BIOGEOCHEMICAL CHARACTERIZATION OF SUSPENDED PARTICULATE MATTER IN THE COLUMBIA RIVER ESTUARY

In order to understand what controls the composition of suspended part iculate material (SPM) in estuarine turbidity maxima (ETM), a set of S PM samples collected in the Columbia River and estuary (northwestern U SA) during 3 seasons (fall 1990, summer 1991, spring 1992) was analyze d for detrital mineral (Min), total organic matter [OM, as 2 x particu late organic carbon (POC)], biogenic silica (BSi), chlorophyll a, delt a(13)C, and lignin. In most samples, Min, OM and BSi collectively acco unted for 100% of total SPM mass, although their relative importance c hanged seasonally. The ETM was a trap for organic matter during all 3 seasons, which can explain the intense microbial activity and microcru stacean grazing observed previously. The organic matter was particular ly rich in chlorophyll a in late spring to early summer. The source of this seasonal enrichment was mainly riverine phytoplankton. The organ ic matter contribution to ETM from the ocean was minor compared to the river, but apparently not negligible. Despite large seasonal variatio ns in chlorophyll content, the delta(13)C of POC concentrated in ETM r emained nearly constant between -26 and -25.5 parts per thousand. Vasc ular plant debris, as depicted by lignin phenol content, always compri sed a minor fraction of the organic matter in ETM, although ETM had hi gher lignin levels than the OM of surrounding waters. Intertidal mudfl ats are if not an additional source of organic matter at least an impo rtant site for transforming riverine organic matter that is ultimately concentrated in ETM.