Ecology and Distributional Patterns of Marine Macro-Invertebrates, Northern Gulf of Mexico1

Abstract

The macro-invertebrate assemblages in the northern Gulf of Mexico can be separated into those occurring in the shallow lagoons and estuaries and those on the continental shelf and slope. Distributional patterns of faunal assemblages in lagoons and estuaries are considerably modified by prevailing climates. Climatic control is exerted to a lesser degree in the shallow waters on the continental shelf. Eleven macrofaunal assemblages are recognized in the lagoons and estuaries along the northern coast of the Gulf of Mexico. Within each climatic zone certain of these assemblages are predominant, whereas others may be completely absent. Those assemblages which tolerate extended periods of low salinity, such as fresh-water marsh, river-influenced, and low-salinity oyster-reef assemblages, cover extensive areas in the humid zone. The number of species is small and populations are high in these environments. High-salinity assemblages characterize the semiarid zone, and here, also, large populations and a small number of species are typical. Within the subhumid zone, and its corresponding wide range of physical factors, most of the eleven macrofaunal assemblages are present. The extent of hese assemblages in the subhumid zone is dependent entirely upon the prevailing climate at the time of sampling; during droughts, high salinity, open bay and sound assemblages predominate, and during wet periods, oyster reef and inter-reef assemblages are the most extensive. Eight macrofaunal assemblages occur on the continental shelf and upper continental slope to 500 fathoms. Although six of these assemblages appear to be confined to rather narrow depth ranges, they are considerably modified by sediment type and average bottom-water temperature conditions. The general limits of the six level-bottom assemblages are (1) surf zone, (2) 2-12 fathoms, on the open coast, (3) 2-12 fathoms off the Mississippi Delta, (4) 12 to 30-40 fathoms, (5) 40-65 fathoms, and (6) 65-500 fathoms. One assemblage is restricted to rocky or calcareous bottom regardless of depth, and another consists of pelagic mollusks which indicate offshore surface waters. Changes in climate and associated shallow-water environments are demonstrated for the Holocene transgression across the continental shelf of the northwestern Gulf of Mexico by using carbon-14-dated shells of species with restricted depth and environmental range, and temperature restrictive geographic ranges. At about 11,000 and again 9,000 years before present, the distribution of dated mollusk shells indicates that the climate along the now submerged shore lines changed from warm-temperate to subtropical and tropical. These conditions were synchronous with major circulation changes in the northern Gulf of Mexico as deduced from other evidence. The presence of extensive deposits of low salinity and surf-zone shells on the continental shelf also permits an interpretation of the locatio of large lagoons and estuaries and extensive stretches of barrier beaches during various periods of the Holocene transgression.