The Importance of and Spatial Variation in Open Channel Respiration as a Proportion of Ecosystem Metabolism in the Yakima River Basin, Washington, USA

Aerobic ecosystem metabolism—the balance between gross primary production and ecosystem respiration (ER)—is a key process influencing biogeochemical cycling rates through the processing of organic matter in aquatic systems. Ecosystem metabolism responds to environmental and anthropogenic perturbations, which can have deleterious effects on downstream water quality. Spatial heterogeneity in rivers also influences stream metabolism, including variation in the amount and quality of organic matter inputs, discharge, temperature, radiant energy, stream morphology, stream order, and land use and land cover. Recent advances in sensor technology have enabled the collection of high spatial and temporal resolution measurements of dissolved oxygen for estimating stream metabolism across ecosystem types in many locations around the world. Many of these studies provide estimates of ecosystem-scale respiration, however, they do not typically distinguish between contributions from the water column and benthic sediments. Our study addressed two primary research questions: 1) how important are contributions from open channel respiration relative to ER in the mainstem and tributaries to the Yakima River; 2) how does open channel respiration vary across stream orders within the Yakima River Basin (YRB). We measured open channel respiration at 47 sites across the YRB using 2-hour semi-in-situ triplicate dark bottle incubations. Sites were selected using machine learning to identify which geospatial parameters most influenced modeled respiration rates, and again in selecting sites across six orders of streams to capture geospatial variation throughout the YRB. Water chemistry samples were collected at each site to identify potential drivers of spatial variation in open channel respiration rates. Preliminary results revealed open channel respiration rates are mostly quite low compared to published values. Linear regression model results show that open channel respiration magnitude increased with rising stream order and rising temperature, but predictive power is low. Comparisons with many other physical and chemical parameters showed no statistically-significant correlations. An intensive three week field campaign is underway to expand our current understanding of ecosystem metabolism in the YRB.