An indicator for the photo-degradation degree of dissolved organic matter in open ocean
Yohei Yamashita, Hokkaido University
Dissolved organic matter (DOM) is defined as the organic matter found in seawater using a 0.2-1.0 µm filter, and is mainly composed of non-living organic matter (Ogawa and Tanoue, 2003). DOM in the ocean plays important roles in the marine biogeochemical cycle, e.g., as a factor determining light penetration and as the substrate for heterotrophic bacteria (Hansell and Carlson, 2013), and thus, DOM in marine ecosystems can be considered as corresponding with the soil organic matter in terrestrial ecosystems. Therefore, a good grasp of spatial and temporal distributions of DOM is crucial for a better understanding of marine ecosystem functions. The major fraction of DOM is recalcitrant (Hansell, 2013), so if one wishes to compare the DOM distribution with other biological and/or biogeochemical parameters, it is important to clarify spatial and distributional patterns of the reactive fraction of DOM rather than bulk DOM concentration.
In the NEOPS project, we have aimed to establish a qualitative parameter of DOM that represents its reactivity. Since the photo-degradation products of DOM are possibly one of the important labile substrates for heterotrophic bacteria in the surface ocean, we have tried to establish a parameter that indicates the degree of photo-degradation of DOM. Figure 1 shows the typical absorption spectrum of oceanic DOM. A fraction of DOM absorbs UV-visible light, and the absorption coefficient of DOM exponentially decreases with wavelength. The absorption spectrum of oceanic DOM has usually been fit to an exponential function (Yamashita and Tanoue, 2009) as follows:
a(λ) = a(λi)e -S(λ - λi)
where a(λ) and a(λi) are the absorption coefficients at wavelength λ and reference wavelength λi (m-1), respectively. S is the spectral slope parameter (nm-1), and S between 275 nm and 295 nm (S275-295) has been used as an indicator of molecular weight of DOM and/or the photo-degradation history of DOM in terrestrial aquatic and coastal environments (Helms et al., 2008). However, factors controlling the S275-295 of oceanic DOM have not been the subject of adequate evaluation. Thus, we evaluated whether photo-degradation is a major factor controlling the S275-295 of the oceanic DOM via photo-degradation experiments using DOM in the deep ocean (Yamashita et al., 2013).
Figure 2 shows changes in levels of light absorbing DOM (a320) and S275-295 along with irradiation of sunlight. Levels of a320 were decreased for light treatments but not for dark experiments during photo-irradiation experiments, indicating the photo-degradation of DOM during natural sunlight irradiation. The photo-degradation of DOM was accompanied by a continuous increase in S275-295. On the other hand, the values of S275-295 were constant throughout the dark experiments. The results of this experiment imply that S275-295 can be used as a tracer of photochemical history of DOM in the open ocean. Thus, we have decided to use the S275-295 as one of the parameters of DOM quality for evaluating the basin-scale distribution pattern of reactive DOM in the ocean.
Fig. 1.Typical absorption spectrum of DOM in the ocean.
Fig. 2. Changes in a320 and S275-295 during photo-irradiation experiments.
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