Optical Impacts of Emiliania huxleyi

For most types of phytoplankton it is the chloroplasts (photosynthesising parts of the cell) that make the most difference to the light, absorbing photons and trapping them to form chemical energy for use by the cell. For coccolithophore species though, the coccoliths as well as the chloroplasts are optically important. The coccoliths do not absorb photons, terminating their travel through the water, but rather they act like tiny mirrors, reflecting or scattering the photons, causing deflections in their directions of travel. This is why coccolithophore blooms change the brightness of the water, and why blooms can be seen from the space shuttle and from satellites. The brightening effect of mass-release of coccoliths was also strikingly apparent following an artificial experiment (called "Chalk-Ex") in which chalk rocks (fossilised coccoliths!) were ground up before being added to the surface ocean, as shown in the picture below.

In the Chalk-Ex experiment a 2 km2 patch of the Gulf of Maine had 13 tons of powdered chalk added to it, in November 2001. Note the pale colour of the water either side of the ship's wake. The powdered chalk was obtained by grinding up Cretaceous-age rocks made of coccolith chalk, shipped especially from the UK. The mean particle size was about 2 microns, the same size as Ehux coccoliths! By comparison typical natural Ehux blooms in the Gulf of Maine consist of hundreds of thousands of tons of chalk, spread over a larger area (Picture courtesy of Barney Balch).

Although each coccolith is invisibly small, when they are present in enormous numbers in the water they cause large changes in the way that light is transmitted or reflected by the water. Figures 1 and 2 below show the affect that different components in the water have on the inherent optical properties of the water, i.e. on the probabilities of absorption (termination) and of scattering (reflection) of photons. It can be seen that an intense bloom of Ehux (100 mg CaCO3-C m-3) causes a large increase in the scattering at all wavelengths of light (400-700nm) relevant to phytoplankton, but no change to the absorption [Balch et al, 1991; Balch et al, 1997a; Balch et al, 1997b]. 400-700nm is also the human-visible part of the spectrum, with red light at ~650nm, green light at ~550nm, and blue light at ~450nm.

Components contributing to absorption and scattering, in typical proportions for mid-summer at 60N in the Atlantic, during a bloom of Ehux. The effects of individual components are summed to obtain total absorption and scattering. Effects of chlorophyll and calcite are proportional to the amounts of the substances in the water.

The figure above shows the impact of coccoliths on the scattering at each wavelength, but what difference does this make to the overall behaviour of light in a coccolith-filled ocean? A Monte-Carlo optical model has been developed to obtain answers to these questions, with the following results:

Firstly, the increased scattering causes the ocean to become more reflective, i.e. the ocean's albedo increases. A greater proportion of the photons which pass through the water-surface from air to water subsequently leave again, from water to air, after having been scattered by coccolith particles in the water. For a typical ocean, the proportion of photons re-emitted by the water increases from 1% when there are no coccoliths in the water, to 3% with 100 mg CaCO3-C m-3 of coccoliths in the water, to 7% with 300 mg CaCO3-C m-3 of coccoliths.

The "nadir radiance" is the intensity of light travelling vertically upwards just above the ocean surface. It is this signal that is picked up by satellites, after transmission upwards through the atmosphere. The figure below shows how the nadir radiance changes with increasing coccolith concentration. From this diagram it is easy to understand why Ehux blooms are so easily visible in satellite images.

Nadir radiance, Lu (uEin m-2 sr-1 s-1) as a function of coccolith calcite concentration, for water containing 0.75 mg chl-a m-3, and with the sun at 45 degrees from the vertical. Model parameters are otherwise typical for the NE Atlantic at 60N, during mid-summer.

Secondly, the addition of coccoliths to the water makes that water brighter near to the surface and darker deeper down. Scalar irradiance is a measure of total light intensity (photons are counted equally regardless of the direction they are travelling in), and the figure below shows how this changes with depth as coccolith concentration increases. The coccoliths shade the deeper water, reducing the amount of light available there. This reduces the depth of water which is habitable for the phytoplankton, because they require a minimum light intensity to survive.

Scalar irradiance depth profiles for three different calcite concentrations, with the same model parameters as for the diagram above.

The coccolith impacts can also be seen more graphically in this model-derived comparison of water colour/brightness for no coccoliths and for 300 mg CaCO3-C m-3 of coccoliths. These model results help us to understand the "white waters" of coccolith blooms, alternatively described as the fairy glow coming from these bloom waters.

Model estimated water colour: comparison of 0 (left) and 300 mg CaCO3-C m-3 (right). Parameters for the model were as for the two diagrams above. The top of each column corresponds to the water surface, the bottom to a depth of 10 metres.

paper describing the optical impacts in detail


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Toby Tyrrell : T.Tyrrell@noc.soton.ac.uk