Reef Metabolism Monitoring Methods and Potential Applications for Coral Restoration.

Coral reef metabolism measurements have been used by scientists for decades to track reef responses to the globe's changing carbon budget and project shifts in reef function. Here, we propose that metabolism measurement tools and methods could also be used to monitor reef ecosystem change in response to coral restoration. This review paper provides a general introduction to net ecosystem metabolism and carbon chemistry for coral reef ecosystems, followed by a review of five metabolism monitoring methods with potential for application to coral reef restoration monitoring. Selected methodologies included those with measurement scales appropriate to assess outplant arrays and whole reef ecosystem outcomes associated with restoration interventions. Subsequently we discuss how water column and CO2 chemistry could be used to address coral restoration monitoring research gaps and scale up from biological, colony-level metrics to ecosystem-scale function and performance assessments. Such function-based measurements could potentially be used to inform several goal-based monitoring objectives highlighted in the Coral Reef Restoration Monitoring Guide. Lastly, this review discusses important methodological factors, such as scale, reef type, and flow environment, that should be considered when determining which metabolism monitoring technique would be most appropriate for a reef restoration project.

Methane and Carbon Dioxide Emissions From Reservoirs: Controls and Upscaling.

Estimating carbon dioxide (CO2) and methane (CH4) emission rates from reservoirs is important for regional and national greenhouse gas inventories. A lack of methodologically consistent data sets for many parts of the world, including agriculturally intensive areas of the United States, poses a major challenge to the development of models for predicting emission rates. In this study, we used a systematic approach to measure CO2 and CH4 diffusive and ebullitive emission rates from 32 reservoirs distributed across an agricultural to forested land use gradient in the United States. We found that all reservoirs were a source of CH4 to the atmosphere, with ebullition being the dominant emission pathway in 75% of the systems. Ebullition was a negligible emission pathway for CO2, and 65% of sampled reservoirs were a net CO2 sink. Boosted regression trees (BRTs), a type of machine learning algorithm, identified reservoir morphology and watershed agricultural land use as important predictors of emission rates. We used the BRT to predict CH4 emission rates for reservoirs in the U.S. state of Ohio and estimate they are the fourth largest anthropogenic CH4 source in the state. Our work demonstrates that CH4 emission rates for reservoirs in our study region can be predicted from information in readily available national geodatabases. Expanded sampling campaigns could generate the data needed to train models for upscaling in other U.S. regions or nationally.

Effects of an Experimental Water-level Drawdown on Methane Emissions from a Eutrophic Reservoir.

Reservoirs are a globally significant source of methane (CH4) to the atmosphere. However, emission rate estimates may be biased low due to inadequate monitoring during brief periods of elevated emission rates (that is, hot moments). Here we investigate CH4 bubbling (that is, ebullition) during periods of falling water levels in a eutrophic reservoir in the Midwestern USA. We hypothesized that periods of water-level decline trigger the release of CH4-rich bubbles from the sediments and that these emissions constitute a substantial fraction of the annual CH4 flux. We explored this hypothesis by monitoring CH4 ebullition in a eutrophic reservoir over a 7-month period, which included an experimental water-level drawdown. We found that the ebullitive CH4 flux rate was among the highest ever reported for a reservoir (mean = 32.3 mg CH4 m-2 h-1). The already high ebullitive flux rates increased by factors of 1.4-77 across the nine monitoring sites during the 24-h experimental water-level drawdown, but these emissions constituted only 3% of the CH4 flux during the 7-month monitoring period due to the naturally high ebullitive CH4 flux rates that persist throughout the warm weather season. Although drawdown emissions were found to be a minor component of annual CH4 emissions in this reservoir, our findings demonstrate a link between water-level change and CH4 ebullition, suggesting that CH4 emissions may be mitigated through water-level management in some reservoirs.