Coralline Algal Microatolls near Cozumel Island, Mexico

Microatolls of coralline algal encrustations were discovered during a reconnaissance study of environments around Cozurnel Island, 14 miles off the northeast coast of the Yucatan Peninsula. Onehundred-sixteen of them were counted in one small area along the northeastern shore in a belt approximately 100 feet wide, 150 yards offshore, and parallel to shore. The microatolls are roughly circular with diameters between 12 and 25 feet. The outer margin of each is outlined by an algal ridge several inches wide and a few inches above sea level. This rim, enclosing a shallow "lagoon", is especially prominent because it supports a luxuriant growth of upright, non-calcareous algae. Many of the microatolls are within 15 feet of one another; the deepest passages observed between them are 14 feet deep. The microatolls form the upper rims of cylindrical structures with vertical to slightly overhanging sides. The sides are covered by coralline algae including Gtmiolithon solubile . Archaeolilholhamnium cpisporum, Lilholhamnium scjunctum , Epilithoit membranaceum , Lilhnphyllum sp. , and Lithopurella sp. Living coral colonies cover only a small, percent of the vertical sides, but most samples show coral beneath the algal encrustation. The origin of the microatoll foundations is speculative.