consisting of gravel seaward ridges, leeward sand ridges,
and central mangrove filled lagoons are also found on the
peripheral rim of some mid-ocean atolls. However, because
these latter islands are a component of a larger reef structure, with lagoons and numerous other types of islands,
they are not considered low wooded islands.
Detailed descriptions of low wooded islands show that
there are differences in morphological expression of
biogeomorphic units between reef settings. Stoddart
(1965) termed the Belize examples “moat islands” due to
their morphological simplicity compared to those from
the Great Barrier Reef, which are considered to have the
greatest topographic complexity (Stoddart, 1965; Stoddart
et al., 1978a). Much of the detailed understanding of the
morphological components, sedimentology and evolution
of low wooded islands is due to investigations in the northern Great Barrier Reef. This work was the result of two
Royal Society Expeditions in 1929 and 1973, which produced comprehensive maps and descriptions of many
islands. As a result of this intense scientific effort low
wooded islands have become synonymous with the subset
of 34 reef platform islands in the northern Great Barrier
Reef. The data from these early investigations of low
wooded islands have been collated and synthesised by
Hopley (1982) and Hopley et al. (2007).
Geomorphic characteristics
Low wooded islands are associated with relatively small
reef platforms that range in area from 1 to more than
500 ha. Reef platforms generally have the following properties: high elevation, are located in regions with small
tidal range, are sheltered from high-energy oceanic swell,
and are in close proximity to mainland coasts.
While the basic distinguishing features of low wooded
islands are the three biogeomorphic units, there is considerable variety in the size and morphology of each unit.
Indeed, Stoddart et al. (1978a) argue that low wooded
islands exhibit the greatest complexity of all reef islands.
Typical examples and characteristic features of these
islands, based on investigation of the Great Barrier Reef
examples, are shown in Figures 1 and 2. These characteristics are described below following a cross-reef sequence
from windward to leeward reef platform.
Emergent reef flat
The reef flat, which is approximately 100 m in width, is
emergent at low water levels and is laterally continuous
enclosing the reef top (Figure 2a and b). On some islands
the central reef top may also be emergent while on others it
is an enclosed depression (Stoddart et al., 1978a). Due to
the moderate energy environment of low wooded islands
algal ridges are absent. Living coral is absent from the
emergent reef flat, which is dominated by algae. However,
many of these emergent reef flats have moated pools with
living microatolls.
Shingle ramparts
Asymmetric ridges of coral shingle occur parallel to the
reef perimeter (Figures 1 and 2a). These ridges have gentle
seaward slopes and steep landward slopes indicative of
episodic storm-driven landward movement. In planform
the inner edge of ramparts are arcuate with occasional
shingle tongues extending on the order of 100 m across
the reef. Eroding ramparts form bassett edges
(Figure 2b). Most islands have multiple ramparts, which
when coalesced form shingle islands.
Moats
Shingle ramparts can impound shallow pools on the inner
reef flat. Drainage is impeded in the moats with ponded
water supporting coral growth above the elevation of
corals on the outer reef.
Platforms or promenades
Conglomerate platform is commonly found on the outer
seaward margins of mangrove and shingle ramparts
(Figure 2c). Their genesis is related to migration and
cementation of ramparts. On the Great Barrier Reef
islands two distinct platform levels have been identified
and linked to differing sea level stages (Figure 1g). On
a number of low wooded islands dead microatolls also
outcrop beneath the cemented platforms.
Shingle islands
Shingle islands are commonly deposited upon the
cemented conglomerate platforms. These islands consist
of a series of shingle ridges, which may have coalesced,
and which are colonised with vegetation (Figures 1 and
2d). The islands vary greatly in width and elevation. Dominant vegetation on these windward shingle islands
includes: low mangrove (Avicennia marina, Aegialitis
annulata); succulents (Sesuvium, Salicornia, Suaeda),
and salt tolerant Phemphis bushes (Hopley, 1982; Stoddart
et al., 1978a).
Mangrove swamp
Mangrove communities of varying size occupy the central
reef top. Stoddart (1980) recorded 15 species of mangrove
from the low wooded islands of the northern Great Barrier
Reef. Dominant species include Rhizophora stylosa and at
higher elevations this is replaced by Ceriops tagal,
Bruguiera sp., Xylocarpus sp. and Osbornia sp.
Rhizophora and Avicennia are also identified as dominant
mangrove species in descriptions of analogous islands on
the Belize barrier reef (Stoddart, 1965) and Jamaica
(Steers and Lofthouse, 1940).
Mangrove stands vary in extent from 1 to more than
125 ha (in the case of Bewick Island, GBR, Stoddart et al.,
1978a). However, there is no relation between reef platform area and proportion of mangrove occupying the reef
surface. These communities are considered opportunistic
in that their presence and distribution depends on the position and dynamics of shingle ramparts and islands, and the
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LOW WOODED ISLANDS
and central mangrove filled lagoons are also found on the
peripheral rim of some mid-ocean atolls. However, because
these latter islands are a component of a larger reef structure, with lagoons and numerous other types of islands,
they are not considered low wooded islands.
Detailed descriptions of low wooded islands show that
there are differences in morphological expression of
biogeomorphic units between reef settings. Stoddart
(1965) termed the Belize examples “moat islands” due to
their morphological simplicity compared to those from
the Great Barrier Reef, which are considered to have the
greatest topographic complexity (Stoddart, 1965; Stoddart
et al., 1978a). Much of the detailed understanding of the
morphological components, sedimentology and evolution
of low wooded islands is due to investigations in the northern Great Barrier Reef. This work was the result of two
Royal Society Expeditions in 1929 and 1973, which produced comprehensive maps and descriptions of many
islands. As a result of this intense scientific effort low
wooded islands have become synonymous with the subset
of 34 reef platform islands in the northern Great Barrier
Reef. The data from these early investigations of low
wooded islands have been collated and synthesised by
Hopley (1982) and Hopley et al. (2007).
Geomorphic characteristics
Low wooded islands are associated with relatively small
reef platforms that range in area from 1 to more than
500 ha. Reef platforms generally have the following properties: high elevation, are located in regions with small
tidal range, are sheltered from high-energy oceanic swell,
and are in close proximity to mainland coasts.
While the basic distinguishing features of low wooded
islands are the three biogeomorphic units, there is considerable variety in the size and morphology of each unit.
Indeed, Stoddart et al. (1978a) argue that low wooded
islands exhibit the greatest complexity of all reef islands.
Typical examples and characteristic features of these
islands, based on investigation of the Great Barrier Reef
examples, are shown in Figures 1 and 2. These characteristics are described below following a cross-reef sequence
from windward to leeward reef platform.
Emergent reef flat
The reef flat, which is approximately 100 m in width, is
emergent at low water levels and is laterally continuous
enclosing the reef top (Figure 2a and b). On some islands
the central reef top may also be emergent while on others it
is an enclosed depression (Stoddart et al., 1978a). Due to
the moderate energy environment of low wooded islands
algal ridges are absent. Living coral is absent from the
emergent reef flat, which is dominated by algae. However,
many of these emergent reef flats have moated pools with
living microatolls.
Shingle ramparts
Asymmetric ridges of coral shingle occur parallel to the
reef perimeter (Figures 1 and 2a). These ridges have gentle
seaward slopes and steep landward slopes indicative of
episodic storm-driven landward movement. In planform
the inner edge of ramparts are arcuate with occasional
shingle tongues extending on the order of 100 m across
the reef. Eroding ramparts form bassett edges
(Figure 2b). Most islands have multiple ramparts, which
when coalesced form shingle islands.
Moats
Shingle ramparts can impound shallow pools on the inner
reef flat. Drainage is impeded in the moats with ponded
water supporting coral growth above the elevation of
corals on the outer reef.
Platforms or promenades
Conglomerate platform is commonly found on the outer
seaward margins of mangrove and shingle ramparts
(Figure 2c). Their genesis is related to migration and
cementation of ramparts. On the Great Barrier Reef
islands two distinct platform levels have been identified
and linked to differing sea level stages (Figure 1g). On
a number of low wooded islands dead microatolls also
outcrop beneath the cemented platforms.
Shingle islands
Shingle islands are commonly deposited upon the
cemented conglomerate platforms. These islands consist
of a series of shingle ridges, which may have coalesced,
and which are colonised with vegetation (Figures 1 and
2d). The islands vary greatly in width and elevation. Dominant vegetation on these windward shingle islands
includes: low mangrove (Avicennia marina, Aegialitis
annulata); succulents (Sesuvium, Salicornia, Suaeda),
and salt tolerant Phemphis bushes (Hopley, 1982; Stoddart
et al., 1978a).
Mangrove swamp
Mangrove communities of varying size occupy the central
reef top. Stoddart (1980) recorded 15 species of mangrove
from the low wooded islands of the northern Great Barrier
Reef. Dominant species include Rhizophora stylosa and at
higher elevations this is replaced by Ceriops tagal,
Bruguiera sp., Xylocarpus sp. and Osbornia sp.
Rhizophora and Avicennia are also identified as dominant
mangrove species in descriptions of analogous islands on
the Belize barrier reef (Stoddart, 1965) and Jamaica
(Steers and Lofthouse, 1940).
Mangrove stands vary in extent from 1 to more than
125 ha (in the case of Bewick Island, GBR, Stoddart et al.,
1978a). However, there is no relation between reef platform area and proportion of mangrove occupying the reef
surface. These communities are considered opportunistic
in that their presence and distribution depends on the position and dynamics of shingle ramparts and islands, and the
640
LOW WOODED ISLANDS