Seaweed’s come in many different shapes and sizes varying from the microscopic seaweeds that encrust rocks on the sea shore to the huge mega kelps that can be many metres long.
Seaweeds need light to photosynthesise. This very important factor restricts seaweed to an area called the photic zone. This is the maximum depth that light can penetrate to in the sea. Any deeper and then the available light is insufficient for photosynthesis.
The easiest place to find seaweeds is on most rocky shores in the world. Although you can find seaweed almost everywhere in the sea in the form of drift (free floating forms), as bits of rotting matter on the strand line or attached to rock surfaces on the seashore.
Seaweeds are found throughout the world, in tropical climes right through to arctic temperatures. My studies are centred mostly in the North East Atlantic, where vast beds of kelp dominate subtidal areas and most sheltered rocky shores are dominated by huge stands of the algae Ascophyllum nodosum.