Seaweed is the common name for countless species of marine plants and algae that grow in the ocean as well as in rivers, lakes, and other water bodies.Kelp forest in the Channel Islands National Marine Sanctuary and National Park.Some seaweeds are microscopic, such as the phytoplankton that live suspended in the water column and provide the base for most marine food chains.Some are enormous, like the giant kelp that grow in abundant “forests” and tower like underwater redwoods from their roots at the bottom of the sea.elp forest in the Channel Islands National Marine Sanctuary and National Park. Some seaweeds are microscopic, such as the phytoplankton that live suspended in the water column and provide the base for most marine food chains.