mon océan & moi

What is it  and why is it measured?

Chlorophyll a is a pigment present in every plant cell. It is the chlorophyll a which gives the green color to plants. This pigment is really important for the process named photosynthesis because it enables plant cell to absorb sunlight. In fact, plants transform the carbon dioxide (CO₂) into organic matter thanks to sunlight and chlorophyll a through photosynthesis. Every year, the photosynthetic organisms would assimilate about 100 billion carbon tones, the half would be due to phytoplankton photosynthesis! In oceanography, the phytoplankton quantity in ocean can be estimate by the chlorophyll a. In fact, the higher is the chlorophyll a concentration, the more phytoplankton there is.

How is it measured by floats?

On a float, the chlorophyll a concentration is measured thanks to a sensor that measures the fluorescence and which is named fluorometer. When chlorophyll a molecule absorbs light (blue), it becomes excited. The molecule has to deactivate (because the excited state is unstable). One of the ways for chlorophyll a deactivation is the emission of fluorescence (i.e. emission of red light). Therefore, the fluorometer emits a blue light in a seawater sample and measures the fluorescence intensity (red light) produced by phytoplankton. Finally, the more there is chlorophyll a in water, the more the fluorescence measured will be high.

Typical examples

• Vertical profiles

The phytoplankton needs light and nutrients to growth. The light intensity is higher at the surface and decreases with depth, whereas nutrients are in higher concentrations in deep waters.

 In the case of a stable water column, the profile shape corresponds to the better compromise between the nutrients availability and the light. In the surface layer, there are not enough nutrients for the phytoplankton growth. In deep waters, the light becomes a limiting factor. The most of the biomass is then in an intermediary optimal layer, where there is both enough light and nutrients (Fig. 1A). Then, a deep maximum of chlorophyll a (DCM) can be observed.

Whether the water column is mixed (by strong winds for example), all the constituents of the upper layer are mixed and homogenized. Therefore, the phytoplankton is mixed in the entire mixed layer (see the mixed layer sheet) and the chlorophyll a concentration is constant within this layer (Fig. 1B). Moreover, the deepening of the mixed layer enables the nutrients supply to surface layer (see the nitrates sheet), which leads to an increase of the chlorophyll a concentration.

• Spatial distribution : Surface chlorophyll a concentration satellite image
The global chlorophyll a spatial distribution can be obtained satellite ocean color images. The satellite sensor SeaWIFS measured the surface chlorophyll a every day at the earth's surface from September 1997 to December 2010. Then, it is possible to obtain the annual average distribution of surface chlorophyll a (Figure 3 below). The surface chlorophyll a differs between around 0.01 and 15 mg.m-3. The wide subtropical areas where there is little chlorophyll a are called "marine deserts".