Honors Theses

A Re-discovery of Microatolls in the Galápagos Islands, Ecuador

Amber Orr, Nova Southeastern University


Microatolls are shallow-water coral colonies shaped like miniature versions of their namesake. They develop their signature ring-like atoll form due to water depth limitations that prevent living tissue from growing any higher, encouraging outward growth instead. Since incipient sub-aerial exposure is an important component of their formation, they are utilized as indicators of current and past sea level. Additional contributors to their formation include accumulation of sediment on the colonies, which smothers polyps in the center of the structure, overgrowth by algae, and upper surface abrasion by bioeroding organisms. Microatolls are described from numerous locations and form from several species in the Indo-West Pacific. However, microatolls occur at only two known sites in the tropical Eastern Pacific: Caño Island, Costa Rica and the Galápagos Islands, Ecuador, and form from one species, Porites lobata. In the Galápagos Islands, early researchers (1970s-1980s) observed microatolls at Urvina Bay, Isabela Island and Devil’s Crown, Floreana Island; however, they are no longer present at either location. In 2019 several microatolls were discovered in a small embayment on the northern coast of Champion Island, located 4.7km ESE of Devil’s Crown. More than 50 P. lobata microatolls were observed during the most recent field observations in 2021. The colonies exhibited an irregular microatoll morphology, with a more elongated, scallop-shaped boundary than the typical rounded form. Colony sizes ranged from 0.2m2 to more than 3.0m2. Upper surface depressions were covered in sediment and filamentous algae, surrounded by an outer ring of living tissue. The shallowest living portions of colonies were at Mean Lower Low Water (MLLW), consistent with other reports of these formations. Additionally, a resident colony of sea lions (Zalophus wollebaeki) in this embayment may be responsible for a novel contribution to microatoll formation. Abrasion by passing sea lions and the opportunistic placement of damselfish (Stegastes spp.) algal lawns may further limit vertical coral growth. The volcanic origins and generally steeply sloped, basalt shorelines of the Galápagos Archipelago are not conducive to microatoll formation. However, the embayment at Champion Island provides the critical combination of shallow depth, gentle slope, sedimentation stress, and biotic disturbances to form these distinctive coral structures.