Tower-based conditional sampling for measuring ecosystem-scale carbon dioxide exchange in coastal wetlands

Long-term measurements of CO2 exchange between coastal wetlands and the atm osphere are necessary to improve our understanding of the role these ecosys tems play in the global carbon cycle, and the response of these systems to environmental change. We conducted research to adapt and evaluate tower-bas ed 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 v elocity, the difference in CO2 concentration between updrafts and downdraft s in the constant flux portion of the boundary layer above the surface, and an empirical coefficient. We constructed a system that used a sonic anemom eter to measure vertical wind velocity (w) and control a high-speed three-w ay valve that diverted air from updrafts and down,drafts into separate samp le lines, depending on the direction of w. An infrared gas analyzer was use d to measure the concentration difference. The conditional sampling system was installed and tested in a marsh in the Nueces River Delta near Corpus C hristi, Texas, as part of a long-term study of effects of freshwater inflow on CO2 flux. System accuracy was evaluated by comparing conditional sampli ng measurements of water vapor flux with independent estimates obtained wit h the Bowen ratio method. Average daily flux estimates for the two methods agreed to within 13%. Measurements showed that freshwater inflow due to flo oding 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 mol 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 conditiona l sampling was well suited for quantifying CO2 exchange in coastal wetlands on a diel, daily, and seasonal basis.