River metabolic rates measured using a transparent tunnel

Abstract To calculate ecosystem metabolism, with the aim of conserving a riverine environment, gross primary production (GPP) of the riverbed and ecosystem respiration (ER) of the aquatic community need to be accurately quantified. The prevalent open‐channel metabolism method requires the estimation of the oxygen transfer rate between the atmosphere and river water, which presents a multitude of challenges. Overcoming these challenges requires the creation of a water mass in the river that is isolated from the atmosphere. Therefore, by constructing an underwater tunnel using transparent sheets, we developed a method for GPP and ER measurement that requires no oxygen transfer rate. Gross primary production obtained in the midstream river using the transparent tunnel method averaged 6.59 and 4.65 g C m −2 d −1 in summer and winter, respectively, whereas ER averaged 3.46 and 2.77 g C m −2 d −1 in summer and winter, respectively. Moreover, these measurements were six to several dozen times higher than those quantified using traditional methods relying on experimentation, such as the light–dark bottle and bag method, which were closer to previous values quantified using the open‐channel metabolism method. The transparent tunnel method reflected the respiration rates of the benthic community across the entire riverbed, requires no oxygen transfer rate, and can estimate ecosystem metabolism with one fewer term.