Impeded Macrofaunal Colonization and Recovery Following Marine Deposition of Inert and Organically Modified Mine-Tailings

Abstract

Fine-grain, waste rock (or tailings) produced during mining processes are the main waste product following the extraction of valuable minerals and metals from ores. The proximity of mineral resources to coastal regions in some countries has meant that the organically inert mine tailings are often deposited into the marine environment as a submarine tailings placement or STP. This creates a real environmental challenge since the deposition of tailings at the seafloor can completely smother the benthic environment and reduce benthic biodiversity and alter ecosystem function. A limited number of studies have attempted to quantify the speed of seafloor recovery following tailings disposal and have suggested that one of the main factors limiting seafloor colonization is the lack of organic matter of the tailings deposited. In this experimental study, we used hydrodynamically unbiased settlement trays within a complete randomized block design to test whether macrofaunal colonization of mine tailings, and resemblance toward ambient assemblages, is enhanced when tailings become more enriched in organic C (Corg). Our experiment showed that tailings with Corg contents ≥ 1% exhibited lower faunal abundance and biomass and different macrofaunal community composition after 1 year compared to background sediments. Macrofaunal species richness (defined as the total number of different taxa) in the 0 and 0.5% Corg tailings treatments, however, was statistically indistinguishable from ambient intertidal sediments after 1 year. Furthermore, macrofaunal community structure was more similar to that of the background intertidal sediments after 1 year, which collectively suggest partial recovery of seafloor biodiversity in tailings with zero to low Corg levels. Nevertheless, macrofaunal abundance and biomass in these treatments remained significantly less than the background community after 1 year suggesting factors other than the Corg content of tailings may be structuring macrofaunal colonization and impeding seafloor recovery of tailings deposits. We propose that the non-marine physical structure of the tailings particles, which show great angularity is predominantly responsible for the observed delayed recovery.