Tracking bioaccumulation

Report: Modelling ecological impacts of metal bioaccumulation from mid-water plume discharges

• An Ecotracer model effectively tracks metal movement through food webs and provides a tool for assessing ecological risks from mining-related discharges.
• Metal bioaccumulation from midwater plumes generated by deep-sea mining is likely to be low under realistic discharge scenarios, with only minor increases in contaminant levels across marine food webs.
• Shallower discharge depths (less than 1000 m) pose a greater risk of metal exposure to marine life than deeper discharges (between 1000 to 3000 m), especially for species in upper ocean layers.

This report presents the results of a study using the Ecotracer routine within the Ecopath with Ecosim (EwE) ecosystem modelling framework to assess the potential for metal bioaccumulation in oceanic food webs due to mid-water discharge plumes from deep-sea mining. The study focused on three categories of metals: highly toxic and persistent (e.g., mercury, lead), moderately toxic (e.g., copper, zinc), and essential but low-toxicity metals (e.g., iron, aluminium). The modelling work also distinguished between the food web in the surface, midwater, and deeper ocean zones and accounted for predators that travel between these layers, linking different parts of the ecosystem.

The model simulated how these metals move through the ocean food web under different discharge depths and concentrations. It updated an ecosystem model for the Eastern Tropical Pacific, including detailed representations of tuna species, mesopelagic fish, and zooplankton.

Results showed that under model scenarios with conservative metal discharges dispersed across the entire model domain, increases in metal concentrations were small—typically less than 2% for highly toxic metals. Trophic magnification factors (TMFs) confirmed that highly toxic metals biomagnify more strongly through the food web than less toxic ones.

The model also found that shallow discharges led to broader ecosystem exposure, with more species accumulating metals, while deep discharges resulted in more localised effects. Modelling suggests that if higher levels of contaminants were released in shallower waters over 20 years, large sharks could show the biggest increase in toxic metals—up to 0.019 ug/g. In contrast, if the same amount was released in deeper waters, the highest increase would be much smaller, around 0.0007 ug/g, estimated for deep-sea fish.

While the model provides useful insights, it has limitations. It assumes uniform metal dispersion and does not account for metal speciation, sediment mixing, dilution, migratory behaviour or seasonal changes in species distribution. It also underscores the need for more data on metal concentrations across a broader range of marine taxa. Uncertainty remains about whether deep-sea releases could affect surface species, highlighting the need for research into feeding links, particularly the potential role of midwater species in transferring contaminants between deep and surface ecosystems.

Despite these caveats, the study offers a robust starting point for understanding how midwater discharge plumes from deep-sea mining might affect marine ecosystems, and it supports the development of science-based environmental safeguards.