Particulate organic matter export by two contrasting small mountainous rivers from the Pacific Northwest, U.S.A.

Abstract

We investigated the export of particulate organic matter (POM) to the ocean by two contrasting small, mountainous rivers, the Umpqua and Eel Rivers, by collecting suspended sediment samples over a range of discharges and analyzing them for a variety of constituents, including organic carbon, nitrogen, biomarkers with distinct biochemical sources, and isotopic compositions (δ13C and Δ14C). Concentrations of all measured constituents in both rivers increased as a function of discharge, resulting in their export being dominated by short‐lived, wintertime high‐discharge events. In the Umpqua River, marked compositional contrasts between low‐ and high‐discharge conditions were consistent with a shift in the provenance of POM from biogenic sources dominated by non‐vascular plant sources at low flows to contributions from vascular plant sources of moderate 14C ages (~300 years before present) dominating at high flows. In contrast, POM from the Eel River, which was highly diluted by mineral sediment at all discharges, had significant contributions from petrogenic sources and displayed lower concentrations of recognizable biomarkers. Both rivers had comparable yields of biogenic POM, which appeared to be moderately degraded and originated primarily from surface soils in erosion prone areas of the watersheds. While tectonic/geologic differences help explain the contrasts in sediment and petrogenic POM yields between the two watersheds, ecological factors such as vegetation coverage, productivity, and soil carbon are more important in influencing the composition of biogenic POM mobilized from these systems.