Application of conditional sampling for measuring ecosystem-scale carbon dioxide exchange in coastal wetlands

Abstract

Carbon dioxide exchange rate (CER) is an indicator of an ecosystem's response to changing environmental conditions. Long-term measurements of CO₂ exchange between coastal wetlands and the atmosphere will improve our understanding of daily and seasonal carbon cycles in these ecosystems as well as the response of these systems to environmental change. One convenient method for continuously measuring CER in remote ecosystems is tower-based conditional sampling. With conditional sampling, CER is determined from the product of the standard deviation of vertical windspeed, the difference in CO[] concentration between updrafts and downdraft, and an empirical relaxation coefficient. Tower-based measurements integrate all sources and sinks of CO₂ that vary with time and space, yielding an ecosystem scale measurement. A conditional sampling system was constructed using a sonic anemometer to measure fluctuations in vertical windspeed, and to trigger the opening and closing of a three-way valve. The three-way valve separated air samples, taken continuously from the acoustic path of the sonic anemometer, into two pathways corresponding to updrafts and downdraft. An infrared gas analyzer was used to determine the difference in CO₂ concentration between the two samples, from which CER was determined. Measurements were also made of many environmental variables which could affect CER. The conditional sampling system was deployed and tested in an estuarine marsh in the upper Nueces River Delta near Corpus Christi, TX. Many environmental factors were found to abed CER the most Important or which are freshwater availability, and photosynthetic photon flux density (PPFD). Due to high salinity and pH in the upper Nueces River Delta, freshwater inflow increases photosynthesis of emergent plants. The presence of water also ads as a barrier to the diffusion of CO₂ from the soil, decreasing net efflux. In the presence of sufficient moisture, availability of PPFD becomes an important factor which governs photosynthesis. Measurements indicate that long term tower-based CO₂ flux measurements are a viable means of evaluating ecosystem scale CO₂ exchange on a diurnal, daily, and seasonal basis.