Chlorophyll concentrations are derived from ocean colour. The green-plant community that is the basis of the ocean food chain consists almost entirely of microscopic organisms called phytoplankton. These phytoplankton contain chlorophyll which modify the ocean colour. By detecting the presence of pigments in particular parts of the visible light spectrum using optical instruments on satellites, we can measure the amount and distribution of phytoplankton in the ocean. This is then calibrated in terms of chlorophyll concentration, usually given in units of milligrams per cubic meter of water.
Besides being the basis of the food chain, phytoplankton have other important functions. They grow by photosynthesis, a process which consumes carbon dioxide. Hence they are important in the ocean carbon cycle. The ocean carbon cycle in highly relevant to the greenhouse effect and climate change. On a global scale, phytoplankton process fifty thousand million tonnes of carbon per annum, playing a significant role in the planetary carbon cycle. This activity of utilising carbon dioxide is known as primary production and this can be computed from chlorophyll concentrations given knowledge of some other parameters.
In addition, chlorophyll concentration is the major determinant of the transmissibility of visible light through the ocean. This property is an important element in physical models that calculate the depth and temperature of the mixed layer, and therefore, ultimately, in numerical weather prediction. At large scales, ocean-colour data provide an ideal vehicle for study of the effects of major events such as El Niño on the global marine ecosystem.
The pigment fields derived from ocean colour can be used to estimate primary production, which is the ecological term for the rate of photosynthesis by the green plants in an ecosystem. Photosynthesis is the process by which new biological tissue is formed, and provides the basis for sustenance of the entire marine food web.
To estimate primary production we need, in addition to the pigment field, the solar irradiance and the photosynthetic performance of the phytoplankton in a given light field. Photosynthetic performance can be measured in experiments conducted in ships at sea. It should be understood that the primary production fields are estimates only: they represent our best effort to combine data from satellites and from ships to calculate this important property of the marine ecosystem.
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