Seasonal and Event Scale Forcings on the Magnitude and Composition of Particulate Organic Matter Fluxes Across a Small Mountainous River Estuary

Abstract:

Small mountainous rivers along the west coast margin of North America display intense seasonal contrasts in runoff, with event-scale peaks in particulate transport associated with winter-time floods. In-situ observations and rapid-response sampling along a typical river-estuary system (Alsea River/Bay, Oregon) were carried out to investigate variations in the magnitude and composition of particulate organic matter transported across the estuary at seasonal, tidal and event scales. Marked contrasts in the overall concentration of particulate organic matter in the estuary are observed during different river discharge conditions (base flow to 3-yr flood), with distinct trends associated with contrasting tidal stage (spring/neap). Elemental, isotopic and biomarker analyses of individual samples reveal large differences in the provenance (phytoplankton, vascular plants, soil) 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.