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.
Four and one-half months early in 1946 were spent in the field at Bikini and Rongelap Atolls to study the circulation systems within the lagoons. Later, in the laboratory, synoptic studies were made of kinimatically similar models based on early field data. The results of these studies were then tested against further observations made in the field. It was found that both lagoons exhibited a primary circulation (overturning winddriven circulation) and a secondary circulation (rotary circulation composed of two counter-rotating compartments). The direction of the primary circulation and the division between the counter-rotating compartments of the secondary circulation were always contained in a vertical plane having roughly the azimuth of the prevailing wind. The vigor of both circulations varied with the average wind strength taken over the previous 24 hours. Perturbations of the primary and secondary modes of circulation were produced by tides and waves and by the North Equatorial Current, which surrounds the atolls of the northern Marshall Islands group. Seasonal changes also were noted: the circulation of the winter months, when the northeast trade winds prevail, showed a marked stability and strength which was lacking in the summer season, when the atolls are on the edge of the belt of doldrums. In addition to the qualitative synopsis of the circulations, numerical data are presented in tabular form which show the volumetric exchanges of water between the lagoons and the ocean. An estimate is made of the rates of refreshment and overturning of each lagoon. A chart of the values of horizontal diffusion in the surface plane is given for Bikini atoll. NOTE.-Contribution from The Seripps Institution of Oceanography, New Series No. 700.
The eastern reefs of Rongelap Atoll, composed of animals and attached algae, produce more organic matter than they consume. The productivity per unit area is considerably higher than that of adjacent waters or any^other open marine areas. The zooxanthellae of the corals make a substantial contribution to the whole production, so that per unit of weight, actively growing reef corals may photosynthesize as fast as lithothamnia Review of work on other coral reefs indicates that these communities are generally self-maintaining. PLATE 80. Crania sp. and Thecidellina congregata.81. Thecidellina maxilla and Argyrotheca sp.
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