Large CO ₂ effluxes at night and during synoptic weather events significantly contribute to CO ₂ emissions from a reservoir

Abstract

CO _2 emissions from inland waters are commonly determined by indirect methods that are based on the product of a gas transfer coefficient and the concentration gradient at the air water interface (e.g., wind-based gas transfer models). The measurements of concentration gradient are typically collected during the day in fair weather throughout the course of a year. Direct measurements of eddy covariance CO _2 fluxes from a large inland water body (Ross Barnett reservoir, Mississippi, USA) show that CO _2 effluxes at night are approximately 70% greater than those during the day. At longer time scales, frequent synoptic weather events associated with extratropical cyclones induce CO _2 flux pulses, resulting in further increase in annual CO _2 effluxes by 16%. Therefore, CO _2 emission rates from this reservoir, if these diel and synoptic processes are under-sampled, are likely to be underestimated by approximately 40%. Our results also indicate that the CO _2 emission rates from global inland waters reported in the literature, when based on indirect methods, are likely underestimated. Field samplings and indirect modeling frameworks that estimate CO _2 emissions should account for both daytime–nighttime efflux difference and enhanced emissions during synoptic weather events. The analysis here can guide carbon emission sampling to improve regional carbon estimates.