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Home , Atoll, Coral island

...what is an ?

by Jan Newhouse

It is characteristic of humans that we invested by photosynthetic algae (a) Truncation. This theory is based attempt to define precisely and/or (zooxanthellae) which can only survive, on the supposition that a high, categorise all that surrounds us. It is also grow and complete their role(s) within volcanic has slowly eroded characteristic that such attempts often the euphotic zone — the water layer away because of atmospheric and lead to difficulty and disagreement (e.g., which is penetrated by sufficient radiant -generated forces. After at what height does a hill become a energy to permit these activities. Though complete had taken place, ?). So it is with the application the thickness of this water layer varies the resulting surface was covered of terms to types. Definitions and from place to place depending on by reef-building . Sufficient classifications have often resulted from turbidity, a generally accepted figure is evidence exists to discount this limited personal experience or bias, rather 80 metres. Hence, relatively shallow reefs theory. than from a consideration of the broad (fringing and barrier) bordering volcanic (b) Antecedent platform. This theory range which exists. Thus it seems wise to are automatically excluded, as are supposes that, at some time in the seek an operational definition for "atoll". patch reefs found in atoll . past, the floor of the ocean was, The word atoll is taken from the In any case, since the interest here is or came, within the euphotic Malayalam atolu, "reef, which is the mainly focused on the primarily cal­ zone. Corals began to grow on the native name for the Maldive Islands. How­ careous materials found above the high bottom and, eventually, broke (or ever, many of us do not consider all levels {motu or ), whether they came close to) the ocean's surface to be exactly like those of the . be on an atoll or barrier reef, it is this to become an atoll. It is possible Neither would we wish to be as restrictive focus which should dominate. that several of the as required by the dictionary definition: The following discussion of the origin atolls arose in this manner. "A ringlike island and reef that of both barrier and atoll reefs is theo­ (c) Subsidence. A rift, a crack or a nearly or entirely encloses a ". (By retical but well supported by knowledge "hot spot" opens on the ocean such a definition, one would have to pieced together from the clues we have at floor and, through this, lava is exclude from consideration places such as the present time. extruded. If this process should Makatea, Helen Reef and Johnston Island. continue for a very long ­ Besides, what is meant by "nearly"?). Geological origin logical period, two phenomena Given the full range of atoll topo­ The numerous theories which have result. For one thing, the floor graphy, features such as and lagoons been proposed to account for atolls are will slowly sink because of the should not be considered in the definition. about as profuse as the definitions. All weight of the new volcanic mass It also follows that we wish to exclude of them presuppose one or both of two and, secondly, the mass may islands with lithic features which are possible conditions: either the submarine eventually reach or rise above the volcanic in origin, and other reef types basement on which the atoll reef stands surface. such as fringing, barrier and patch. Such a and the water level of the ocean have If the new island is in an oceanic area definition is as follows: been static during atoll formation, or where reef growth can take place, four they have undergone relative shifts up activities follow: there may be continued "An atoll is a living reef which is or down. island growth (as on the island of Hawaii), separated from the nearest land of Since some of these theories stretch subsidence is an ongoing process, fring­ volcanic origin by water having a the bounds of credibility (e.g., floating ing reefs will develop, and atmospheric depth greater than that at which coral platforms, coral pounded by waves erosion will inhibit reef growth where hermatypic corals can grow". into volcanic fissures to depths greater there is freshwater run-off. Hermatypic — that is, reef-building — than 1300 metres), only the major Should the island continue to sink corals are symbiotic organisms. They are theories will be taken up here: more rapidly than it is being built up by

Page 4 SOUTH PACIFIC BULLETIN THIRD QUARTER 1980 (1) Let us suppose that an island has sunk to the reef stage since the latest great rise in ocean level. We would expect to find a - topped reef without a lagoon, e.g., Oeno. (2) If the island has been sinking dur­ ing both the drop and rise, yet is still above the ocean level, one would expect a barrier reef to have developed at the margin of the old eroded , e.g., , Truk, Huahine. (3) If all volcanic portions had dis­ appeared before the latest great drop and rise, one would expect a new reef margin to have grown up from the former table reef. In this case, there would be a classic lagoon in an atoll fitting the dic­ tionary definition, e.g., , Puluwat, Manihi. View to the lagoon, Huahine, French . (4) If, however, there have been tectonic forces as well as the eustatic rise and fall, we could expect a number of varied forms, e.g., Oroluk, Nauru, Swain's. continued lava flow, the ultimate result and rise being 100 and 80 metres Since our interest here is with cultiva­ would be a broad "table reef with no respectively. tion on islets, it is important to emphasise material of volcanic origin above the With this thought behind us, it is now that there are six means by which mater­ ocean surface. possible to lay out the four oceanic ials can arrive on a reef in such a way as We suppose that such processes features mentioned above: to be found above the high tide level: started millions of years ago, and are still continuing today. The resulting oceanic features we see today are of four kinds but, before taking these up, it would be well to discuss eustatic shifts in the level of the ocean. Carbon dioxide (C02) acts as a blan­ ket, holding in the heat from the radiant energy reaching earth's surface. It is found in both the atmosphere and as a solute in the ocean. Let us suppose that we are in a period when the ocean level is low — that is, a cold period when much of the water is held in solid form at the poles. This means that there is less water in the , and a relatively large quantity of C02 is in the atmosphere. With such a condition, the temperature of earth will begin to increase, glaciers and polar regions will melt, and the level of the oceans will rise. As this occurs, more liquid water will be available for the solution of C02, the quantity of C02 in the atmosphere will become less, and the earth cools. All available evidence suggests that earth has gone through just such a series of pulsations, with the latest great drop Making a canoe in , .

SOUTH PACIFIC BULLETIN THIRD QUARTER 1980 Page 5 (1) In rare instances, alluvium from tures are moderated by the cooling precipitation/year for the major island the adjacent can flow breezes present on atolls. Atoll groups with atolls. All figures are out upon its fringing reef. Temae weather seldom seems oppressive rounded off: on Moorea is such an area, as was where there is exposure to the pre­ The variability from season to season Fetuna on Raiatea following the vailing winds". and year to year is extremely significant, hurricane of May 1978. Surface winds in the Northern Hemis­ almost as much so as the general average (2) There can be erosional remnants phere tend to be dominated by those rainfall. Since drought occurs on even the from the latest drop in level of which are easterly, and which rarely reach wettest atolls and the range of variation about two metres some 3000 gale force. Though gales are somewhat can be great, it is mandatory that these years ago. These remnants are more frequent in the Southern Hemis- extremes be taken into account when called rock or conglomerate, and are found on many atoll rims or even on patch reefs (e.g., TABLE 1: AVERAGE RAINFALL IN PACIFIC ISLAND ATOLL GROUPS Takapoto). (3) Due to shifting of the ocean's Group Rainfall (cm) floor, reef materials can be raised considerably above the present Marshall Islands 140 (Eniwetok) to 400 (Jaluit): there is a decrease from water level (e.g., Makatea). south to north. (4) Incidental introduction from else­ 200-300 where — as, for example, 70-330, with the southern atolls drier than those to the or stones caught in the roots of a north. log, or pumice which has drifted 200-280 in. 300 (5) Through human introduction; e.g., Phoenix 40-130 soil to Eniwetok, Wake, Canton 100-300 (often arriving as ballast) and 230-360 basalt artifacts prized by abori­ 120-190 ginal inhabitants. (6) Storm action. This is by far the prime origin of islet materials — phere, and storms occur more often in cultivation is considered. As examples of the reef-derived magnesium and the western Pacific, here, too, the winds seasonality: dry months in Tokelau are calcium carbonates which are are generally from an easterly direction. from April to September, and in the deposited by strong wave action. Rainfall is considerably more variable: Tuamotus from May through October. It is ironic that the conditions from one part of the Pacific to another, Severe yearly droughts are most feared most by atoll inhabitants by season, and from year to year. Table 1 marked of all. On the yearly rain­ are the same ones responsible for gives the relative magnitude (average) of fall has ranged from 17 cm to 220 cm. the land on which they live.

Climatic characteristics Though, in theory, atolls should have some influence on their local weather and climate, studies have not been sufficiently definitive, long-term or accurate enough to document these effects statistically. Since, in any case, these effects could be only minor, it is sufficient to state that atolls are subject to the general con­ ditions of the area in which they are located. "The climate of the Pacific atoll realm is marine and tropical in character. The range in air temperature is about 2°F annually, with a slightly greater range as one approaches 15 to 20° N and S latitudes. Winds are mostly those of the trades and associated cyclonic depressions. In the western extremities of the realm, the Asiatic and Australian continental air masses are a factor also. Average relative humidity is high, but sensible tempera- Reclamation land with coconut and fruit trees in , .

Page 6 SOUTH PACIFIC BULLETIN THIRD QUARTER 1980 Other examples are Fanning (70-530 cm), Penrhyn (90-380 cm) and Maiden (10—240 cm). As we all realise, the m& jQw^pf extremes of weather on atolls have much more significance than they do on high islands. Atoll dwellers have long known that the two most feared natural phenomena are storms, usually accom­ panied by too much water, and droughts, with too little. Although weather records have been much more complete these past few decades, I am not aware of any agency or institution which has compiled and co­ ordinated these with atoll cultivation in mind. Since it is obvious that comparative crop studies made on several atolls should include weather records before and dur­ ing the cultivation periods, it would be well to have a centralised clearing house for these data. Humidity, temperature, incident sun­ light, evaporation, rainfall and wind records should be taken daily, then analysed for both short-term correlations Fresh water is often scarce on atolls. and cyclic phenomena (these latter may permit predictive ability).

Freshwater resources For completeness, six sources will be and the cisterns are covered to dis­ sidered in dealing with . . . water inven­ considered, though, from the point of courage mosquitoes. For direct tories. In particular, because of tidal view of magnitude, several of these are human purposes, this technique is mixing, there may exist only a relatively quite insufficient for either in situ cultiva­ more widely practised on atolls thin layer of very fresh water, underlain tion or irrigation: than any other. by a deep transition zone of increasingly (a) The coconut. This certainly served (d) On an atoll, the water may either (with depth) brackish water". (This mix­ as the prime source of drinking be caught as above, evaporate ing may be affected by the geological water for several thousand years. from the land surface, be lost as structure of an islet). All things considered, including evapotranspiration from plants, "There are some fundamental limita­ the future, it is the most depend­ run off to the sea or lagoon (in tions to the exploitation of island ground able for this purpose. small quantity), contribute to water for a ... . water supply. Most saturation of the matrix making (b) Atoll peoples developed a tech­ obviously, average annual withdrawal up an islet, or percolate down to nique for channelling the water must be less than average annual recharge, the water table. running down the underside of since some of the input is inevitably lost coconut trees into containers. Depending on three variables, islet to natural mixing processes, etc. Perhaps Generally speaking, a coconut size and shape (shape at least as important less obvious is the problem created by leaflet was tied on the underside, as size), rainfall (both frequency and withdrawing all or most of the potable close to the ground level. This quantity) and permeability, a freshwater water from the lens or some portion of it. was introduced into containers lens of varying size and stability will form This water is replaced in part by more such as a coconut shell, a wooden over the underlying salt water. This lens saline water from the transition zone, and bowl or the shell of Tridacna. (often called Gyben-Herzberg) will tend when the lens is recharged some of the (c) Water catchment above ground to float because of the differences in salt remains behind in the sediments to level is still practised today, but density between fresh and salt water. It mix with the incoming fresh water. If more often and with considerably has one part above mean for repeated, this will result in a gradual rise more efficiency. Galvanised roof­ every forty below, is in dynamic equilib­ in surface salinity, even if the actual ing and rain gutters catch and rium (a function of tidal change, rainfall, volume of fresh water is replaced each direct water into cisterns of vari­ evaporation and withdrawal by plants and year. ous sizes and construction. In humans), and has a relatively short resi­ "A related problem is that of upconing some cases, this water is passed dence time compared with water tables of saline water if withdrawal rates are too through screens to remove foreign on high islands. high; a wide, thin lens of potable water material (e.g., lizards and leaves), "Further complications must be con­ cannot be fully utilised by pumping hard

SOUTH PACIFIC BULLETIN THIRD QUARTER 1980 Page 7 at a few isolated points, as this will pull Given these measurements, and accurate stocks, there are three monitoring pro­ up brackish water from below faster than figures for drawdown by humans, a fresh­ grammes and/or studies which are recom­ the fresh water can recharge (the well water budget can be constructed. mended here in connection with atoll laterally). Slow withdrawal from as wide The thickest part of a lens is generally cultivation: an area as possible is the appropriate nearer the lagoon than that of the • Records of precipitation, evaporation technique. ocean. This is because the smaller parti­ and incident sunlight. "Finally, it must be realised that if a cles making up an islet are found nearer • Measures on the lens itself, as well as lens system contains as its equilibrium the lagoon: islets having their origin in the factors which affect it. inventory about 10 years' worth of storm-carried materials, and the smaller • The feasibility of distillation. This recharge, it will be possible to withdraw materials are carried further. As well, should include the reliability of water in excess of the recharge for several many lagoon are subject to local sources of supply for all required years before depletion becomes obvious currents which bring and deposit finer materials. . . . Without careful management, it will particles. be possible to come to rely on a higher (f) Finally, there is the possibility of (e) There is a distinct possibility that production rate than is actually sustain­ barging fresh water from outside distillation might be feasible to able on a long-term basis". the atoll in question. It should be augment natural freshwater sup­ The management of the lens is vital, mentioned that, for those who plies. Highly technical means whether the concern be for use by plants make a living by studying resour­ which rely on fuels are neither which are growing on the surface (e.g., ces, water, though possibly the economic nor advisable, but breadfruit) as in pits (e.g., taro), or for most important, is considered to simple yet possibly extensive water to be used in irrigation. As a be the least economic resource. systems, using black paint, cloth natural system, it needs to be understood The plan to tow an iceberg from or plastic, could provide water for in terms of present characteristics, the Antarctic to Saudi Arabia human consumption, thereby per­ seasonal variation and long-term averages. foundered on economic grounds mitting the natural water to be Parameters of particular interest are rain­ even though the Saudis have used exclusively for crops. fall, evaporation, approximate "head", plenty of money and icebergs are tidal responses and the chloride content. Thus, in consideration of freshwater free! D

Page 8 SOUTH PACIFIC BULLETIN THIRD QUARTER 1980