Photobleaching as a factor controlling spectral characteristics of chromophoric dissolved organic matter in open ocean

Abstract

Abstract. Chromophoric dissolved organic matter (CDOM) ubiquitously occurs in marine environments and plays a significant role in the marine biogeochemical cycles. Basin scale distributions of CDOM have recently been surveyed in the global ocean and indicate that quantity and quality of oceanic CDOM are mainly controlled by in situ production and photobleaching. However, factors controlling the spectral parameters of CDOM in the UV region, i.e., spectral slope of CDOM determined at 275-295 nm (S 275−295 ) and the ratio of two spectral slope parameters (S R ); the ratio of S 275−295 to S 350−400 , have not been well documented. To evaluate the factor controlling the spectral characteristics of CDOM in the UV region in the open ocean, we determined the quantitative and qualitative characteristics of CDOM in the subarctic and subtropical surface waters (5-300 m) of the western North Pacific. Absorption coefficients at 320 nm in the subarctic region were higher than those in the subtropical region throughout surface waters, suggesting that magnitudes of photobleaching were different between the two regions. The values of S 275−295 and S R were also higher in the subtropical region than the subarctic region. The dark microbial incubation showed biodegradation of DOM little affected S 275−295 , but slightly decreased S R . On the other hand, increases in S 275−295 and relative stableness of S R were observed during photo-irradiation incubations respectively. These experimental results indicated that photobleaching of CDOM mainly induced qualitative differences in CDOM at UV region between the subarctic and subtropical surface waters. The results of this study imply that S 275−295 can be used as a tracer of photochemical history of CDOM in the open ocean.

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

“…However, for the autochthonous treatment, it decreased conspicuously until the third day (from 0.0087 to 0.0051 m −1 ) but then increased again from day 3 to day 7 until it almost returned to its original value (from 0.0051 to 0.0084 m −1 ). The increase in slope matches an increase in a CDOM (440) and has been considered by Helms et al (2008) to be a result of transformation from high to low molecular weight CDOM and is considered to be a response to photoinduced decomposition (Moran et al, 2000;Grzybowski, 2000;Yamashita et al, 2013). The initial decrease in slope during the early part of the experiment echoes observations by Yamashita et al (2013) and Fichot and Benner (2012), who attributed this phenomena to microbial degradation of bioavailable CDOM (Nelson et al, 2004).…”

“…The structural modifications in DOM and its coloured fractions that are in part conveyed through variation in S CDOM result from interplay between the input of allochthonous DOC from the catchment, the production of autochthonous DOC from the microbial digestion of phytoplankton cells and the rate at which these materials are degraded biologically and photochemically (Zhang et al, 2009b;Yamashita and Tanoue, 2004;Vantrepotte et al, 2007;Nelson et al, 1998;Yamashita et al, 2013). Newly produced autochthonous CDOM typically has a higher S CDOM coefficient compared to humic-rich allochthonous material (Bracchini et al, 2010).…”

“…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.…”

Bio-Optical Properties of Two Neigboring Coastal Regions of Tropical Northern Australia: The Van Diemen Gulf and Darwin Harbour

“…The underlying assumption that CDOM properties can be inferred from DOC concentrations (Shanmugam, 2011;Hestir et al, 2015) was, however, not verified in this dataset. Other recent studies also reported the complete lack of relationship between DOC and CDOM (e.g., Nelson and Siegel, 2013) because the controlling factors for DOC and CDOM were different (Yamashita et al, 2013).…”

Absorption and fluorescence properties of chromophoric dissolved organic matter of the eastern Bering Sea in the summer with special reference to the influence of a cold pool

“…The use of spectral slopes over narrow wavelength intervals, e.g., 275-295 nm (S or S 275−295 ) have been shown to provide information on C(DOM) source, photooxidative degradation, molecular size distribution, and microbial activity with steeper slopes for example signifying lower molecular weight material and vice-versa (Blough and Del Vecchio, 2002;Helms et al, 2008). Shorter wavelength spectral slope parameters have been found to be the best indicators of photodegradation with increases in the spectral slope S strongly correlating to apparent removal of CDOM absorption (Granskog, 2012;Fichot and Benner, 2012;Yamashita et al, 2013).…”