Abstract
Many coral reef systems are shelf-based and consist of reefs that are arranged in rows parallel to the coastline. They usually show an increase in species richness in the offshore direction, coinciding with decreasing terrigenous impact and a deeper seafloor. These two conditions usually concur, which makes it less easy to distinguish how each of them influences coral diversity separately. Since reefs off Jakarta (in the Thousand Islands archipelago) are arranged in an 80 km long string perpendicular to the coastline in south-to-north direction, with a maximum shelf depth halfway along (instead of at the end of) the string, this archipelago is very suitable for studies on inshore–offshore gradients. In the present study, mushroom corals (Fungiidae; n = 31) were used to examine diversity patterns on 38 reef sites along such a gradient, involving species richness over their entire depth range from reef flat to reef base (2–30 m) and separately at shallow depths (2–6 m). Total species diversity was highest in the central part of the archipelago, with unique species occurring in deep habitats. Diversity at shallow depths was only slightly higher here than at reefs located more nearshore and offshore, which both had less clear water. Therefore, shelf depth and distance from the mainland can be considered separate determinants of coral diversity off Jakarta.
Highlights
Shelf-based reef systems are ideal model areas in which to study the influence of inshore-to-offshore environmental gradients on the diversity of reef coral assemblages
The present results show that the highest diversity of mushroom corals off Jakarta can be found on the reefs surrounded by a deep seafloor in the central section of the archipelago and not on the northernmost reefs, which are farthest away from the mainland
The offshore reefs have shown a large reduction in water transparency since the 1920s, while the turbidity on the inshore reefs has not changed much until 2005, after which offshore reefs started to show a better visibility compared to inshore reefs [57,120,121,122]
Summary
Shelf-based reef systems are ideal model areas in which to study the influence of inshore-to-offshore environmental gradients on the diversity of reef coral assemblages. To serve as a model reef system and to enable the distinction of clear diversity patterns, the reefs should be abundant, evenly distributed, and be geomorphologically uniform. Most such reef systems are predominantly arranged alongside and parallel to the mainland shoreline, such as the Great Barrier Reef, located off the north-east Australian coast [11,12,13,14,15], and various smaller shelf-based reef complexes, such as the Spermonde Archipelago off Southwest Sulawesi [5,16,17,18,19], the Diversity 2019, 11, 46; doi:10.3390/d11030046 www.mdpi.com/journal/diversity
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