Biogeochemical gradients in the lower Columbia River

Abstract

Water, suspended particulate materials (SPM), and biota were sampled between June 14 and 22, 1992 at forty-five mid-channel sites along a downstream gradient in the lower 350 km of the Columbia River drainage, at four mid-channel sites in the lower 27 km of the Willamette drainage and at the mouths of nine smaller tributaries to the Columbia. Water samples were analyzed for nutrient (ammonium, nitrate, phosphate, silicate), dissolved organic carbon (DOC) and SPM concentrations, bacterial cell density and activity, and zooplankton composition and density. The SPM samples were analyzed for particulate organic carbon (POC), plant pigment (chlorophyll and pheopigment) and major metal (Al, Mn, Ti, V) content as well as stable isotopic ( $$\delta ^1 $$ 3C) composition of the POC. Willamette waters displayed significantly higher nutrient and DOC concentrations than those in the mainstem Columbia or any of the smaller tributaries. Elevated nutrient and DOC concentrations are attributed to runoff from the extensive agricultural lands found throughout the Willamette Valley. Regardless of collection site, total Al content of all riverborne particles was high (5.8 ± 0.5% by weight), indicating total SPM mass was predominantly (∼ 70±) detrital mineral. Nonetheless, the majority of riverborne organic matter was not allochthonous but rather derived from healthy phytoplankton as indicated by high chlorophyll a to POC (Chl:POC) values. Chlorophyll a concentration increased by almost 100% downstream in the mainstem Columbia between Bonneville Dam and the estuary. This apparent increase in phytoplankton biomass was not accompanied by a parallel decrease in any nutrient concentration probably because non-point source additions occurred all along the drainage and compensated for nutrient loss due to phytoplankton 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 Willamette and the mainstem Columbia is likely due to light limitation imposed on the phytoplankton by specific differences in the mixing dynamics of the two flow regimes. POC in Willamette waters displayed a 50% reduced chlorophyll content and 2–2.5% 13C-depletion relative to that present in waters from the mainstem Columbia. These compositional dissimilarities may simply reflect physiological difference between the diatom communities that comprised the bulk of phytoplankton in these two systems at the time of sampling. Alternatively, they may be caused by greater contribution of POC from non-phytoplankton sources in the Willamette. An argument is advanced suggesting that methanotrophy has contributed up to 5% of the POC that was measured in the Willamette River at the time of our study.