Coral Reefs of the Southern Gulf of Mexico

Chapter 5. Reef Zonation and Ecology: Veracruz Shelf and Campeche Bank

Reef Zonation and Ecology: Veracruz Shelf and Campeche Bank Ernesto Chávez, John W. Tunnell Jr., and Kim Withers For decades, the ecology and zonation of coral reefs have dominated ecological studies in tropical regions of the world. Along with the geologic history of an area, physical environmental parameters govern ecological and geographical distribution of reef organisms. Benthic habitats and communities are usually similar and typical in various geographic regions, but understanding the reef types characteristic of any given region can be critical to understanding the ecological processes. Platform reefs are the characteristic reef type in the southern Gulf of Mexico. Coral reefs are generally classified by their shape and proximity to the shoreline . Major types include atoll reefs, shelf or platform reefs, fringing reefs, and barrier reefs. Atolls are the common reef structure in the central Pacific Ocean. These reefs are typically ring-shaped with a central lagoon and develop on igneous rock emerging from the deep ocean. Atolls often have emergent margins (reef flats and islands that are also known as keys or cays) and may also have patch reefs in their sediment-dominated lagoons. A shelf or platform reef is a reef bank emerging from a continental shelf rather than the deep ocean. Located near or far from the mainland coast, the reef platform may include a shallow lagoon with sand keys. In cross section, these reefs look like flat-topped mountains (Fig. 5.1), and when viewed from above, their outline is often ellipsoidal. Fringing reefs are found on or near the shoreline and are composed of only a reef front or forereef slope. A barrier reef can be a simple offshore barrier or it can be a complex system of reefs that may include all the reef types described above. Barrier reefs protect island and mainland coasts from the direct impact of wave action and erosion. There are only two major barrier reef systems, the Great Barrier Reef in eastern Australia and the Meso-American Barrier Reef System (MBRS), running south from northeastern Yucatán to Honduras in the eastern Caribbean Sea. The northern part of the MBRS is a series of fringing-barrier reefs along the Quintana Roo, Mexico, coastline with fringing reefs occurring at rocky headlands and barrier reefs occurring between the headlands along coastal embayments . To the south, off Belize and Honduras, the MBRS becomes a more typical offshore barrier reef. Because the MBRS is outside the Gulf of Mexico, its description is beyond the scope of this book. However, the MBRS is linked to Gulf of Mexico reefs, particularly as a larval source (Jordán-Dahlgren 2002; Jordán-Dahlgren and Rodríguez-Martínez 2003). In the strictest sense, there are no atolls in the Gulf of Mexico or Caribbean Sea. Although superficially similar to Pacific atolls, Banco Chinchorro, Glover ’s Reef, Lighthouse Reef, and Turneffe Atoll (MBRS) are not built on igneous rock. Alacrán Reef, a platform reef on Campeche Bank, has been considered an 5 A4339.indb 41 A4339.indb 41 7/6/07 11:18:56 AM 7/6/07 11:18:56 AM 42 Chávez, Tunnell, and Withers atoll or “atoll-like” because of its shape (Fosberg 1962), but its geological origin is quite different (see Liddell, chapter 3, this volume). Most coral reefs in the Gulf of Mexico are platform reefs. The actively growing portion of Gulf platform reefs is oriented toward the northeast, east, or southeast, depending on geographic location, and faces the dominant wave swell, where a well-defined reef crest outlines the reef front. Although some are well enough developed to have a reef lagoon and small cays (e.g., Isla de Enmedio Reef, Alacrán Reef, Cayos Arcas), many are just shallow platforms without a lagoon (e.g., Topatillo, Anegadilla). The length of the emergent reef crest ranges from a few hundred or thousand meters in the Tuxpan Reef System (TRS) and Figure 5.1. Bottom profile between Progreso, Yucatán and Alácran Reef, showing typical shape of platform reef (adapted from Kornicker et al. 1959). Figure 5.2. Profiles of Isla de Enmedio Reef radiating from four cardinal compass directions starting at Enmedio Lighthouse (adapted from Lehman 1993). A4339.indb 42 A4339.indb 42 7/6/07 11:18:57 AM 7/6/07 11:18:57 AM Reef Zonation and Ecology 43 Veracruz Reef System (VRS), to about 22 km on Alacr...