Response of resource excavations in Mobile Bay, Alabama, to extreme forcing

Abstract

Fossil reef deposits of the American oyster,Crassostrea virginica, are a common component of the near surface sediments in the middle and upper reaches of Mobile Bay, northern Gulf of Mexico. Mining of these deposits occurred from 1946 through 1982 in open areas of undisturbed bay bottom, outside of the shipping channel corridors, in water depths of 3–5 m. The mining process resulted in the formation of pit to furrow-shaped depressions with elevated rims at some sites and troughs and ridges in other areas. Studies carried out in the early 1970s predicted normal physical processes would restore the bottom to pre-shell mining conditions within 1 yr, thus minimizing any long-term effects on the Mobile Bay estuary. However, over the period 1974–1976 unfilled excavations, some with raised rims and ridges, were observed where mining had occurred 3–5 yr earlier. In addition, the depressions tended to be sites of relatively high salinity, hypoxic to anoxic water. In 1992–1993, close grid bathymetric surveys produced no indication of either depressions or raised features at any of the old mining sites. Evidence suggests this leveling of the bay’s bathymetry was the result of two major hurricanes, Frederic in 1979 and Elena in 1985, which mobilized and redistributed significant quantities of sediments within Mobile Bay. These findings indicate currents and waves associated with Mobile Bay’s normal tidal activity and annual recurrent storms were acting very slowly to refill depressions and flatten elevated features. In contrast, the direct impact of major hurricanes appears to have served as an effetive agent to return Mobile Bay’s bathymetry to a state similar to pre-shell mining conditions.