The grass of the sea seen by satellite

This MERIS image shows a large aquamarine-coloured phytoplankton bloom in the Bay of Biscay, between Spain and France.

Phytoplankton are microscopic marine plants which also use green-pigmented chlorophyll to perform photosynthesis.

When the concentration of phytoplankton is high, the plankton chlorophyll collectively tints the ocean waters. This bloom phenomena may be caused by massive nutrients supply and favourable light conditions.

Question : How can satellite imagery be used to monitor algal blooms ?