Coupling Concentration- and Process-Discharge Relationships Integrates Water Chemistry and Metabolism in Streams

Stream ecosystem processes, such as metabolism, are dynamically impacted by flow intensity. Therefore, without integrating ecosystem processes with water quality, we miss opportunities to develop frameworks to understand metabolic responses to changing flow. Flow simultaneously affects the material transport and biological opportunities for material transformation. Combining the strengths of ecohydrology and stream ecology to understand how flow variation alters ecosystem processes, we analyzed more than 5 years of water quality and stream metabolism data. We created segmented process-discharge (P-Q) relationships to examine how metabolism rates vary across discharge and compared them to concentration-discharge (C-Q) relationships to explore the dynamic effects of discharge on processes and physicochemical parameters. Within the segmented P-Q relationships, we found the behavior of ecosystem respiration (ER), gross primary production (GPP), and net ecosystem production (NEP) to be different at high and low flows with varying degrees of statistical significance, demonstrating the potential for divergent metabolic responses across changing flows. GPP declined with increasing discharge. The rate of ER declined with discharge initially but then became unchanging at higher flows. NEP reflected the divergent trends between ER and GPP, as the relationship of NEP to Q was flat at lower discharge and declined at higher flows. Interrelated physicochemical parameters and ecosystem processes, such as pH and NEP, had mirrored responses to discharge. Coupling analyses of flow, water quality, and metabolism offers a more complete picture of interrelated ecosystem processes, allowing for a better understanding of ecosystem response to the physical and chemical changes that occur across flows.