Variations in the magnitude and composition of particulate organic matter export across a small, mountainous river estuary along Pacific Northwest margin of North America

Abstract:

The Pacific Northwest margin of North America is characterized by numerous small mountainous rivers that mobilize the bulk of particulate materials (i.e., inorganic sediment, organic matter) from surrounding watersheds during short-lived, winter-time floods. However, it is unclear how estuarine processes alter the magnitude and composition of the particulate load prior to transport to the ocean. In this presentation, we present results from in-situ observations and rapid-response sampling along a typical river-estuary system (Alsea River/Bay, Oregon) designed to address this issue. We found marked contrasts in the concentrations of sediment and particulate organic matter in the estuary during different river discharge conditions (flood vs. non-flood), with distinct trends associated with contrasting tidal stage (spring/neap). Elemental, isotopic and biomarker analyses of individual samples revealed large differences in the provenance and composition of organic materials mobilized under different discharge/tidal conditions. Continuous, in-situ measurements of current, salinity, temperature and turbidity, combined with the compositions from individual samples allow us to determine instantaneous material fluxes into and out of Alsea Bay. We use these data to derive net fluxes during wintertime conditions and assess the role of floods in the retention and transport of particulate organic matter across the estuarine interface.