Particulate organic matter (POM) composition in stream runoff following large storm events: role of POM sources, particle size, and event characteristics

Abstract

Large storm events have tremendous erosive energy which is capable of mobilizing large amounts of material from watershed sources into fluvial systems. This complex mixture of sediments and particulate organic matter (POM) are important nutrient sources that have the potential to impact downstream water quality. This study investigates how the composition and quality of storm events mobilized POM varies as a function of the source, particle size, event characteristics, and drainage location. Storm event POM and source sediments were collected from a forested headwater (second order stream) catchment in the Piedmont region of Maryland. Streamwater extractions were performed for three particle class sizes and bulk source materials, and the resulting fluorescent organic matter was analyzed. Carbon (C) and Nitrogen (N) amount, C:N ratio and isotopic analysis of 13C and 15N were performed on solid state materials from events and sources. Key findings from this study are: (1) Composition and quality varied greatly between sources. Carbon/nitrogen rich and isotopically depleted sources have the ability to contribute large amounts of labile material to the stream, while the carbon/nitrogen poor and isotopically enriched sources likely contribute more recalcitrant material. (2) Event characteristics such as storm magnitude, intensity, and antecedent conditions regulated the sources that were mobilized. Summer storms mobilized more carbon and nitrogen rich labile sources, while winter events mobilized more carbon and nitrogen poor refractory material from near stream sources. (3) POM quality was distinct in different size classes, with the Coarse size showing more C/N rich and labile properties while the Fine size showing more degraded and refractory properties. (4) POM quality varied by drainage location and was most strongly regulated by the sources available at each location within the stream network. Future climate variability will likely increase the frequency and intensity of large storm events, thus mobilizing more POM into aquatic systems. POM has the potential to impact water quality, aquatic nutrient cycling, and greenhouse gas fluxes and therefore should be accounted for in future management decisions.