Sizes, shapes, and patterns of coral reefs in the Maldives, Indian Ocean: the influence of wind, storms, and precipitation on a major tropical carbonate platform

Abstract

The Maldives are among the largest reef areas. The archipelago consists of 21 atolls (14 larger and 7 smaller ones) and 4 reef-fringed islands (small atolls with completely sediment-filled lagoons, i.e., filled buckets) that cover a total area of 21,370 km2. In order to get a better understanding of the variety of reef forms in this major carbonate platform location, reef areas, reef types, and other geomorphological elements such as channels through marginal reefs, lagoons, lagoon patch reefs, and faroes (smaller circular reefs that are found in lagoons and at marginal reefs) were quantified using satellite images and British admiralty sea charts. These numbers were compared to each other and related to environmental parameters such as precipitation, wind stress, wind speed, current strength, and storm impact. Sizes and abundances of geomorphological elements show many statistically significant correlations. For example, total reef area covaries with reef and channel areas and abundances. Lagoon depth in larger atolls increases from north to south and correlates with atoll area and precipitation rates, probably as a result of limestone dissolution during sea-level lowstands and the fact that larger catchment areas may produce higher karst reliefs. Apart from precipitation, the influence of wind and wind-induced waves appears to be of great importance for spatial patterns of reef types in the Maldives. From south to north within the archipelago, i.e., away from the equator the influence of the Indian Monsoon and storms gets stronger. In the same direction, the abundance of lagoonal reefs and channels through marginal reefs increases. The pattern is not as clear for faroes, which are so typical for the Maldives, and whose mode of formation has been disputed. Both the numbers of lagoonal and marginal faroes reach maxima in the north-central part of the archipelago and decrease towards the north and south, for reasons not entirely clear. It is suggested that faroes form via processes of lateral sand transport during the reversing monsoon and subsequent colonization by corals. Wind stress appears to be correlated with faro development as it increases from south to north and slightly decreases again in the very north, just like the abundance of faroes. In addition, the destructive influence of cyclones, which only reaches as far south as the northern Maldives, might be detrimental to faro development. Given that dissolution during glacial sea-level lowstands also plays a role in faro development, the decreasing precipitation rates (as proxy for glacial limestone dissolution) might hamper faro formation towards the north of the archipelago.