Apparent Quantum Yield Of Chromophoric Dissolved Organic Matter Photobleaching In The Yangtze River Estuary

Dissolved organic matter (DOM) in natural waters is an important active organic carbon pool on the earth. Chromophoric dissolved organic matter (CDOM) is the light-ab-sorbing component of DOM whose photodegradation (including photobleaching) plays a key role in aquatic optics and carbon and contaminants cycling. CDOM experiences intense physical, chemical, and biological transformations in land-ocean transitional zones (e.g.estuaries) where freshwater and seawater mixes.The estuary of the Yangtze River is China's largest estuary with both estuarine and continental shelf characteristics. Using water samples collected from the Yangtze River Estuary (YRE) and its adjacent waters and prepared from commercially available humic(HA) and fulvic (FA) acids, this study investigated the spatial and seasonal variations in the apparent quantum yield of CDOM photobleaching (AQY) in the YRE, the relationship of the AQY with salinity,the absorption coefficient of CDOM (aCODOM,330),the concentration of dissolved organic carbon (DOC), and the specific absorbance ratio (SUVA254) in the YRE,the effect of dilution on the AQY of HA and FA,the effect of mixing of sea water on the AQY. The key results obtained from this study are summarized as follows.In both summer and winter, the photobleaching AQY decreased from the land to the sea, showing positive correlations with aCDOM,330, DOC and SUVA254, and a negative correlation with salinity.Dilution decreased the AQY of HA and FA. The relationship of the AQY with the dilution factor can be described by the Michaelis-Menten kinetic curve, showing first-order character at high dilution factors (i.e. low concentrations) and approaching saturation at low dilution factors (i.e. high concentrations). The dilution effect increased with decreasing wavelength for both HA and FA and overall was stronger for FA than for HA.Mixing of HA and FA with the shelf seawater (salinity: 31.3) increased the AQY in comparison with those of the original seawater and humic substances, showing nonconser-vative behavior with respect to aCDOM,330. Mixing of the water in the head of the YRE(salinity: 0.1) with the shelf seawater (salinity: 31.3) led to a linear increase in the AQY with increasing aCDOM,330 (and decreasing salinity), manifesting conservative mixing behavior.This study demonstrates that the AQY in Estuaries is likely controlled by various factors such as the chemical property of CDOM (terrigenous vs. marine; fresh vs. old),mixing and dilution of sea water (DOM concentration, ionic strength, pH and salinity), and temperature (seasonal and spatial).