Ecosystem metabolism in the lower Columbia and Willamette Rivers, USA: Insights into the juvenile salmonid food web

Tawnya D Peterson , Nikolai M Danilchik, Joseph A Needoba

The Columbia River and its major tributary, the Willamette River, are large systems impacted by human activities, including the installation of hydroelectric dams. Hourly measurements of dissolved oxygen (DO) from in situ sensors and laboratory incubations showed that daily net ecosystem production (NEP) was slightly positive in the Columbia, with annual NEP of 121.30 gC m-2 y-1. In the Willamette, we observed high gross primary production (GPP) and positive daily NEP during the summer months but slightly negative daily NEP during winter and spring, with annual NEP equal to -16.67 gC m-2 y-1. Our estimates of annual NEP in both the Columbia and the Willamette far exceeded the global average reported for temperate rivers of -260 gC m-2 y-1. Generalized additive regression modeling showed that variation GPP and ecosystem respiration (ER) could be explained by river discharge, chlorophyll concentration, temperature, and nitrate concentration in the Columbia River (83.0% and 73.8% of deviance in GPP and ER explained, respectively). In the Willamette, variation in GPP was associated with chlorophyll and temperature (41.6% of deviance explained), while ER variation tracked temperature alone (31.6% of deviance explained). The light attenuation coefficient (Kd) determined from vertical irradiance profiles was influenced by turbidity in both rivers, with chlorophyll accounting for 7.7% of the variation in turbidity in the Willamette and 43.5% in the Columbia. A comparison of GPP rates determined from in situ and bottle data suggest that the relative contribution of phytoplankton versus vascular plants to GPP is modulated by hydrologic variation. The data lend support for classifying the Columbia as a ‘green’ river, where annual areal primary production exceeds respiration and food webs are supported by autochthonous production. The Willamette displayed characteristics of both a green and a brown river, depending on the season, with a shift from negative to positive NEP values from winter to summer and net negative values at an annual scale.