Zhu Xiaohui, Weiser Matthew W, Harringmeyer Joshua P, Kaiser Karl, Walker Brett D, Bélanger Simon, Anderson Chloe H, Fichot Cédric G
Department of Earth and Environment, Boston University, Boston, MA 02215, USA.
Department of Earth and Environment, Boston University, Boston, MA 02215, USA.
Sci Total Environ. 2024 Feb 20;912:168670. doi: 10.1016/j.scitotenv.2023.168670. Epub 2023 Nov 22.
The photochemical degradation of chromophoric dissolved organic matter (CDOM) upon solar exposure, known as photobleaching, can significantly alter the optical properties of the surface ocean. By leading to the breakdown of UV- and visible-radiation-absorbing moieties within dissolved organic matter, photobleaching regulates solar heating, the vertical distribution of photochemical processes, and UV exposure and light availability to the biota in surface waters. Despite its biogeochemical and ecological relevance, this sink of CDOM remains poorly quantified. Efforts to quantify photobleaching globally have long been hampered by the inherent challenge of determining representative apparent quantum yields (AQYs) for this process, and by the resulting lack of understanding of their variability in natural waters. Measuring photobleaching AQY is made challenging by the need to determine AQY matrices (AQY-M) that capture the dual spectral dependency of this process (i.e., magnitude varies with both excitation wavelength and response wavelength). A new experimental approach now greatly facilitates the quantification of AQY-M for natural waters, and can help address this problem. Here, we conducted controlled photochemical experiments and applied this new approach to determine the AQY-M of 27 contrasting water samples collected globally along the land-ocean aquatic continuum (i.e., rivers, estuaries, coastal ocean, and open ocean). The experiments and analyses revealed considerable variability in the magnitude and spectral characteristics of the AQY-M among samples, with strong dependencies on CDOM composition/origin (as indicated by the CDOM 275-295-nm spectral slope coefficient, S), solar exposure duration, and water temperature. The experimental data facilitated the development and validation of a statistical model capable of accurately predicting the AQY-M from three simple predictor variables: 1) S, 2) water temperature, and 3) a standardized measure of solar exposure. The model will help constrain the variability of the AQY-M when modeling photobleaching rates on regional and global scales.
发色溶解有机物（CDOM）在太阳照射下发生的光化学降解，即光漂白，会显著改变海洋表层的光学性质。通过导致溶解有机物中吸收紫外线和可见光辐射的部分发生分解，光漂白调节着太阳加热、光化学过程的垂直分布以及表层水体中生物群落所受的紫外线照射和光照情况。尽管其具有生物地球化学和生态相关性，但这种CDOM汇的量化程度仍然很低。长期以来，全球范围内量化光漂白的努力一直受到确定该过程代表性表观量子产率（AQY）这一固有挑战的阻碍，以及由此导致的对其在天然水体中变异性缺乏了解的影响。测量光漂白AQY具有挑战性，因为需要确定能够捕捉该过程双重光谱依赖性的AQY矩阵（AQY-M）（即其大小随激发波长和响应波长而变化）。一种新的实验方法现在极大地促进了天然水体中AQY-M的量化，并有助于解决这个问题。在这里，我们进行了受控光化学实验，并应用这种新方法来确定沿陆地-海洋水生连续体（即河流、河口、沿海海洋和开阔海洋）全球采集的27个对比水样的AQY-M。实验和分析揭示了样品之间AQY-M大小和光谱特征的显著变异性，强烈依赖于CDOM组成/来源（由CDOM 275 - 295纳米光谱斜率系数S表示）、太阳照射持续时间和水温。实验数据有助于开发和验证一个统计模型，该模型能够从三个简单的预测变量准确预测AQY-M：1）S，2）水温，以及3）太阳照射的标准化度量。该模型将有助于在区域和全球尺度上对光漂白速率进行建模时限制AQY-M的变异性。
