Mangrove islands

Duke, Norman C. (2011) Mangrove islands. In: Hopley, David, (ed.) Encyclopedia of Modern Coral Reefs. Structure, Form and Process. Springer, Dordrecht, The Netherlands, pp. 653-655.

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Abstract

[Extract] Mangrove Islands are atypical islands where the bulk of land area, if not all, is below tidal highwater. The term, 'Mangrove Island', is applied broadly however to include any 'low' island with a substantive portion of mangrove vegetation. Since such 'islands' depend on mangrove presence, their occurrence is similarly restricted to the tropics and subtropics where sea temperatures rarely drop below 20° C, except in Australia and New Zealand.

Substrates of mangrove islands vary widely, including: banks of fine estuarine mud, sheltered sandy berms, rocky outcrops and coral reef enclaves. While mangroves favour soft sediments, nutrients and sheltered conditions, they can be amazingly adaptable, clinging tenaciously to harder substrate in periodically exposed locations.

Biodiversity and structural complexity of mangrove islands is influenced by global and regional factors. Overall, mangrove assemblages are comprised of downstream marine species: in the Indo West Pacific - Aegialitis annulata, Avicennia marina, Bruguiera gymnorhiza, Lumnitzera racemosa, Osbornia octodonta, Pemphis acidula, Rhizophora stylosa; and, in the Atlantic East Pacific - Avicennia germinans, Conocarpus erectus, Laguncularia racemosa, Rhizophora mangle. Specific diversity depends on terrigenous catchment and annual rainfall, showing the importance of fresh water. In general, the larger the catchment water supply, the greater the number of mangrove species.

There are two predominant types of mangrove islands depending on substrate type and geomorphic setting, including: 1) coral reef islands based on carbonate sediments, clear waters and low nutrients; and, 2) river mouth islands based on fine terrigenous sediments, turbid waters and runoff nutrients. Both settings are characterised by active processes where banks are continually eroded, formed and reformed. Coral reef mangrove islands form when mangroves grow on 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, like Low Isles, occur behind shingle ramparts on windward, south-eastern shores of reefs. Mangrove islands of Montgomery Island in northern Western Australia, show tidal range can be extreme, to 10 metre. Overall, the biomass of mangroves on coral reefs is usually modest to low, below 5 metres in height. River mouth mangrove islands form when mangroves occupy depositional banks downstream in coastal estuaries. Low energy conditions occur during seasonal lulls in river flow and storms. Mangroves readily colonise deep sediments, rich in organic matter, with nutrient-loaded river runoff. The biomass of resulting mangrove trees is maximal, in some cases up to 50 metres at the Mahe River mouth, Darien, Pacific coast of Panama, Central America. The number and size of mangrove islands have expanded greatly following clearing of catchment vegetation, seen in the Fitzroy River estuary north-eastern Australia.

Mangrove islands provide valuable habitat, plus areas of respite and final refuge, for key groups of displaced coastal fauna, including: insects, fish, crustaceans, molluscs, birds, flying foxes, and various other bats. In north-eastern Australia, river mangrove islands are the last refuge for flying fox colonies. In the same region, Torres Strait pigeons have taken refuge on the coral reef mangrove islands at Low Isles.

Item ID: 48519
Item Type: Book Chapter (Reference)
ISBN: 978-90-481-2638-5
Keywords: mangrove islands; habitat; ecosystem; global; depositional gain; overwash; sand islands; IWP; AEP
Date Deposited: 09 May 2017 01:23
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050202 Conservation and Biodiversity @ 40%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 30%
05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050206 Environmental Monitoring @ 30%
SEO Codes: 96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960503 Ecosystem Assessment and Management of Coastal and Estuarine Environments @ 40%
96 ENVIRONMENT > 9610 Natural Hazards > 961002 Natural Hazards in Coastal and Estuarine Environments @ 30%
97 EXPANDING KNOWLEDGE > 970105 Expanding Knowledge in the Environmental Sciences @ 30%
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