Recent foraminifera around petroleum production platforms on the southwest Louisiana shelf

The Gulf of Mexico outer continental shelf off the southwest Louisiana coast is an area of heavy offshore petroleum production. Six hundred and seventy-three samples were taken from around petroleum production platforms and control sites in the area, in order to assess the effects of petroleum exploration, drilling, and production activity on the marine organisms. A total of 64,326 living foraminifera were recovered, representing 51 species in 35 genera. Densities up to 372,154 foraminifera per 10 cm2 were estimated for the summer, partly caused by the large quantities of available food supplied by the Mississippi River, and partly caused by the wet picking method which preserved delicate juvenile tests. Nonionella basiloba and Buliminella bassendorfensis together account for almost 76% of the total live foraminiferal population. Slight seasonal variations in the foraminifera population were found. Changes in species composition at the collecting sites are closely related to water depth. Low standing crops at the shallow inshore stations in the summer were primarily caused by low dissolved oxygen. Low diversity values, generally not exceeding [alpha]=5, were found for the foraminifera populations. The low diversity is caused by the large range of seasonal temperatures, movement of sediment by currents and tides, and, to a lesser degree, the variations in salinity. Negative effects of petroleum drilling operations on the benthonic foraminifera are not supported by this study. The controlling physical factors on the foraminifera populations are those related to the natural environment, water depth, temperature and salinity, tides and currents, and dissolved oxygen.