Artificial oxygen fluxes measured by the eddy correlation method using stirring-sensitive oxygen microsensor and oxygen optodes in a flume experiment

In the last decade, the aquatic eddy correlation (EC) technique has proven to be a powerful approach for non-invasive measurements of oxygen fluxes across the sediment water interface. Fundamental to the EC approach is the correlation of turbulent velocity and oxygen concentration fluctuations measured with high frequencies in the same sampling volume. Oxygen concentrations are commonly measured with fast responding electrochemical microsensors. However, due to their own oxygen consumption, electrochemical microsensors are sensitive to changes of the diffusive boundary layer surrounding the probe and thus to changes in the ambient flow velocity. The so-called stirring sensitivity of microsensors constitutes an inherent correlation of flow velocity and oxygen sensing and thus an artificial flux which can confound the benthic flux determination. To assess the artificial flux we measured the correlation between the turbulent flow velocity and the signal of oxygen microsensors in a sealed annular flume without any oxygen sinks and sources. Experiments revealed significant correlations, even for sensors designed to have low stirring sensitivities of ~0.7%. The artificial fluxes depended on ambient flow conditions and, counter intuitively, increased at higher velocities because of the nonlinear contribution of turbulent velocity fluctuations. The measured artificial fluxes ranged from 2 - 70 mmol m**-2 d**-1 for weak and very strong turbulent flow, respectively. Further, the stirring sensitivity depended on the sensor orientation towards the flow. Optical microsensors (optodes) that should not exhibit a stirring sensitivity were tested in parallel and did not show any significant correlation between O2 signals and turbulent flow. In conclusion, EC data obtained with electrochemical sensors can be affected by artificial flux and we recommend using optical microsensors in future EC-studies. […]

近十年来，水生涡旋相关法（aquatic eddy correlation，EC）已被证明是一种用于无创测量沉积物-水界面氧气通量的高效研究手段。该方法的核心是对同一采样体积内高频测得的湍流速度与氧气浓度脉动进行相关分析。当前氧气浓度通常通过响应速度优异的电化学微传感器（electrochemical microsensors）进行测量。然而，由于这类传感器自身会消耗氧气，其对探针周围扩散边界层的变化十分敏感，进而会受到周围水流流速变化的影响。微传感器的所谓搅拌灵敏度（stirring sensitivity）本质上是流速与氧气传感信号之间的固有相关性，由此产生的人工通量会干扰底栖通量（benthic flux）的测定。为评估该人工通量，我们在一个无任何氧气源与汇的密封环形水槽中，测量了湍流流速与氧气微传感器信号之间的相关性。实验结果显示，即便针对设计时搅拌灵敏度约为0.7%的低灵敏度传感器，二者间仍存在显著相关。人工通量受周围水流条件影响，且与直觉相悖的是，由于湍流速度脉动的非线性贡献，其在更高流速下反而会升高。针对弱湍流与极强湍流工况，测得的人工通量范围分别为2~70 mmol·m⁻²·d⁻¹。此外，搅拌灵敏度还与传感器朝向水流的方位有关。研究同时对本应不具备搅拌灵敏度的光学微传感器（光极，optodes）进行了平行测试，结果未发现氧气信号与湍流流速间存在任何显著相关性。综上，采用电化学传感器获取的EC数据可能会受到人工通量的影响，我们建议在未来的EC相关研究中使用光学微传感器。……