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 (S275–295) and the ratio of two spectral slope parameters (SR); the ratio of S275–295 to S350–400, 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 S275–295 and SR were also higher in the subtropical region than the subarctic region. The dark microbial incubation showed biodegradation of DOM little affected S275–295, but slightly decreased SR. On the other hand, increases in S275–295 and relative stableness of SR 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 S275–295 can be used as a tracer of photochemical history of CDOM in the open ocean.
Highlights
Chromophoric dissolved organic matter (CDOM) ubiquitously occurs in the open ocean (Kitidis et al, 2006; Nelson et al, 2007, 2010; Swan et al, 2009; Yamashita and Tanoue, 2009)
Since S275−295 can be measured with high precision due to relatively high absorbance levels in this wavelength range (Helms et al, 2008), these spectral parameters may be used as a tracer of water masses and biogeochemical history of CDOM in the open ocean where CDOM levels are quite low (Nelson et al, 2007, 2010; Swan et al, 2009; Yamashita and Tanoue, 2009)
The different behavior, i.e., increase or decrease trend, of S350−400 among samples submitted to photo-irradiation was found (Helms et al, 2008). These results suggest that changes in S350−400 during photobleaching of CDOM might depend on CDOM sources that related to compositions
Summary
Chromophoric dissolved organic matter (CDOM) ubiquitously occurs in the open ocean (Kitidis et al, 2006; Nelson et al, 2007, 2010; Swan et al, 2009; Yamashita and Tanoue, 2009). Y. Yamashita et al.: Photobleaching as a factor controlling spectral characteristics of CDOM considered to be produced autochthonously through various biological processes (Nelson et al, 2004; Steinberg et al 2004; Ortega-Retuerta et al, 2009; Shank et al, 2010), and the primary local source of CDOM has been assigned to microbial activity (Nelson et al, 2004; Yamashita and Tanoue, 2004). From the global distribution of CDOM, microbially produced CDOM has been considered biologically refractory components with the time scale of the thermohaline circulation (Nelson et al, 2007, 2010; Yamashita and Tanoue, 2008; Swan et al, 2009), and CDOM has been suggested as a tracer of deep ocean biogeochemical processes and circulation (Nelson et al, 2007, 2010). The distribution of CDOM in the open ocean is basically controlled by a balance between autochthonous production and photobleaching under the influence of transport by vertical mixing, ventilation, and upwelling of water masses
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