Quantifying chromophoric dissolved organic matter (CDOM) photobleaching in the global surface ocean
Embargo Date
2026-02-06
OA Version
Citation
Abstract
The photobleaching of chromophoric dissolved organic matter (CDOM) is considered an important loss process for CDOM absorption in sunlit natural waters, where it can regulate the biota’s exposure to sunlight, surface solar heating, and dissolved organic matter (DOC) dynamics. Understanding the global significance of CDOM photobleaching is essential for assessing its impact on the light field of the surface ocean, related biogeochemical processes, and climate dynamics. Despite its importance, this sink remains poorly quantified, primarily due to the difficulty of: 1. determining photobleaching apparent quantum yields (AQY) that capture the dual spectral dependency of this process and are applicable to polychromatic sunlight; 2. constraining the variability of AQY across the land-ocean aquatic continuum under diverse environmental conditions. This dissertation seeks to quantify the global role of photobleaching as a sink of CDOM.Chapter 1 presents a simple method to determine CDOM photobleaching AQY matrix (AQY-M) for natural water samples that does not require any a-priori assumptions about the spectral dependency of photobleaching. It combines controlled irradiation experiments, a partial-least-square regression (PLSR) and an optimization procedure to produce AQY matrices that are spectrally coherent and optimized for modeling accurate photobleaching rates in natural waters. Water temperature and the solar-exposure history of CDOM had a major influence on the magnitude and spectral characteristics of the AQY-M. These factors should be considered when determining the AQY-M of samples, and provide constraints when modeling photobleaching rates in natural waters.
Chapter 2 builds a model to constrain the natural variability of CDOM photobleaching AQY-M in natural waters. This model was developed using AQY-Ms determined for a set of water samples (n=27) collected from surface waters along the land-ocean aquatic continuum (range of salinity: 0-35.79 PSU). The analysis investigated the dependencies of the measured AQY-M on CDOM composition, water temperature, and exposure duration. The results demonstrated a strong relationship between the magnitude and spectral characteristics of the AQY-M and the CDOM composition and solar exposure duration. Terrigenous CDOM exhibited a higher overall magnitude of AQY-M compared to oceanic CDOM. Additionally, water temperature was found to influence the extent of photobleaching. The model used principle components (PCs) to re-present the spectral characteristics in the AQY-M, and used a series of linear regressions to describe PCs’ dependencies on these variables. The AQY-M prediction model perform well in the independent cross-validation with training samples (n=27) and in the validation with extra samples (n=12), showing promise for global applicability in estimating photobleaching AQY-M.
Chapter 3 expands the investigation to a global scale, aiming to develop a comprehensive understanding of CDOM photobleaching significant, seasonal dynamics and sensitivities to climate change in the surface ocean. The chapter combines remote sensing and modeling approaches to create a global climatology of CDOM photobleaching rates. The chapter explores the seasonal variability (high in the summer and low in the winter) of photobleaching rates, calculates the annual global sink (~1.5% to 6.5%) of CDOM due to photobleaching, and examines the sensitivity (increase ~2% to 6% due to sea surface temperature increase in this century) of this process to climate change-induced variations in ocean physical properties. The annual CDOM photobleaching turnover rate in the mixed layer (~1 to 6 times) varied seasonally and, with the highest rates observed near the equator (~25 times at 350 nm) in the open ocean.
Overall, this dissertation contributes to the field of CDOM photobleaching research by providing insights into the quantification of photobleaching rates, the natural variability of AQY-M in natural waters, and the global dynamics of CDOM photobleaching in the surface ocean. The methodologies and findings presented in these chapters enhance our understanding of CDOM photobleaching processes, their dependencies, and their implications for aquatic ecosystems and climate dynamics.
Description
2023
License
Attribution-NonCommercial-ShareAlike 4.0 International