= Solution
The principal non-water visible-light absorbers can be grouped into <phytoplankton pigments>, <coloured dissolved organic matter> (CDOM), and <non-algal particles> (NAP), including organic detritus and absorbing mineral material. A useful additive decomposition of the <absorption coefficient> is
$$
a(\lambda)=a_w(\lambda)+a_{\mathrm{ph}}(\lambda)+a_{\mathrm{CDOM}}(\lambda)+a_{\mathrm{NAP}}(\lambda).
$$
The supplied sketch separates the three non-water contributions; its amplitudes are arbitrary and are not concentrations or measured spectra.
\Image[/past-exam-of-the-year-2-of-the-earth-sciences-geology-course-of-the-university-of-oxford/2025/paper-3-q6-ocean-absorption.png]
{title=Qualitative marine absorption spectra and deep-light spectral filtering}
{description=Original synthetic sketches: phytoplankton have blue and red pigment absorption bands, while CDOM and non-algal particles preferentially absorb short wavelengths. The right panel illustrates blue-rich deep light in low-chlorophyll clear water and a greener spectrum in more absorbing high-chlorophyll water. Spectra are normalized separately; curves and representative depths are illustrative, not observations or a calibrated radiative-transfer calculation.}
<Phytoplankton pigments> give prominent absorption maxima around $440$ and $675\ \mathrm{nm}$, associated with <chlorophyll a>, and a relative minimum in the green. <Accessory pigments>, including <carotenoids> and <phycobilins>, broaden or add absorption bands, so an assemblage is not just a pure chlorophyll-a solution. Intracellular packaging and cell size also modify the shape and absorption per unit pigment.
<Coloured dissolved organic matter> consists of dissolved light-absorbing organic compounds, often including humic material from terrestrial runoff and marine degradation. Its absorption is strongest at the blue/ultraviolet end and declines approximately exponentially toward red, without the narrow chlorophyll red peak. A common local parameterization is $a_g(\lambda)=a_g(\lambda_0)e^{-S_g(\lambda-\lambda_0)}$ with positive spectral slope $S_g$; here $S_g$ is an optical slope, not gas saturation.
<Non-algal particles> include nonliving organic detritus, pigment degradation products and absorbing suspended mineral particles. Their aggregate absorption often also decreases approximately exponentially from blue to red, with variable slope and mineral-dependent departures. Suspended sediments can strongly scatter as well as absorb; bright sediment-laden water is not evidence of a red absorption peak. Chlorophyll-containing living cells belong in $a_{\mathrm{ph}}$, so these categories should not be double counted.<Ocean optics protocols by NASA>[Revision 4, Volume I, sections defining chlorophyll, CDOM and non-pigmented-particle absorption, and the importance of blue absorption to ocean-colour retrieval.]{ref}
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