The apparent quantum yield matrix (AQY-M) of CDOM photobleaching in estuarine, coastal, and oceanic surface waters

Xiaohui Zhu; Matthew W Weiser; Joshua P Harringmeyer; Karl Kaiser; Brett D Walker; Simon Bélanger; Chloe H Anderson; Cédric G Fichot

doi:10.1016/j.scitotenv.2023.168670

The apparent quantum yield matrix (AQY-M) of CDOM photobleaching in estuarine, coastal, and oceanic surface waters

Sci Total Environ. 2024 Feb 20:912:168670. doi: 10.1016/j.scitotenv.2023.168670. Epub 2023 Nov 22.

Authors

Xiaohui Zhu¹, Matthew W Weiser², Joshua P Harringmeyer², Karl Kaiser³, Brett D Walker⁴, Simon Bélanger⁵, Chloe H Anderson⁶, Cédric G Fichot²

Affiliations

¹ Department of Earth and Environment, Boston University, Boston, MA 02215, USA. Electronic address: zhuxh@bu.edu.
² Department of Earth and Environment, Boston University, Boston, MA 02215, USA.
³ Department of Marine and Coastal Environmental Science, Texas A&M University, Galveston Campus, Galveston, TX, USA.
⁴ Department of Earth and Environmental Sciences, University of Ottawa, Ottawa, ON, Canada.
⁵ Department of Biology, Chemistry and Geography, BOREAS, Université du Québec à Rimouski, Rimouski, Québec G5L 3A1, Canada.
⁶ MARUM Center for Marine Environmental Sciences, University of Bremen, Leobener Str. 8, 28359 Bremen, Germany; Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA.

PMID: 37996032
DOI: 10.1016/j.scitotenv.2023.168670

Abstract

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, S₂₇₅_-₂₉₅), 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) S₂₇₅_-₂₉₅, 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.

Keywords: AQY; Apparent quantum yields; CDOM; Photobleaching.