You may not even be able to see the plankton, one of the most important communities of organisms in the sanctuary. Plankton is the group of organisms that float in the ocean and are carried around by currents because they are very weak swimmers. Most of them are so small that a microscope is needed to see them. Part of the food web linking the organisms at Gray's Reef. Green plants make their food from carbon dioxide and water by photosynthesis.
Plants produce most of the food in the ocean, as they do on land, and so they are called producers.
Photosynthesis is powered by light energy from the sun. Gray's Reef lies only about 60 feet beneath the ocean's surface and so enough light reaches it for plants to live. However, more than half the sanctuary's food energy comes from phytoplankton, producers floating in the plankton. Most members of the phytoplankton are microscopic but a few are larger, notably sargassum weed, which drifts through the sanctuary in summer and also acts as a raft carrying its own little community of organisms. Plants give off oxygen during photosynthesis, and about half the oxygen in the air today has been produced by phytoplankton.