Tracking ecological recovery in real-time using genomics: impact of a high-magnitude earthquake on intertidal kelp

Figure 1: Photo taken shortly after the 2016 Kaikōura earthquake, with many dead D. antarctica within the uplifted intertidal zone.

By: Felix Vaux

When someone asks you to think of a large-scale ecological disturbance event, what do you think of? Forest fires, landslides or floods? Such disasters easily spring to mind because their effects on ecosystems often overlap with the threats they pose to human society. A less obvious example is earthquake uplift along coastlines. Here, the previous intertidal zone is lifted out of the sea and intertidal organism with limited mobility are left high-and-dry to perish out of water. At the same time, terrain previously submerged below the waves is lifted up to form a new intertidal zone. The overall outcome is devastation for most intertidal marine organisms and vacant habitat in the newly formed intertidal zone that is ripe for fresh colonisation.

In 2016, a devastating, high magnitude earthquake (7.8 Mw) struck central New Zealand (Aotearoa). Coastline across the Kaikōura region was lifted up in a chaotic fashion, with some shoreline lifting up as much as 6 m! Along the coastline, swathes of indigenous seaweed were devastated by this uplift – including the charismatic, brown macroalga Durvillaea antarctica (specifically the ‘NZ North’ lineage). Also known as southern bull kelp or rimurapa in te reo Māori, D. antarctica is large seaweed (often growing >10 m long!) that thrives in exposed intertidal zones across New Zealand and provides valuable habitat for numerous invertebrates and epiphytic algae. Notably, D. antarctica has honeycomb-like structures in its fronds, that allow individuals to raft and disperse over long distances when they break away from rocks.

Shortly after the earthquake in 2016, we travelled to the uplift zone and sampled many individuals of D. antarctica that perished due to the tectonic uplift. These samples represented the pre-uplift populations of D. antarctica. Over the next four years, we revisited the region and sampled new recruits of D. antarctica that colonised the newly formed intertidal zone. We also sampled wider populations across central New Zealand that were not affected by the earthquake.

Figure 2: Photo taken during fieldwork collections for D. antarctica, years after the earthquake, showing new recruits of D. antarctica growing in the new intertidal zone.

Using genomic sequencing (genotyping-by-sequencing), we investigated genetic diversity among the pre- and post-uplift populations. Specifically, we used the earthquake as a natural experiment to investigate recolonisation processes. We were particularly interested if new genotypes travelled from further afield had managed to establish themselves within the uplift zone.

Figure 3: Not all fieldwork! A photo of an electrophoresis gel used for one of the genotyping-by-sequencing libraries. Hundreds of individuals across 17 locations were sequenced for the study.

Our genomic results indicate that within the first four years, very little change has occurred in population structure. This result seems perplexing at first, but these findings are the first, early ‘snapshots’ of an ongoing recolonisation process that will likely take hundreds of years to complete. Using oceanographic connectivity modelling, we also estimate the that northbound dispersal is most likely for the species, which aligned with results for a modest number of likely dispersers.

Figure 4: Figure 3 from the paper, showing our sampling across central New Zealand (a), the lack of significant genetic change between the pre- and post-uplift populations of D. antarctica (b), and our estimates for oceanographic connectivity that indicated that northbound dispersal should dominate (c). See the paper for a full explanation.

Firstly, previous surveillance research and our own fieldwork has made it clear that many areas of the coastline are still extirpated and D. antarctica hasn’t yet recolonised every section of coastline it previously inhabited. In those areas, we don’t yet know which genotypes may arrive – and change remains possible. In order to exploit this exciting natural experiment further – future researchers will need to conduct ongoing sampling over multiple decades.

Secondly, the 2016 earthquake occurred across a complex series of fault lines with uneven and complicated patterns of uplift across our sampled locations. So, while many locations are extirpated, there are others where D. antarctica managed to hold in the lowest areas of the intertidal and shallow subtidal zones. These survivors are often sparse, but they’ve clearly managed to dominate the early recolonisation process at the sites we sampled. In such cases, it therefore makes sense that genetic diversity hasn’t yet changed significantly between the pre- and post-uplift populations.

Figure 5: Juveniles of D. antarctica growing in the new intertidal zone at one the sample locations.

This natural experiment and our findings are fascinating though, because we’ve managed to capture genetic diversity prior a very unpredictable natural disturbance event, and we’ve been able to track the earliest stages of a large-scale recolonisation process in real-time! This study system with D. antarctica will allow future researchers to investigate hypotheses regarding recolonisation that straddle the boundaries of ecology and evolution, as well as micro- and macro-evolutionary change. Will the initial recolonisation from refugial populations always dominate the population structure of recolonised coastline, or will long-distance dispersal eventually lead to distinct genetic populations or admixture? Likewise, previous research has demonstrated that D. antarctica is susceptible to marine heatwaves – and sparse populations devasted by the earthquake are particularly at risk. How will the fragile D. antarctica populations within the Kaikoura region change over time?

Figure 6: Illustration showing potential scenarios for the future recolonisation of D. antarctica. See paper Figure 4 for full explanation.

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