Abstract
Fine-grained, 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 colonisation is the lack of organic matter of the tailings deposited. In this experimental study, we used hydrodynamically unbiased settlement trays within a complete randomised block design to test whether macrofaunal colonisation of mine tailings, and resemblance towards 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 one 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 one year. Furthermore, macrofaunal community structure was more similar to that of the background intertidal sediments after one 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 one year suggesting factors other than the Corg content of tailings may be structuring macrofaunal colonisation 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.
Highlights
The disposal of sedimentary material into the marine environment constitutes a significant problem in coastal zone management and, albeit often localized, represents a major anthropogenic disturbance on the benthos (Bolam et al, 2004; Ramirez-Llodra et al, 2015; Trannum et al, 2019)
Corg content in the sampling control’ (SC) plots was significantly higher than the Corg content in the 0, 0.5, and 1% treatments at all sampling points, and despite Corg content increasing in the 0 and 0.5% treatments over time, which was possibly due to greater organic C deposition rates from the adjacent mudflat compared to the rate of mineralization, there were no significant changes in Corg content in these, nor in the 1% treatment during the experiment (Figure 3)
There was a statistically significant decrease in Corg content in the 2.5% treatment between 45 and 180 days compared to the SC plots, as well as in the 5% treatment between 45 days and later sampling times (Figure 3) possibly due to C-remineralization rates being higher than C-deposition rates
Summary
The disposal of sedimentary material into the marine environment constitutes a significant problem in coastal zone management and, albeit often localized, represents a major anthropogenic disturbance on the benthos (Bolam et al, 2004; Ramirez-Llodra et al, 2015; Trannum et al, 2019). The millions of tons of waste produced annually limits potential transport options so tailings tend to be disposed of in slurry form directly to the coastal environment through de-aerated pipelines (Kvassnes and Iversen, 2013; Ramirez-Llodra et al, 2015). In Norway, where this practice is routinely carried out, the coastal environments regularly affected by the deposition of mine tailings include soft-sediment fjord habitats (Kvassnes and Iversen, 2013; Trannum et al, 2018, 2019, 2020). Mine tailings deposited into coastal water bodies in Norway have included both chemically inert or reactive (e.g., containing sulfides) forms, while tailings deposited today are largely chemically inert (Kvassnes and Iversen, 2013)
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