Biotic Assemblages and Ecological Controls on Reefs and Banks of the Northwest Gulf of Mexico

Abstract

This paper summarizes the results of investigations of the ecology of reefs and banks in the northwestern Gulf of Mexico. Nearshore surface waters are turbid out to approximately the 10 m isobath. A turbid nepheloid layer up to 20 m in thickness exists beyond this depth. Seasonal temperature and salinity variability is high in nearshore waters. At the shelf edge, the temperature to 50 m depth remains above 19°C. Topographic prominences are common in the region. Most are surface expressions of the diapirism of underlying salt domes derived from deeply buried Jurassic salt deposits. Biotic zonation on the banks is primarily depth related. Zones of major reef-building include the Diploria- Montastrea-Porites Zone (15–36 m), the Madracis Zone (28–46 m), the Stephanocoenia-Millepora Zone (36–52 m), and the Algal-Sponge Zone (45–98 m). A zone of minor reefbuilding on some banks is the Millepora Zone (18–52 m). An Antipatharian Transitional Zone exists from 56 to over 100 m on some banks. The Nepheloid Zone near the base of the banks is a zone of no reef-building. Not all zones are present on all banks. The community structure and depths of these zones depend on and are modified by the regional current regime, depth of the bank crests, substrate characteristics, winter temperature minima, river influences, and the relative depth and thickness of the nepheloid layer. Nearshore communities are warm temperate in nature, though tropical organisms are occasionally abundant. Progressing offshore, the benthos becomes increasingly tropical. The northerly location and isolated nature of the Flower Gardens have lead to reduced community diversity ( e.g. , only 18 of the 55 Western Atlantic hermatypic coral species exist), but not reduced abundances or growth rates of the species present. Assemblages on other banks exist below the tolerance limits of thriving tropical reefs due to their frequent immersion in turbid water, excessive crest depths, or lower minimum winter temperatures.