BIOGEOCHEMICAL GRADIENTS IN THE LOWER COLUMBIA RIVER

Water, suspended particulate materials (SPM), and biota were sampled b etween June 14 and 22, 1992 at forty-five mid-channel sites along a do wnstream gradient in the lower 350 km of the Columbia River drainage, at four mid-channel sites in the lower 27 km of the Willamette drainag e and at the mouths of nine smaller tributaries to the Columbia. Water samples were analyzed for nutrient (ammonium, nitrate, phosphate, sil icate), dissolved organic carbon (DOC) and SPM concentrations, bacteri al cell density and activity, and zooplankton composition and density. The SPM samples were analyzed for particulate organic carbon (POC), p lant pigment (chlorophyll and pheopigment) and major metal (Al, Mn, Ti , V) content as well as stable isotopic (delta(1) 3C) composition of t he POC. Willamette waters displayed significantly higher nutrient and DOC concentrations than those in the mainstem Columbia or any of the s maller tributaries. Elevated nutrient and DOC concentrations are attri buted to runoff from the extensive agricultural lands found throughout the Willamette Valley. Regardless of collection site, total Al conten t of all riverborne particles was high (5.8 +/- 0.5% by weight), indic ating total SPM mass was predominantly (similar to 70%) detrital miner al. Nonetheless, the majority of riverborne organic matter was not all ochthonous but rather derived from healthy phytoplankton as indicated by high chlorophyll a to POC (Chl:POC) values. Chlorophyll a concentra tion increased by almost 100% downstream in the mainstem Columbia betw een Bonneville Dam and the estuary. This apparent increase in phytopla nkton biomass was not accompanied by a parallel decrease in any nutrie nt concentration probably because non-point source additions occurred all along the drainage and compensated for nutrient loss due to phytop lankton growth. Despite nutrient concentrations near eutrophic levels, phytoplankton biomass in the Willamette was significantly lower than that in the mainstem Columbia. This particular contrast between the Wi llamette and the mainstem Columbia is likely due to light limitation i mposed on the phytoplankton by specific differences in the mixing dyna mics of the two flow regimes. POC in Willamette waters displayed a 50% reduced chlorophyll content and 2-2.5% C-13-depletion relative to tha t present in waters from the mainstem Columbia. These compositional di ssimilarities may simply reflect physiological difference between the diatom communities that comprised the bulk of phytoplankton in these t wo systems at the time of sampling. Alternatively, they may be caused by greater contribution of POC from non-phytoplankton sources in the W illamette. An argument is advanced suggesting that methanotrophy has c ontributed up to 5% of the POC that was measured in the Willamette Riv er at the time of our study.