TERRESTRIAL ENVIRONMENTS AND ECOLOGY 



191 



not available. Extensive radiological surveys were con- 

 ducted in the northern Marshall Islands, including 

 Enewetak, and provide information on the radionuclides in 

 the soil, vegetation, and animals (Robison et al., 1981, 

 1982). 



The two most common and troublesome radionuclides 

 in the soils arc cesium-137 and strontium-90 because they 

 are picked up by plants, such as the coconut palm Cocos 

 nucifer, and are concentrated in the leaves and nuts which 

 may subsequently be consumed by man (Bastian and Jack- 

 son, 1975; Jackson and Carpenter, 1967). Radionuclides 

 have a cumulative effect in the diet: the more you eat, the 

 more you get. Surface material, soil, debris, and vegetation 

 containing these and other radionuclides were collected, 

 removed from the site of contamination, and entombed in 

 a slurry of concrete in two atomic craters at the north end 

 of Runit Island. Because of the transuranium nuclides, 

 chiefly plutonium, at this former test site and because of 

 the entombment of other radioactive materials on this 

 island, Runit is permanently off-limits to humans. It is 

 interesting to note that it took only a few years for 

 seabirds to recognize Runit as an ideal nesting site. 

 Seabirds returned to islands denuded by nuclear testing in 

 less than two years (Held, 1960). The people of Enewetak 

 prey heavily on birds and eggs, but they do not forage on 

 Runit. In 1985, B. Ristvet (personal communication) gave a 

 rough estimate of 10,000 birds nesting on Runit. He 

 reported that the smell of guano was perceptible about a 

 mile west of Runit in the lagoon. 



Currently, at Bikini Atoll the United States government 

 is making an effort to eliminate the radionuclides from the 

 soil by means other than total removal of the contaminated 

 soil. The soil is relatively poor in potassium, and to com- 

 pensate, the plants pick up cesium-137. Adding 

 potassium-rich fertilizers reduces the uptake of cesium-137. 

 Although there is no clay in atoll soils which would serve 

 to trap cesium, adding a mineral silicate such as mica 

 tends to have the same effect. These findings offer hope 

 that a solution can be found short of soil removal for Bikini 

 Atoll. Fortunately, Bikini does not have the transuranium 

 nuclides found at Enewetak which necessitated the 

 extremely thorough cleanup of Enewetak Atoll. 



From my observations at Enewetak, the A horizon 

 varies in thickness from a few centimeters to 40 to 50 cm 

 on the larger islands where it may be covered with a layer 

 of decomposing vegetation. The soils are usually well 

 drained and feel dry to the touch. Where they are poorly 

 drained, for example in depressions where the water table 

 is close to the surface, they have a wet sticky clay or 

 muck-like consistency. 



On Ikuren Island the A horizon is about 40-cm thick on 

 the lagoon side. This part of the island is covered with a 

 dense growth of coconut and Pisonia trees. Small meadows 

 of grass, Lepturus repens, grow in open areas of the for- 

 est. Toward the ocean or south side of Ikuren, the soil 

 grades into coral rubble mixed with organic debris but 

 hardly any humus. This seems similar to the situation at 

 Bonriki Island, Tarawa Atoll, described by Seru and Morri- 



son (1985). The coconut and Pisonia vegetation gives wau 

 to Scaevola and Tournefortia as the soil gets coarser. The 

 coral rubble becomes increasingly coarse until it ends 

 abruptly on a steep seaward berm of unconsolidated coral 

 rubble. The limestone rubble and sandy soil are typically 

 gray in color due to the blue-green algae, Brach]^thchia 

 quoyi, which may be important in nitrogen fixation (Nier- 

 ing, 1963; Wiens, 1962). 



According to Trudgill (1979), there are three primary 

 sources of the soil of Aldabra Atoll: 



1. Mechanically derived carbonate fragments 



2. Chemically derived solution residues 



3. Leaf litter 



The composition of the vegetation and the phosphates 

 and nitrates derived from fecal material of birds, crabs, and 

 rats — all more abundant where there is more 

 vegetation — have considerable influence on the nature of 

 the atoll soils where the influence of organic materials is 

 especially significant. This is an important observation 

 because it means that the soil of an island such as Ikuren, 

 which has had good vegetation cover during the recent 

 past, should be richer than that of Enjebi and other islands 

 on the east and north rim of Enewetak, where the vegeta- 

 tion has undergone much disturbance during the past 40 

 years. 



Soil is a precious terrestrial resource. In the atoll situa- 

 tion, the influence of organic material is all important. It 

 not only carries out the normal role of soil organic matter 

 in storing and recycling nutrients, but it is also the major 

 moisture storage component in the soils, since coral sand 

 and rocks have an extremely limited moisture storage 

 capacity. The fertility of atoll soils, therefore, is almost 

 entirely dependent on the content of organic matter (Seru 

 and Morrison, 1985). Every effort must, therefore, be 

 made to protect the organic-rich layers from erosion and 

 conserve them through cover of native vegetation. 



TERRESTRIAL BIOTA (EXCLUDING MAN) 



The terrestrial biota and ecology of Enewetak were not 

 studied as thoroughly as the marine ecosystem. From the 

 inception of the laboratory, the emphasis was placed on 

 marine organisms. The historical reasons for this are not 

 altogether clear (Helfrich and Ray, Chapter 1, this volume; 

 Hincs, 1962). In part it was because of the magnitude of 

 the marine environment when compared to the terrestrial 

 one and because there was an early concensus that a great 

 deal more was known about the terrestrial organisms than 

 about the multitude of unfamiliar marine organisms. In any 

 event, the opportunity to conduct research on a tropical 

 coral atoll apparently was much more appealing to marine 

 biologists than to other scientists. As a result far fewer 

 studies were made on the terrestrial biota and ecosystem. 



We lack comprehensive, long-term studies of the plants 

 of Enewetak. What we do know has been summarized by 

 Lamberson (Chapter 3, Volume II). Ecological processes 

 have not been studied in the terrestrial ecosystem of 

 Enewetak. Other than species lists, we know very little 



