Eutrophication, ocean colour and algal blooms
Earth-observing satellite Envisat MERIS image of a phytoplankton bloom in the South Atlantic Ocean: different types and quantities of phytoplankton exhibit different colours, such as the blues and greens in this image(Photo Credit: ESA)
The main reason to measure ocean colour is to study phytoplankton, the microscopic algae which are at the base of the oceanic food web. Remote sensing plays an important role in the detection, monitoring and prediction of algal blooms in the marine environment as these algae are considered a potential threat when they form so called harmful algal blooms and so appropriate measures can be taken. In situ measurements are useful when more information is required on the type of algae present but when there is a sudden shift in time and location these methods become too expensive. Satellite sensors detect the reflected light by the sea surface in different wavelengths. The "colour" of the ocean is determined by the impact of light with the water and any colored particles or dissolved chemicals in the water. Colour is the light reflected by the water and the substances present in it. When light hits a water molecule or a coloured substrate in it, the different colours (wavelengths) can be absorbed or scattered in differing intensities. The colour we see results from the colours that are reflected. The substances in seawater which most affect the water colour are: phytoplankton, inorganic particles, dissolved organic chemicals, and the water molecules themselves. Phytoplankton contains green-coloured chlorophyll-a (necessary to produce organic carbon using light and carbon dioxide during photosynthesis) which absorbs red and blue light and reflects green light. The ocean colour is also an indicator of the health of oceans. The chlorophyll concentrations can be derived from satellite data by calculating the ratio blue / green of the ocean. When blue is more absorbed, green is more reflected which indicates a higher concentration of phytoplankton in the water and vice versa. Remote sensing can thus provide a wide visual picture and allows us to create more insight into the eutrophication processes.
Examples of modern colour satellites sensors are SeaWiFS (Sea-viewing Wide Field of view Sensor), MODIS (Moderate Resolution Imaging Spectroradiometer) and MERIS (Medium Resolution Imaging Spectrometer). Once