Oysters, prawns, crabs and other bottom organisms are limited in distribution by depth/oxygen content of water/food availability/water temperature and other factors. They are not spread across the Gulf but are in a zone nearshore; many fish are limited by food supply/ oxygen content of water, temperature, etc. as well.
Marine scientists are worried that the oil plumes might affect the zones with valuable food supplies, reducing oxygen content, micro-organisms on which seafood species depend, and other needs of species harvested in the Gulf.

The total area and volume of the Gulf is not pertinent.
The area concerned is near-offshore Texas, Alabama, Mississippi, Florida and perhaps Cuba. Mexico seems to be well away from plume projections.