Increases in the longwave photobleaching of chromophoric dissolved organic matter in coastal waters

Abstract

Salinity effects on the photobleaching of chromophoric dissolved organic matter (CDOM) due to coastal mixing were investigated through a comparative study of surrogate and surface-water CDOM. Suwannee River humic acid (SRHA) and ultrafiltered river dissolved organic matter (UDOM) added to mixtures of river and seawater permeates (,1 kDa) that varied in salinity from 0 to 33 to mimic coastal mixing. Surface-water CDOM was collected from the Chesapeake Bay in January, June, and September 2002. Shortwave CDOM absorption loss (e.g., 280 nm) did not change with salinity; however, longwave CDOM absorption loss (e.g., 440 nm) often decreased by 10% to 40% with salinity. Apparent quantum yields for average absorption loss from 280 to 550 nm (Q avg ) increased with salinity for both surrogate and surface-water CDOM, providing evidence for an effect of salinity independent of light absorption among different samples. Further, hydrogen peroxide photoproduction from UDOM increased from 15 to 368 nmol L 21 h 21 with salinity, even though pH values were circumneutral. A kinetic model demonstrated that, at circumneutral pH and iron concentrations expected for the Chesapeake Bay, photo-Fenton chemistry could not explain the increase in hydrogen peroxide production quantum yields (Q hp ) with salinity. Using Q avg for the SRHA and UDOM surrogates, a model of the change in surface-water CDOM photoreactivity in the Chesapeake Bay as a function of salinity suggested additional CDOM inputs for the lower Chesapeake Bay. Because estuarine mixing increases photobleaching of longwave CDOM absorption, the modeling of absorption coefficients above 400 nm may underestimate dissolved organic matter in coastal waters.

Exposure of dissolved organic matter to UV-radiation increases bacterial growth efficiency in a clear-water Alpine stream and its adjacent groundwater

“…We also calculated the specific UV absorbance SUVA254 and SUVA300 (mg C L -1 m -1 ). The apparent quantum yield (AQY) was calculated for each treatment and experiment as a proxy for photoreactivity (Osburn et al 2009). The AQY is defined as the average decrease in the absorption coefficient over the 280-550 nm region divided by the total mol photons absorbed per m 2 .…”

“…The AQY is defined as the average decrease in the absorption coefficient over the 280-550 nm region divided by the total mol photons absorbed per m 2 . The average decrease in absorption was calculated according to Osburn et al (2009) as…”

Spatio-seasonal variability of chromophoric dissolved organic matter absorption and responses to photobleaching in a large shallow temperate lake

“…Dilution processes alone cannot explain the loss of DOC; therefore, it must also be due to inlake transformation. The processing and transformation of DOC by photobleaching not only influences carbon cycling, but it also is accompanied by an increase in the transparency of the water column (Osburn et al, 2009) and changes in the optical properties (Yamashita et al, 2013) that have wider implications for the underwater light climate and primary production.…”

Variability of carbon monoxide and carbon dioxide apparent quantum yield spectra in three coastal estuaries of the South Atlantic Bight

“…Despite the continuous input of CDOM to the coastal ocean via continental sources, ocean water as a whole is not highly coloured, due largely to photochemical transformations that are known to alter the absorptive characteristics of coloured components within the DOC pool (Andrews et al, 2000;Del Vecchio and Blough, 2002;Goldstone et al, 4280 H. E. Reader and W. L. Miller: Variability of CO and CO 2 apparent quantum yield spectra 2004; Osburn et al, 2009). The absorption of solar radiation by CDOM can lead to a whole host of chemical reactions, including formation of reactive oxygen species such as hydroxyl radical (OH q ) and superoxide (O − 2 ) (Micinski et al, 1993;Moffett and Zafiriou, 1993;Zika et al, 1985;Blough and Zepp, 1995), breakdown of large organic molecules into lower molecular weight carbon compounds (Wetzel et al, 1995;Kieber et al, 1989Kieber et al, , 1990, alteration of the redox state of biologically important metals (White et al, 2003;Barbeau, 2006;Barbeau et al, 2003), and formation of oxidized inorganic carbon species, such as CO and CO 2 (measured as DIC) (Clark et al, 2004;Johannessen and Miller, 2001;Miller and Zepp, 1995;White et al, 2010;Ziolkowski and Miller, 2007;Wang et al, 2009;Xie et al, 2004;Zafiriou et al, 2003).…”