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Aim Ecologists traditionally study how contemporary local processes, such as biological interactions and physical stressors, affect the distribution and abundance of organisms. By comparison, biogeographers study the distribution of the same organisms, but focus on historic, larger-scale processes that can cause mass mortalities, such as earthquakes. Here we document cascading effects of rare biogeographical (seismic) and more common ecological (temperature-related) processes on the distribution and abundances of coastal foundation species. Location Intertidal wave-exposed rocky reefs around Kaikōura, New Zealand, dominated by large, long-lived, and iconic southern bull kelps (Durvillaea antarctica and Durvillaea willana). Methods In November 2016, a 7.8 Mw earthquake uplifted the coastline around Kaikōura by up to 2 m, and a year later the region experienced the hottest summer on record. Extensive sampling of intertidal communities over 15 km coastline were done shortly after the earthquake and heatwaves and 4 years after the earthquake. Results Durvillaea lost 75% of its canopy to uplift and the heatwaves reduced canopies that had survived the uplift by an additional 35%. The survey done 4 years after the earthquake showed that Durvillaea had not recovered and that the intertidal zone in many places now was dominated by small turfs and foliose seaweed. Main conclusions Cascading impacts from seismic uplift and heatwaves have destroyed populations of Durvillaea around Kaikōura. Surviving smaller and sparser Durvillaea patches will likely compromise capacity for self-replacement and lower resilience to future stressors. These results are discussed in a global biogeographical-ecological context of seismic activity and extreme heatwaves and highlight that these events, which are not particularly rare in a geological context, may have common long-lasting ecological legacies.
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Diversity and Distributions. 2021;27:2369–2383.
Received: 1 May 2021 
  Revised: 17 August 2021 
  Accepted: 18 August 2021
DOI : 10.1111/ddi .13407
Cascading impacts of earthquakes and extreme heatwaves
have destroyed populations of an iconic marine foundation
Mads S. Thomsen1,2 | Luca Mondardini1| François Thoral1| Derek Gerber1|
Shinae Montie1| Paul M. South3| Leigh Tait4| Shane Orchard1| Tommaso Alestra1|
David R. Schiel1
This is an op en access article under the ter ms of the Creative Commons Attribution License, which pe rmits use, distribution and reproduction in any medium,
provide d the original wor k is properly cited.
© 2021 The Authors. Diversity and Distributions publish ed by John Wiley & Sons Ltd.
1Marine Ecology Research Group, Centre
for Integrative Ecology, School of Biological
Science s, University of Canterbur y,
Christchurch, New Zealand
2Depar tment of Bioscie nce, Aarhus
University, Roskilde, Denmark
3Cawthron Institute, Nelson, New Zealand
4NIWA, Christchurch, New Zealand
Mads S. Thomsen , Depar tment of
Bioscience, Aarhus Uni versit y, Roskild e
400 0, Denmark.
Funding information
Ministry of Primary I ndust ries and the
Ministry of Business, Innovation and
Employment, Grant/Award Number:
UOCX1704; New Zealand Ministry of
Business, Innovation and Employ ment,
Grant/Award Number: CAW X1801; Brian
Mason Trust
Editor: April Blakeslee
Aim: Ecologists traditionally study how contemporary local processes, such as bio-
logical interactions and physical stressors, affect the distribution and abundance of
organisms. By comparison, biogeographers study the distribution of the same organ-
isms, but focus on historic, larger- scale processes that can cause mass mortalities,
such as earthquakes. Here we document cascading effects of rare biogeographical
(seismic) and more common ecological (temperature- related) processes on the distri-
bution and abundances of coastal foundation species.
Location: Intertidal wave- exposed rocky reefs around Kaikōura, New Zealand, domi-
nated by large, long- lived, and iconic southern bull kelps (Durvillaea antarctica and
Durvillaea willana).
Methods: In November 2016, a 7.8 Mw earthquake uplifted the coastline around
Kaikōura by up to 2 m, and a year later the region experienced the hottest summer
on record. Extensive sampling of intertidal communities over 15 km coastline were
done shortly after the earthquake and heatwaves and 4 years after the earthquake.
Results: Durvillaea lost 75% of its canopy to uplift and the heatwaves reduced cano-
pies that had survived the uplift by an additional 35%. The survey done 4 years after
the earthquake showed that Durvillaea had not recovered and that the intertidal zone
in many places now was dominated by small turfs and foliose seaweed.
Main conclusions: Cascading impacts from seismic uplift and heatwaves have de-
stroyed populations of Durvillaea around Kaikōura. Surviving smaller and sparser
Durvillaea patches will likely compromise capacity for self- replacement and lower
resilience to future stressors. These results are discussed in a global biogeographical-
ecological context of seismic activity and extreme heatwaves and highlight that these
events, which are not particularly rare in a geological context, may have common
long- lasting ecological legacies.
Ecologist s traditionally study how ubiquitous processes such as
competition, predation, disturbance and abiotic stress affect the
distributions and abundances of populations, species and communi-
ties (Begon et al., 1986; Smith & Smith, 2015). These processes typi-
cally take place over shor t temporal (days- decades) and small spatial
(cm- km) scales. Similarly, larger- scale events, such as hurricanes,
fires, droughts and heatwaves, which are often associated with cli-
mate change, can have important effects on ecosystem structure
(Buma, 2015; Lugo, 2008; Lundholm, 2009; Sippel et al., 2018).
For example, atmospheric heatwaves have altered population de-
mographics and species interactions and caused poleward range
movements and localized extinctions of foundation species, which
provide critical biogenic habitat for associated species and ecosys-
tem functioning (Chen et al., 2011; Dillon et al., 2010; Ellison, 2019;
Thomas et al., 2004). Like ecologists, palaeontologists and bioge-
ographers also study processes that affect distributions and abun-
dances of organisms but focus on rarer events and longer temporal
(millennia to millions of years) and larger spatial (km to global) scales,
such as stochastic long- range dispersal incidents or cataclysmic
tectonic events that can cause extinctions and dispersal barriers
(Black & Black, 1988; Brown, 2009; Cox et al., 2016; Crisci, 2001).
However, these traditional disciplinary boundaries are human-
made constructs, and it is increasingly recognized that studies
across spatiotemporal scales are required for better understanding
of present- day distributional patterns (Crisci et al., 2006; Wiens &
Donoghue, 2004).
In the marine environment, heatwaves have intensified in recent
de cade s, and mo del s pr oject tha t the y wi ll co nti nue to be come str on-
ger and more frequent (Frölicher et al., 2018; Hobday et al., 2018;
Oliver et al., 2019; Sen Gupta et al., 2020). Over the last few de-
cades, marine heatwaves have caused coral bleaching, localized ex-
tinctions, poleward range- shifts of many species of fish and seaweed
and altered species interactions (Cheung & Frölicher, 2020; Smale
& Wernberg, 2013; Thomsen et al., 2019; Wernberg et al., 2013).
At longer time scales, many coastal ecosystems have also experi-
enced extreme disturbances associated with seismic activity such as
earthquakes and volcanic eruptions (Castilla & Oliva, 1990; Castilla
et al., 2010). However, traditional marine ecology (e.g. textbooks
like Castro & Huber, 2019; Kaiser et al., 2011) do not typically dis-
cuss large- scale disturbances associated with seismic activity in
a context of contemporar y species distributions and biodiversit y.
Nevertheless, rare mega- disturbances do affect local biodiversity
with ramifications for many years to come. For example, historic and
contemporary earthquakes have affected present- day species dis-
tributions (Haven, 1964; Noda et al., 2016; Rodil et al., 2016). Such
examples show that the extreme events traditionally studied by
biogeographers and palaeontologists can be relevant for ecologists
and the study of contemporary ecosystems.
On November 14, 2016, a 7.8 Mw (on the moment magnitude
scale) earthquake struck the northeast South Island of New Zealand,
with the epicentre located 60 km south- west of the small coastal
town of Kaikōura (Clark et al., 2017; Hamling et al., 2017; Kaiser
et al., 2017). With a shallow hypocentre of 15 km and many complex
inshore and offshore faults, slips, vertical displacements, coastal
uplift s of up to 6.5 m, and >2,000 aftershocks in only 3 days, four
of which had magnitude >6 Mw, these earthquakes directly af-
fected c. 130 km coastline (Clark et al., 2017; Hamling et al., 2017;
Xu et al., 2018). Over the next few months, we observed extensive
loss of habitat- forming seaweeds and slow- moving benthic inverte-
brates, with associated losses of primary produc tivity, biogenic hab-
itat and altered food webs (Gerrity et al., 2020; Schiel et al., 2019;
Thomsen et al., 2020). A year later, over the austral summer of
2017/18, New Zealand experienced the strongest marine heatwave
on record (Salinger et al., 2019, 2020). This large- scale extreme
marine event, coincided with high air temperatures, low tides and
calm sea conditions (Salinger et al., 2020; Thomsen et al., 2019),
and had widespread effects such as high glacial melting, losses of
habitat- forming seaweeds, and movement of fish into warm waters
(Salinger et al., 2020; Schiel et al., 2019; Tait et al., 2021; Thomsen
et al., 2019; Thomsen & South, 2019). Following Hobday et al. (2018),
this large- scale temperature anomaly was named the “Tasman Sea
2017/18 marine heatwave” (Perkins- Kirkpatrick et al., 2019; Salinger
et al., 2019). However, analyses of local sea temperature data show
that this heatwave was comprised of multiple consecutive events
(see Result section), and the term is hereafter pluralized and referred
to as “heatwaves.”
Some of the major coastal species that appeared to be imme-
diately affected by the earthquakes were the southern bull kelps,
Durvillaea spp. (see Figure 1, hereafter just bull kelps). Bull kelps
are the world’s largest fucoid algae, with individuals of some spe-
cies reaching up to 10 m long, weighing up to 70 kg and living up
to 10 years (Hay, 1979, Nelson 2013, Hay, 2020). Bull kelps are
dominant conspicuous habitat- forming seaweeds on the intertidal–
subtidal fringe of rugged wave- exposed reefs in New Zealand and
many other places in the southern hemisphere where they con-
trol local diversity and ecological functioning (Schiel, 2019; Taylor
& Schiel, 2005; Thomsen & South, 2019). Southern bull kelps are
also iconic, culturally important seaweeds in New Zealand and other
parts of the world, such as Chile. For example, there is a long tradi-
tional use of Durvillaea spp. as storage bags , known as “p ōh ā” in New
Zealand, and as a source of food in Chile for at least 14,000 years
(Dillehay et al., 2008). The iconic nature of bull kelp is reflected in
its use in cultural and spiritual symbolism (Pérez- Lloréns et al., 2020)
such as being emblematic of the knowledge and status of the people
alternative foundation species, cataclysmic disturbances, habitat- formers, regime shift, seismic
uplift, turf algae
of Kaikōura (Anon., 2007). We examined changes to the distribu-
tion and abundance of bull kelp around Kaikōura, following the 2016
earthquakes and 2017/18 heatwaves and whether they returned
or were replaced by alternative foundation species (large canopy-
forming seaweed, such as Lessonia variegata and Carpophyllum
maschalocarpum) that were also common in the region (Schiel, 2006;
Thomsen & South, 2019). These results are discussed in the context
of other large- scale impacts over long time periods.
2.1 | Study system and marine heatwaves in the
Kaikōura region
The bull kelp species in this study were Durvillaea antarctica, which
occurs at the lowest tidal zone, and D. willana, which occurs slightly
deeper in the subtidal zone and is only partially exposed at the
lowest tides. A third bull kelp species, D. poha, can be morphologi-
cally similar to D. antarctica, but inhabits slightly less wave- exposed
habitats and is absent (or very rare) along the Kaikōura coastline
(Fraser et al., 2012; Peters et al., 2020; Vaux et al., 2021; Velásquez
et al., 2020). Additionally, there are two recognized clades of D. ant-
arctica in New Zealand that are distributed to the north and south
of Banks Peninsula, and therefore, most of the D. antarctica popula-
tions studied here belonged to the D. antarcticaNZ north” clade,
whereas the Moeraki samples used as controls in survey 1 were of
the D. antarctica “NZ south” clade (Fraser et al., 2020). Bull kelps
have exceptionally strong and large holdfasts and their heavy fronds
affect the subcanopy environment and biodiversity through shading
and whiplash (Schiel, 2019; Thomsen & South, 2019). The physical
attributes of the Tasman Sea 17/18 marine heat waves, and covary-
ing environmental factors, such as low wind speed, fewer waves and
high land temperatures, as well as co- occurring spring tides, have
FIGURE 1 (a) Landscape photograph
of healthy bull kelp at non- uplifted
reef (Oaro) before the heatwaves. (b)
Landscape photo of uplifted reef showing
the high band with white boulders
dominated by dead encrusting algae, mid
band with green boulders dominated by
newly colonized Ulva spp. and a distant
low band with brown boulders dominated
by live bull kelp. (c) Drone image showing
a ca. 5 m high (white), 8 m mid (green)
and 3 m low (brown) shore bands. (d)
Decaying ghost holdfast peeling off the
reef to reveal a new rock scar surrounded
by dead (white) calcifying algae. (e,f)
Examples of mid bands with circles
around a new grey holdfast scar (e) and
a ghost holdfast without stipe (f). (g)
Mid band dominated by Ulva with circle
around ghost holdfasts with stipes but no
fronds, (h). Low band showing living (pink)
encrusting algae and living (brown) fronds
of unbranched (lef t circle = Durvillaea
antarctica) and branched (right
circle = D. willana) bull kelp stipes.
Quadrat size = 0.5 × 0.5 m
been analysed and discussed in great detail (Perkins- Kirkpatrick
et al., 2019; Salinger et al., 2019, 2020; Thomsen et al., 2019). To
highlight that the Kaikōura coastal region was embedded in this
heatwave event, we produced a large- scale heat map showing the
maximum extent of the Tasman Sea 17/18 MHWs (i.e. for January
27, 2018, Figure 2a). To show that the 17/18 event was extreme
compared to past MHWs, we added local- scale temporal analysis
of sea surface temperatures describing all MHWs near Oaro and
Kaikōura peninsula (grid centred around 173.625, −42.625 and
173.875; −42.375, respectively) between 1/1- 1982 and 1/1- 2020.
The distance from the reef sites to the grid centre was <25 km. For
each of the two sites, we identified all MHWs and calculated their
maximum intensity (temperature above the 90% threshold in °C),
durations (in days) and cumulative intensity (duration × intensity).
For brevity, we only show maximum intensity (Figure 2b), whereas
duration and accumulated intensity are reported in the Appendix S1.
The MHW analyses were done using the R package heatwaver ver-
sion 0.4.5 (Schlegel & Smit, 2018; Smit et al., 2018), which defines a
MHW as a period of 5 or more consecutive days where the sea sur-
face temperature is greater than the 90th percentile calculated from
a 30 year climatology (period between 1/1- 1983 and 31/12- 2012).
Analyses were based on NOA A high- resolution blended analysis of
daily sea surface temperature data in ¼ degree grids derived from
satellites and in situ data (oisst v2.1, accessed from https://coast p/index.html) (Huang et al., 2021). More
details, including codes and outputs, are available at: https://rpubs.
com/FranT oto/Thoms en2021_MHW and
oto/Thoms en_etal_2021_MHW_EQ.
FIGURE 2 (a) Maximal areal extent of
the Tasman Sea marine heatwave (January
27, 2018) covering c. 5.4 million km2. (b)
Maximum intensity of marine heatwaves
recorded offshore of the Kaikōura
Peninsula and Oaro between 1982 and
2020 (see Appendix S1 for similar data
for other heatwave metrics). (c) Sample
sites along the Kaikōura coastline. Oaro
is the non- uplifted control reef, and
the blue markers show locations of 15
uplifted reefs (0.5– 2 m uplift) numbered
with distance from Oaro. All reefs were
sampled in Survey 1, reefs 1, 2, 4, 6, 7
and 8 were sampled in Survey 2, and
reefs 1, 2, 4, 6, 9, and 15 were sampled in
Survey 3
82 84 86 88 90 92 94 96 98 00 02 04 06 08 10 12 14 16 18 20 22
Kaikoura Peninsula
Oaro 2 km
2.2 | Survey 1: Loss of bull kelp within
months of the earthquake
The control reef at Oaro was selected due to its proximit y to uplifted
reefs and because it had similar high cover of bull kelp beds prior to
the earthquake (Figure 1a). Oa ro and the 15 uplif te d re ef s (0.5– 2 m),
were surveyed in February– March 2017 during low spring tides,
when access was logistically possible. For this study, the zone that
had been inhabited by bull kelp prior to the uplift was divided into
three elevation bands. The high band was visibly “white” because
all the calcified understory red algae had died and were bleached
by the sun (Figure 1b– f). There were also many obvious holdfast
scars of dead bull kelp in this zone (Figure 1d,e). The mid band was
a bright “green” because it was dominated by the ephemeral green
algae Ulva spp. (Figure 1b,g ) (Gerrity et al., 2020; Schiel et al., 2019).
In this band, most Durvillaea holdfasts were still present with their
stipes or stipe- remnants (Figure 1f,g), but their fronds had decom-
posed (i.e. "ghost- holdfasts" as described in Thomsen et al., 2019).
The low band was visibly brown and red or pink because bull kelp
canopies were still present, and the understory was still dominated
by living encrusting red algae (Figure 1h). Generally, what was
previously lower on the intertidal shore became the high band at
around >1.75 m above the lowest astronomical tide datum (LAT),
the mid band was uplifted from the shallow subtidal zone into the
mid- intertidal (~0.75– 1.75 m), and the low band (< ~0.75 m above
LAT) was uplifted from deeper than around – 0.5 m. At each reef
and elevation band, photographs were taken of haphazardly posi-
tioned 0.5 × 0.5 m quadrats and we quantified: (1) number of bull
kelp stipes with intact blades, (2) number of stipes without blades,
(3) per ce nt age cove r of int act holdfast s, (4) number of ho ld fast sc ars
and (5) per cent cover of holdfast scars. Stipes were counted be-
cause they represent individuals, whereas holdfasts can consist of
several coalesced individuals. Holdfast scars were near- circular, bare
reef sections (Schiel, 2019; Thomsen et al., 2019). Data for D. antarc-
tica and D. willana were pooled because these species cannot be
separated from observations of scars or holdfasts without stipes.
Previous analyses have shown that small- scale, reef- specific degree
of uplift did not correlate with any of the five measured responses
(Mondardini, 2018) and this aspect was therefore not explored.
No ne of the re spon ses coul d be tra nsf orme d to va ria nce ho mogene -
ity (Levine’s tests, p < .001), and sampling was unbalanced between
elevation bands and uplifted and control reefs. Standard or per-
mutational based ANOVA could therefore not be used (Anderson
et al., 2017). Instead, we used pairwise Mann– Whitney tests on rel-
evant data subsets to test for differences between the control site
at Oaro versus up lif ted reef s (a tot al of 10 test s for the 5 responses;
excluding the high band, which was not present at Oaro) and be-
tween the three elevation bands at the uplifted reefs only (a total
of 15 tests for the 5 responses). We note that 1 in 20 tests may be
inflated based on an a priori significant level of 0.05 when multiple
analyses are done (Anderson, 2005). All bull kelp and algae data are
reported as means and standard errors (SE).
To relate abundances of bull kelp holdfasts (with or without
blades as measured above) to pre- ear thquake healthy live bull kelp
canopies, percent cover of bull kelp holdfasts was estimated in 40
50 × 50 cm quadrats haphazardly positioned within dense bull kelp
beds (i.e. with 100% canopy cover) at Moeraki, a southern site not
affected by earthquakes and less affected by the Tasman Sea 17/18
heatwave (Thomsen et al., 2019; Thomsen & South, 2019). The 40
quadrat s were collected over a period of 2 years with ten quadrats
collected in Autumn and Spring in 2019 and 2020.
2.3 | Survey 2: Cascading loss of bull kelp after the
earthquakes and heatwaves
Six of the uplifted reefs (Figure 2c) were sampled with an Advanced
Phantom 3 drone equipped with a 12 MP HD camera, before and
after the summer period of the Tasman Sea 2017/18 marine heat-
waves. Images were taken 10 m above each reef during low tide,
with each image covering c. 95 m2, estimated from survey tapes and
1 m2 fixed quadrats positioned on each reef (Murfitt et al., 2017;
Thomsen et al., 2019). At this height, individual live bull kelps are
easily differentiated from rocks or other seaweeds that have dif-
ferent sizes, shapes and coloration. A total of 161 geotagged drone
images (12– 35 per reef) were captured between April– July 2017,
covering the period from shortly after the ear thquakes but before
the onset of the heatwaves. Another set of 161 images with similar
geocoordinates was collected on these reefs in April 2018 after the
heatwaves. In processing images, percent cover of white (calcified
dead algae), green (Ulva spp.) and bull kelp areas were estimated
visually for each image with a superimposed grid of 100 cells. Based
on results from Survey 1, the white and green areas observed in
the 2017 drone images on the uplifted reefs were confidently in-
terpreted as representing recently dead bull kelp because there was
high percent cover of new reef scars and ghost holdfasts (Figures 1
and 3). Percent loss of bull kelp following the earthquakes was calcu-
lated as cover of (white + green areas)/(white + green + bull − kelp
areas) × 100 for each 2017 image. Then, for each image, the of bull
kelp lost following the heatwaves was calculated as the area of bull
kelp2018/bull kelp2017 × 100. The number of drone images for the
heatwave impact was less than for uplift analyses because images
with 100% loss of bull kelp from the earthquake were removed from
the heatwave analyses. Data had, like for survey 1, strong variance
heterogeneity (Levine’s tests, p < .001) and we therefore used three
Mann– Whitney tests to investigate whether (a) per cent canopy loss
from the uplift was greater at the uplifted reefs compared to at Oaro
(i.e. an uplift ef fec t due to th e ear thqu ake) , (b ) th e los s from th e heat-
waves was greater at the uplifted reefs compared to Oaro (i.e. if a
previous uplift effect modified heatwave effects) and (c) whether
the loss reported after the earthquakes was greater than the loss
reported from the heat waves (i.e. the earthquake effects relative to
the later heatwave effects, here excluding the Oaro data because
this reef did not experience uplift).
2.4 | Survey 3: Recovery or replacement of bull kelp
with alternative foundation species after 4 years
Bull kelps are competitively superior foundation species that control
biodiversity (Schiel, 2019). However, there are a range of species on
the Kaikōura coastline that are also foundation species and, although
they are typically subordinate to bull kelps, it is possible that they
benefited from large- scale losses of bull kelp to become more domi-
nant . We evaluated whethe r bull kelp had rec over ed or bee n re placed
by alternative foundation species such as the laminarian Lessonia var-
iegata and the fucoids Carpophyllum maschalocarpum, Cystophora sca-
laris and Marginariella boryana, a new photograph survey was done in
December 2020 at six of the uplifted reefs (Figure 2c). At each reef,
bet we en 10 and 15 geotagge d digit al photog ra phs wer e taken at ran-
dom at ca. 1 m from the reef surface in the three elevation bands
(high, mid and low) previously dominated by bull kelp. Each of these
photos was taken at low spring tide and covered approximately 1 m2
reef surface (Thomsen et al., 2020). For each photograph, percent
cover was quantified for live D. antarctica and D. willana (Figure 1h),
other fucoids and kelp species, and smaller seaweeds grouped into
the categories of white/dead encrusting algae, encrusting reds, green
and other brown algae. Survey 3 data were only evaluated graphi-
cally due to the absence of “before” data and because statistical tests
among shore bands were unnecessary due the almost total absence
of alternative foundation species in the high and mid bands.
2.5 | Earthquakes and heatwaves in a global context
To consider our results in a global context, rare and potentially cata-
clysmic event s that could have implications for coastal ecosystems
were identified by combining the positions of the Kaikōura 2016
earthquake, the Tasman Sea 2017/18 MHW, historically recorded
earthquakes and volcanic eruptions, and the 62 most extreme ma-
rine heatwaves since 1982 in a single map. Earthquake data were
downloaded from the Significant Earthquake Database that contains
information on the most destructive earthquakes from 2,150 BCE to
the present that had (a) at least moderate damage (≥US $1 million),
(b) 10 or more human deaths, (c) Magnitude 7.5 or greater (Modified
Mercalli Intensity) and/or (d) generated a tsunami (accessed 1/11-
2020) (Anon., 2020). To map all the earthquakes from the data-
base, a few missing intensities were replaced with a conservative
low Magnitude value of 5 and a few missing geocoordinates with
the latitude- longitude provided by Google map when typing in the
“earthquake location” information. Volcanic eruption data were
downloaded from the Global Volcanism Program (Volcanoes of the
World 4.9.1, accessed confirmed cases 28/11 2020). To map all erup-
tions in this database, missing Volcanic Explosivity Index (VEI) cells
were replaced with a conservative low value of 1.5 VEI. Raster im-
ages were produced where all earthquake and eruption events were
superimposed on a global coastline, and then, the distance (in km)
from each seismic event to the nearest coastline was calculated.
We also quantified the extent of coastline affected by of each
of the 62 most extreme marine heatwaves (covering global satellite
data from 1982 to 2017). First, shape- polygons were extracted from
Figure 5 in Sen Gupta et al. (2020). These were overlaid onto a map of
the world’s coastline, converted to raster- format and the distance of
coastline affected by each ex treme marine heatwave was calculated.
The same methodolog y described in Sen Gupt a et al. (2020) was used
to quantify the attributes of the Tasman Sea 2017/18 heatwaves and
calculate the length of affected coastline to allow for direct compari-
sons with the 62 other extreme events. Coastlines have fractal prop-
erties, so calculating absolute coastline distance depend on the scale
of observation. We therefore used the same scale to calculate the
global coastline as well as MHW- impacted coastlines (1:10,000,00 0),
to provide a robust measure of the percentage of coastline affected.
More details, including R- codes related to calculating coastal dis-
tances for earthquakes, volcanic eruptions and extreme MHWs, are
at oto/Thoms en_etal_2021_MHW_EQ and oto/Thoms en2021_MHW.
3.1 | Marine heatwaves in the Kaikōura region
The Kaikōura peninsula and Oaro have experienced 105 and 119 ma-
rine heatwaves, respectively, since 1982 (Figure 2b). At Kaikōura and
Oaro, the three (4.4°C, 40 days, 3.5°C, 37 days and 3.2°C, 20 days)
and two (4.7°C, 40 days, 3.6°C, 20 days) strongest events on record
occurred, respectively over the summer of 2017/18.
3.2 | Survey 1: Loss of bull kelp after the
There were significant differences in the low band between the
uplifted reefs and the Oaro control reef, for all response variables
(p < .001), except “stipe with blades” ( p = .50, Figure 3). However,
in the mid band only “stipes without blades” (p = .001) and “holdfast
percentage cover” (p = .027) were significantly different between
Oaro and the uplifted reefs. Densities of stipes without blades were
greater at uplif ted reefs compared to Oaro in the low (0.07 ± 0.02
SE vs. 1.25 ± 0.15 SE ) and mid (0.125 ± 0.12 vs. 2.16 ± 0.07) bands.
The percentage cover of holdfasts was greater at uplif ted reefs
compared to Oaro, for the mid (13.3 ± 0.42 vs. 4.25 ± 2.89) and
low (18.3 ± 0.91 vs. 11.96 ± 0.65) bands. Densities (0.49 ± 0.07 vs.
0.20 ± 0.04) and per cent cover (2.97 ± 0.47 vs. 1.05 ± 0.25) of hol d-
fast scars in the low band were greater at the uplifted reefs com-
pared to at Oaro.
For the uplifted reefs only, most responses were significantly dif-
ferent between elevation bands ( p < .001). There were significant in-
creases in percentage cover and density of holdfast scar s from low, to
mid, to high bands and significant decreases in the percentage cover
of holdfasts (Figure 3a,b,e). Per cent cover of holdfast scars increased
from to 2.9 7 (±0 . 47 ) to 9.3 8 (±0.49) , densi t y of ho l d fas t sc a r s incre a s e d
from 0.49 (±0 .07) to 2.0 8 (±0.11) whereas per cent cover of remaining
holdfasts decreased from 18.33 (±0.92) to 7.77 (±0.51), from the low
to high bands. For stipes without blades, the greatest densities were
in the mid band (2.17 ± 0.07; Figure 3c) with similar lower densities
between the low and high bands (1.29 ± 0.12). By comparison, den-
sities of stipes with blades where greater in the low compared to the
mid and high bands (Figure 3d, 4.41 ± 0.34 vs. 0.25 ± 0.05). Finally,
the holdfast- canopy survey from Moeraki, showed that lower eleva-
tion bands within extensive closed bull kelp canopies had, on average,
8 ± 1.4% cover of holdfasts (n = 40). This result suggests that the low
and mid bands at the uplifted reefs, prior to the earthquakes likely
had closed bull kelp canopies (Figure 3e). The quadrats at Moeraki
also had 56.7 ± 3.7% cover of encrusting coralline algae showing that
healthy bull kelp beds have high abundances of encrusting coraline
beneath their canopies (see Appendix S1).
3.3 | Survey 2: Cascading loss of bull kelp after the
earthquakes and heatwaves
Loss of bull kelp after the earthquake was significantly greater at
the uplifted reefs compared to at Oaro (75 ± 2.3 vs. 0.0 ± 0.0%,
p < .001; Figure 4; U- Statistics = −8. 24, N = 149). Furthermore, bull
kelp loss from the heatwaves was significantly lower at the uplifted
reefs compared to Oaro across all bands (35.3 ± 3.2 vs. 61.2 ± 6.6%,
p <.001; U- Statistics = −3.53, N = 239). Finally, on the uplifted
reefs only, the loss from the earthquakes was significantly greater
than the subsequent loss following the heatwaves (75.5 ± 2.3 vs.
35.3 ± 3.2%, p <.001; U- Statistics = −8.27, N = 336).
3.4 | Survey 3: Replacement of bull kelp with
alternative foundation species
There were no bull kelps in the high band, a few scattered individu-
als in the mid band, and low cover in the low band (Figure 5, 1.5%
D. antarctica, 7% D. willana). By contrast, small foliose, filamentous,
and turf seaweeds, typically bet ween 3 and 30 cm long, dominated
the low and mid- elevation bands. Encrusting algae were found in all
bands but were less common in the low band (4% cover) compared to
healthy bull kelp beds (typically c. 60% cover, see supporting data).
Six alternative foundation species were found in the mid (with low
cover values) and low elevation bands (with low to mid- cover values)
including Carpophyllum maschalocarpum (most common), followed
Lessonia variegata, Cystophora scalaris, C. torulosa, Landsburgia querci-
folia, Marginariella spp. and Hormosira banksii.
FIGURE 3 Mean (+SE) percent cover
of holdfast scars (a), density of holdfast
scars (b), density of stipes without blades
(c), density of stipe with blades (d) and
percent cover of holdfasts (e) for low, mid
and high elevation bands a few months
after the 2016 uplift. N from left = 118,
8, 0 (i.e. no high band at the control reef),
181, 939, 412. Data were pooled across
15 uplifted reefs. It requires c. 8% cover of
living bull kelp holdfasts to reach a 100%
canopy cover (see Results section). Letters
and numbers refer to significant Mann–
Whitney tests: capital letters = effect
of uplift in the low band, lower case
letters = effects of uplift in the mid band;
number = effects of bands at the uplifted
reefs (note there was no high band at the
control reef)
Holdfast scars
Stipes without blades
Holdfast scars
Density per 0.25 m
Stipe with blades
Density per 0.25 m
Low Mid High Low Mid High
Low Mid High Low Mid High
Uplied reefs
Control reef
3.5 | The Kaikōura earthquakes and marine
heatwaves in a global context
The Significant Earthquake Database showed that >6,200 large
and destructive earthquakes occurred over the last 4,150 years
(Figure 6), of which 453 were of similar or of larger magnitude than
the Kaikōura 7.8 Mw event. Furthermore, a minimum of 9,908 vol-
canic eruptions have been confirmed over the same time period,
with a mean Volcanic Explosivity Index (VEI) of 1.9 and where 529
events had a VEI 4 or more. From these data, it was calculated that
4,186, 2,749, 2,565, 2,606 and 4,002 events occurred 0– 10, 11– 25,
26– 50, 51– 100 or more than 100 km from the coastline, respec-
tively. In other words, 75% (12,106) of the recorded seismic events
occurred with 100 km of a coastline (Figure 6) and may therefore
have affected coastal organisms.
Around 175,597 km of the world’s coastline has been affected
by the 62 most extreme marine heatwaves recorded between 1982
and 2017. Using Sen Gupta et al. (2020) methodology to identify
areas with “Max intensit y, with severity >2,” the maximum areal
extent of the Tasman Sea 2017/18 was estimated to cover 5.4 mil-
lion km2 on the 27/01/2018 (Figure 2a), with a maximum intensity
of 15.8°C M km2 (on 28/01/2018), based on a core date range of
15/11/17- 14/04/18. These data, when sorted by Maximum Intensity
S > 2, make the Tasman Sea 2017/18 event the 5th most extreme
event recorded worldwide since 1982. From these data, we calcu-
lated (as for the 62 previously recorded extreme events) that an ad-
ditional 6,703 km coastline was affected, adding the total amount
of coastline affected by extreme marine heatwaves since 1982 to
182,300 km (Figure 6), representing about 15% of global coastline.
4.1 | Loss of bull kelp
The earthquakes in November 2016 uplifted ca. 50 km of wave-
exposed rocky substrate between 0.5 and 6 m (31 km of boulder
reefs and 16 km of consolidated reef) (Gerrity et al., 2020). This up-
lift caused a 75% canopy loss of bull kelp in the southern Kaikōura
region. Furthermore, we found an additional 35% canopy loss of the
surviving bull kelp populations following consecutive heatwaves
over the summer of 2017/18.
Dramatic uplift- associated loss has also been observed on wave-
protected reef- platforms for smaller intertidal habitat- forming fu-
coids, such as Hormosira banksii and Cystophora spp., following the
same earthquakes (Schiel et al., 2019; Thomsen et al., 2020). It is
likely that the 75% loss of bull kelp encapsulates a disproportional
amount of the inter tidal D. antarctica compared to the shallow sub-
tidal D. willana (Figures 1h and 5) because D. antarctica was instantly
lifted to a very high elevation characterized by extreme emersion and
desiccation stress (Morton & Miller, 1973). By comparison, D. willana
FIGURE 4 Mean (+SE) percent canopy loss of bull kelp
(Durvillaea spp.) after the November 2016 Kaikōura uplift (black)
and after the 2017/18 heatwaves (red) averaged across six uplifted
reefs versus the control reef at Oaro. Number of images analysed
from left to right; 213, 123, 26 and 26. Letters and numbers refer
to significant Mann– Whitney test s: capital letters = effect of uplift
before the heatwaves; lower case letters = effects of heatwaves
between control and uplifted reefs; numbers = effects of
heatwaves on bull kelp populations that survived the uplift
Uplifted reefsControl reef
After uplift
After heatwaves
FIGURE 5 Mean (+SE) percent cover of bull kelp (a), small
seaweeds (b) and perennial canopy- forming seaweeds (c) (fucoids
and kelps) in December 2020 pooled across six uplifted reefs in
three elevation bands that were dominated by bull kelp prior to
the earthquakes. Number of samples for the low, mid and high
bands = 172, 123 and 139, respectively
Small seaweeds
Encrusting whites
Encrusting reds
Canopy-forming seaweeds
Low MidHigh
C. torulosa
C. scalaris
Southern bull kelp
D. antarctica
D. willana
inhabits depths of 0– 5 m, and therefore, many individuals have re-
mained within the vertical range of the species after the 0.5– 2 m
uplift at the study sites (Hay, 1979; 2020; Vaux et al., 2021). Greater
rates of heatwave- associated loss of bull kelp were also proportion-
ally less on the uplif ted reefs compared to Oaro (Figure 4), perhaps
because the uplift had already selected hardier individuals (Bennett
et al., 2015; Coleman & Wernberg, 2020; Gurgel et al., 2020; King
et al., 2018) or possibly because the bull kelp beds contained D. poha ,
a species of bull kelp with a typically southern distribution and likely
less tolerant to temperature stress (Thomsen et al., 2019; Vaux
et al., 2021; Velásquez et al., 2020). Together, the earthquakes and
heatwaves have massively reduced the abundance of bull kelp along
the Kaikōura coastline, ranging from heatwave only induced losses
at no n- uplif ted si tes (>60% at Oaro) to beyond the extent quantified
in this study (north of Kaikoura peninsula to Cape Campbell) where
reefs were uplifted up to 6 m and almost all bull kelp were destroyed
(Gerrit y et al., 2020; Schiel et al., 2019; Thomsen et al., 2020).
To date, most research on impacts from marine heatwaves has
focused on the effects of temperature on subtidal algal forests in
isolation (Smale et al., 2019; Straub et al., 2019). However, grazing,
wave exposure, turbidity or nutrient levels can modify temperature-
induced losses of kelp (Butler et al., 2020; Ling et al., 20 09; Tait et al,
2021; Zimmerman & Robertson, 1985). Multiple co- occurring and
cascading ecological stressors and disturbances often have complex
interactions and it is therefore important to quantify impacts from
marine heat waves in concert with other stressors (Crain et al., 2008;
Harvey et al., 2013; Hawkins et al., 2008). Here, bull kelp losses from
heatwaves were smaller on reefs that had already experienced losses
due to seismic uplift, but these results could have been modified by
other covarying stressors such as high summer air temperatures,
high irradiance, lower- than- usual tides and low wave energy events
(Salinger et al., 2019, 2020; Thomsen et al., 2019). In other words,
impact s from marine heatwaves may often be modified by a complex
set of covarying factors, and therefore, they should be studied as a
multi- factorial stressor.
4.2 | Dominant intertidal seaweeds 4 years after the
No alternative foundation species (sensu Thomsen & South, 2019)—
large perennial habitat- forming fucoids or laminarians— colonized
the high and mid bands of the uplifted coastline previously inhabited
by bull kelp. The high band of the former bull kelp habitat turned
white because the substrate had high cover of encrusting calcar-
eous red algae that were bleached by the sun following the uplift
(Figure 1) (Schiel et al., 2019). This band became inhabited by typi-
cal upper- shore species such as limpets, barnacles and small sea-
weeds (Figure 5). The mid band of green ephemeral turf algae was
initially short- lived (Schiel et al., 2019), but 4 years after the earth-
quakes it returned in late spring and summer (Figure 5). Ephemeral
turf algae are often the first colonizers after disturbances in this
region and around the world (Connell et al., 2014; Filbee- Dexter &
Wernberg, 2018; Lilley & Schiel, 20 06). Finally, the low intertidal to
shallow subtidal band became dominated by a mixture of foliose,
bladed, filamentous and turf algae (e.g. Polysiphonia spp. Sarcophalia
spp.), patches of surviving bull kelp (mainly D. willana) and scattered
alternative foundation species, such as the kelp Lessonia variegata
FIGURE 6 Map showing distribution of high- impact earthquakes (brown) and volcanic activity (orange) over the last c. 4,000 years
and the coastline (green) affected by the 63 most extreme marine heatwaves recorded between 1982 and 2018 (red dotted polygons).
Heatwaves 1– 62 were extracted from Sen Gupta et al. (2020) and 63 is the new Tasman Sea 2017/18 marine heatwave (analysed using the
same methods as in Sen Gupta et al., 2020, see also Figure 2a)
27 28
35 39
21 24
27 28
35 39
and the fucoids Carpophyllum maschalocarpum and Marginariella bo-
ryana (Figure 5).
4.3 | Wider ecological consequences
While it is challenging to determine the exact areal extent of bull kelp
loss following the ear thquakes, preliminary estimates suggest that
aro und 125,00 0 m2 bull kelp forest could have been lost to the uplift
(if 25% of the rocky coastline was dominated by bull kelp and that the
combined mid and high bull kelp band on average was ca. 10 m wide).
The wider cascading impact from the uplift and heatwaves have re-
sulted in the loss of millions of bull kelp individuals and likely an on-
going reduction in bull kelp cover on the Kaikōura coast (Figure 7).
Surviving smaller and more patchy populations can have lower
genetic diversity and may be less resilient to recover from future
disturbances (Buma, 2015; Elmqvist et al., 2003; Frankham, 2005).
Furthermore, new ecological states can arise when severe or cumu-
lative disturbances serve as tipping points from one state to another
and alter patterns of recruitment, habitat dominance and networks
of interactions within an ecosystem (Benedetti- Cecchi et al., 2015;
Dai et al., 2012; Hawkins et al., 2015; Moore, 2018). It is likely that
the positive feedbacks (propagule pressure, habitat maintenance)
that maintained bull kelp forests have been lost and it is possible
that a new stable state dominated by small turf and foliose algae is in
development (Figure 7), as has been seen in many other parts of the
world (Filbee- Dexter et al., 2016; O'Brien & Scheibling, 2018). Such
turf assemblages are typically limited by large canopy- forming algae,
but once established can prohibit colonization by canopy- formers
through habitat modification, competition for primary space and
the resulting reductions in propagule pressure (Jenkins et al., 2004;
Kennelly, 1987; Petraitis & Dudgeon, 2004; Smale, 2020). If such
an alternative state persists, it will likely result in long- term reduc-
tions of bull kelp- associated species and ecosystem services such as
carbon- storage and the dampening of wave action (Filbee- Dexter &
Wernberg, 2018; Smale, 2020).
4.4 | Caveats and limitations of the study
The inherent unpredictability of cataclysmic events such as earth-
quakes often necessitates ad hoc research designs such as the one
presented here. In this instance, we had very few data for the most
impacted zones prior to the earthquake, which would have allowed
for direct before/after contrasts. Instead, we developed a method
based on robust ecological criteria (presence of dead, decaying and
living bull kelp holdfasts) to estimate the extent of bull kelp cover
prior to the earthquakes and heatwaves at our study sites. The te-
nacity of the bull kelp holdfasts, contrasts to an unimpacted control
site, ancillary data from a southern reference site, and our extensive
experience working on the Kaikōura coastline (Schiel et al., 2019;
Schiel et al., 2016; Thomsen et al., 2020) contributed to this being
a robust, if not ideal method. For example, it was impossible to de-
termine whether decayed holdfasts were D. willana or D. antarctica,
preventing us from assessing the pre- and post- earthquake relative
abundance of these species at our study sites. Another limitation of
this study was the lack of multiple control sites to incorporate site–
site variability into our contrasts between uplifted and non- impacted
reefs. However, our control reef at Oaro was the only unimpacted
reef along ~100 km of coastline, and the only accessible bull kelp reef
for hundreds of kilometres and is typical of bull kelp reefs elsewhere
(Schiel, 2019; Schiel et al., 2018; Thomsen et al., 2019).
4.5 | The Kaikōura earthquakes and heatwaves in a
global context
Earthquakes and ex treme heatwaves are often considered unique
and rare disturbances that have little relevance for traditional and
contemporary ecology (e.g. textbooks such as Begon et al., 1986;
Castro & Huber, 2019; Kaiser et al., 2011; Nybakken, 1993; Smith &
Smith, 2015). However, on a global scale alm ost 15,000 high- impact
FIGURE 7 Conceptual diagram showing impacts from the 2016
earthquakes and 2017/18 heatwaves on many uplifted reefs along
the Kaikoura coastline. (a, b) Dense forests of 2– 8 m large bull
kelps are maintained by positive feedbacks characterized by high
propagule production, refugium from consumers, chemical cues
and frond abrasion to facilitate recruitment and growth of new bull
kelp, abalone, crayfish, fish and holdfast fauna. (c) The earthquakes
and heatwaves destroyed many of these forests. (d) Dense bull
kelp forests have been partly replaced by 0.05– 0.3 m foliose and
turfing algae, with interspersed patches of surviving bull kelp
and alternative foundation species such as Carpophyllum spp. and
Cystophora spp. The animal communities, feedback mechanisms,
stability of the new system and mechanisms that may tip it back
to bull kelp forests remain unknown. The red arrow represents
a tipping point and state- change whereas the blue arrows show
possible feedback mechanisms that maintain a state
Tipping point
Kelp forest
Low subcanopy light
High kelp growth/producvity
High kelp whiplash/abrasion
High kelp propagule producon
High kelp recruitment/regeneraon
Predator escape for abalone & crayfish
Compeve release for crusng alga
Chemical cues for juvenile abalone
Turf bed
Few kelp propagules
Low kelp whiplash
Sediments trapped in turf
No rock for propagule aachment
Few encrusng coralline algae
Few unique holdfast habitats
Expanding algal turf
earthquakes and volcanic eruptions have been recorded over the
last 4 millennia, of which 75% were within 100 km of the coastline,
providing circumstantial evidence that seismic activity may have
common legacy effect s. While the individual traits (e.g. uplift, sub-
sidence or horizontal displacement) of these 15,000 earthquakes
have not been studied, it is likely that their impacts varied depend-
ing on their traits and magnitudes, and the physical and biological
characteristics of the affected shoreline. For example, earthquakes
can directly affect organisms by altering their elevation on a shore,
through indirect effects such as increased sedimentation, but also
through the destruction and creation of new habitat, as occurs
when boulders are deposited in the marine environment (Bodin
& Klinger, 1986; Castilla et al., 2010; Schiel et al., 2019; Thomsen
et al., 2020; Vaux et al., 2021). The Kaikōura earthquake provided
evidence to support recent research that has implicated historic
seismic events in the contempora ry dis tribution of bull kelps in New
Zealand (Craw et al., 2020; Hay, 2020; Parvizi et al., 2020; Vaux
et al., 2021). It is likely that strong seismic events have been impor-
tant structural forces that underly present- day coastal ecology in
many parts of the world.
Superimposed on dramatic geological events are an increas-
ing number of unusually hot oceanic conditions that are caused by
heatwaves (Oliver et al., 2019; Sen Gupta et al., 2020). Temperature
affects all aspects of biology, from biochemical rates at subcellu-
lar levels, reproduction rates, control over species ranges and ulti-
mately the distribution of world’s major biomes (Bartsch et al., 2012;
Lüning, 1990; Spalding et al., 2007). It is therefore not surprising
that strong heatwaves have altered the ecolog y of impacted re-
gions (Rogers- Bennett & Catton, 2019; Smale et al., 2019; Straub
et al., 2019). Globally, we estimated that the most extreme of these
events have caused elevated temperatures across ca. 182,300 km
of coastline (ca. 15% of global coastline see Results) since 1982. A
few of these events have been studied in detail, demonstrating sig-
nificant, and often detrimental, impacts on local marine biota (Jones
et al., 2018; Montie et al., 2020; Rogers- Bennett & Catton, 2019;
Smale et al., 2019; Thomsen et al., 2019; Wernberg et al., 2016), but
most events have simply not been studied.
Many types of large- scale extreme disturbances can create
similar ecological legacy effects and are likely important drivers
of contemporary species- distribution patterns. For example, ex-
treme 1000 - year floods, fires and hurricanes can devastate coastal
communities through extreme run- off, water turbidity, enhanced
sedimentation, lowered salinity, wave action, altered biogeochem-
istry and through ash- deposits (Dunbar & McCullough, 2012; Ely
et al., 1993; Flannigan et al., 2006; Kunkel et al., 2013). Furthermore,
some of these events are more important on coastlines that have low
seismic activity (i.e. coastlines that appear less affected in Figure 6).
The possibilit y that many of these events will become stronger and
more frequent in the future, highlights their importance in both
ecology and biogeography and calls for greater inter- disciplinar y
cross- scale approaches (Alfieri et al., 2017; Ely et al., 1993; Flannigan
et al., 2006; Walsh & Pittock, 1998).
This study demonstrated that cumulative effects from seismic up-
lift and a subsequent heatwave caused great mortalit y of a marine
foundation species. The uplif t first caused 75% canopy loss of bull
kelp, and then, the heatwaves killed an additional 35% of those re-
maining, resulting in cascading losses of likely millions of individuals
on the Kaikōura coast. Four years after the uplift, bull kelp had not
recovered and the low zone was instead inhabited by a mixture of
patchy bull kelp beds (dominated by D. willana), a few other peren-
nial habitat- forming species such as Carpophyllum maschalocarpum,
and small ephemeral turf and foliose algae. This represented a dif-
ferent ecological system maintained by new feedback loops that
likely slow down or hinder recovery of bull kelp beds and lower their
resilience to future stressors. Cataclysmic events may be relatively
common on global historical scales because more than 12,000 major
events have been recorded within 100 km of the coastline over the
last four millennia, and because relatively recent extreme marine
heatwaves have occurred along more than 180,000 km coastline or
about 15% of global coastline around the world. Thus, the ecologi-
cal legacy effects of large- scale disturbances, such as earthquakes,
heatwaves and other extreme events, may be relatively common. It
is worthwhile viewing local- scale short- term ecological research in
this wider historical context. Models that incorporate appropriate
legacy effects will likely yield better interpretation of contemporary
local- scale processes.
This research was suppor ted by a grant from Brian Mason (Impacts
of marine heatwaves) and with support from the Ministry of Primary
Industries and the Ministry of Business, Innovation and Employment
(earthquake impacts and recovery- UOCX1704). PMS is suppor ted
by New Zealand Ministry of Business, Innovation and Employment
under Contract CAWX1801.
The authors declare no conflicts of interest.
The peer review history for this article is available at https://publo n/10.1111/ddi.134 07.
The data presented in this study are openly available in DRYAD at zgwq.
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How to cite this article: Thomsen, M. S., Mondardini, L., Thoral,
F., Gerber, D., Montie, S., South, P. M., Tait, L., Orchard, S.,
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ddi.134 07
... grazers supressing kelp recovery following marine heatwaves, McPherson et al., 2021; low water clarity increasing temperature-induced kelp loss, Tait et al., 2021). Co-occurring disturbances can have particularly strong consequences for coastal biota (e.g. the cascading impacts of earthquakes and marine heatwaves on bull kelp communities, Thomsen et al., 2021). In such cases, the cumulative effects of concerted mesoscale catastrophic events can exacerbate the intensity and duration of regional disturbances and result in long-lasting evolutionary shifts in disturbed locations. ...
... Indeed, as such atmospheric disruption events are arguably a defining characteristic of many coastal systems, disturbancerelated phylogeographic processes should perhaps similarly be seen here as the rule more than the exception. Along similar lines, recent studies reveal the pervasive coastal biological effects of geological disturbance in tectonically active regions of the globe (reviewed in Thomsen et al., 2021). For instance, the 1964 Alaskan earthquake which initiated rapid speciation in sticklebacks (Lescak et al., 2015) also eliminated numerous populations of seagrass, Zostera marina (Short & Wyllie-Echeverria, 1996). ...
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Aim: Coastal habitats are among the most dynamic environments on earth and are highly vulnerable to large‐scale physical disturbance. Genetic studies of nearshore marine species are revealing long‐lasting signatures of major coastal disturbance events. We synthesize emerging data to highlight how physical perturbations can impact the phylogeographic patterns of coastal populations. Taxon: Coastal marine and estuarine taxa. Location: Coastal regions around the globe. Methods: We synthesize coastal genetic and genomic literature, focussing particularly on the phylogeographic consequences of natural disturbance events including uplift, tsunami, hurricanes, glaciations and sea‐level fluctuations. We focus on studies with clear physical analytical frameworks constrained by abiotic data. Results: Tectonic and atmospheric disruptions can be considered short‐term events with major impacts on populations adjacent to the centre of disturbance, typically resulting in the evolution of shallow phylogeographic patterns. Long‐lived climate‐driven disturbances such as glaciations, however, operate over vast geographic scales and often drive deep evolutionary patterns in affected populations. We show that studies using genome‐wide data could better identify fine‐scale signatures of both past and contemporary habitat perturbations. Conclusions: Recent data reveal the interplay between physical upheaval and coastal phylogeography, indicating that disturbance can affect diversity, connectivity and demography of coastal populations. The interplay between long‐lived large‐scale disturbance and species‐specific biotic traits has shaped deep phylogeographic patterns of coastal taxa. Additionally, it could be argued that, at least for some regions, short‐term disturbance is the ‘rule’ rather than the ‘exception’, and thus, represents a key driver of coastal genetic patterns in disturbance prone coastal regions. Geo‐genomic approaches that combine genome‐wide data with explicit habitat models or disturbance history information have been particularly successful in explaining the drivers of coastal evolutionary change. We argue that future integration of genomic and physical data will be crucial for tracing evolutionary trends in fast‐changing marine environments.
... Earthquake uplift can therefore cause mortality in D. antarctica if it is sufficient to move individuals above the high tide mark. For example, shores uplifted by several metres during the 2010 Chilean and 2016 Kaikōura earthquakes resulted in large-scale die-offs of intertidal Durvillaea (Castilla et al., 2010;Clark et al., 2017;Peters et al., 2020;Schiel et al., 2019;Tait et al., 2021;Thomsen et al., 2021 (Fraser, Davies, et al., 2018). ...
... The Wellington region immediately west of Turakirae Head expe- Durvillaea populations were heavily reduced at sites uplifted by <2 m, but appeared to have been extirpated at sites of greater uplift (Peters et al., 2020;Tait et al., 2021;Thomsen et al., 2021). ...
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Large‐scale disturbance events have the potential to drastically reshape biodiversity patterns. Notably, newly vacant habitat space cleared by disturbance can be colonised by multiple lineages, which can lead to the evolution of distinct spatial ‘sectors’ of genetic diversity within a species. We test for disturbance‐driven sectoring of genetic diversity in intertidal southern bull kelp, Durvillaea antarctica (Chamisso) Hariot following the high‐magnitude 1855 Wairarapa earthquake in New Zealand. Specifically, we use genotyping‐by‐sequencing (GBS) to analyse fine‐scale population structure across the uplift zone and apply machine learning to assess the fit of alternative recolonisaton models. Our analysis reveals that specimens from the uplift zone carry distinctive genomic signatures potentially linked to post‐earthquake recolonisation processes. Specifically, our analysis identifies two parapatric spatial‐genomic sectors of D. antarctica at Turakirae Head, which experienced the most dramatic uplift. Based on phylogeographic modelling, we infer that bull kelp in the Wellington region was likely a source for recolonisation of the heavily uplifted Turakirae Head coastline, via two parallel, eastward recolonisation events. By identifying multiple parapatric genotypic sectors within a recently recolonised coastal region, the current study provides support for the hypothesis that competing lineage expansions can generate striking spatial structuring of genetic diversity, even in highly dispersive taxa.
... Related post-earthquake studies on the large southern bull kelp, Durvillaea spp., which occupies the intertidal-subtidal fringe on wave-exposed shores, show that it is particularly susceptible to desiccation with prolonged exposure, especially when wave splash is low and temperatures are high during low tide [51]. Other large brown algae such as Carpophyllum maschalocarpum and Cystophora spp. ...
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Widespread mortality of intertidal biota was observed following the 7.8 Mw Kaikoura earthquake in November 2016. To understand drivers of change and recovery in nearshore ecosystems, we quantified the variation in relative sea-level changes caused by tectonic uplift and evaluated their relationships with ecological impacts with a view to establishing the minimum threshold and overall extent of the major effects on rocky shores. Vertical displacement of contiguous 50 m shoreline sections was assessed using comparable LiDAR data to address initial and potential ongoing change across a 100 km study area. Co-seismic uplift accounted for the majority of relative sea-level change at most locations. Only small changes were detected beyond the initial earthquake event, but they included the weathering of reef platforms and accumulation of mobile gravels that continue to shape the coast. Intertidal vegetation losses were evident in equivalent intertidal zones at all uplifted sites despite considerable variation in the vertical displacement they experienced. Nine of ten uplifted sites suffered severe (>80%) loss in habitat-forming algae and included the lowest uplift values (0.6 m). These results show a functional threshold of c.1/4 of the tidal range above which major impacts were sustained. Evidently, compensatory recovery has not occurred—but more notably, previously subtidal algae that were uplifted into the low intertidal zone where they ought to persist (but did not) suggests additional post-disturbance adversities that have contributed to the overall effect. Continuing research will investigate differences in recovery trajectories across the affected area to identify factors and processes that will lead to the regeneration of ecosystems and resources.
... The onshore portion of the Akatore fault has, in the past millennium, uplifted ~30 km of the coast by 2-3 m above the high-tide zone (Litchfield & Norris, 2000). Recent ecological studies of tectonically active shores elsewhere have shown that uplift events approaching this magnitude are sufficient to extirpate habitat-forming macroalgae and their associated epibiota, thus creating empty intertidal habitats for recolonization Jaramillo et al., 2012;Schiel et al., 2019;Thomsen et al., 2020Thomsen et al., , 2021. In the case of the Akatore fault, radiocarbon dating has constrained the most recent rupture event to 800-1000 years BP (Craw et al., 2021;Litchfield & Norris, 2000). ...
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The elimination of lower trophic levels following severe habitat disturbance can trigger new community assembly processes. However, little is known about how past habitat disturbances have affected codependent evolution of trophically-linked and closely interacting taxa. Using genome-wide analysis of a macroalgal community affected by ancient catastrophic coastal uplift, we track the ecological dynamics of past co-dispersal and co-diversification among obligate interacting taxa. Our study reveals rapid and concerted reassembly of an intertidal community following disturbance. Specifically, hierarchical co-demographic analyses of multispecies genomic data support synchronous expansions of four strictly intertidal species in the wake of tectonic disturbance. These data show that tight algal-epifaunal links underpin parallel demographic responses across distinct trophic levels. These results highlight that high-resolution comparative genomic data can elucidate the strength of obligate ecological interactions, and the evolutionary dynamics of past co-dispersal and co-diversification in post-disturbance communities.
Climatic extremes are becoming increasingly common against a background trend of global warming. In the oceans, marine heatwaves (MHWs)—discrete periods of anomalously warm water—have intensified and become more frequent over the past century, impacting the integrity of marine ecosystems globally. We review and synthesize current understanding of MHW impacts at the individual, population, and community levels. We then examine how these impacts affect broader ecosystem services and discuss the current state of research on biological impacts of MHWs. Finally, we explore current and emergent approaches to predicting the occurrence and impacts of future events, along with adaptation and management approaches. With further increases in intensity and frequency projected for coming decades, MHWs are emerging as pervasive stressors to marine ecosystems globally. A deeper mechanistic understanding of their biological impacts is needed to better predict and adapt to increased MHW activity in the Anthropocene. Expected final online publication date for the Annual Review of Marine Science, Volume 15 is January 2023. Please see for revised estimates.
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Marine heatwaves (MHWs) can cause dramatic changes to ecologically, culturally, and economically important coastal ecosystems. To date, MHW studies have focused on geographically isolated regions or broad-scale global oceanic analyses, without considering coastal biogeographical regions and seasons. However, to understand impacts from MHWs on diverse coastal communities, a combined biogeographical-seasonal approach is necessary, because (1) bioregions reflect community-wide temperature tolerances and (2) summer or winter heatwaves likely affect communities differently. We therefore carried out season-specific Theil–Sen robust linear regressions and Pettitt change point analyses from 1982 to 2021 on the number of events, number of MHW days, mean intensity, maximum intensity, and cumulative intensity of MHWs, for each of the world’s 12 major coastal biogeographical realms. We found that 70% of 240 trend analyses increased significantly, 5% decreased and 25% were unaffected. There were clear differences between trends in metrics within biogeographical regions, and among seasons. For the significant increases, most change points occurred between 1998 and 2006. Regression slopes were generally positive across MHW metrics, seasons, and biogeographical realms as well as being highest after change point detection. Trends were highest for the Arctic, Northern Pacific, and Northern Atlantic realms in summer, and lowest for the Southern Ocean and several equatorial realms in other seasons. Our analysis highlights that future case studies should incorporate break point changes and seasonality in MHW analysis, to increase our understanding of how future, more frequent, and stronger MHWs will affect coastal ecosystems.
Major ecological disturbance events can provide opportunities to assess multispecies responses to upheaval. In particular, catastrophic disturbances that regionally extirpate habitat-forming species can potentially influence the genetic diversity of large numbers of co-distributed taxa. However, due to the rarity of such disturbance events over ecological timeframes, the genetic dynamics of multispecies recolonization processes have remained little understood. Here we use single nucleotide polymorphism (SNP) data from multiple coastal species to track the dynamics of co-colonization events in response to ancient earthquake disturbance in southern New Zealand. Specifically, we use a comparative phylogeographic approach to understand the extent to which epifauna (with varying ecological associations with their macroalgal hosts) share comparable spatial and temporal recolonization patterns. Our study reveals concordant disturbance-related phylogeographic breaks in two intertidal macroalgal species along with two associated epibiotic species (a chiton and an isopod). By contrast, two co-distributed species, one of which is an epibiotic amphipod and the other a subtidal macroalga, show few if any genetic effects of palaeoseismic coastal uplift. Phylogeographic model selection reveals similar post-uplift recolonization routes for the epibiotic chiton and isopod and their macroalgal hosts. Additionally, co-demographic analyses support synchronous population expansions of these four phylogeographically similar taxa. Our findings indicate that coastal paleoseismic activity has driven concordant impacts on multiple codistributed species, with concerted recolonization events likely facilitated by macroalgal rafting. These results highlight that high-resolution comparative genomic data can help reconstruct concerted multispecies responses to recent ecological disturbance.
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Marine heatwaves (MHW) are becoming stronger and more frequent across the globe. MHWs affect the thermal physiology of all biological organisms, but wider ecosystem effects are particularly impactful when large habitat-forming foundation species such as kelps are affected. Many studies on impacts from MHWs on kelps have focused on temperature effects in isolation, except for a few studies that have integrated co-occurring stress from grazers, wave exposure and nutrient limitation. It is likely that many stressors act in concert with MHWs and exacerbate their effects. Here we analyzed satellite images over 60 months to assess temporal changes in abundance of surface canopies of the giant kelp Macrocystis pyrifera in the New Zealand coastal zone. The analysis encompassed the most extreme MHW on record (2017/18), across a 6° latitudinal gradient of four regions southward from the northern distributional limit of Macrocystis along mainland New Zealand. We tested the association of surface canopy cover of Macrocystis with sea surface temperature, temperature anomalies, chlorophyll-a (a proxy for nutrient availability) and water clarity (diffuse attenuation coefficient). We found a reduced cover of Macrocystis across all regions during and after the 2017/18 MHW, with least impact at the most southern region where the maximum temperatures did not exceed 18°C. There was also an important and significant interaction between temperature and water clarity, showing that temperature-induced kelp loss was greater when water clarity was poor. These results show that notable negative effects occurred across the coastal range of this foundation species and highlight the importance of studying MHW effects across latitudinal gradients and in concert with other co-occurring stressors.
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Understanding the forces that shape species distributions is increasingly important in a fast‐changing world. Although major disturbance events can adversely affect natural populations, they can also present new opportunities, for example by opening up habitat for colonization by other lineages. Following extensive geographic sampling, we use genomic data to infer a range extension following disturbance for an ecologically important intertidal macroalgal species. Specifically, we genotyped 288 bull‐kelp (Durvillaea) plants from 28 localities across central New Zealand. All specimens from the North Island were expected to be D. antarctica, but unexpectedly 10 samples from four sites were unexpectedly identified as D. poha. Extensive sampling from the northern South Island (105 samples at five locations) confirmed the absence of D. poha north of the Kaikōura Peninsula. The North Island specimens of D. poha therefore reveal a biogeographic disjunction, some 150 km northeast of the nearest (South Island) population of this species. Based on strong geographic correspondence between these North Island samples and historic disturbance, we infer that tectonic upheaval, particularly earthquake‐generated landslides, likely extirpated local D. antarctica and created an opportunity for a northward range expansion event by D. poha. Close phylogenomic relationships between this new North Island population and South Island samples supports a geologically recent northward expansion, rather than a deeper evolutionary origin. These findings indicate the potential of large‐scale disturbances to facilitate sudden biogeographic range expansions, and they emphasise the ability of genomic analyses with fine‐scale sampling to reveal long‐lasting signatures of past disturbance, dispersal and colonization.
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Knowledge of long-term and multi-scale trends in ecological systems is a vital component in understanding their dynamics. We used Landsat satellite imagery to develop the first long-term (1986-2015) data set describing the cover of dense surface canopies of giant kelp Macrocystis pyrifera around the entire coastline of Tasmania, Australia, and assessed the extent to which potential environmental drivers explain the dynamics of surface canopies at multiple spatial and temporal scales. Broad-scale temporal patterns in canopy cover are correlated with El Niño-Southern Oscillation events, while regional patterns are related to sea surface temperature and nutrient regimes are associated with the East Australian Current. Regression models developed to predict the presence or absence of giant kelp canopy emphasise the importance of sea surface temperature in these systems. Long-term decline in canopy cover is clearly evident in most regions, and in light of increasing thermal stress associated with a changing ocean climate, this raises concern for the future of this species as a major habitat-forming kelp in Australia and some other regions worldwide. Given that Tasmania represents the stronghold of the range of this species in Australia, but is a geographic trap in that there is no suitable habitat for M. pyrifera to the south, our findings support the Federal listing of giant kelp communities in Australia as an endangered marine community type.
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Prolonged high-temperature extreme events in the ocean, marine heatwaves, can have severe and long-lasting impacts on marine ecosystems, fisheries and associated services. This study applies a marine heatwave framework to analyse a global sea surface temperature product and identify the most extreme events, based on their intensity, duration and spatial extent. Many of these events have yet to be described in terms of their physical attributes, generation mechanisms, or ecological impacts. Our synthesis identifies commonalities between marine heatwave characteristics and seasonality, links to the El Niño-Southern Oscillation, triggering processes and impacts on ocean productivity. The most intense events preferentially occur in summer, when climatological oceanic mixed layers are shallow and winds are weak, but at a time preceding climatological maximum sea surface temperatures. Most subtropical extreme marine heatwaves were triggered by persistent atmospheric high-pressure systems and anomalously weak wind speeds, associated with increased insolation, and reduced ocean heat losses. Furthermore, the most extreme events tended to coincide with reduced chlorophyll-a concentration at low and mid-latitudes. Understanding the importance of the oceanic background state, local and remote drivers and the ocean productivity response from past events are critical steps toward improving predictions of future marine heatwaves and their impacts.
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Extreme summer marine heatwaves increase chlorophyll a in the Southern Ocean - Shinae Montie, Mads S. Thomsen, Wolfgang Rack, Paul A. Broady
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Theory suggests that catastrophic earth-history events can drive rapid biological evolution, but empirical evidence for such processes is scarce. Destructive geological events such as earthquakes can represent large-scale natural experiments for inferring such evolutionary processes. We capitalized on a major prehistoric (800 yr BP) geological uplift event affecting a southern New Zealand coastline to test for the lasting genomic impacts of disturbance. Genome-wide analyses of three co-distributed keystone kelp taxa revealed that post-earthquake recolonization drove the evolution of novel, large-scale intertidal spatial genetic ‘sectors’ which are tightly linked to geological fault boundaries. Demographic simulations confirmed that, following widespread extirpation, parallel expansions into newly vacant habitats rapidly restructured genome-wide diversity. Interspecific differences in recolonization mode and tempo reflect differing ecological constraints relating to habitat choice and dispersal capacity among taxa. This study highlights the rapid and enduring evolutionary effects of catastrophic ecosystem disturbance and reveals the key role of range expansion in reshaping spatial genetic patterns.
NOAA/NESDIS/NCEI Daily Optimum Interpolation Sea Surface Temperature (SST) version 2.0 (DOISST v2.0) is a blend of in situ ship and buoy SSTs with satellite SSTs derived from Advanced Very High Resolution Radiometer (AVHRR). DOISST v2.0 exhibited a cold bias in the Indian Ocean, South Pacific, and South Atlantic due to a lack of ingested drifting-buoy SSTs in the system, which resulted from a gradual data format change from the Traditional Alphanumeric Codes (TAC) to the Binary Universal Form for the Representation of meteorological data (BUFR). The cold bias against Argo was about -0.14°C on global average and -0.28°C in the Indian Ocean from January 2016 to August 2019. We explored the reasons for these cold biases through six progressive experiments. These experiments showed that the cold biases can be effectively reduced by adjusting ship SSTs with available buoy SSTs, using the latest available ICOADS R3.0.2 derived from merging BUFR and TAC, as well as by including Argo observations above 5 m depth. The impact of using satellite MetOp-B instead of NOAA-19 was notable on high-latitude oceans but small on global average, since their biases are adjusted using in situ SSTs. In addition, the warm SSTs in the Arctic were improved by applying freezing-point instead of regressed ice-SST proxy. This paper describes an upgraded version, DOISST v2.1, which addresses biases in v2.0. Overall, by updating v2.0 to v2.1, the biases are reduced to -0.07°C (-0.04°C) and -0.14°C (-0.08°C) in the global and Indian Ocean, respectively, when compared against independent (dependent) Argo observations. The difference against the Group for High Resolution SST (GHRSST) multi-product ensemble (GMPE) product is reduced from -0.09°C to -0.01°C in the global oceans and from -0.20°C to -0.04°C in the Indian Ocean.
Late Holocene earthquake activity on the reverse Akatore Fault of southeastern South Island has caused uplift along >20 km of nearby coastline, with the last major earthquake event c. 800 years ago. Fault-related uplift has exposed a prominent rocky and sandy terrace along the coastline. We used a combination of approaches including land-based observations, LIDAR remote sensing, and radiocarbon dating of shell deposits, to reveal the extent and nature of past uplift in the region. Cumulative uplift of this terrace varies laterally from <1 m at the northern and southern ends to possibly exceed 5 m in the central portion. The coastal terrace lies below a steep fossil sea cliff. Caves at the base of this cliff contain shells with radiocarbon ages <500 years old, implying persistence of large wave incursion right across the terrace after uplift. A major uplift event of ∼2 m along the whole 20 km of shoreline caused extirpation of some intertidal kelps that now have genetically distinct lineages in the uplift zone.
Extreme climatic events cause devastating impacts to species and ecosystems, precipitating significant mortality. However, emerging empirical evidence is revealing that such mortality can drive directional selection and result in increased tolerance. We discuss the novel opportunities for promoting climate resilience presented by this 'silver lining' of extreme events.
The 2016 Mw7.8 Kaikōura earthquake lifted 140km of coastline on New Zealand’s South Island by up to 6.4m. This caused extensive mortality and destruction of habitat critical for early life stages of blackfoot abalone, Haliotis iris (called pāua), a species of cultural and commercial importance. The fishery for pāua was closed, at considerable financial loss to local communities. This study determined the extent to which habitats and populations of pāua survived along the coastline. With aerial imaging, the coast was categorised into broad habitats at a 10m scale. This was used to select areas for in situ assessments of pāua populations and specific habitat features at 26 sites over 1.5 years. We quantified key habitat features to identify correlates and potential drivers of pāua abundance and distribution. We found that despite extensive habitat degradation from uplift, erosion and sedimentation, abundant pāua in size classes <30mm shell length indicated successful settlement and juvenile recruitment had occurred soon after the earthquake. Pāua up to 170mm shell length had also survived in shallow habitats. A generalized linear mixed model showed that pāua were negatively influenced by the degree of uplift, and positively associated with the cover of unconsolidated layered rocks. Juvenile pāua (<85mm) abundance was greatest at sites with <2.5m of uplift. There was further recruitment 1.5 years post-earthquake and evidence of good growth of the previous year’s cohort. Despite major disruption to this coastline, there appears to be very good potential for recovery of pāua and the fishery.