Visible radiation-driven photomineralization and photoproduction of dissolved organic matter in a large estuary: implications for coastal ocean biogeochemical cycles

Abstract

Photochemistry can convert dissolved organic matter (DOM) to inorganics (mainly CO₂) and “new” DOM, hence impacting aquatic carbon cycling. The apparent quantum yields (AQYs) of these photoprocesses usually decrease with increasing wavelength. This study reports exceptions to this paradigm and discusses the biogeochemical implications of this phenomenon. Specifically, we determined the broadband AQYs over ultraviolet-B (UVB), ultraviolet-A (UVA), and visible (VIS) radiations for photomineralization of dissolved organic carbon (DOC) and photobleaching of chromophoric and florescent DOM (CDOM, FDOM) in the freshwater, brackish water, and seawater samples from the Pearl River estuary. UVB-broadband AQYs of DOC photomineralization (AQY_DOC) and of CDOM and humic-like FDOM photoleaching were considerably higher than their UVA and VIS counterparts for all three water samples. Surprisingly, the broadband AQY_DOC over VIS was significantly higher than that over UVA for the brackish water and seawater samples, contrary to the above-mentioned paradigm. Moreover, exposure of all three water samples to VIS produced protein-like FDOM, while significant losses of this FDOM pool occurred in the presence of UV. Per depth-integrated contributions in the water column, UVB or UVA primarily controlled CDOM and FDOM photobleaching, while VIS dominated DOC photomineralization and protein-like FDOM formation. These results suggest photochemistry may cause CO₂ and biolabile DOM accumulations in VIS-dominated sunlit waters below surface mixed layers, contributing to coastal ocean DOM biogeochemical cycling, acidification, and deoxygenation. This shall be a self-intensified process since UV-driven CDOM photobleaching in surface layers reinforces and extends further deeper the VIS-induced subsurface CO₂ and biolabile DOM photoproduction.