Impacts of riverine heatwaves on rates of ecosystem metabolism in the United States

Abstract Rivers produce and decompose large amounts of carbon globally due, in part, to high rates of gross primary production (GPP) and ecosystem respiration (ER), collectively known as ecosystem metabolism. Water temperature is a major driver of ecosystem metabolism, and in‐stream temperatures are increasing globally, including extreme temperature events called heatwaves. This study used published estimates of daily GPP and ER from 48 stream and river locations in the United States to examine how ecosystem metabolism responds to riverine heatwaves. During low‐severity heatwaves, GPP and ER increase proportionally, resulting in no net difference. However, during severe and extreme heatwaves, GPP declined up to 82% while ER increased up to 47%, resulting in greater rates of heterotrophy (ER > GPP). While rivers were typically heterotrophic outside of heatwave conditions, these results suggest that during heatwaves, rivers become stronger sources of carbon dioxide.