April 10, 2020
Author: Collin Roesler
When light penetrates the ocean, photons are either absorbed or scattered. While scattering redirects the angle of the photon path, absorption removes the photons permanently from the path. Unlike scattering which is weakly spectrally dependent, absorption is highly spectrally dependent. The magnitude and spectral features depend upon the concentration and composition of the particulate and dissolved constituents and water itself. The inherent optical properties (IOPs) are conservative properties and therefore the magnitude of the absorption coeﬃcient varies linearly with the concentration of the absorbing material. Theoretically, the absorption coeﬃcient can be expressed as the sum of the absorption coeﬃcients of each component:
Where N is the total number of components and a˙i indicates the iˆth component, and all absorption is spectral, indicated by (
Practically, however, it is not possible to measure the absorption properties of each individual absorbing component and thus the individual components can be grouped into similarly absorbing constituents based upon similarity in their optical properties and/or analytically-based groupings:
where subscriptions w, phyt, NAP and CDOM indicate water, phytoplankton, non-algal particles, and colored dissolved organic matter, respectively.
Absorption by water (Fig. 1) is weak in the blue and strong in the red and varies with temperature and salinity.
Absorption by particles is separated into phytoplankton and non-algal particles (NAP) using spectrophotometry and an extractive technique. Phytoplankton absorption (Fig. 2) demonstrates the most spectral variations of any of the components due to the individual pigment absorption spectra but in general exhibits peaks in the blue and red regions of the spectrum due to the ubiquitous presence of chlorophyll a.
Non-algal particle absorption (Fig. 3) is strongest in the blue, decreasing approximately exponentially to the red. This component, operationally-deﬁned, includes living zooplankton and bacteria, as well as the non-pigmented parts of phytoplankton (cell walls, membranes etc), detrital material as well as inorganic particles.
Colored dissolved organic matter (CDOM) absorption (Fig. 4) is very similar to that of NAP due in part to the similarity in composition (organic material), but generally exhibits a steeper exponential slope. This material is operationally separated from NAP by ﬁltration; CDOM is measured on the ﬁltrate passing through 0.2mm or 0.7 mm nominal pore sized ﬁlters.
For oligotropic environments with very low concentrations of suspended and dissolved material, the absorption coeﬃcient is dominated by water (Figure 5A) and the wavelength of minimum absorption is in the blue, hence the blue color of the seawater. For eutrophic and/or coastal environments with high concentrations of suspended and dissolved material (Figure 5B), the absorption coeﬃcient is dominated by that material and the wavelength of minimal absorption shifts to the green, lending green color to that environment.