Simulated mining-related sedimentation impacts on the deep-sea macrofauna of the Chatham Rise, New Zealand

With the possibility of deep-sea mining of marine mineral resources occurring in the near future, it is necessary to understand the potential impacts that mining may have on benthic communities. Previous simulated mining experiments have observed direct impacts of deep-sea mining (e.g., faunal mortality); however, indirect impacts of sedimentation were not understood. In New Zealand, there has been interest in mining the seabed of the Chatham Rise, but mining consents have been refused, partly due to the uncertainties of sedimentation impacts on benthic communities. A disturbance experiment conducted in 2019 on the Rise used a modified agricultural plough designed to create a sediment cloud that could result from mining. This disturbance was used to assess the resilience of benthic communities to sedimentation in a proposed future mining area. Macrofaunal and sediment samples were collected with a multicorer before, immediately after and one year after disturbance to assess the impact on the community and its ability to recover. Samplingevents took place in disturbed (physically run over by the plough and subjected to sedimentation) and undisturbed areas (subjected to sedimentation only) at each sampling period. Macrofaunal abundance significantly decreased in disturbed areas after disturbancebut not in undisturbed areas. However, community structure changed in both areas after disturbance; in disturbed areas this was mostly driven by changes in numerically dominant fauna, but in undisturbed areas by the more sensitive fauna which may provide an early warning sign for further changes under increased sedimentation. One year after disturbance, community structure had recovered in both areas. Abundance-based community structure correlated most strongly with C:N molar ratios in the sediment which increased after disturbance. Ecosystem function was measured by sediment community oxygen consumption (SCOC) which increased similarly in both disturbed and undisturbed areas after disturbance; SCOC may be a more sensitive measure than community structure in assessing sedimentation impacts. No correlations were found between SCOC and macrofaunal abundance, biomass, diversity or bacterial abundance. The results of this research are useful for managing the impacts of industries where sedimentation is an issue, such as for bottom trawl fisheries and deep-sea mining. The results highlight the importance of leaving unmined patches of seabed adjacent to or within mined areas, to aid the recovery of macrofaunal communities subjected to mining disturbance.