The production of organic matter by phytoplankton in the waters in and around the northern Marshall Islands is extremely low. The amount of plankton swept across the reefs of these islands by the wind‐driven currents is grossly inadequate to support the animals living on the reefs. The reef community, consisting of attached algae and animals, is self‐supporting; all the algae produce at least as much organic matter as all the animals consume. The rate of production of organic matter by an atoll as a whole ‐ water, reef, and bottom ‐ is several times as high as that of the surrounding sea, and permits maintenance of a marine population considerably denser than that of the ocean. The maximum possible rate of growth of the reef is found to be 1.4 cm per year.