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

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）已被证实为一种高效可靠的非侵入式测量手段，可用于沉积物-水界面的氧气通量测定。该方法的核心原理为：在同一采样体积内，高频采集得到的湍流速度与氧气浓度脉动之间的相关性。氧气浓度通常通过快速响应电化学微传感器（electrochemical microsensors）进行测定，但这类传感器自身会消耗氧气，因此对探针周围扩散边界层的变化极为敏感，进而响应环境流速的波动。微传感器的所谓“搅拌灵敏度”，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的准确测定。 为评估该人工通量，我们在无任何氧汇与氧源的密封环形水槽中，开展了湍流流速与氧气微传感器信号相关性的测量实验。实验结果表明，即便针对搅拌灵敏度设计值约为0.7%的低灵敏度传感器，二者仍存在显著相关性。人工通量随环境流动条件动态变化，且与直觉相悖的是，在更高流速下，由于湍流速度脉动的非线性贡献，人工通量反而会升高。在弱湍流至极强湍流的工况下，测得的人工通量范围为2~70毫摩尔每平方米每天。此外，搅拌灵敏度还与传感器相对于流向的朝向有关。 本研究同时测试了本应不具备搅拌灵敏度的光学微传感器（光极，optodes），结果未发现氧气信号与湍流流速之间存在任何显著相关性。综上，采用电化学传感器获取的EC数据可能受到人工通量的干扰，因此我们建议在未来的EC相关研究中使用光学微传感器。[…]