Chapter 8. In Wilkinson, C. (ed.), Status of Coral Reefs of
the World: 2004. Australian Institute of Marine Science, 1,
213–234.
Sewell, R. B. S., 1936a. An account of Addu Atoll. Scientific
Reports, John Murray Expedition 1933–1934, 1, 63–93.
Sewell, R. B. S., 1936b. An account of Horsburgh or
Goifurfehendhu Atoll. Scientific Reports, John Murray Expedition 1933–1934, 1, 109–125.
Stoddart, D. R. (ed.), 1966. Reef studies at Addu atoll, Maldive
Islands. Preliminary results of an expedition to Addu atoll in
1964. Atoll Research Bulletin, 116, 1–122.
Tiwari, V. M., Grevemeyer, I., Singh, B., and Morgan, J. P., 2007.
Variation of effective elastic thickness and melt production along
the Deccan-Reunion hotspot track. Earth and Planetary Science
Letters, 264, 9–21.
Wallace, C. C., and Zahir, H., 2007. The ‘Xarifa’ expedition and the
atolls of the Maldives 50 years on. Coral Reefs, 26, 3–5.
Cross-references
Atolls
Coral Cay Classification and Evolution
Faroes Reefs
Gardiner, John Stanley (1872–1946)
Indian Ocean Reefs
MANGROVE ISLANDS
Norman C. Duke
University of Queensland, Brisbane, QLD, Australia
Definition
“Mangrove islands” is a descriptive term applied usually
to overwash banks or outcrops vegetated notably by mangrove plants (cp. Woodroffe, 2002). Considering the geological definition of islands, mangrove islands are atypical
since they lack a terrestrial land mass surrounded by water.
In the absence of mangrove plants, the base geomorphic
feature would be described as a mud bank or sand cay.
Therefore, it is the mangroves themselves that give mangrove islands their island-like appearance. The term is
sometimes applied more broadly, however, to include
other “low” islands with relatively small terrestrial parts
amongst the mangroves.
Mangrove islands are a distinct type of island where the
bulk of land area, if not all, is below tidal high water. Since
mangrove islands depend on mangrove presence, their
occurrence is restricted, as are mangroves, to the tropics
and subtropics where sea temperatures rarely drop below
20
C, except in Australia and New Zealand (see Mangroves). In addition, the substrate of mangrove islands
varies widely, including banks of fine estuarine mud, peat,
sheltered sandy berms, rocky outcrops, and coral reef
enclaves (e.g., see, Hopley, 1982; Duke et al., 1998;
Macintyre et al., 2004). While mangroves favor soft sediments and nutrients in sheltered conditions, they can be
very adaptable, clinging tenaciously to hard or soft substrate in periodically exposed locations.
The biodiversity and structural complexity of vegetation on mangrove islands is influenced by global
and regional factors (Duke et al., 1998). Overall, mangrove assemblages are comprised chiefly of downstream
marine species: in the Indo-West Pacific (see Duke
and Larkum, 2008) – Aegialitis annulata, Avicennia
marina, Bruguiera gymnorhiza, Lumnitzera racemosa,
Osbornia octodonta, Pemphis acidula, Rhizophora
stylosa, Rhizophora mucronata; and in the Atlantic East
Pacific (see Tomlinson, 1986) – Avicennia germinans,
Conocarpus erectus, Laguncularia racemosa, Rhizophora
mangle. Specific diversity depends on terrigenous catchment and annual rainfall, reflecting the importance of
fresh water. In general, the larger the catchment water
supply, the greater the number of mangrove species. So,
larger islands often have greater numbers of mangrove
species.
There are two predominant types of mangrove islands,
as distinct from continental or oceanic islands. Mangrove
islands are influenced by sea level, substrate type, and
geomorphic setting, to form as either: (1) reefal mangrove
islands based on carbonate or peat sediments (Hopley,
1982; Macintyre et al., 2004), clear waters, and low nutrients; or (2) river mouth mangrove islands (Duke et al.,
2005) based on fine terrigenous sediments, turbid waters,
and runoff nutrients. Both settings are characterized
by active processes where over time the tidal banks are
continually formed, eroded, and reformed. Reefal mangrove islands form when mangroves grow on coral reef
flats or behind shingle ramparts. Low energy conditions
may arise within exposed locations. On Australia’s Great
Barrier Reef, “low wooded islands” with mangroves
(Hopley, 1982), like Low Isles (Figure 1), often occur
behind shingle ramparts on windward, southeastern
shores of reefs. Mangrove islands form Montgomery
Island in northern Western Australia, and they show that
tidal range can be extreme, to 10 m. Overall, the height
of mangrove stands on coral reefs is usually modest to
low, below 5 m. River mouth mangrove islands form when
mangroves occupy depositional banks downstream in the
mouths of river-influenced estuaries. Low energy conditions occur during seasonal lulls in river flow and storms.
Mangroves readily colonize fine deltaic sediments, rich in
organic matter, washed in nutrient-loaded river runoff.
The height of resulting mangrove trees is maximal, in
some cases attaining 50 m; for example, at the mouth of
the Mahe River, Darien, Pacific coast of Panama, and Central America. Because these mangrove islands are formed
from riverine sediments, their number and size have
expanded greatly following the clearing and disturbance
of catchment vegetation, as seen in the Fitzroy River estuary northeastern Australia (Duke et al., 2005; see
Figure 2).
Mangrove islands provide valuable structure and habitat, plus areas of respite and refuge, for key groups of
MANGROVE ISLANDS
653
the World: 2004. Australian Institute of Marine Science, 1,
213–234.
Sewell, R. B. S., 1936a. An account of Addu Atoll. Scientific
Reports, John Murray Expedition 1933–1934, 1, 63–93.
Sewell, R. B. S., 1936b. An account of Horsburgh or
Goifurfehendhu Atoll. Scientific Reports, John Murray Expedition 1933–1934, 1, 109–125.
Stoddart, D. R. (ed.), 1966. Reef studies at Addu atoll, Maldive
Islands. Preliminary results of an expedition to Addu atoll in
1964. Atoll Research Bulletin, 116, 1–122.
Tiwari, V. M., Grevemeyer, I., Singh, B., and Morgan, J. P., 2007.
Variation of effective elastic thickness and melt production along
the Deccan-Reunion hotspot track. Earth and Planetary Science
Letters, 264, 9–21.
Wallace, C. C., and Zahir, H., 2007. The ‘Xarifa’ expedition and the
atolls of the Maldives 50 years on. Coral Reefs, 26, 3–5.
Cross-references
Atolls
Coral Cay Classification and Evolution
Faroes Reefs
Gardiner, John Stanley (1872–1946)
Indian Ocean Reefs
MANGROVE ISLANDS
Norman C. Duke
University of Queensland, Brisbane, QLD, Australia
Definition
“Mangrove islands” is a descriptive term applied usually
to overwash banks or outcrops vegetated notably by mangrove plants (cp. Woodroffe, 2002). Considering the geological definition of islands, mangrove islands are atypical
since they lack a terrestrial land mass surrounded by water.
In the absence of mangrove plants, the base geomorphic
feature would be described as a mud bank or sand cay.
Therefore, it is the mangroves themselves that give mangrove islands their island-like appearance. The term is
sometimes applied more broadly, however, to include
other “low” islands with relatively small terrestrial parts
amongst the mangroves.
Mangrove islands are a distinct type of island where the
bulk of land area, if not all, is below tidal high water. Since
mangrove islands depend on mangrove presence, their
occurrence is restricted, as are mangroves, to the tropics
and subtropics where sea temperatures rarely drop below
20
C, except in Australia and New Zealand (see Mangroves). In addition, the substrate of mangrove islands
varies widely, including banks of fine estuarine mud, peat,
sheltered sandy berms, rocky outcrops, and coral reef
enclaves (e.g., see, Hopley, 1982; Duke et al., 1998;
Macintyre et al., 2004). While mangroves favor soft sediments and nutrients in sheltered conditions, they can be
very adaptable, clinging tenaciously to hard or soft substrate in periodically exposed locations.
The biodiversity and structural complexity of vegetation on mangrove islands is influenced by global
and regional factors (Duke et al., 1998). Overall, mangrove assemblages are comprised chiefly of downstream
marine species: in the Indo-West Pacific (see Duke
and Larkum, 2008) – Aegialitis annulata, Avicennia
marina, Bruguiera gymnorhiza, Lumnitzera racemosa,
Osbornia octodonta, Pemphis acidula, Rhizophora
stylosa, Rhizophora mucronata; and in the Atlantic East
Pacific (see Tomlinson, 1986) – Avicennia germinans,
Conocarpus erectus, Laguncularia racemosa, Rhizophora
mangle. Specific diversity depends on terrigenous catchment and annual rainfall, reflecting the importance of
fresh water. In general, the larger the catchment water
supply, the greater the number of mangrove species. So,
larger islands often have greater numbers of mangrove
species.
There are two predominant types of mangrove islands,
as distinct from continental or oceanic islands. Mangrove
islands are influenced by sea level, substrate type, and
geomorphic setting, to form as either: (1) reefal mangrove
islands based on carbonate or peat sediments (Hopley,
1982; Macintyre et al., 2004), clear waters, and low nutrients; or (2) river mouth mangrove islands (Duke et al.,
2005) based on fine terrigenous sediments, turbid waters,
and runoff nutrients. Both settings are characterized
by active processes where over time the tidal banks are
continually formed, eroded, and reformed. Reefal mangrove islands form when mangroves grow on coral reef
flats or behind shingle ramparts. Low energy conditions
may arise within exposed locations. On Australia’s Great
Barrier Reef, “low wooded islands” with mangroves
(Hopley, 1982), like Low Isles (Figure 1), often occur
behind shingle ramparts on windward, southeastern
shores of reefs. Mangrove islands form Montgomery
Island in northern Western Australia, and they show that
tidal range can be extreme, to 10 m. Overall, the height
of mangrove stands on coral reefs is usually modest to
low, below 5 m. River mouth mangrove islands form when
mangroves occupy depositional banks downstream in the
mouths of river-influenced estuaries. Low energy conditions occur during seasonal lulls in river flow and storms.
Mangroves readily colonize fine deltaic sediments, rich in
organic matter, washed in nutrient-loaded river runoff.
The height of resulting mangrove trees is maximal, in
some cases attaining 50 m; for example, at the mouth of
the Mahe River, Darien, Pacific coast of Panama, and Central America. Because these mangrove islands are formed
from riverine sediments, their number and size have
expanded greatly following the clearing and disturbance
of catchment vegetation, as seen in the Fitzroy River estuary northeastern Australia (Duke et al., 2005; see
Figure 2).
Mangrove islands provide valuable structure and habitat, plus areas of respite and refuge, for key groups of
MANGROVE ISLANDS
653