Impact of Red Mud Deposits in the Canyon of Cassidaigne on the Macrobenthos of the Mediterranean Continental Slope

Abstract

Since 1967, red mud produced by an aluminum plant by the process of aluminum extraction from bauxite has been discharged into the canyon of Cassidaigne (North Mediterranean Sea) at 320 m depth. To study the effect of these dumpings on the macrobenthic fauna of the continental slope, sediments were sampled in September 1991, 1997, and 2002 from stations located on both sides of the canyon, at between 250 and 2,200 m depth. For each station, a faunal analysis was coupled with analyses of sediment granulometry and heavy metal content. The species collected belong to deep mud assemblages characteristic of the bathyal zone of the Mediterranean Sea. We observed a progressive decrease with depth in species richness and average population density. Such a decrease is usual on the continental slope and cannot be attributed to a particular anthropogenic disturbance of the ecosystem. Hierarchical analyses do not differentiate communities under the dependence of the deposits from communities not impacted by red mud. Correlations between environmental variables and communities fail to demonstrate a particular incidence of titanium, the optimal marker for red mud. The distribution of the benthic populations is directly dependent on changes in bathymetry and associated parameters, such as sedimentation rate and food availability. There is a significant, long-term variability in the qualitative and quantitative composition of the communities of the continental slope, which are directly under the influence of the Mediterranean Northern Current and the global evolution of climatic conditions.