Deep-sea benthic foraminifera in the Cassidaigne Canyon (NW Mediterranean): assessing the ecological recovery six years after the cessation of red mud dumping at a bauxite industrial waste site

During an environmental survey performed in winter and spring 2022, living (Rose Bengal stained) benthic foraminiferal faunas were investigated at 13 stations sampled within the Cassidaigne Canyon (NW Mediterranean Sea) and surrounding area. These stations are located between 265–2300 m water depth. For many decades, industrial bauxite residues of red mud have been dumped into the canyon via a submarine pipe, causing physical disturbance and chemical contamination. In January 2016, solid waste underwater dispersal ceased and was replaced with the dumping of a low-density liquid effluent. Six years after the cessation of red mud dispersal, our observations at the 725 m-depth station closest to the Cassidaigne Canyon submarine outlet show a better ecological quality compared to the 2012 (during the red mud dumping) and 2016 (ten months after the cessation of dumping) sampling, suggesting a putative biotic recovery at the seafloor. That being said, this station still presents the highest abundance of opportunistic species, and a noticeably altered benthic diversity. At the other twelve stations, foraminiferal standing stocks and simple diversity decrease with decreasing food input to the seafloor and increasing water depth. There foraminiferal composition, with a minor contribution of opportunistic and stress-tolerant species, echoes (1) the overall meso-oligotrophic patterns of a relatively stable ecosystem, and (2) the putative trophic effect of phytodetritus exportation in spring 2022.