Deep-sea macrofauna community recovery in Kaikōura canyon following an earthquake-triggered turbidity flow

Abstract

Sediment density flows can transport massive amounts of sediment across large distances and can have dramatic, long-lasting impacts on deep-sea benthic communities. A canyon-flushing event in Kaikōura Canyon, New Zealand, triggered by the 2016 Mw 7.8 Kaikōura Earthquake, which included significant submarine mass wasting, debris and turbidity flows, provides an excellent opportunity to investigate the effects of this type of large-scale natural disturbance on benthic communities. The macrofauna community in the canyon was well characterised before the disturbance and the canyon described as a deep-sea benthic biomass hotspot. Macrofauna community structure before and after the event was analysed in a time series of sediment cores from 8 sites (∼400–1300 m water depth) collected 10 years and 6 years before, and 10 weeks, 10 months, and 4 years after the disturbance. The benthic community structure showed significant variability at all time points, but immediately after the 2016 disturbance abundances of all macrofauna dramatically decreased. Four years after the disturbance the benthic community was similar to, but not yet comparable to the pre-event community. The average abundances of most macrofauna increased over the 4 years post-earthquake, with polychaetes and bivalves being key taxa in the recovery process, especially 10 months and 4 years after the disturbance. Infauna abundances >5 cm below the seabed, remained low even 4 years after the event but the overall pattern of vertical faunal distribution was trending towards a pre-disturbance state. Community recovery was investigated in relation to changes in the physical characteristics of the habitat caused by the disturbance, using variables derived from sediment cores (e.g., sediment texture, organic matter, and pigment content). While the environmental variables explained relatively little of the overall variability in macrofauna community structure, key variables identified were related to particle size structure and food availability and quality. The minimum threshold time for the macrofauna community to fully recover, based on within-community similarity, was estimated to be between 5.6 and 6.7 years, although recovery was predicted to take up to 9 years if higher recovery thresholds were used. The management implications of this study are considered in comparison to the few studies of large-scale disturbances in the deep sea, in terms of their relevance to the efficacy of the marine reserve that encompasses Kaikōura Canyon, and potential implications for our understanding of the impacts of anthropogenic disturbances such as seabed mining.