This load of organic matter can deplete the oxygen and make them smell like sewage. Again these must be in the upper reaches of the water to receive sunlight. These organisms can locally deplete the minerals for growth. Most productive areas are where waters from deep currents come up to the surface bringing minerals to the area with photosynthesis. Such areas occur along our coast.
Thermal vents occur in places in the deep ocean.
These vents bring heat and minerals to the water. Sulfides can be used for energy instead of sunlight and there is a whole ecosystem that depends on this energy source for life. Coral Reefs Coral is disappearing at an amazing rate around the world. Some disappearance may be due to warming water or human waste from the streams. Coral provides a place for a very rich community of algae that are the basis for the community.