Carbon loss and optical property changes during long‐term photochemical and biological degradation of estuarine dissolved organic matter

Abstract

Terrestrially derived dissolved organic matter (DOM) impacts the optical properties of coastal seawater and affects carbon cycling on a global scale. We studied sequential long-term photochemical and biological degradation of estuarine dissolved organic matter from the Satilla River, an estuary in the southeastern United States that is dominated by vascular plant-derived organic matter. During photodegradation, dissolved organic carbon (DOC) loss (amounting to 31% of the initial DOC) was much less extensive than colored dissolved organic matter (CDOM) or fluorescent dissolved organic matter (FDOM) loss (50% and 56% of the initial CDOM and FDOM), and analysis of kinetics suggested a reservoir of DOC that was resistant to photodegradation. In contrast, CDOM photodegradation closely followed first-order kinetics over two half-lives with no indication of a nondegradable component. FDOM loss was slightly biased toward fluorophores considered representative of terrestrial humic substances. Additional changes in optical properties included increases in spectral slope and shifts in fluorescence excitation/ emission maxima that were generally consistent with previous observations from field studies of photobleached DOM. Biological degradation of photobleached DOM was more rapid than that of unbleached material, and this net positive effect was evident even for extensively photodegraded material. Bacterial degradation caused shifts in the opposite direction from photochemical degradation for both spectral slope and excitation/emission maxima and thus dampened but did not eliminate changes in optical properties caused by photobleaching.Photochemical processes that induce changes in natural dissolved organic matter (DOM) can influence many aspects of carbon cycling in marine environments. The sunlight-mediated conversion of DOM into inorganic forms (dissolved inorganic carbon, carbon monoxide) and into bacterial substrates (low-molecular-weight carbonyl compounds and others) increases turnover rates of DOM in surface waters (Bushaw et al. 1996;Miller and Moran 1997;Moran and Zepp 1997). The loss of color (i.e., photobleaching) that accompanies photochemical modification of DOM affects the optical properties of seawater and influences penetration of ultraviolet and photosynthetically active wavelengths (Vodacek et al. 1997). Changes in light penetration depth subsequently affect exposure of marine microorganisms to sunlight and can modify the activities of autotrophs and heterotrophs in both positive and negative ways (Herndl et al. 1997;Jeffrey et al. 1999;Moran and Zepp 2000). Photoreactions of DOM therefore play key roles in oceanic carbon cycling, potentially controlling the degradation of terrestrially derived DOM in estuaries and shallow continental shelf systems (Miller and Moran 1997;Vodacek et al. 1997) and driving the turnover of marine-derived DOM in the surface ocean (Opsahl and Benner 1998;Cherrier et al. 1999).Several models have been used to predict the effects of Acknowledgments We thank L. R. Pomeroy...

Photodegraded dissolved organic matter from peak freshet river discharge as a substrate for bacterial production in a lake-rich great Arctic delta

“…Consistent with other studies (Moran et al 2000;Vahatalo and Wetzel 2004;Spencer et al 2009), these C losses via mineralization were outpaced by losses of chromophoric DOM, as reflected by a greater cumulative decrease in a350 (−62%) throughout the incubation period as compared to either DOC or Σ 8 .…”

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

“…Because both CDOM absorption and solar irradiance changed dramatically from 280 to 550 nm, we integrated Da over that waveband to calculate an averaged apparent quantum yield for photobleaching, Q avg (Moran et al 2000;Osburn et al 2001;Tzortziou et al 2007). The computation of Q avg provided equivalent units in which to compare photobleaching results that might be biased by variations in optical thickness (Hu et al 2002).…”

Relationship between the seasonal change in fluorescent dissolved organic matter and mixed layer depth in the subtropical western North Pacific

“…[33] The depletion of FDOM M due to photodegradation in the surface layer, however, probably does not imply a substantial loss of organic carbon by direct mineralization to inorganic carbon such as carbon monoxide and carbon dioxide [Kieber et al, 1989;Mopper et al, 1991;Moran and Zepp, 1997;Moran et al, 2000]. The photodegradation of the fluorophore (photobleaching) of FDOM M actually means that DOM which loses fluorescent properties is not categorized as FDOM M .…”