Biogeochemical characterization of suspended particulate matter in the Columbia River estuary

In order to understand what controls the composition of suspended particulate material (SPM) in estuarine turbidity maxima (ETM), a set of SPM samples collected in the Columbia River and estuary (northwestern USA) during 3 seasons (fall 1990. summer 1991, spring 1992) -.as analyzed for detrital mineral (Min), total organic matter [OM, as 2 X particulate organic carbon (POC)], biogenic silica (BSI), chlorophyll a, 6' .' ~:, and lignin. In most samples, Mm, OM and BSI collectivc,ly accounted for 100'.':, of total SPM mass, although their relative importance changed seasonally. The ETM was a trap for organlc matter during all 3 seasons, which can explaln the intense microbial activity and microcrustacean grazing observed previously. The organic matter was particularly rich in chlorophyll a in late spring to early summer The source of this seasonal enrichment was mainly rivenne phytoplankton. The organic matter contribution to ETM from the ocean was mlnor compared to the river, but apparently not negligible. Despite large seasonal variations in chlorophyll content, the 6C of POC concentrated in ETM remained nearly constant between -26 and -25.S4':,,. Vascular plant debris, as depicted by lignin phenol content, always comprised a minor fraction of the organic matter in ETM, dllhuuyh ET>l had h ~g h e r lignin levels than the OM of surrounding waters. Intertidal mudflats arc. I! not an additional source of organic mattclr at least a n i m p o r t ~~n t site for transforming riverine organic matter that is ultimately concentrated in ETh4.