ENVIRONMENTAL IMPACTS OF A SHIPPING CHANNEL ON FORAMINIFERA AND SEDIMENTS, MOBILE BAY, AL, USA

Mobile Bay is a complex Gulf of Mexico estuary where salinity fluctuations, anthropogenic activities, and riverine inflows greatly affect the present environmental conditions. The center of the bay, with an average water depth of 3 m, is dissected by a dredged shipping channel of ~14 m water depth and ~122 m width, with spoil banks deposited along the western side that influence tidal circulation. Prior to channel dredging, incoming tidal flow was generally counterclockwise, but presently, the majority of the tidal wedge travels up the center of the bay through the ship channel.

The development and expansion of the shipping channel over the last ~150 years have had a pronounced effect on the marine microfauna in Mobile Bay. Foraminifers and sediments from seven box cores with ²¹⁰Pb_xschronology document the effects of the altered circulation, reduced marine tidal exchange, and changed sedimentation patterns, which have caused a faunal turnover within the bay. Environmental changes, beginning in the late 1800s, were caused by the reduced marine influence and consequent greater influence of low-pH freshwater in the bay. Decreasing numbers of calcareous foraminifers in the box cores are recorded from the early 1900s to the present, most likely caused by lowered pH of the bay water.

Since the completion of the current dredged channel in the 1950s, agglutinated foraminiferal densities have increased in a response to the reduced tidal flushing and increased terrestrial organic matter. However, the youngest sediments in the deepest areas of the bay also record accelerated losses of foraminiferal tests and densities as a result of hypoxic water formation. Comparisons of the present-day foraminiferal assemblage with foraminifers collected in the early 1970s further document the continued loss of calcareous foraminifers in the bay. This biologic degradation agrees with the observed loss of the oysters and submerged aquatic vegetation from Mobile Bay during the last century.