Tower-based conditional sampling for measuring ecosystem-scale carbon dioxide exchange in coastal wetlands
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Long-term measurements of CO2 exchange between coastal wetlands and the atmosphere are necessary to improve our understanding of the role these ecosystems play in the global carbon cycle, and the response of these systems to environmental change. We conducted research to adapt and evaluate tower-based conditional sampling as a method for measuring net CO2 exchange (NCE) at the ecosystem scale on a continuous basis. With conditional sampling, NCE is determined from the product of the standard deviation of vertical wind velocity, the difference in CO2 concentration between updrafts and downdrafts in the constant flux portion of the boundary layer above the surface, and an empirical coefficient. We constructed a system that used a sonic anemometer to measure vertical wind velocity (w) and control a high-speed three-way valve that diverted air from updrafts and downdrafts into separate sample lines, depending on the direction of w. An infrared gas analyzer was used to measure the concentration difference. The conditional sampling system was installed and tested in a marsh in the Nueces River Delta near Corpus Christi, Texas, as part of a long-term study of effects of freshwater inflow on CO2 flux. System accuracy was evaluated by comparing conditional sampling measurements of water vapor flux with independent estimates obtained with the Bowen ratio method. Average daily flux estimates for the two methods agreed to within 13%. Measurements showed that freshwater inflow due to flooding of the Nueces River increased NCE by increasing CO2 assimilation and decreasing CO2 efflux. Over a 65-d Period, daily NCE varied from a maximum gain of 0.16 tool CO2 m-2 d-1 during flooding to a maximum loss of -0.14 mol CO2 m-2 d-1 when the marsh dried. Our study showed that conditional sampling was well suited for quantifying CO2 exchange in coastal wetlands on a diel, daily, and seasonal basis.
