9An Overview of The Nature and Dynamics of Reef Islands
Hiroya YAMANO,1,2 Hajime KAYANNE3 and Masashi CHIKAMORI4
1

Social and Environmental Systems Division, National Institute for Environmental Studies,
16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japane-mail: hyamano@nies.go.jp
2
UR 128 CoRéUs, Institut de Recherche pour le Développement,
BP A5 98848 Nouméa Cedex, New Caledonia
3
Department of Earth and Planetary Science, The University of Tokyo,
3-7-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
4
Faculty of Informatics, Teikyo Heisei University, 2289-23 Uruido-Otani, Ichihara, Chiba 290-0171, Japan

Abstract
Reef islands are formed entirely of bioclasts produced in adjacent reefs that formed in response to
Holocene sea-level changes. Thus reef islands are regarded as combined products of geological, physical, and ecological processes. They have diversity in size, constituents, sedimentation pattern, anddynamics, which would be a result of finer-scale variations in environmental factors. We provide an
overview of the developmental history of reef islands and factors controlling their formation andmaintenance. The factors are: 1) Geological: relative fall in sea level in the late Holocene, 2) Physical:
waves, currents and cyclones, and 3) Ecological: distribution of reef-building organisms and land
vegetation, and factors that influence coral-reef development. We show how the factors are changing or
predicted to change in the future, and propose two integrated studies: a process-based modeling study
and a mapping-based comparative study.
Key words: atoll, coral reef, global climate change, reef islands, sea-level rise

1. Introduction
Reef islands on atolls and Platform reefs (Fig. 1)
are low and flat with a maximum altitude of several
meters. Thus they are highly vulnerable to extreme
events (e.g., cyclones) and environmental changes(e.g., sea-level rise) (Roy & Connell, 1991; Sherwood& Howorth, 1996; Leatherman, 1997; Nurse et al.,2001). Understanding the controlling factors for reefisland formation and maintenance would contribute toa better ability to predict the future status of islands
and to establish countermeasures to the changes.
These islands are formed entirely of bioclasts produced in adjacent reefs that formed in response to
Holocene sea-level changes. They are naturally dynamic, and erosion, deposition and cementation of
sediment on islands can occur concurrently(Woodroffe, 2002a). Some islands are vegetated, andthe vegetation protects the islands from erosion(Wiens, 1962; Chikamori, 2001). Thus, reef islands
are regarded as combined products of geological,
physical, and ecological processes. They can be split
into two major groups: motus and sand cays (Stoddart& Steers, 1977). The seaward beach of the motus is
Global Environmental Research9(1)/2005: 9-20
printed in Japan

formed of cobbles and gravel that have generally been
carried from the reef front by cyclones (e.g. Wiens,
1962; Maragos et al., 1973; Scoffin, 1993). On theother hand, sand cays and the lagoonward beach of themotus are generally composed of sand thought to havebeen deposited under normal, not stormy, conditions(Wiens, 1962; Schofield, 1977) (Fig. 2). Gourlay(1988) studied sand cays in the Great Barrier Reef ofAustralia, and found that reef geology (Holocene sealevel changes and reef evolution), wave action, reef
shape and orientation, and climatic variations (winds
and cyclones) were factors influencing the formationand stability of the cays.
While their structure is classified into two general
types (motus and sand cays), reef islands have diversity in size (Fig. 1), constituents (Table 1), sedimentation history (Woodroffe, 2002b), and dynamics (Floodand Heatwole, 1986; Bayliss-Smith, 1988) (Fig. 3).
The diversity of reef islands would be a result of finerscale variations in environmental factors (Woodroffe,
2002a). In this paper, we describe the history of reefisland formation and present the controlling factors.
Then, based on the factors, we depict the islands’
response to environmental changes, and propose©2005 AIRIES10

H. YAMANO et al.

integrated studies on reef islands. We focus on naturalenvironmental factors that are not a direct result of
human activities.

2. Evolution of Atolls and Reef Islands
It should be pointed out the evolution of atolls has
two contrasting aspects, the structure of atoll foundations and the surface morphology of atolls, based on
the formative processes and timescales involved(McLean & Woodroffe, 1994). Atoll structure is explained as a result of long-term (~1 million years) subsidence of a volcanic island, in accordance with what
is called Darwin’s theory. The gradual subsidence ofthe volcanic basement produces an evolutionary
sequence from fringing reefs to atolls. The subsidence
is attributed to the motion of the lithospheric plate

Fig. 1(Scott & Rotondo, 1983) (Fig. 4).
Though subsidence controls the structure of atolls,
the surface morphology is closely influenced by Quaternary and contemporary environmental factors.
Modern coral reefs have formed in association with
Holocene sea-level changes (e.g., Kayanne, 1992;
Montaggioni, 2000) (Fig. 5). The thickness of
Holocene reefs in atolls is 8-28 m (Dickinson, 2004),
indicating aerial exposure of the atolls during the glacial and early Holocene periods (Fig. 6). In most regions of the Indo-Pacific, the sea level reached its present level at around 7,000-6,000 years BP. The reef
crest generally caught up with the sea level before
4,000 years BP, the vertical accommodation space wasfilled due to a stable sea level, and the reef framework
expanded laterally. Subsequently, backreefs werefilled with bioclasts, and reef islands formed (Tudhope,

A Global distribution of coral reefs. Numbers indicate the location of coral-reef areas
referred to in this study. 1: Maldives, 2: Cocos (Keeling), 3: Great Barrier Reef of Australia,
4: Caroline Islands, 5: Marshall Islands, 6: Kiribati, 7: Tuvalu, 8: Hawaii, 9: Northern Cook
Islands, 10: Tuamotu Archipelago, French Polynesia, and 11: Belize. B An example of reefislands on an atoll (Majuro Atoll, Marshall Islands), indicating reef islands have diversity insize and shape within an atoll. Patchy, small motus (i) are located on the windward side of
the atoll, while long (ii) or large, vegetated (iii) motus are located on the leeward side of theatoll. Coral reef distribution data are from the Reef Base (http://www.reefbase.org).
Coastlines are from the Global Self-consistent, Hierarchical, High-resolution Shoreline
database (GSHHS) (Wessel & Smith, 1996). Contours are for the isothermal line in thecoldest month, 18 ˚C, which is considered to be needed for coral reef formation. The map
was prepared by Hiroto Shimazaki.

Nature and Dynamics of Reef Islands

11

ca. 200m

ca. 20 cm

Fig. 2 Schematic figure of a reef flat structure as indicated by an oblique view and cross section of a
Pacific atoll rim example. Photos indicate present-day morphology of a motu and reef-building
organisms contributing to the island sediment mass. The oceanward ridge is formed by coral
cobbles transported during cyclones, while the lagoonward beach is formed by sands. Arrows
indicate foraminifera that dominate beach sands in the western Pacific region (see Table 1).
Photos of coralline algae, Halimeda and foraminifera are from Ui et al. (1987).
Table 1 Constituents of reef island beach sediments
Sea

Island type

Reef type

RegionSite

Constituent*
Location
number C CA H
F
M
Referencein Fig. 1A
BC O
1◎Woodroffe (1992)2○ ○◎Woodroffe et al. (1999)***

Indian Ocean

Cay
Motu

Atoll
Atoll

Maldives
Cocos (Keeling)**
Cocos (Keeling)

Pacific Ocean

Cay

Platform reef

Green Island3○ ○◎ ○ ○ Yamano et al. (2000)

Cay

Platform reef

Great Barrier Reef ofAustralia
Great Barrier Reef ofAustralia

****

3

◎

○

Motu
Motu
Motu
Motu
Motu
Motu
Motu
Motu
Motu
Motu
Motu

Atoll
Atoll
Atoll
Atoll
Atoll
Table reef
Atoll
Atoll
Atoll
Atoll
Atoll

Southern Carolines
Marshall Islands
Marshall Islands
Marshall Islands
Marshall Islands
Kiribati
Kiribati
Kiribati
Kiribati
TuvaluNorthern Cook Islands

Kapingamarangi
Eniwetok
BikiniRongelap
Rongerik
Makin
TarawaTarawaButaritari
Funafuti
Suwarrow

4
5
5
5
5
6
6
6
6
7
9

○

Atoll
Atoll
Atoll
Atoll
Atoll

MexicoBelizeBelizeBelizeBahamaAlacran Reef
Turneffe Islands
Lighthouse Reef
Glover's Reef
Hogsty Reef

11
11
11
11
11

○
○
○
○
○ ○

Caribbean Sea Cay
Cay
Cay
Cay
Cay

McLean and Stoddart (1978)◎ ○ McKee et al. (1959)◎
Emery et al. (1954), Todd (1960)○ ○○Emery et al. (1954), Todd (1960)◎○ Emery et al. (1954), Todd (1960)◎
Emery et al. (1954), Todd (1960)◎ ○Woodroffe and Morrison (2001)◎
Weber and Woodhead (1972)◎○ Schofield (1977)◎○ Schofield (1977)○ ○ ○ ◎ ○Collen and Garton (2004)○ ○ ○◎Tudhope et al. (1985)◎◎◎◎

Folk and Robles (1964), Folk and Cotera (1971)
Stoddart (1962b)
Stoddart (1962b, 1964)
Stoddart (1962b)○ Milliman (1967)* C: corals, CA: Coralline algae, H: Halimeda, F(BC) Baculogypsina and Calcarina, F(O): Other foraminifera, M: Molluscs◎: Major constitutent, ○: Other constitunets** General information*** Foraminifera species were assumed from the distribution of marine biota described in Kench (1997)**** Summary of ~200 samples from 31 islands

12

H. YAMANO et al.

A. Frequent storms

B. Occasional storms

C. Storm freeFig. 3Model of reef-top morphology and response to storm perturbation. The structure anddynamics of reef islands are constrained by the intensity of storms and cyclones.
Reproduced after Bayliss-Smith (1988), McLean and Woodroffe (1994) and Woodroffe(2002a).

Asthenosphere

Melting anomaly

Fig. 4 Model of atoll formation (Scott & Rotondo, 1983).
Island-atoll types on the Pacific plate result primarily
from the subsidence of volcanic islands due totangential motion of the lithospheric plate surface.
Reproduced after Scott and Rotondo (1983) andHopley (1982).

Fig. 5 Caribbean and Indo-Pacific schematic sea-level
curve and evolution of reef facies. Reproduced after
McLean and Woodroffe (1994). Note that in someareas no evidence of sea-level highstand was shown(e.g., Western Indian Ocean; Camoin et al., 2004),
and the period of sea-level highstand may differ(e.g., Grossman et al., 1998; Dickinson, 2001,
2003).

Nature and Dynamics of Reef Islands

Fig. 613

A sea-level curve of the last 140,000 years and the evolution of atolls and reef islands in
response to Quaternary sea-level changes. Reproduced after McLean and Woodroffe(1994).

1989; McLean & Woodroffe, 1994; Woodroffe et al.,
1999; Yamano et al., 2001; Kayanne et al., 2002) (Fig.
6). The existing atoll surface landforms have a time
depth of generally <1,000-2,000 years (Dickinson,2004). Anatomies of reef islands from three regions(Cocos (Keeling) in the eastern Indian Ocean,
Warraber Island in the Great Barrier Reef of Australia,
and Makin in the western Pacific Ocean) revealed remarkably individualistic accumulation patterns of theislands (Woodroffe, 2002b). In the Cocos (Keeling),
the predominant mode of continuous accretion has
been oceanward. On Makin, accretion commenced at
the central part of the island. On Warraber, growth of
the island seems to have stopped at 2,000 years BP.
In the Caribbean region, the sea level continued torise throughout the Holocene without stabilization,and reefs grew up in accordance with the rising sea
level in general (e.g., Lighty et al., 1982; Neumann &Macintyre, 1985; Gischler & Hudson, 2004), whiledrowning of the reefs due possibly to a jump in sea
level was suggested (Blanchon & Shaw, 1995).
Despite the presence of detailed studies of reef evolution, studies on the evolution of reef islands in the
Caribbean region are scarce. Thus overall, the formation process of islands is still enigmatic, though a general developmental history has been presented for the
Pacific region.

3. Environmental Controls
3.1 Factors controlling coral reef formation
Reef islands on atolls have their foundation oncoral reefs, which act as a natural breakwater and produce a depositional environment suitable for island
maintenance. The bioclasts that form reef islands arederived from reef-building organisms (e.g., corals,
calcareous algae and foraminifera) distributed on coral
reefs. Thus, understanding the evolution and controlling factors of coral reefs provides a background forstudies on the islands. The first-order determination
factors for global reef distribution are sea surface temperature (SST), salinity, nutrients, light availability,
and aragonite saturation state (Kleypas et al., 1999b).
The aragonite saturation state is determined by theconcentration of CO2 in the atmosphere. A second-order, but regionally important constraint on reefdevelopment may be wave energy that leads to a conspicuous contrast in reef geomorphology between
windward and leeward margins, with better framework development along exposed windward than protected leeward reefs (Blanchon & Jones, 1995;
Yamano et al., 2003). This could be because vigorous
water motion replenishes food and nutrients, removes
sediment, and moderates extremes in temperature,salinity, and dissolved oxygen concentration (Wells,
1957).
3.2 Factors controlling reef-island formation
3.2.1 Geological perspective
Relative sea-level fall in the late Holocene
In most coral reef regions of the Indo-Pacific, inthe late Holocene after sea-level stabilization, a relative fall in the sea level occurred (Eastern Indian
Ocean: Eisenhauer et al., 1993; Woodroffe et al., 1999,
Pacific Ocean: Grossman et al., 1998; Dickinson,
2001) (Fig. 5), which was attributed to global isostatic
adjustment (Mitrovica & Milne, 2002). Though reefislands are found in the Caribbean and western Indian
region where no sea-level fall was observed(Caribbean Sea: Toscano & Macintyre, 2003; western
Indian Ocean: Camoin et al., 2004), this relative
sea-level fall is suggested to have had a significant
effect on reef island evolution (Yamano, 2002). It issuggested that both the depositional environment andthe distribution of reef-building organisms were modified by this relative sea-level fall (Buddemeier et al.,
1975; Scoffin, 1993). Scoffin (1993) suggested that

14

H. YAMANO et al.

storm-induced coral rubble was left on reef flats because of the relative sea-level fall, thus forming shingle ramparts, while storm intensity was assumed to beconstant throughout the late Holocene. At present,
larger benthic foraminifera are abundant on some reef
crests in the western Pacific region (e.g., Jell et al.,
1965; Sakai & Nishihira, 1981; Hohenegger, 1994;
Yamano et al., 2001). The occurrence of abundant
foraminifera on reef crests was hypothesized to havebeen induced by subaerial exposure of the reef crest in
low tides due to the relative sea-level fall, and theforaminifera were transported and ultimately produced
the sand cays and sand beaches of the motus (Yamano
et al., 2000).
Thus, the timing of the relative sea-level fall could
be related to the timing of island formation. The timing differs from region to region (e.g., 2,000 years BP
in the Marshall Islands and 1,250 years BP in the
Tuamotu Archipelago; Grossman et al., 1998;
Dickinson, 2003); and this could affect the evolutionof the islands. Dickinson (2003, 2004) reviewed thesea-level history in the late Holocene and indicatedthat the timing of complete emergence of paleoreef
flats is important because they produced stable islands
for human settlement. Unfortunately, in comparison
with the ages of human settlement on reef islands,
information on the ages for the islands themselves is
scarce, so the timing of reef-island formation,
sedimentation history and their constraints remain to
be examined, based on the ages of island sediments. In
addition, mechanisms of island evolution in regionswith no relative sea-level fall have not been presented.
3.2.2 Physical perspective
Waves and currents
Hydrodynamic environments on reef flats are afactor in the transport and deposition of bioclastic
sediments, and active transport of the sediments has
been observed (Flood, 1974; Kench, 1997; Yamano etal., 2000). Currents in coral reefs are produced bywinds, tides, and waves breaking at the reef edge
(Andrews & Pickard, 1990). On the oceanward side of
the islands, waves and currents derived from wind
waves and ocean swells determine beach profile(Cowell & Kench, 2001; Kench & Cowell, 2001). The
effect of waves breaking depends on water level,
which is determined by tides (Kraines et al., 1998).
On the lagoonward side, transport of sediments iscaused by refracted ocean swells and wind waves developed in the lagoon (Yokoki et al., submitted).
The shape of reefs is important locally. Gourlay(1988) indicated that diffracted waves around the platform reefs are responsible for the creation and maintenance of coral cays, and suggested that the size, shapeand orientation of the reef platform determine whether
a cay will form and be stable. This could also be trueof atoll islands, as large, vegetated islands formed in
the corner of atoll rims (Wiens, 1962; Yamano et al.,submitted) (Fig. 1).

Tropical cyclones
The intensity of tropical cyclones appears to affectthe evolution of reef islands (Bayliss-Smith, 1988;
Woodroffe, 1992) (Fig. 3). While coral communities
suffer catastrophic damage from cyclones (Woodley
et al., 1981; Harmelin-Vivien & Laboute, 1986),
cyclones have both depositional (positive) and erosional (negative) effects on reef islands (Blumenstock,
1958; Harmelin-Vivien, 1994). While disappearanceof cays was observed in Belize (Stoddart, 1962a),
coral rubble was deposited to form new islands in
Tuvalu (Maragos et al., 1973). Sequential studies of
the islands revealed that these new islands formed bya cyclone were attached to the original islands andcontributed to an increase in land area (Baines &McLean, 1976). Oceanward ridges composed of coral
cobbles are common on most reef islands (McKee,
1959; Wiens, 1962; Siddiquie, 1980; Delesalle et al.,
1985; Scoffin, 1993), but sand dominates island sediments outside the cyclone belt (Woodroffe, 1992;
Woodroffe & Morrison, 2001) (Fig. 3), both indicating
the importance of the cyclones.
3.2.3 Ecological perspective
Distribution of reef-building organisms and theircalcification
Because reef islands are composed of bioclastic
sediments, distribution of reef-building organisms isthe primary factor determining the constituents of reefislands. Reef islands are composed mainly of corals,
calcareous algae (mainly coralline algae and Halimeda),
molluscs and foraminifera (Fig. 2), the proportions ofwhich differ from region to region (Table 1). Thus, thebiogeography of these reef-building organisms is important. In the western Pacific, dense populations ofbenthic foraminifera (Baculogypsina and Calcarina)
are found on shallow reef flats (Sakai & Nishihira,1981), and the tests contribute to island sediment mass
(Table 1). In the Pacific, there are differences in dominant foraminifera species on the reef flats. Calcarina
and Baculogypsina are specific to western Pacific,
while Amphistegina are common throughout the
Pacific (Murray, 1991; Langer & Hottinger, 1999).
These differences should cause the variation in theisland’s constituents. Sediment constituents reflect this,though we admit that the samples are insufficient(Table 1). Amphistegina tests contribute to the sediment mass of the Suwarrow Atoll, central Pacific(Tudhope et al., 1985), while Baculogypsina and/or
Calcarina tests dominate the sediments of western
Pacific atoll islands. Even if species are common,
physio-chemical factors described in the previous
chapter (SST, salinity, nutrients, light availability, andaragonite saturation state) could affect the CaCO3 production of the reef-building organisms, the substanceforming the reef islands. In addition, differences inthese factors could be correlated, in part, with thebiogeography of the reef-building organisms.
On the reef flats, the distribution of reef-buildingNature and Dynamics of Reef Islands

organisms is constrained by wave energy and reef
geomorphology. Wave energy is a major factor determining distribution of reef-building organisms, anddistinct ecological zonation in response to wave
energy is found throughout the world (Geister, 1977;
Done, 1983). On shallow reef flats, another significant
effect is subaerial exposure during low tides (Pugh &Rayner, 1981). In the western Pacific, corals cannot
live on reef crests subaerially exposed during low
tides, but abundant foraminifera occur instead (Sakai& Nishihira, 1981). Halimeda seems to be abundant in
the Caribbean region, where no sea-level fall occurred
and the reef flat would not have been exposed to air,and contributes to sediment mass (Folk & Robles,1964). The degree of exposure would be a function of
tidal range, reef development and relative sea-level
fall in the late Holocene.
Distribution of land vegetation
Stabilization of coastlines by coastal vegetation
may be another important ecological factor in the formation and maintenance of reef islands (Chikamori,2001). He suggested that there was a zonation in natural vegetation on islands in relation to the distancefrom the shoreline. Because beaches are affected bysalt spray, only salt-tolerant species (Pemphis acidulaand Scaevola frutescens) can occur there. These species could have initially settled on the islands and contributed to their stabilization (Fig. 7).
Globally, distribution of flora is constrained by the15

amount of precipitation (Fosberg, 1956; Stoddart,
1992; Mueller-Dombois & Fosberg, 1998). From the
Marshall Islands to Tuvalu, Fosberg (1956) identified
nine vegetation zones based on annual rainfall, andshowed the correlation to vegetation. Pemphis appears
only in regions that receive mean annual rainfall of
more than 1,000 mm.

4. Changing Environments and Their Possible Impacts on Reef Islands
As described above, reef islands are combined
products of various factors. Many of these factors arechanging, or are predicted to change in the future(IPCC, 2001). The response of reef islands could be
discussed based on the synthesized knowledge of
IPCC climate change evidence and scenarios (e.g.,
Sem et al., 1996). According to the IPCC Third Assessment Report (2001), emissions of CO2 due to fossil fuel burning are virtually certain. The concentrationof CO2 in 2100 is projected to be between 540 and
970 ppm, while the concentration in 2000 was 370
ppm. The rate of sea-level rise from 1990 to 2100 will
be between 0.09 and 0.88 m. Though there are stillinsufficient data regarding the intensity of cyclones,
an increase in cyclone peak wind intensities, as wellas mean and peak precipitation intensities, is ‘likely’
to occur in the future. Larger year-to-year variations inprecipitation are ‘very likely.’ SST was shown to haverisen at a rate of 0.044˚C/decade from 1861 to 2000.

Fig. 7 Scaevola frutescens settled on a storm ridge formed
during Cyclone Bebe in 1972 (cf. Maragos et al.,1973). Photos courtesy of Hiroto Shimazaki.

16

H. YAMANO et al.

In addition, winds and sea-level fluctuation couldchange due to the changes in atmospheric circulation(e.g., El Niño and the monsoon).
These environmental changes could affect thestatus of the islands. McLean et al. (2001) suggested
the increasing CO2 concentrations and the SST rise
would result in a coral-reef decline. Increasing CO2
concentration would reduce the aragonite saturation
state, resulting in the lower calcification rates of thereef-building organisms (Kleypas et al., 1999a). The
reduced production of CaCO3 could result in a loss of
island sediments. The SST rise would cause more frequent coral bleaching (Hoegh-Guldberg, 1999;
Sheppard, 2004). Guinotte et al. (2003) simulated thestatus of coral reefs under a climate change scenario,
and predicted that rising atmospheric CO2 and SST
will threaten the all the coral reefs in the Pacific
Ocean. The decline of corals could result in the possible disappearance of shingle ramparts because of noformation of new ramparts due to the disappearance ofcorals, as predicted by Williams et al. (1999).
A sea-level rise could have two effects on
reef-island maintenance (Fig. 8). One is increased
wave activity due to drowning of the modern coral
reef that breaks waves on the reef edge (Roy &Connell, 1991). The lagoonward beach would also be
inundated (Sem et al., 1996). The threshold value of
sea-level rise for reef drowning was estimated to be
0.4 m/century (Kayanne, 1992). The second is redistribution of reef-building organisms on the reef crest
due to the sea-level rise (Yamano, 2002). This could
be true of the Pacific region where larger benthic
foraminifera, which comprise beaches and sand cays
(e.g., Todd, 1960; Schofield, 1977; McLean &Stoddart, 1978), densely populate reef crests that are
subaerially exposed at low tides (Fig. 1). The habitat
was shown to have been produced by the relative
sea-level fall in the late Holocene (Yamano et al.,2001). Thus, the habitat of foraminifera could
decrease as the sea level rises at any rate, and may
disappear when the low water level is situated above
the top of the reef crests. On the other hand, underhigher sea-level conditions than that at present, corals
would flourish again instead of foraminifera. While
the CaCO3 production by coral is larger than that offoraminifera, sand production could be reduced, because coral contributes to coral-reef framework development (Yamano, 2002) (Fig. 8). Using a Shoreline
Translation Model, Kench and Cowell (2002) simulated the retreat of oceanward coastlines due toFig. 8 Possible scenarios of the response of reef flats to rising sea level. The threshold rate of sea-level rise (0.4m/century) for keep-up/give-up of the reef crest is from Kayanne (1992). A1 Pessimistic scenario. The reef cankeep up with the rising sea level because of the small rate of rising and vigorous growth of corals, and it canprotect the islands from waves. However, foraminifera disappear, and the beach composed of foraminifera tests
now is eroded. A2 Most pessimistic scenario. The reef cannot keep up with the rising sea level because of large
rate of rising. Waves increase and foraminifera disappear, resulting in a severe damage to the reef islands. B
Optimistic scenario. The reef can keep up with the rising sea level because of the small rate of rising andvigorous growth of corals, and it can protect the islands from waves. Foraminifera disappear, but a substitute,such as Halimeda, appears and contributes to the maintenance of the beach. This scenario might be possible
also, because reef islands, composed of Halimeda, are found in the Caribbean region (Table 1) where the sea
level has been rising and continues to rise (Fig. 5). Reproduced after Yamano (2002).

Nature and Dynamics of Reef Islands

sea-level rise, considering the change in both wave
action and sand production on reefs. They indicatedthat reduction of sediment supply on the reef flat
would result in a two-fold increase in the rate ofshoreline retreat in cases of 0.5 and 0.9 m increases in
sea level. This result indicates the importance of considering the effects of both physical and ecologicalchanges on changes in reef islands.
Changes in climate, including winds, intensity ofcyclones, and precipitation, affect the dynamics ofreef islands. Wind-induced waves appear to affect the
dynamics of sand beaches (e.g., Yokoki et al., submitted). Flood (1986) showed evidence that a long-term(18 years) change in wind direction caused a change
in the shoreline position of a sand cay in the Great
Barrier Reef of Australia. In addition to wind, achange in the intensity of cyclones may affect the
dynamics of the islands as suggested by comparative
studies of reef-island structure (Bayliss-Smith, 1988;
Woodroffe, 1992) (Fig. 3). Changes in the amount ofprecipitation and its variability could cause changes invegetation. Evidence or quantitative predictions ofislands’ responses to these changes in cyclone intensity and precipitation, however, have yet to be presented.

5. Toward Integrated Research on Reef
Islands
We have presented factors important in reef island
formation and maintenance, and shown how theislands could be prone to changes in response to environmental changes. In order to understand the natureand dynamics of reef islands, interdisciplinary studies
that integrate geological, physical, and ecological factors are needed. As we have pointed out, more sedimentation studies will be needed. Then, a process-based study and a comparative study may be proposed.
For the process-based study, an estimate of thesediment budget of each reef-building organism will
be needed in order to explain variations in sedimentconstituents. Integrating the budget to hydrodynamics
would help to clarify the maintenance mechanism of
the islands. A simple examination of CaCO3 production, sediment constituents and the transport mechanism on a platform reef revealed the importance of
foraminifera to the formation and maintenance of acoral sand cay in the Pacific (Yamano et al., 2000).
This approach, accompanied with a more detailed
sediment budget as represented by Harney and
Fletcher (2003), may be applied to reef islands inother regions. In order to make predictions, quantitative modeling will be needed. A novel attempt tounderstand the future change was made by Kench andCowell (2002) who examined both sediment production and the physical environment to quantitatively
model shoreline changes due to sea-level rise. In addition to normal processes, processes in stormy condi-

17

tions should also be considered. Quantitative estimation of the threshold of mechanical breakdown (Dennyet al., 1985) and modeling transport and deposition ofcorals during cyclones (e.g., Massel & Done, 1993)
would be needed.
Because reef islands have diversity as a result ofvariations in environmental factors, a comparative
study to extract important factor(s) for specific typesof reef islands will be another important approach.
Though we showed that data for sediment constituentsand historical sedimentation patterns of reef islands
are insufficient, the size and shape and the environment adjacent to the islands can be analyzed. Recent
advances in satellite remote sensing and data assimilation could contribute to a global-scale analysis. Usingsatellite data, nationwide coral reef mapping was accomplished for the Maldives (Naseer & Hatcher,2003), Tuamotu Archipelago (Andréfouët et al., 2001),
and the Marshall Islands (Yamano et al., submitted),
and is in progress for coral reefs globally (Andréfouët
et al., in press). For the controlling factors, recent
availability of global-scale data sets for SST, salinity,
nutrients, light availability, and aragonite saturation
state have made it possible to discuss environmental
limits to coral reef development (Kleypas et al.,1999b). In addition to the factors controlling coralreef distribution, most factors controlling reef-island
formation arise on a global scale (Shimazaki et al.,2005). Of particular interest is comparative studies onregions with different sea-level histories (e.g., thePacific, western Indian Ocean and Caribbean Sea) anddifferent intensities of cyclones. This mapping- based
comparative approach could help to explain themechanism of reef-island evolution.
Upon understanding the primary factors and processes of reef-island formation and maintenance, thefuture status of reef islands could be assessed precisely using simulated values of the factors (sea-levelrise, Kench & Cowell, 2002, see above). Assessmentof the future status of coral reefs is in progress, using
climate models to simulate CO2 concentration and
SST (Kleypas et al., 1999a; Guinotte et al., 2003).
Climate models are being improved and applied toestimating future reef island status on a global scale(e.g., Lal et al., 2002). These approaches should contribute to a better understanding of reef island evolution and future status of these islands.
Note added in proof: Recently, a new model ofreef-island formation was presented for Maldives in
the Indian Ocean (Kench et al., 2005). They showed
the islands formed since between 5,500-4,500 years
BP when the reef surface was as much as 2.5 m below
modern sea level. The internal facies was dominatedby Halimida. This example might correspond to the
Caribbean examples (Table 1) and should also be examined further by a comparative study on various environmental factors.

18

H. YAMANO et al.

Acknowledgements
This research was supported partly by the Global
Environment Research Fund of the Ministry of theEnvironment, Japan (Project No. B15, Principal Investigator: HK).
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(Received 2 February 2005, Accepted 11 March 2005)