Coral diversity and mode of growth of lateral expansion patch reefs at Mexico Rocks, northern Belize shelf, Central America

Mexico Rocks is a large patch reef complex on the outer shelf of northern Belize, to the lee of the barrier reef. The complex consists of approximately 100 patch reefs, clustered on a topographic ridge of Pleistocene limestone, and is composed predominantly (83%) of head corals (Montastrea annularis). Biotic zonation is not apparent on any of the patch reefs. Concomitant with increasing area of individual reefs is an increase in surficial areas of dead coral, areas degraded by physical and bio-erosion, and cavities. Such areas enhance habitat complexity and provide zooid-free substrates that are colonized rapidly by more cryptic coral species. The result of such processes is an increase in spatial heterogeneity and species richness on larger reefs relative to smaller patch reefs in the complex. Initial colonization of the rocky substratum (Pleistocene Limestone bedrock) by single head corals began about 3.5 Ka BP, and the patch reefs mostly accreted vertically as growth kept pace with slowly rising sealevel. Continued growth involved lateral expansion and coalescence of adjoining patch reefs to form larger composite reef masses as a consequence of the rapid growth rates of Montastrea annularis and lack of vertical accommodation space. Such a lateral-expansion style of reef growth, together with dominance of head corals, may mimic the biotic composition and geometry of give-up (drowned) reefs common in early transgressive systems tracts in modem and ancient shelf settings.