Parameterizing Air‐Water Gas Exchange in the Shallow, Microtidal New River Estuary

Estuarine CO 2 emissions are important components of regional and global carbon budgets, but assessments of this flux are plagued by uncertainties associated with gas transfer velocity ( k ) parameterization. We combined direct eddy covariance measurements of CO 2 flux with waterside pCO 2 determinations to generate more reliable k parameterizations for use in small estuaries. When all data were aggregated, k was described well by a linear relationship with wind speed ( U 10 ), in a manner consistent with prior open ocean and estuarine k parameterizations. However, k was significantly greater at night and under low wind speed, and nighttime k was best predicted by a parabolic, rather than linear, relationship with U 10 . We explored the effect of waterside thermal convection but found only a weak correlation between convective scale and k . Hence, while convective forcing may be important at times, it appears that factors besides waterside thermal convection were likely responsible for the bulk of the observed nighttime enhancement in k . Regardless of source, we show that these day‐night differences in k should be accounted for when CO 2 emissions are assessed over short time scales or when pCO 2 is constant and U 10 varies. On the other hand, when temporal variability in pCO 2 is large, it exerts greater control over CO 2 fluxes than does k parameterization. In these cases, the use of a single k value or a simple linear relationship with U 10 is often sufficient. This study provides important guidance for k parameterization in shallow or microtidal estuaries, especially when diel processes are considered.