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. Without the oceans' phytoplankton, we should be unable to breathe.
Primary consumers are animals that eat producers.
On the Reef, primary consumers include members of the zooplankton which eat phytoplankton. Among these are planktonic arthropods and the larvae of relatively large sessile benthic animals such as sponges, and corals. Sponges and corals, along with tunicates, hydroids, and several types of worms that live in tubes, are filter feeders, living on plankton that they filter out of the water and on detritus, particles of dead organic matter. Secondary consumers are animals that eat primary consumers. For instance, small reef fishes, such as wrasses and tomtates, feed on benthic invertebrates.