Water column respiration in the Yakima River basin is explained by temperature, nutrients, and suspended solids

Understanding aquatic ecosystem metabolism involves the study of two key processes: carbon fixation via primary production and organic C mineralization as total ecosystem respiration (ERtot). In streams and rivers, ERtot includes respiration in the water column (ERwc) and in the sediments (ERsed). While literature surveys suggest that ERsed is often a dominant contributor to ERtot, 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 ERwc and aqueous chemistry across 47 sites in the Yakima River basin, Washington, USA. We find that ERwc rates vary throughout the basin during baseflow conditions, ranging from 0 to -7.38 g O2 m-3 d-1, and encompass the entire range of ERwc rates from previous work. Additionally, by comparing to ERtot estimates for rivers across the contiguous United States, we suggest that the contribution of ERwc rates to reach-scale ERtot rates across the Yakima River basin is likely highly variable, but we do not test this directly. We observe that ERwc is locally controlled by temperature, dissolved organic carbon, total dissolved nitrogen, and total suspended solids, which explains 49 % of ERwc variability across the basin using Least Absolute Shrinkage and Selection Operator (LASSO) regression. 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 by knowing the 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 ERwc are potentially transferable across basins.