Chemical characteristics of particulate organic matter from a small, mountainous river system in the Oregon Coast Range, USA

To better understand the transfer of particulate organic matter (POM) by small mountainous river systems (SMRS) to the ocean, we measured the concentration and composition of suspended particles from the Alsea River, a SMRS in the Oregon Coast Range, over a wide range of discharges that included several floods. All particulate constituents measured, including organic carbon, nitrogen and biomarkers such as lignin-derived phenols, cutin acids and amino acid-derived products, displayed concentrations that increased as a power function of discharge. In contrast to other SMRS, virtually all POM in the Alsea River samples was modern and had an average age < 60 year. In spite of their similar (14)C ages, marked contrasts in elemental and biomarker compositions were evident in particles collected at low and high discharges. Particles at low flows were primarily composed of organic detritus from non-vegetation sources (e.g., algal cells) whereas mineral-rich particles containing vegetation and soil-derived POM were predominant at elevated flows. Biomarker compositions of these latter particles suggest most of the POM originated from areas if the watershed with measurable hardwood contributions, which include areas affected by shallow landslides and riparian zones. We infer that the low uplift rates, lush vegetation, high net primary production and relatively thick soils that characterize the Alsea watershed are responsible for these trends. Comparison of our results with those from other systems, including the well-studied Santa Clara River (southern California), demonstrates that SMRS are part of a continuum whereby contrasts in hydroclimate, geology and vegetation lead to significant differences in the age and composition of POM exported to the ocean.