Morphology and Marine Habitats of Two Southwestern Caribbean Atolls: Albuquerque and Courtown Juan

Albuquerque and Courtown are two small, uninhabited oceanic atolls, located in the southwestern Caribbean Sea, belonging to the San Andres and Providencia archipelago, Colombia. These atolls have a volcanic basement and are surrounded by deep water. Based on photo-interpretation of geomorphological and ecological features as well as on data collected during field work, the gross morphology, marine bottom habitats and reef structures of both atolls are described down to a depth of 50 in. Distributions of morphological and bottom habitat units are presented in thematic maps showing the overall zonational patterns in the two atolls. Morphological and ecological zonation; in both atolls are primarily controlled by both wave exposure in a windward-leeward gradient and depth. The presence of an ample windward fore-reef terrace, a well developed windward barrier reef with spur-and-groove system, an extensive lagoonal terrace with sudden transition to the lagoon basin, and profuse development of ribbon and anastomosing patch reefs in the lagoon are characteristics common to both atolls. As in other Caribbean atolls, the outer slope in Albuquerque and Courtown is outlined by a sandy step or bench at 35 to 45 m depth. Significant differences between the two atolls exist in the degree of development and structure of leeward peripheral reefs, as well as in the amplitude of the leeward fore-reef terrace. At Albuquerque, peripheral reefs grow on a shallow flat and enclose the lagoon along a wide semicircle, whereas at Courtown such reefs have in part developed algal ridge-like structures and are unevenly distributed, leading to an open lagoon to the east. The broad leeward terrace in Albuquerque contrasts markedly with the rapidly dipping leeward slope towards the outer shelf margin in Courtown. Accumulations of sand and rubble have led to the formation of cays and small islands on the lagoonal terrace in both atolls, but also on leeward peripheral reefs in ' Institnto de Investigaciones Marinas y Costeras, INVEMAR, Apartado 1016, Santa Marta, Colombia Universidad Nacionat de Colombia (Departamento de Biologia) Manuscript received 31 March 1995; revised 21 November 1995 Courtown, some of which have experienced remarkable changes in their size and shape in the last 25 years. Biological composition and structure of reefs in both atolls show a great resemblance to one another and to the better-known reef complexes around the nearby islands of San Andres and Providencia. Although no urban development exists in these atolls, recent decline of living coral and over-exploitation of marine resources were