Temporal and spatial variations in the polychaete (Annelida) populations on the upper continental slope of the northern Gulf of Mexico

Polychaete worms (Annelida), the dominant macrofaunal taxon in most fine-grained marine sediments, were sampled in 1983-85 and then again in 2000-02 at nine locations at depths of 324-1454 rn. on the upper continental slope of the northern Gulf of Mexico (GoM). The assemblages exhibited relative stability in abundance and diversity, but fell into six separate groups of species (>35% similarity) that were related to time of-sampling, location, depth. This depth gradient experiences an increase in oxygen from 2.5 to 4.5 ml/L, a six degree decrease in temperature (10-12 degrees down to 4 degrees C) and a decline of 30-37 mg C m(-2) day(-1) down to 7 mg C m(-2) day(-1) in estimates of the particulate organic carbon (POC) input to the sea floor, but these steep gradients had secondary effects on species turnover or depth-related zonation (Beta diversity). The species composition of four of the six groups was separated on the basis of sampling between 1983-85 and 2000-02 as opposed to depth or location. The species composition of the two groups on the eastern transect was different from the western sites and the two eastern groups differed in species composition from each other between 1983-85 and 2000-02. The two groups of species at the three deeper sites to the west (864-1410 m) were also separated on the basis of time-of-sampling but the group of species located at the three shallow locations (324-625 m) was not; it was a mixture of the two sampling periods. Significantly higher densities (p < 0.05) in April 1984, on the eastern transect, suggest that seasonal recruitment may have occurred but the higher densities were attributed to only two species.