A Revision and Critique of Atoll Numbers Based on Satellite Imagery

Atolls are individually small but collectively form regionally widespread reef systems that are geomorphically varied. Although many have been described, many others have not. Recent work has explored the recurrent question of how many atolls exist in the tropical and subtropical oceans of the world. To address this question, we reexamined available satellite imagery and proposed a global list with defined criteria. In doing so, we found a number of atolls listed by others that exhibited poor image quality. Others were rejected due to known volcanics on their surfaces (making them almost-atolls), and a few were tentatively eliminated due to island elevations of 20 m or more, or for other reasons listed herein. We also note a total of 96 atolls whose attributes (except location) have not been described, and we illustrate them in this contribution. In addition, we report 38 atolls that are new to any compilation. The results of these adjustments and additions bring an air of uncertainty concerning an exact number of atolls, but our data indicate that the number currently resolvable by satellite imagery and otherwise not excluded by our criteria is 611. However, recognizing atolls with poor image resolution as well as those with elevations above 20 m could bring the total well above this figure. Our compilation also reveals Indonesia as the most atoll-rich region known thus far, with 95 of these features, 22 of which are newly described here, suggesting that more may be found in this complex region.