Comment on Demars et al. 2015, Stream metabolism and the open diel oxygen method: Principles, practice, and perspectives

Demars et al. present a review of stream ecosystem metabolism theory that provides the basis for a set of best practices. Here we clarify two technical and conceptual errors in reviewing the current state of research regarding inverse modeling of ecosystem metabolism. Specifically, the assertions by Demars et al. that inverse modeling of metabolism is limited to low reaereation, high productivity ecosystems and, when feasible, measurement of gas transfer by volatile tracer injection is the preferred method, are based on an incomplete conceptualization of current ecosystem metabolism models and does not recognize the unconstrained biases in tracer injection methods. In addition to these corrections, we highlight how inverse model fitting is a robust methodology to constrain key metabolic parameters by capturing the information contained in the dynamics of diel O-2 data. When done in a Bayesian context, inverse model fitting also provides a means to incorporate multiple sources of available data, including those from tracer studies, and formally propagate uncertainties in parameter estimates. Last, we describe how model fitting of diel O-2 data can also provide information on temperature sensitivity of ecosystem respiration.