Estimate of gas transfer velocity in the presence of emergent vegetation using argon as a tracer: Implications for whole-system denitrification measurements

Denitrification associated with emergent macrophytes is a pivotal process underlying the treatment performance of wetlands and slow-flow waterways. Laboratory scale experiments targeting N losses via denitrification in sediments colonized by emergent macrophytes require the use of mesocosms that are necessarily open to the atmosphere. Thus, the proper quantification of N2 effluxes relies on the accurate characterization of the air-water gas exchanges. In this study, we present a simple approach for direct measurements of the gas transfer velocity, in open-top mesocosms with Phragmites australis, by using argon as a tracer. Different conditions of water velocity (0, 1.5, 3, and 6 cm s−1) and temperature (8.5, 16, and 28 °C), were tested, along with, for the first time, the presence of emergent vegetation. The outcomes demonstrated that water velocity and temperature are not the only factors regulating aeration at the mesocosm scale. Indeed, the gas transfer velocity was systematically higher, in the range of 42-53%, in vegetated compared to unvegetated sediments. The increase of small-local turbulence patterns created within water parcels moving around plant stems translated into significant modifications of the reaeration process. The adopted approach may be used to improve the accuracy of denitrification measurements by N2 efflux-based methods in wetland and slow-flow waterway sediments colonized by emergent macrophytes. Moreover, the present outcomes may have multiple implications for whole-system metabolism estimations from which largely depend our understanding of biogeochemical dynamics in inland waters that have strong connections to worldwide issues, such as nitrate contamination and greenhouse gas emissions.