upstream locations. FPOM was derived from a more variable mixture of sources with stream beds and stream banks playing a greater role at larger drainage locations (up to similar to 50 and similar to 30%, respectively). Contributions from both forest floor litter and humus to CPOM increased with increasing event runoff, and litter contributions increased during events with higher rainfall intensities. Higher C and N content was noted in coarse sediments and finer POM fractions appeared to be more degraded based on C:N and isotope ratios. Climate-change projections predict intensification of large storm events in the Northeastern US. Results of this study suggest that large storms will increase the fluvial exports of coarse, labile, C- and N-rich POM with subsequent impacts on receiving aquatic ecosystems.