| Summary: | [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 ...
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