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

Fossil reef deposits of the American oyster, Crassostrea virginica, are a c ommon component of the near surface sediments in the middle and upper reach es of Mobile Bay, northern Gulf of Mexico. Mining of these deposits occurre d from 1946 through 1982 in open areas of undisturbed bay bottom, outside o f the shipping channel corridors, in water depths of 3-5 m. The mining proc ess resulted in the formation of pit to furrow-shaped depressions with elev ated rims at some sites and troughs and ridges in other areas. Studies carr ied out in the early 1970s predicted normal physical processes would restor e the bottom to pre-shell mining conditions within 1 yr, thus minimizing an y long-term effects on the Mobile Bay estuary. However, over the period 197 4-1976 unfilled excavations, some with raised rims and ridges, were observe d where mining had occurred 3-5 yr earlier. in addition, the depressions te nded to be sites of relatively high salinity, hypoxic to anoxic water. In 1 992-1993, close grid bathymetric surveys produced no indication of either d epressions or raised features at any of the old mining sites. Evidence sugg ests this leveling of the hay's bathymetry was the result of two major hurr icanes, Frederic in 1979 and Elena in 1985, which mobilized and redistribut ed significant quantities of sediments within Mobile Bay. These findings in dicate currents and waves associated with Mobile Bay's normal tidal activit y and annual recurrent storms were acting very slowly to refill depressions and flatten elevated features. In contrast, the direct impact of major hur ricanes appears to have served as an effective agent to return Mobile Bay's bathymetry to a state similar to pre-shell mining conditions.