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

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.