CO2 Flux in Tidal Flats and Salt Marshes

Abstract

Tidal flats and salt marshes are sites where CO2 is released to the environment by decomposing organic matter and CO2 is absorbed by vegetation through photosynthesis. It is thought that on balance, carbon is stored in tidal flats and salt marshes. To explore this topic, we reviewed published estimates of air–water, air–sediment, water–sediment, and air–marsh fluxes of CO2 in tidal flats and salt marshes. We also carried out multiyear measurements of CO2 flux and related parameters at two field sites in Osaka Bay, Japan, having flat intertidal and salt marsh areas. The CO2 fluxes were measured using the eddy correlation and chamber methods. The air–sediment CO2 flux data from tidal flats indicated net absorption of atmospheric CO2 into the sediment during daytime hours. The air–water CO2 flux data indicated that CO2 was emitted from the water surface in small amounts, with temporal fluctuations and seasonal changes that were strongly related to salinity, as has been documented in the literature. We found that CO2 was absorbed into salt marsh and intertidal sediment and that CO2 was emitted from subtidal sediment as well as from the water surface of the tidal flat ecosystem during periods of submersion. The air–sediment CO2 flux and its temporal fluctuation at the field sites appear to be regulated by vegetation such as the reed Phragmites australis and microphytobenthos.