Water Column Respiration in the Yakima River Basin is Explained by Temperature, Nutrients and Suspended Solids

Abstract. Understanding aquatic ecosystem metabolism involves the study of two key processes: carbon fixation via primary production and organic C mineralization as total ecosystem respiration (ER_tot). In streams and rivers, ER_tot includes respiration in the water column (ER_wc) and in the sediments (ER_sed). While literature surveys suggest that ER_sed is often a dominant contributor to ER_tot, recent studies indicate that the relative influence of sediment-associated processes versus water column processes can fluctuate along the river continuum. Still, a comprehensive understanding of the factors contributing to these shifts within basins and across stream orders is needed. Here we contribute to this need by measuring ER_wc and aqueous chemistry across 47 sites in the Yakima River basin, Washington, USA. We found that ER_wc rates varied throughout the basin during baseflow conditions, ranging from –7.38 to 0.36 g O₂ m⁻³ d⁻¹, and encompassed the entire range of ER_wc rates from previous work. Additionally, by comparing to ER_tot estimates for rivers across the contiguous United States, we suggest that the contribution of ER_wc rates to reach-scale ER_tot rates across the Yakima River basin are likely highly variable, but we did not test this directly. We did not observe clear increases in ER_wc moving down the stream network, and instead observed that ER_wc is locally controlled by temperature, dissolved organic carbon, total dissolved nitrogen, and total suspended solids, which explained 40 % of ER_wc variability across the basin. Our findings highlight the potential relevance of water column processes in aquatic ecosystem metabolism across the entire stream network and that these influences are likely not predictable simply via position in the stream network. Our results are generally congruent with previous work in terms of locally-influential variables, suggesting that the observed variability and suite of associated environmental factors influencing ER_wc are potentially transferable across basins.