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

Abstract

[1] Spatial and temporal distributions of marine humic-like fluorescent dissolved organic matter (FDOM M ) were determined in the subtropical western North Pacific to evaluate the controlling factors of FDOM M behaviors. The observations were conducted at 4 stations (15-30°N) along 137°E in a subtropical area between January 2006 and April 2007. The florescence intensity of FDOM M was low (0.14-0.25 quinine sulfate units (QSU)) in the surface layer probably due to photodegradation, and increased with depth (0.90-1.10 QSU at 1000 m), irrespective of season and station. In the surface layer, the thickness of the water mass with low fluorescence intensity (<0.3 QSU) showed the seasonal change by being deeper in winter and shallower in summer, depending on the mixed layer depth (MLD). The average fluorescence intensity within the mixed layer also varied seasonally at midlatitudes; the intensity in summer was 40.8-53.8% of that in winter. Since the MLD was very much shallower in summer than in winter, FDOM M in the mixed layer would be kept within a shallow depth during the summer where intensive photodegradation could occur. The concentration of total organic carbon (TOC) was at its maximum at the water surface and decreased with depth, being adverse to FDOM M . Thus, the ratio of fluorescence intensity to TOC concentration was lowest (0.002-0.003) in surface water, which implies that FDOM M is not quantitatively important to the dissolved organic carbon pool. However, considering the possible difference in the stabilities of FDOM M against photochemical and microbial degradation, it is conceivable that photobleached FDOM M is one of the important organic groups constituting marine dissolved organic matter.Citation: Omori, Y., T. Hama, M. Ishii, and S. Saito (2010), Relationship between the seasonal change in fluorescent dissolved organic matter and mixed layer depth in the subtropical western North Pacific,

Drivers of fluorescent dissolved organic matter in the global epipelagic ocean

“…The increase with depth of the humic-like FDOM components in the upper 200 m followed the regular pattern previously reported for open ocean waters (Fig. 3) of the Atlantic (Mopper et al 1991;Chen and Bada 1992;Determann et al 1996;Kowalczuk et al 2013;Lønborg et al 2015;Timko et al 2015), the Southern ocean (Wedborg et al 1998;Yamashita et al 2007), the equatorial Pacific (Chen and Bada 1992;Hayase and Shinozuoka 1995), the Okhotsk Sea and the northwestern North Pacific (Tani et al 2003;Omori et al 2010;Yamashita et al 2010Yamashita et al , 2015, the Arabian Sea (Coble et al 1998) or the global cruise of Jørgensen et al (2011) that covered the Atlantic, Pacific, Indian, and Southern Oceans. Generally, the humic-like FDOM intensity is lower in surface waters where sunlight penetrates and photolyses the involved compounds, which have been shown to be very vulnerable to natural solar radiation (Omori et al 2010(Omori et al , 2011.…”

“…3) of the Atlantic (Mopper et al 1991;Chen and Bada 1992;Determann et al 1996;Kowalczuk et al 2013;Lønborg et al 2015;Timko et al 2015), the Southern ocean (Wedborg et al 1998;Yamashita et al 2007), the equatorial Pacific (Chen and Bada 1992;Hayase and Shinozuoka 1995), the Okhotsk Sea and the northwestern North Pacific (Tani et al 2003;Omori et al 2010;Yamashita et al 2010Yamashita et al , 2015, the Arabian Sea (Coble et al 1998) or the global cruise of Jørgensen et al (2011) that covered the Atlantic, Pacific, Indian, and Southern Oceans. Generally, the humic-like FDOM intensity is lower in surface waters where sunlight penetrates and photolyses the involved compounds, which have been shown to be very vulnerable to natural solar radiation (Omori et al 2010(Omori et al , 2011. Photobleaching has a significant influence on the optical properties of DOM in the open ocean (Helms et al 2013) and its vertical effect is limited to the penetration of the ultraviolet radiation in the ocean (Kowalczuk et al 2013).…”

The most oligotrophic subtropical zones of the global ocean: similarities and differences in terms of chlorophyll and yellow substance

“…The area near Mariana islands, with a slightly higher mean [Chl] content and a similar low CDOM content (see also fluorescence measurements in Omori et al, 2010), is probably of exceptional clarity regardless of the season. According to the present results, no other oligotrophic gyres, beside these Pacific systems, exhibit such low pigment and yellow substance contents.…”

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

“…The low levels of CDOM found in the subtropical region has been explained by extensive photobleaching of CDOM due to the long residence time of surface waters in the subtropical gyres with high insolation (Swan et al, 2009). Seasonal variability of vertical profiles and characteristics of fluorescent DOM in the subtropical region of the western Pacific were also explained by deep winter mixed layer and the extensive photobleaching (Omori et al, 2010(Omori et al, , 2011. Thus, different levels of CDOM found between the subarctic and the subtropical regions in this study can be explained mainly by differences in photobleaching degree of CDOM between the regions.…”