Resilience of deep-sea benthic communities to turbidity flows following the 2016 Kaikōura Earthquake

Sediment density flows are complex events that contain multiple flow types which can transport massive amounts of sediment across large distances. In this thesis, the term “turbidity flow” will mainly be used to refer to these events. Turbidity flows are believed to have profound and lasting impacts on benthic communities in the deep sea, with hypothesised effects on both productivity and diversity. To better understand these large-scale disturbances, the physical characteristics of turbidity flows and the mechanisms by which they influence deep-sea benthic communities are reviewed. In addition, data from turbidity flows that occurred hundreds to thousands of years ago as well as three more recent events were compiled and analysed to assess support for published hypotheses (Chapter 1). A canyon-flushing event in Kaikōura Canyon, New Zealand, triggered by the 2016 Mw 7.8 Kaikōura Earthquake, included significant submarine mass wasting, debris, and turbidity flows, and provided an opportunity to investigate the effects of this disturbance. The mega- (Chapter 2), macro- (Chapter 3), and meiofauna (Chapter 4) community structure, abundance, biomass, and diversity before and after the event were analysed in a time series of imagery and sediment cores. The results showed that all fauna dramatically decreased 10 weeks after the turbidity flow event, and by 4 years after the disturbance the benthic communities were similar to, but not yet the same as, the pre-event communities. The minimum threshold time for each size component of the benthic community to recover was estimated, with macrofauna projected to take the longest to recover (5.6-6.7 years), and megafauna and meiofauna estimated to recover in a similar time period (4.6-5.2 years, and 4.6-5.0 years, respectively). Community recovery was investigated in relation to changes in the physical characteristics of the habitat caused by the disturbance, using environmental variables from images and bathymetric variables (Chapter 2) and sediment cores (Chapter 3 and 4). For the megafauna community, bioturbation was identified as a key explanatory variable, while sediment fineness and food quantity were key for macrofauna, and food quantity along with the food quality variables were key for meiofauna. The implications of the resilience of the deep-sea benthic community in Kaikōura Canyon are discussed in the context of the local marine protected area, the surrounding fishery, and global seabed mining (Chapter 2, 3, and 4). Key findings from the preceding chapters were synthesised, and their implications for the previously published hypotheses and future research were discussed (Chapter 5).