Indonesian reefs

Abstract

Indonesia’s territorial seas are the world’s largest contiguous coral reef domain under the sovereignty of a single country. All major morphological forms of coral reef occur in Indonesian waters: atolls, barrier, fringing, platform, as well as coral communities (not building reefs), and incipient coral reefs. Many of the major barrier reefs are submerged structures that rise to depths of 15–10 m from near the 200 m isobath, reflecting their former existence as fringing reefs during periods of low sea level, brought to an end by the 120 m rise in sea level following the last glacial maximum, 18–17 kya. Most, if not all, “atolls” occur on sea-floor highs whose geology is not known, but is likely in most cases not to be volcanic. In Indonesia, atoll forms may, therefore, be better described as lagoonal platform reefs. Volcanic islands are extremely abundant, many with good reef development, but many also too active and/or too young to have permitted substantial reef development. There are abundant raised fossil reef terraces across the country, a result of upward displacement of islands by subduction beneath them of the Indian Ocean plate, by Pleistocene and Holocene sea-level fluctuations, and by seismic uplift. There are large and thriving modern reefs in both the deep eastern and shallow western parts of Indonesia’s enclosed seas, as well as along the south and westward shores facing the Indian Ocean. The biodiversity of Indonesia’s reefs – the highest of anywhere in the world – represents the overlap of four types of distributions: Indian Ocean, Pacific Ocean, Indian plus Pacific Ocean, and endemics (Wallace and Wolstenholme, 1998; Hoeksema, 2007). These overlaps are believed to be outcomes of geological history, sea-level fluctuations, and the barrier to west to east dispersal provided by the Indonesian Throughflow, which moves five times as much water from east to west than in the opposite direction (Gordon et al., 2003). Current reef diversity and productivity are sustained by an extremely complex physical and oceanographic setting.