Continuous measurement of air-water gas exchange by underwater eddy covariance
Continuous measurement of air-water gas exchange by underwater eddy covariance
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DOI：
10.5194/bg-14-5595-2017
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发表时间：
2017-12-11
期刊：
影响因子：
4.9
通讯作者：
Pace, Michael L.
中科院分区：
文献类型：
--
作者：
Berg, Peter;Pace, Michael L.
气体交换，如O-2，CO2和CH 4，在空气-水界面上是水生生态系统研究的一个重要组成部分，但交换率通常测量或估计有很大的不确定性。这降低了与初级生产、呼吸和温室气体排放等气体交换相关的共同生态系统评估的准确性。在这里，我们使用的水生涡度相关技术-最初开发的海底O-2通量测量-下面的空气-水界面（类似于4厘米），以确定气体交换率和系数。使用声学多普勒测速仪和安装在浮动平台上的快速响应的双O-2温度传感器的3-D水的速度，O-2浓度和温度进行了测量，在高速（64 Hz）。通过结合这些数据，并通过空气-水界面的O-2和热量的并发垂直通量推导，并从前者计算的气体交换系数。在不同河流站点的概念验证部署给出了在公布值范围内的标准气体交换系数（k（600））。一个40小时长的部署揭示了一个独特的昼夜模式，在空气-水交换的O-2，这是控制主要是由物理过程（例如，日变化的空气温度和相关的空气-水热通量），而不是由生物活动（初级生产和呼吸）。这种气体交换的物理控制在LOTIC系统中可能很普遍，并增加了基于测量的水柱O-2浓度变化的生物活性评估的不确定性。例如，在40小时的部署中，有几乎恒定的河流流量和微不足道的风两个主要驱动因素，但我们发现气体交换系数变化了几倍。这可能是由于地表水中垂直温度-密度梯度的形成和侵蚀造成的，该梯度是由流入或流出河流的热通量驱动的，影响了湍流混合。这种效应在广泛使用的气体交换系数的经验相关性中没有考虑，并且是气体交换估计中不确定性的另一个来源。水涡度相关技术可以在一个无与伦比的详细程度上研究空气-水气体交换过程及其控制。与新方法相关的一个发现是，在空气-水界面处的热通量可以，相反，在底栖环境中通常发现的，是相当大的，需要使用高速并行温度测量的O-2传感器读数的校正。快速响应的O-2传感器对温度变化具有固有的敏感性，如果忽略这种校正，则与湍流热通量相关的温度波动将被错误地记录为O-2波动，并使O-2涡流通量计算产生偏差。
Exchange of gases, such as O-2, CO2, and CH4, over the air-water interface is an important component in aquatic ecosystem studies, but exchange rates are typically measured or estimated with substantial uncertainties. This diminishes the precision of common ecosystem assessments associated with gas exchanges such as primary production, respiration, and greenhouse gas emission. Here, we used the aquatic eddy covariance technique - originally developed for benthic O-2 flux measurements - right below the air-water interface (similar to 4 cm) to determine gas exchange rates and coefficients. Using an acoustic Doppler velocimeter and a fast-responding dual O-2-temperature sensor mounted on a floating platform the 3-D water velocity, O-2 concentration, and temperature were measured at high-speed (64 Hz). By combining these data, concurrent vertical fluxes of O-2 and heat across the air-water interface were derived, and gas exchange coefficients were calculated from the former. Proof-of-concept deployments at different river sites gave standard gas exchange coefficients (k(600)) in the range of published values. A 40 h long deployment revealed a distinct diurnal pattern in air-water exchange of O-2 that was controlled largely by physical processes (e.g., diurnal variations in air temperature and associated air-water heat fluxes) and not by biological activity (primary production and respiration). This physical control of gas exchange can be prevalent in lotic systems and adds uncertainty to assessments of biological activity that are based on measured water column O-2 concentration changes. For example, in the 40 h deployment, there was near-constant river flow and insignificant winds two main drivers of lotic gas exchange - but we found gas exchange coefficients that varied by several fold. This was presumably caused by the formation and erosion of vertical temperature-density gradients in the surface water driven by the heat flux into or out of the river that affected the turbulent mixing. This effect is unaccounted for in widely used empirical correlations for gas exchange coefficients and is another source of uncertainty in gas exchange estimates. The aquatic eddy covariance technique allows studies of air-water gas exchange processes and their controls at an unparalleled level of detail.A finding related to the new approach is that heat fluxes at the air-water interface can, contrary to those typically found in the benthic environment, be substantial and require correction of O-2 sensor readings using high-speed parallel temperature measurements. Fast-responding O-2 sensors are inherently sensitive to temperature changes, and if this correction is omitted, temperature fluctuations associated with the turbulent heat flux will mistakenly be recorded as O-2 fluctuations and bias the O-2 eddy flux calculation.