Open oceans have extremely low productivity. The Shallow Ocean Oceanic productivity is concentrated in shallow waters, partly because nutrients are washed into the water from nearby land, partly because the shallow water allows nutrients to be retained within the community instead of sinking to the bottom of the deep, deep ocean. Mainly N of Tropic of Cancer, S of Tropic of Capricorn. Prefer rocky shores. Kelp is a type of brown algae.
Kelp beds have a complex vertical structure, like a forest.
Major component of reefs is coral. The stony framework of a coral reef is built by animals, who have a mutualistic relationship with photosynthetic algae. Coral reefs are extremely productive. Thus, coral reefs can hold large concentrations of nutrients in their biomass although the open ocean around them may be nearly devoid of nutrients. Reefs occur in mainly tropical seas, but can be killed if the ocean temp gets too high.