The July 1996 flash flood in the Saguenay region (Quebec). 2. Impact on the sediment and biota of the Baie des Ha!Ha! and the Saguenay Fjord

Several million tons of sediments and various debris were transported to th e north arm of the Saguenay Fjord and the Baie des Ha! Ha! following the to rrential rains and accompanying flash flood of July 1996 in the Saguenay re gion (Quebec). The flood deposits covered the indigenous sediments and buri ed the benthic community. The objective of this work is to determine the be st chemical and biological indicators of the changes that occurred in the b enthic habitat of the fjord. The new sediment layer is poor in organic matt er but rich in detrital carbonates and contains low mercury and lead concen trations reflecting the absence of industrial contamination. Ongoing geolog ical processes leading to the re-establishment of steady-state conditions w ere evidenced by measurements of porosity, redox potential (Eh), and salini ty of interstitial waters taken a few weeks and a few months after the even ts. The meiofauna were severely affected with a massive reduction of the av erage density of organisms relative to values recorded before the flood, bu t recolonisation was already in progress 1 year after the flood. Important losses of macrobenthic species were recorded in 1997 relative to 1996. Howe ver, the abundance and the diversity of the polycheate taxa were higher in 1997, indicating a recolonisation of the new sediment layer by pioneer spec ies.