Aspects of the Ecology of the Deep-water Fauna of the Gulf of Mexico

Abstract

Recent transects across the continental slope off western Louisiana, the Mississippi River delta, and the Florida peninsula in the general depth range of 300–3,000 m have provided information on habitat variables and on faunal composition, density, and depth zonation. In the meiofauna (retained by 63μm screens) nematodes, harpacticoid copepods, nauplii, polychaetes, ostracods, and kinorynchs were numerically dominant, in that order, and together these groups made up 98% of the fauna. The macrofauna (retained by 0.3 mm screens) was dominated by polychaetes, ostracods, bivalves, tanaids, bryozoans, and isopods in that order, and together these made up 86% of the fauna. Densities of both groups were highest on the Central Transect, and densities of both tended to decrease with depth. Between the depths of 300 m and 3,000 m there was a threefold decrease in meiofaunal and a twofold decrease in macrofaunal density. Among the megafauna (collected by otter trawl) invertebrate densities, dominated by crustaceans, were four to five times as great as fish densities at all depths and on all transects. Densities were greatest on the Eastern and least on the Central Transect, and on all transects they decreased with depth. On the slope off Louisiana and East Texas, in the depth range of 400–900 m, dense biological communities have been encountered at about 40 locations aggregated around oil and gas seeps. These organisms include clusters of large tube worms (vestimentiferans), vesicomyid clams, mussels, galatheid crabs, bresiliid shrimps, neogastropods, limpets, and fishes. This community is trophically dependent upon chemoautotrophic bacteria (which utilize hydrogen sulfide), although some mussels directly utilize methane as a carbon source. This community is closely related to that of the hydrothermal vent systems of the East Pacific Rise and to the seep communities at the base of the Florida escarpment. The megafauna of the northern and eastern Gulf of Mexico falls naturally into the following depth distribution pattern: Shelf/Slope Transition Zone (118–475 m), Archibenthal Zone—Horizon A (500–775 m), Archibenthal Zone—Horizon B (800–975 m), Upper Abyssal Zone (1,000–2,275 m), Mesoabyssal Zone (2,300–3,225 m), and Lower Abyssal Zone (3,250–3,850 m). Biological characteristics of each zone are discussed.