Distribution of benthic foraminiferal populations in surface sediments of the Saguenay Fjord, before and after the 1996 flood

In the summer of 1996, a 'flash' flood occurred in the Saguenay-Lac-St-Jean region (Quebec) leading to the deposition, in less than 2 days, of an estimated 6-15 x 10(6) m(3) of sediments at the head of the Saguenay Fjord. In order to evaluate the impact of such a sedimentary event on the benthic meiofauna, foraminiferal analyses were performed in surface sediment at eight stations in 1994, 1997, 1998 and 1999 along the main axis of the fjord. A 10-cm sedimentary sequence taken from a box core collected in 1997 from the deepest part of the fjord and spanning the last ca. 35 years provides a reference state for pre-flood foraminiferal assemblages. A significant change in foraminiferal assemblages is observed downcore, with the decrease of Spiroplectammina biformis relative to Adercotryma glomerata. Pb-210 measurements suggest that this change occurred during the late 1970s. Surface samples (0-1 cm) collected between 1997 and 1999 in the inner basin also reveal a decrease of S. biformis percentages. Given the relative stability of this environment, such a change in faunal assemblages could be related to reduced industrial waste inputs or to a differential preservation of taxa. The 1996 flood had a major negative impact on the concentration and species diversity of benthic populations near the head of the fjord (Baie des Ha! Ha!), where the flood material is up to 50 cm, thick. However, two years after the flood, benthic foraminiferal populations recolonized the sediments in the Saguenay Fjord. (C) 2002 Elsevier Science B.V. All rights reserved.