yasmina ourradi

Shelf seas are important for global carbon cycling, but their carbonate system dynamics remain poorly understood due to complex spatial and temporal variability driven by interacting biological, physical and hydrological processes. To understand these complex dynamics, we focused on the Marsdiep channel at the Wadden Sea-North Sea interface, where strong tidal exchange creates ideal conditions for investigating carbonate system variability across multiple timescales. High-resolution (10-min) measurements of pH, combined with discrete sampling of dissolved inorganic carbon (DIC), total alkalinity (TA) and pH, were conducted over approximately one year between February 2022 and January 2023 at this location. We developed a multi-linear regression (MLR) model based on salinity and tidal data to predict TA (TApred) (RMSD = 17.5 µmol kg⁻¹), and subsequently calculated DIC from the TApred and pH (RMSD = 19.8 ± 1.9 µmol kg⁻¹). To evaluate the performance and long-term stability of the pH sensor, we applied a semivariogram approach - an approach not commonly used in this context - to estimate pH sensor uncertainty and drift patterns over time. We propose this method as a robust approach for assessing sensors performance in short and long-term deployments at sea, particularly when calibration sampling frequency is irregular. Results revealed pronounced diel and seasonal variability in pH (seasonal range: 0.6), DIC (419 µmol kg⁻¹), and TA (213.7 µmol kg⁻¹). These fluctuations reflected the interplay of biological and hydrological processes, with pH mainly controlled by biological process, TA by hydrological processes, and DIC influenced by the combination of both. We used the Wasserstein distance to quantify the balance of processes driving DIC at any given time. During this study period, the Wadden Sea acted as a net CO2 source to the atmosphere, with an annual release of 4.7 g-C m−2. A deeper understanding of the influence of biological and hydrological controls on the marine carbonate system is still needed to unravel the relative importance of the processes involved, especially in regions of higher complexities such as the Wadden Sea. Continuous high-frequency measurements may provide a tool to capture these dynamics across multiple time scales, from hourly to seasonal and interannual, in order to refine our understanding of their role in driving the carbonate system and in regional carbon cycling under changing climatic and hydrological conditions.

陆架海对全球碳循环至关重要，但受生物、物理与水文过程相互作用驱动的复杂时空变异影响，其碳酸盐系统动力学机制仍未得到充分认知。为阐明此类复杂动力学机制，本研究聚焦瓦登海（Wadden Sea）与北海交界带的马斯迪普（Marsdiep）水道——该区域因强烈的潮汐交换，为多时间尺度下的碳酸盐系统变异研究提供了理想条件。本研究于2022年2月至2023年1月期间，在该站位开展了为期约1年的高频（10分钟间隔）pH原位监测，并同步采集溶解无机碳（dissolved inorganic carbon, DIC）、总碱度（total alkalinity, TA）及pH的离散水样。本研究基于盐度与潮汐数据构建了多元线性回归（multi-linear regression, MLR）模型以预测总碱度（记为TApred），其均方根偏差（root mean square deviation, RMSD）为17.5 µmol kg⁻¹；随后基于TApred与pH计算溶解无机碳，计算的均方根偏差为19.8 ± 1.9 µmol kg⁻¹。为评估pH传感器的性能与长期稳定性，本研究采用了半变异函数（semivariogram）方法——此类方法在该研究场景中并不常见——以估算pH传感器随时间变化的不确定性与漂移模式。本研究提出该方法可作为一种稳健的评估手段，用于近海短期与长期部署的传感器性能评估，尤其适用于校准采样频率不规律的场景。研究结果显示，pH（季节变化幅度达0.6）、DIC（419 µmol kg⁻¹）与TA（213.7 µmol kg⁻¹）均呈现显著的昼夜与季节变异。此类波动反映了生物与水文过程的相互作用：pH主要受生物过程调控，TA由水文过程主导，而DIC则受两者共同影响。本研究采用瓦瑟斯坦距离（Wasserstein distance）量化任意时刻驱动DIC变化的过程平衡关系。研究期间，瓦登海整体表现为大气CO₂的净源，年释放量达4.7 g-C m⁻²。仍需进一步深入解析生物与水文调控过程对海洋碳酸盐系统的影响，以阐明各相关过程的相对重要性，在瓦登海这类复杂程度较高的海域尤为如此。持续的高频监测手段可用于捕捉从小时、季节到年际的多时间尺度动力学特征，从而在气候变化与水文条件改变的背景下，深化我们对这些过程在驱动碳酸盐系统及区域碳循环中作用的认知。