Mississippi-Alabama Continental Shelf Ecosystem Study Data Summary and Synthesis.Volume 1. Executive Summary

摘要

Biological, physical, chemical, and geological characteristics were studied in aseries of five cruises between March 1987 to March 1988 along three north-south transects across the continental shelf of Mississippi and Alabama. Four stations in depths of approximately 50, 100, 150 and 200m were sampled along each of these transects. Side-scan, ROV, and underwater color photographs and video data were collected around topographic features in the study area. Subbottom profiler records indicate that the shelf edge is built upon delta-front forset beds that were truncated by erosion during the last low stand of sea level in the Pleistocene. Topographic features constructed on top of these sediments were generally of three classes: (1) pinnacles; (2) linear ridges; and (3) enigmatic features. Sediments contained a mixture of biological and petroleum hydrocarbons. Biological hydrocarbons were predominantly plant biowaxes with a possible minor planktonic input. Petroleum hydrocarbons were present as polynuclear aromatic compounds (PAH), a complete suite of n-C, and an unresolved complex mixture. Sediment PAH's on the shelf are on average six times lower than PAH's analyzed in sediments in adjacent bays. High hydrocarbon concentrations were generally at the seaward ends of the transects between the 100 and 200 m isobaths with stations closest to the delta containing the highest concentration of hydrocarbons. Sediments varied greatly in iron and trace metal content, but the variations seem to be largely the result of natural variability in grain size and mineralogy. Deep water sediments were more enriched in iron and trace metals than those in shallow water, but all were typical of unpolluted Gulf of Mexico shelf sediment. Satellite data positioned fronts associated with the Loop Current, warm core eddy, warm intrusions reaching into the region from the top of the Loop, warm intrusions from the Loop, and a cold ridge extending southward from the study area. Biological studies showed polychaetes were the dominant benthic macroinfauna taxon, both in numbers of species and numbers of individuals. No species appeared to dominate the community. There were no discernible patterns of diversity or abundance that could be attributed to inshore-offshore or east-west gradients. The largest numbers of species collected in the macroepifauna samples were at stations in 100m depths and the largest numbers of individuals were collected at the 150 and 200m stations. Demersal fish trawls sampled 2,839 specimens representing 98 species and 37 families.