offish and invertebrates (Dixon, 1884; Fowler, 1901; Pilsbry 

 &Vanatta. 1905a.b). However, preliminary studies on mollusks, 

 benthic invertebrates, and lagoon plankton were begun by 

 Soviet scientists in 1988 (Tsyban & Smith, 1988; Sirenko & 

 Koltun. Subchapter 1.4. this volume). The lagoon and reefs are 

 remarkably pristine, having changed little since their first 

 discovery; all early travelers remarked on their beauty, 

 abundance, and variety (Bennett, 1840; Markham, 1904). 



The usual assemblage of tropical invertebrates — 

 echinoderms. mollusks, crustaceans, porifera, corals, tunicates, 

 et cetera — are present. Large numbers of black sea cucumbers 

 (PI. 10). about 20 cm long, are particularly abundant on the 

 lagoon reef flats of the southern windward islands. They have 

 been tentatively identified as Ludwigothuria sp. (B. Sirenko, 

 personal communication). Conspicuous fish families include 

 paiTot fish (Scaridae), butterfly fish (Chaetodontidae), 

 surgeonfish (Acanthuridae), damselfish (Pomacentridae), 

 pufferfish (Tetraodontidae). and wrasses (Labridae). 



Substrata 



Throughout the atoll, the substrata reflect a coralline 

 origin. There is little "soil" in the accepted sense. Various 

 grades of jagged, eroded coral and mollusean rubble (from fist- 

 sized to tiny pebbles), together with sand, coralline algae, and 

 small proportions of organic litter, humus, and guano, are 

 present. Such accumulations of reef and terrestrial debris are 

 similar to those of other low, coral atolls ( Fosberg, 1953; 

 Stone. 1953: Wiens, 1962; Niering. 1963; Stoddart & Sachet. 

 1969; Reese, 1987; Garnett, 1983). 



Generally speaking, atoll soils are calcareous and extremely 

 immature, a consequence of their limited age and frequent 

 disturbance by storms. Barely modified beyond the reef that 

 spawned their presence, they are composed primarily of calcium 

 and magnesium carbonates. Water retention, if any, is due to 

 accumulated organic matter and its associated chemical changes. 

 This is particularly important with respect to guano, which 

 reacts with coral sand and humus to form phosphatic hardpan 

 and nitrogen-rich "soils" (Fosberg. 1953). 



Reese (1987) categorizes atoll "soils" into five types, all of 

 which occur, in different proportions, at Caroline. The degree 

 of organic matter, decomposition, amount of humus, and the 

 depth of the "soil" strata are directly correlated with age and 

 size of the motus. 



/. Accumulations of coral rubble, mainly of stone size. 

 These youngest of "soils" are most evident around the edges of 

 the motus. acting as a substrate for pioneer herb mats. Often 

 extending well inland, they can support surprisingly lush 

 Tournefortia scrub. 



2. Unaltered coral sand and gravel. Although exposed 

 sand was uncommon at Caroline in 1 988, this substrate occurred 

 intertidally where the lagoon was filling in and on actively 

 growing sandbars, primarily in the upper lagoon (Pis. 23,28), 

 northeast and northwest South (Pis. 24,32), and the lagoon ward 

 edge of Shark (PI. 29). In 1990, sand was more ubiquitous on 

 Caroline as a result of the deposition of tons of sand during the 

 severe February storm. 



3. Soils with a weakly developed A-horizon, with color 

 i mly slightly darker than the unaltered sand below, but with no 



evidence of structural development. Especially evident in 

 1988 within the ancient interislet channels that compose Long 

 Island (PI. 33), much of this substrate is now storm-eroded and 

 overlain by fresh sand. 



4. Soils with a more developed A-horizon. deeper and 

 darker than above, with some structural development. This 

 stage defines areas where the rubbly/sandy substrate approaches 

 a true, but poor, "soil." As such, it represents older, more stable 

 parts of each island. It is common within the islet interiors 

 where Pisonia is (or was) present. Humus and guano fill the 

 gaps within the irregular shapes of eroded coral. Its composition 

 may be likened to a coarse mixture of gravel, sand, bones, and 

 shells, all mixed with sparse amounts of partly-decomposed 

 litter. Land crabs are particularly numerous, helping break 

 down organic matter into finer particles. 



5. Soils with an accumulation of raw humus on the surface 

 and with a relatively deep A-horizon. During this stage 

 phosphatic hardpan may develop. These true soils, though 

 somewhat depleted by guano diggers, cover significant areas 

 on South and Nake. Cocos and/or Pisonia debris adds greatly 

 to their dark color and moisture content. This earthy substrate 

 is composed primarily of rotting Cocos fronds and fibers that 

 have been shredded by coconut crabs. Patches of blackish 

 muck on South Island support local patches of Polynesian 

 arrowroot (Tacca leontopetaloides). 



On Caroline, hardpan (PI. 76) was present in several areas 

 (primarily South, Nake, Long, Emerald, and Mannikiba), 

 supporting herb mats and Tournefortia scrub. This substrate 

 may be likened to an old asphalt road. 



Caroline provides an excellent example of the progression 

 of soil development through islets of different age and size 

 classes (see Ecological Succession section). From a wave- 

 washed mound of coral rubble, barely above sea level (Fig. 5), 

 the substrate gradually improves in texture and fertility as the 

 emerging islet ages and organic matter accumulates. Pioneer 

 plants are hardy, salt-tolerant, low-lying mats consisting 

 primarily of Heliotropium, and later, Tournefortia. Increasing 

 numbers of shrubs provide shade and branches for nesting 

 seabirds. Larger trees (Pisonia. Cordia, Morinda) add more 

 shade and thereby increase humidity, as well as provide 

 opportunities for additional organic "fallout": leaves and bird 

 remains (nests, eggs, chicks, droppings, regurgitated food, 

 dead adults). 



Each stage of substrate development accelerates the 

 accumulation of organic material and helps to define an 

 emerging, deeper A-horizon. Soil maturity is indicated by 

 more organic matter, improved soil texture, and a lowered 

 volume of coralline and mollusean debris. Caroline's soils 

 barely exceed several centimeters in depth and are always 

 intermingled with coral fragments. As a result, they are 

 unsuitable for burrowing seabirds, such as petrels and 

 shearwaters. 



Hydrology 



Hydrological information is essentially lacking. No 

 standing fresh water exists. The quality, extent, and salinity of 

 the freshwater lenses, as well as their variability according to 

 tide, season, and rainfall, are unknown. At the time of Caroline's 



13 



