95 



2.2.16 



ECOSYSTEMS OF THE MISSISSIPPI-ALABAMA OCS REGION 



Dr. William W. Schroeder 



Marine Science Program 



The University of Alabama 



Dauphin Island, AL 36528 



Gulf of Mexico. It 

 east it extends to the 



The Mississippi-Alabama shelf is one of the five major continental shelf subprovinces in the ' 

 is separated, to the west, from the TEXLA shelf by the Mississippi River delta while to the easi u exienus io uu 

 western rim of the DeSoto Canyon and abuts the West Florida shelf north of the head of the DeSoto Canyon 

 The shelf break occurs at depths ranging from 60 to 100 m. 



From east to west the sediment regime grades from the extensive MAFLA sand sheet through a transitional zone 

 to the north-south trending silts and clays of the St. Bernard prodelta deposit through another transitional zone 

 and back into sands of the Chandeleur Islands. South of Mobile Bay and Mississippi Sound the inner shelf is 

 covered with a complex of barrier island sands and silts and clays from the adjacent estuaries. To the southeast 

 along the outer shelf and shelf break, surficial sediments consist of a mixture River birdfoot delta region delta- 

 front silt and silty-clay deposits merge with the adjacent shelf deposits. 



Polychaetous annelids dominate the macroinfaunal taxon and constitute the majority of biomass in most areas 

 covered with unconsolidated sediments. An unusual characteristic of this shelf area, compared with other shelf 

 regions of the northern Gulf of Mexico, is the lack of dominance by any one species within given depth zones. 

 The numerically dominant macroepifaunal taxon (excluding heart urchins) is most often Crustacea. 



Topographic features of a hardbottom nature are common on this shelf. They range from low relief (<2 m) 

 northwest trending linear ridges constructed of sand and shell gravel and occasional rock rubble to isolated 

 outcrops or erosional remnants of indurated materials on the inner shelf to low to high (up to 15-18 m) relief 

 patch reefs, linear ridges and pinnacles on the outer shelf and shelfbreak. These hardbottom features provide 

 substrate for sessile epifauna not ordinarily found on the more extensive areas of unconsolidated sediment. 



