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

Abstract

The photochemical oxidation of oceanic dissolved organic carbon (DOC) to carbon monoxide (CO) and carbon dioxide (CO<sub>2</sub>) has been estimated to be a significant process with global photoproduction transforming petagrams of DOC to inorganic carbon annually. To further quantify the importance of these two photoproducts in coastal DOC cycling, 38 paired apparent quantum yield (AQY) spectra for CO and CO<sub>2</sub> were determined at three locations along the coast of Georgia, USA over the course of one year. The AQY spectra for CO<sub>2</sub> were considerably more varied than CO. CO AQY spectra exhibited a seasonal shift in spectrally integrated (260 nm–490 nm) AQY from higher efficiencies in the fall to less efficient photoproduction in the summer. While full-spectrum photoproduction rates for both products showed positive correlation with pre-irradiation UV-B sample absorption (i.e. chromophoric dissolved organic matter, CDOM) as expected, we found no correlation between AQY and CDOM for either product at any site. Molecular size, approximated with pre-irradiation spectral slope coefficients, and aromatic content, approximated by the specific ultraviolet absorption of the pre-irradiated samples, were also not correlated with AQY in either data set. The ratios of CO<sub>2</sub> to CO photoproduction determined using both an AQY model and direct production comparisons were 23.2 ± 12.5 and 22.5 ± 9.0, respectively. Combined, both products represent a loss of 2.2 to 2.6 % of the DOC delivered to the estuaries and inner shelf of the South Atlantic Bight yearly, and 5 to 6 % of the total annual degassing of CO<sub>2</sub> to the atmosphere. This result suggests that direct photochemical production of CO and CO<sub>2</sub> is a small, yet significant contributor to both DOC cycling and CO<sub>2</sub> gas exchange in this coastal system

Marine Photochemistry of Organic Matter

“…They estimated that during their ~1-year sampling period, 40% of the tDOC was mineralized to CO 2 , mainly by bacterial mineralization of photoproduced labile DOM. In contrast, Reader and Miller (2012) estimated that abiotic, photochemical production of DIC and CO accounted for about 3% of the DOC delivered to the coastal waters of the South Atlantic Bight; however, this estimate was based on the residence time of DOC in a very confined area (inshore and estuarine) and did not apply to further oxidation during transit through the entire South Atlantic Bight. Furthermore, microbial mineralization of photoproduced labile DOM was not examined in that study.…”

“…For example, at 350 nm, AQYs for DIC photoproduction range from ~6 × 10 −5 mol DIC (mol quanta) -1 for inshore waters (Johannessen et al, 2007) to ~2 × 10 −3 mol DIC (mol quanta) -1 for DOM-rich coastal river water (Gao and Zepp, 1998;Osburn et al, 2009) and ~5 × 10 −3 mol DIC (mol quanta) −1 on the Mackenzie Shelf, Beaufort Sea (Osburn et al, 2009). Within this wide range, there is no apparent relation of AQYs to DOM type (e.g., fresh vs. inshore vs. coastal vs. open ocean) or to CDOM absorbance, temperature, or salinity (Reader and Miller, 2012;White et al, 2010). For example, in a salinity transect in the Delaware Estuary (from salinity 3 to 21), AQYs at 350 nm were remarkably constant, at about 1 × 10 −4 mol DIC (mol quanta) −1 (±~10% RSD), while corresponding CO AQYs decreased by a factor of about 1.6 with increasing salinity (from ~1.6 × 10 −5 mol CO (mol quanta) −1 to ~1.0 × 10 −5 mol CO (mol quanta) −1 ), suggesting that DIC and CO are formed by different photochemical mechanisms (White et al, 2010).…”

“…This variability appears to be due to differences in DOM/CDOM type, e.g., terrestrial vs. microbial, prior photobleaching history and irradiation wavelength (Boyle et al, 2009;Del Vecchio and Blough, 2004;Golanoski et al, 2012;Ma et al, 2010;Reader and Miller, 2012;Stubbins et al, 2011;White et al, 2010; see discussion of CDOM sources in Chapter 10). Similar variability was also observed for DIC photoproduction by broadband irradiations (Reader and Miller, 2012;White et al, 2010). Despite this variability, using this approximate relation between DIC and CO photoproduction, coupled with the relative ease in measuring the latter, global DIC photoproduction has been estimated from CO photoproduction Stubbins et al, 2006a;Wang et al, 2009); see Section V for further discussion.…”

“…The AQYs for DIC photoproduction are about 4-30 (or more) times greater than for CO photoproduction (White et al, 2010), with a mean of about 23 for coastal waters (Reader and Miller, 2012). This variability appears to be due to differences in DOM/CDOM type, e.g., terrestrial vs. microbial, prior photobleaching history and irradiation wavelength (Boyle et al, 2009;Del Vecchio and Blough, 2004;Golanoski et al, 2012;Ma et al, 2010;Reader and Miller, 2012;Stubbins et al, 2011;White et al, 2010; see discussion of CDOM sources in Chapter 10).…”

“…The fundamental equations used to describe direct and indirect photochemical reactions, and their application to aquatic systems, are described elsewhere (e.g., Leifer, 1988) and will not be detailed here. In addition, evaluation of errors associated with AQY determinations and with the assumption that AQYs are constant with photon exposure (i.e., irradiance integrated over time of exposure) are discussed in detail elsewhere (Neale and Kieber, 2000;Reader and Miller, 2012). Understanding and evaluating these errors and assumptions are key to accurately modeling photochemical processes in seawater on a global scale (Fichot and Miller, 2010;Powers and Miller, 2014; see Section V in this chapter).…”