Tatsuki Tokoro

Net uptake of atmospheric CO2 by coastal submerged aquatic vegetation

‘Blue Carbon’, which is carbon captured by marine living organisms, has recently been highlighted as a new option for climate change mitigation initiatives. In particular, coastal ecosystems have been recognized as significant carbon stocks because of their high burial rates and long-term sequestration of carbon. However, the direct contribution of Blue Carbon to the uptake of atmospheric CO2 through air-sea gas exchange remains unclear. We performed in situ measurements of carbon flows, including air-sea CO2 fluxes, dissolved inorganic carbon changes, net ecosystem production, and carbon burial rates in the boreal (Furen), temperate (Kurihama), and subtropical (Fukido) seagrass meadows of Japan from 2010 to 2013. In particular, the air-sea CO2 flux was measured using three methods: the bulk formula method, the floating chamber method, and the eddy covariance method. Our empirical results show that submerged autotrophic vegetation in shallow coastal waters can be functionally a sink for atmospheric CO2. This finding is contrary to the conventional perception that most near-shore ecosystems are sources of atmospheric CO2. The key factor determining whether or not coastal ecosystems directly decrease the concentration of atmospheric CO2 may be net ecosystem production. This study thus identifies a new ecosystem function of coastal vegetated systems; they are direct sinks of atmospheric CO2.

Improved Post-processing of Eddy-Covariance Data to Quantify Atmosphere–Aquatic Ecosystem CO2 Exchanges

The capture of carbon by aquatic ecosystems and its sequestration in sediments has been studied as a potential method for mitigating the adverse effects of climate change. However, the evaluation of in situ atmospheric CO2 fluxes is challenging because of the difficulty in making continuous measurements over areas and for periods of time that are environmentally relevant. The eddy covariance method for estimating atmospheric CO2 fluxes is the most promising approach to address this concern. However, methods to process the data obtained from eddy covariance measurements are still being developed, and the estimated air-water CO2 fluxes have large uncertainties and differ from those obtained using conventional methods. In this study, we improved the post-processing procedure for the eddy covariance method to reduce the uncertainty in the measured air-water CO2 fluxes. Our procedure efficiently removes low-quality fluxes using a combination of filtering methods based on the received signal strength indicator of the eddy covariance sensor, the normalized standard deviation of atmospheric CO2 and water vapor concentrations, and a high-pass filter. The improved eddy covariance fluxes revealed diurnal and semi-diurnal cycles and a significant relationship with water fCO2, patterns that were not observed from the results before filtering. Although there were still differences with indirect conventional measurements like the bulk formula method, the methods used in this study should improve the accuracy of carbon flow estimates at sites with complex terrains like coastal areas.

Air–Water CO 2 Flux in Shallow Coastal Waters: Theory, Methods, and Empirical Studies

The fact that the ocean is one of the largest sinks of atmospheric CO2 on Earth is an important consideration in the prediction of future climate changes and identification of possible mitigation strategies for global climate change. Recently, carbon storage in vegetated coastal habitats (blue carbon ecosystems) has been explored as a new option to mitigate climate change. However, the complexity of the mechanisms that control air–water CO2 fluxes in shallow coastal ecosystems has precluded their adequate quantification. Spatiotemporal extension of accurate values of these fluxes will be an important milestone for assessing the contribution of blue carbon ecosystems to mitigation of climate change. In this chapter, we explain the theoretical understanding of air–water CO2 fluxes and methods for their measurement. We then discuss results of empirical measurements of air–water CO2 fluxes in shallow coastal waters. We conclude that statistical analyses of augmented air–coastal-water CO2 flux data based on long-term measurements and multiple methods should lead to a quantitative understanding of the current status and future air–water CO2 fluxes in shallow coastal waters at national and global scales.