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A cay (also spelled caye or key and or, due to its spelling, ) is a small, low-elevation, sandy island formed on the surface of coral reefs. Cays occur in tropical environments throughout the Pacific, Atlantic and Indian Oceans (including in the Caribbean and on the Great Barrier Reef and Belize Barrier Reef), where they provide habitable and agricultural land for hundreds of thousands of people. Their surrounding reef ecosystems also provide food and building materials for island inhabitants.

Formation and composition
A cay is formed when ocean currents transport lose sediment across the surface of a reef to a depositional node, where the current slows or converges with another current, releasing its sediment load. Gradually, layers of deposited sediment build up on the reef surface (Hopley 1981, Gorlay 1998). Such nodes occur in windward or leeward areas of reef surfaces and sometimes occur around an emergent outcrop of old reef or beach rock.

The island resulting from sediment accumulation is made up almost entirely of biogenic sediment – the skeletal remains of plants and animals – from the surrounding reef ecosystems (Hopley 1982). If the accumulated sediments are predominantly sand, then the island is called a cay; if they are predominantly gravel, the island is called a motu.

Cay sediments are largely composed of calcium carbonate (CaCO3), primarily of aragonite, calcite, and high magnesium calcite. They are produced by myriad plants (e.g., coralline algae, species of the green algae Halimeda) and animals (e.g., coral, molluscs, foraminifera). Small amounts of silicate sediment are also contributed by sponges and other creatures (Chave 1964, Folk and Robles 1964, Scoffin 1987, Yamano 2000). Over time, soil and vegetation may develop on a cay surface, assisted by the deposition of sea bird guano.

Development and stability
A range of physical, biological and chemical influences determines the ongoing development or erosion of cay environments. These influences include: the extent of reef surface sand accumulations, changes in ocean waves, currents, tides, sea levels and weather conditions, the shape of the underlying reef, the types and abundance of carbonate producing biota and other organisms such as binders, bioeroders and bioturbators (creatures which bind, erode and mix sediments) living in surrounding reef ecosystems (Harney and Fletcher 2003, Hart and Kench 2007).

Significant changes in cays and their surrounding ecosystems can result from natural phenomena such as severe El Niño Southern Oscillation (ENSO) cycles. Also, tropical cyclones can help build or destroy these islands (Scoffin 1993, Woodroffe 2003).

There is much debate and concern over the future stability of cays in the face of growing human populations and pressures on reef ecosystems, and predicted climate changes and sea level rise (Kench and Cowell 2003, Hart 2003). There is also debate around whether these islands are relict features which effectively stopped expanding two thousand years ago during the late Holocene or, as recent research suggests, they are currently still growing with significant new additions of reef sediments (Woodroffe et al. 2007).

Understanding the potential for change in the sediment sources and supply of cay beaches with environmental change is an important key to predicting their present and future stability. Despite, or perhaps because of all the debate around the future of cays, there is consensus that these island environments are very complex and somewhat fragile.

Examples
Examples of cays include:
 * Heron Island, a coral cay on the southern Great Barrier Reef
 * Warraber Island in central Torres Strait (10°12' S, 142°49' E), Australia, a small ‘vegetated sand cay’ according to the classification schemes of McLean and Stoddart (1978) and Hopley (1982). Approximately 750 by 1500 m wide, this island is situated on the leeward surface of a large 11 km2 emergent reef platform. This cay and the surrounding reef flat are Holocene in origin, having formed over an antecedent Pleistocene platform (Woodroffe et al. 2000).
 * The Florida Keys are composed primarily of exposed ancient coral reefs and oolite beds formed behind reefs. A few of the Florida Keys, such as Sand Key, are "cays" as defined above.
 * Tobacco Caye, Dangriga, Belize
 * Prickly Pear Cays