Variation of the macrobenthic infaunal density in a bathymetric gradient: western gulf of mexico

Abstract

The variability of the macrobenthic infaunal density is analyzed along a depth gradient, from the shelf break down to the abyssal plain in the western Gulf of Mexico. Samples were obtained at 10 sites in June 1997. Results show a decrease in density with increasing depth. Greatest density was recorded at 1230 m depth (2713.6 ± 1469.6 ind m^–2), just below the minimum oxygen layer. This value is 1.5 times higher than values observed at the shelf break. Density decreases with increasing depth, attaining values three times smaller (880 ± 176.7 ind m^–2) on the lower slope. The density recorded in the abyssal plain shows high variability (794.7 ± 43.7 to 1669.3 ± 536.3 ind m^–2); the highest values recorded in the abyssal plain are similar to those recorded at the shelf break (1865.6 ± 1132.6 to 1932.8 ± 685.9 ind m^–2). Density values recorded herein are higher than those previously published for the Gulf of Mexico and are more largely attributed to the hydrodynamics and input of organic matter than to the sediment factors. Chlorophyll a concentration in the water column is the parameter most closely related to density variability, having a similar effect as depth. The localities fall into three groups: the first includes the abyssal zone and continental rise (>3000 m), the second includes the sites located on the slope (>1500 to <3000 m) and the third includes the sites with depths less than 1500 m. The low concentration of dissolved oxygen defines the lower density on the shelf break and upper slope. Grain size and organic matter content in surficial sediment have a lesser effect on density at this depth. In contrast, the stations located on the lower slope and abyss were defined by the organic matter content in surficial sediment, which together with depth affect the macrobenthic infauna at different space scales.