3.6 Metrics

• \(S_{300-600}\) linked to DOM molecular weight (Colin A. Stedmon and Nelson 2015).

3.6.1 Slope ratio

Equation (??) shows how the slope ratio (\(S_R\)) is calculated.

\[\begin{equation} S_R = \frac{S_{275-295}}{S_{350-400}} \label{eq:sr} \end{equation}\]

By calculating the ratio of the slope of the shorter wavelength region (275–295 nm) to that of the longer wavelength region (350–400 nm), a dimensionless parameter called “slope ratio” or \(S_R\) is defined. This approach avoids the use of spectral data near the detection limit of the instruments used, and focuses on absorbance values that shift dramatically during estuarine transit and photochemical alteration of CDOM (Helms et al. 2008).

Figure 3.5 shows in red the 275-295 and 350-400 nm spectral range.

FIGURE 3.5: Spectral range used to calculate the slope ratio.

References

Stedmon, Colin A., and Norman B. Nelson. 2015. “The Optical Properties of DOM in the Ocean.” In Biogeochemistry of Marine Dissolved Organic Matter, edited by Dennis A. Hansell and Craig A. Carlson, Academic P, 481–508. Burlington: Elsevier. doi:10.1016/B978-0-12-405940-5.00010-8.

Helms, John R., Aron Stubbins, Jason D. Ritchie, Elizabeth C. Minor, David J. Kieber, and Kenneth Mopper. 2008. “Absorption spectral slopes and slope ratios as indicators of molecular weight, source, and photobleaching of chromophoric dissolved organic matter.” Limnology and Oceanography 53 (3): 955–69. doi:10.4319/lo.2008.53.3.0955.