Past exam of the year 2 of the earth sciences (geology) course of the University of Oxford / 2025 / Paper 3 / 6

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 isThe supplied sketch separates the three non-water contributions; its amplitudes are arbitrary and are not concentrations or measured spectra.

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.^[ref]

Many open-ocean chlorophyll retrieval algorithms infer chlorophyll a from the blue-to-green remote-sensing reflectance ratio. Increasing pigment generally depresses blue reflectance relative to green. In coastal waters, however, coloured dissolved organic matter and non-algal particles can independently absorb blue light, as in the sketches, and therefore mimic an increase in pigment. Their concentrations need not covary with phytoplankton biomass: river runoff and sediment resuspension can change them separately.

Moreover, the satellite measures emerging radiance, not absorption directly. Schematically,where $b_b$ is the backscattering coefficient; this is an explanatory approximation, not a complete retrieval formula. Suspended particles change both absorption and backscattering, and different mixtures can produce similar band ratios. Shallow-bottom reflection, land-adjacency effects and more difficult atmospheric correction introduce further coastal errors. Consequently a simple blue/green ratio can overestimate chlorophyll a when excess CDOM removes blue light, although sediment scattering and other effects mean the bias is not universally positive. Multiband or hyperspectral methods that separately constrain pigment, dissolved absorption and particles, supported by local observations, are preferable in optically complex water.^[ref]

Chlorophyll a occurs in nearly all oxygen-producing phytoplankton groups and is central to their photosynthesis. Its concentration can be measured reproducibly by pigment extraction or calibrated optical methods, and its absorption affects ocean colour, making it observable over large areas. It is therefore a practical common index of photosynthetic standing stock across diverse communities, unlike a pigment limited to one group.

It is a proxy, not a fixed conversion to phytoplankton biomass or primary production. The carbon-to-chlorophyll ratio varies with species, light acclimation, nutrient supply and physiological condition; low-light cells often contain more pigment per unit carbon. Dead pigment and methodological differences can also affect interpretation, and nonphotosynthetic plankton are not represented. A pigment increase need not mean the same proportional increase in living carbon biomass.^[ref]

The photic zone is the illuminated layer supporting photosynthesis; its euphotic depth is conventionally associated with about 1% of surface photosynthetically active radiation, integrated over approximately $400$–$700\mathrm{nm}$. This is a useful convention rather than a universal organism-specific compensation depth. Spectral filtering can be represented locally bywhere $K_d$ is the wavelength-dependent diffuse attenuation coefficient. It includes effects of absorption and scattering and is not simply equal to the absorption coefficient $a$.

In clear, low-chlorophyll water, pure water removes red light strongly, while weak blue pigment and CDOM absorption permits blue light, typically around $450$–$490\mathrm{nm}$, to penetrate furthest. The deepest usable light is consequently dim and blue-rich, and the euphotic zone can extend relatively deep. At high pigment concentrations, phytoplankton pigments preferentially remove blue and red light; associated detritus or CDOM can remove still more blue. The remaining transmission window commonly shifts toward blue-green or green wavelengths, roughly $500$–$570\mathrm{nm}$, and the total light falls off faster, so the euphotic depth is shallower. Exact window and depth depend on the full particle/dissolved mixture, not chlorophyll alone. The diagram normalizes the two deep spectra separately to show their different shapes, not equal absolute light at a common depth.

This confines photosynthetic growth to a thinner illuminated interval in high-chlorophyll water and allows a deeper potential habitat in clear low-chlorophyll water, subject also to nutrient supply, mixing and the organism's compensation requirement. Cells near the base benefit from low-light acclimation and pigment systems that capture the remaining wavelengths. In blue-rich deep ocean water, blue-absorbing pigments and some low-light-adapted picoplankton can be favored; where the surviving light is greener, groups with effective green-absorbing accessory pigments, such as certain phycobilins, gain an advantage. A pigment appears coloured by light it fails to absorb, so green-absorbing accessory pigments need not themselves look green. There is no universal deep species determined by chlorophyll concentration alone. A deep chlorophyll maximum can partly reflect increased pigment per cell rather than a proportional maximum in carbon biomass.^[ref]