A Multicomponent Model of Chromophoric Dissolved Organic Matter Photobleaching¶§

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

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.

Impacts of Polar Changes on the UV-induced Mineralization of Terrigenous Dissolved Organic Matter

Local climates in the Northern and Southern Hemisphere are influenced by Arctic Amplification and by interactions of the Antarctic ozone hole with climate change, respectively. Polar changes may affect hydroclimatic conditions in temperate regions, for example, by increasing the length and intensity of precipitation events at Northern Hemisphere midlatitudes. Additionally, global warming has led to the thawing of ancient permafrost soils, particularly in Arctic regions, due to Arctic Amplification. Both heavy precipitation events and thawing of permafrost are increasing the net transfer of terrestrially derived dissolved organic matter (DOM) from land to surface waters. In aquatic ecosystems, UV-induced oxidation of terrigenous DOM (tDOM) produces atmospheric CO2 and this process is one of several mechanisms by which natural organic matter in aquatic and soil environments may play an important role in climate feedbacks. The Arctic is particularly affected by these processes: for example, melting of Arctic sea ice allows solar UV radiation to penetrate into the ice-free Arctic Ocean and to cause photochemical reactions that result in bleaching and mineralization of tDOM. Open questions, in addition to those shown in the Graphical Abstract, remain regarding the resulting contributions of tDOM photomineralization to CO2 production and global warming.