The tiny algae and bacteria that live in the ocean form the base of the food chain there. The primary producers in a food chain are always autotrophs, organisms that can manufacture their own food, often simple sugars, from the energy and materials in their environments.   In order to do this, almost all the autotrophs in the ocean need to have access to light.

     When  sunlight enters the water, it is absorbed and reflected back by particles in the water.  The deeper the water gets, the less light penetrates it.  Even under ideal conditions, the light barely reaches down 200 meters (about 600 feet) and very, very little of it gets that far down.

     When the water is muddy or has lots of tiny photosynthesizers in  it, sometimes the light gets down only a few feet.

     Seaweeds need light.  They use chlorophyll to manufacture their food, and some auxiliary (helper) pigments to soak up light and give it to the chlorophyll when the water is deep and light is scarce.  Check the page on colors of seaweed to learn more about this.