# Tolerance of organisms composing an Arctic kelp community to ocean warming and marine heatwaves

Anaïs Lebrun, Cale Miller, Frédéric Gazeau, Pierre Urrutti, Samir Alliouane,

Jean-Pierre Gattuso, Steeve Comeau

► To cite this version:

Anaïs Lebrun, Cale Miller, Frédéric Gazeau, Pierre Urrutti, Samir Alliouane, et al. Tolerance of organisms composing an Arctic kelp community to ocean warming and marine heatwaves. Journal of Ecology, 2025, 10.1111/1365-2745.70074. hal-05103984

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RESEARCH ARTICLE

Marine Heatwaves

# Tolerance of organisms composing an Arctic kelp community to ocean warming and marine heatwaves

Anaïs Lebrun<fnref n="1" /> ID

<figure><img src="image_3.png" /></figure>

Cale A. Miller<fnref n="1" />,<fnref n="2" /> ID

<figure><img src="image_4.png" /></figure>

Frédéric Gazeau<fnref n="1" />

Pierre Urrutti<fnref n="1" />

Samir Alliouane<fnref n="1" />

Jean-Pierre Gattuso<fnref n="1" />,<fnref n="3" />

Steeve Comeau<fnref n="1" />

**Correspondence**

Cale A. Miller

Email: cale.miller@imev-mer.fr

**Funding information**

Prince Albert II of Monaco Foundation, Grant/Award Number: 3051; H2020

Environment, Grant/Award Number: 869154

Handling Editor: Pol Capdevila

## Abstract

1. The Arctic region is experiencing rapid warming and an increase in the frequency and intensity of marine heatwaves (MHWs), posing an increasing risk to coastal ecosystems. Kelp communities in the Arctic are vital habitats that support biodiversity and resilience but are particularly vulnerable to increasing temperature anomalies.

2. In this study, we conducted a mesocosm experiment over 23 days in summer to assess the effects of warming and MHWs on three kelp species (Saccharina latissima, Laminaria digitata and Alaria esculenta) and associated community members: coralline algae, snails, sea urchins and mussels.

3. Three warming scenarios were tested, all of which were offset from a naturally variable control. The four conditions were a control (i.e. ambient seawater) which ranged from ~7.5 to 10°C (naturally variable), a constant high-temperature treatment (+1.8°C from the control), a treatment simulating a single 13-day heatwave with a maximum offset temperature of +2.8°C from the control, and a treatment simulating two consecutive 5-day MHWs with a maximum offset of +3.9°C.

4. For the three kelp species, net photosynthetic rate, maximum quantum yield, chlorophyll a content, carbon to nitrogen ratio and growth rate were investigated. For the associated species, only growth rates were evaluated.

5. Laminaria digitata showed no significant changes in any of the measured parameters, suggesting a robust tolerance to heat stress. Conversely, the maximum quantum yield for S. latissima decreased in the two heatwaves treatment. In addition, A. esculenta displayed a decrease in chlorophyll a content and net photosynthesis over time in the two heatwaves treatment.

6. The growth rates of coralline algae, mussels, snails and sea urchins were not affected by the treatments.

7. Synthesis: These results suggest that Arctic kelp communities are fairly tolerant to warming and short-term MHWs, despite the few minor effects on photosynthesis by some of the kelp species. We note that the future response of kelp communities must account for the potential indirect effects of climate change, such

This is an open access article under the terms of the [Creative Commons Attribution License](https://creativecommons.org/licenses/by/4.0/), which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2025 The Author(s). *Journal of Ecology* published by John Wiley & Sons Ltd on behalf of British Ecological Society.

<figure><img src="image_5.png" /></figure>

<footnote n="1">¹Laboratoire d'Océanographie de Villefranche, Sorbonne Université, CNRS, Villefranche-sur-Mer, France</footnote>

<footnote n="2">²Department of Earth Sciences, Utrecht University, Utrecht, The Netherlands</footnote>

<footnote n="3">³Institute for Sustainable Development and International Relations, Sciences Po, Paris, France</footnote>
as predation and competition, along with any consequential effects of warming anomalies, as those presented here.

KEYWORDS

Arctic, climate change, kelp, marine heatwaves, mesocosm

# 1 | INTRODUCTION

Temperature is one of the main drivers of marine ecosystem structure and functioning (e.g. Drewnik et al., 2017; Wernberg et al., 2013). Temperature is known to affect the distribution of marine organisms and impact various physiological processes at the organismal level (e.g. photosynthesis, reproduction and respiration), and large-scale ecological processes such as nutrient cycling. For example, increased temperature significantly enhances the metabolic rates of marine organisms, so long as it does not exceed thermal tolerance limits (Nguyen et al., 2011; Peck et al., 2009). The thermal scope of heat tolerance, however, varies across heterotrophic and autotrophic marine taxa, and often according to a geographical distribution. Large brown macroalgae, often referred to as kelp, can be characterized as such organisms that display a varying range of thermal tolerance as they employ the ability to regulate physiological processes in response to temperature variations by adjusting their pigment content, or utilizing storage molecules (Britton et al., 2020; Davison et al., 1991; Diehl et al., 2020). However, beyond a thermal tolerance limit, detrimental physiological consequences appear and can eventually lead to death (e.g. Reed et al., 2016; Scheschonk et al., 2019).

Increasing global sea surface temperature has been linked to an increase in the frequency, intensity and duration of marine heatwaves (MHWs; Oliver et al., 2018). The increasing baseline of sea surface temperature has resulted in physiological stress for some marine organisms, resulting in a poleward shift north (Sunday et al., 2012). MHWs are prolonged, discrete events of anomalous warm water, occurring when sea surface temperature (SST) exceeds the 90th percentile of the average daily temperature over the past 30 years for at least five consecutive days (Hobday et al., 2016). The impact of MHWs can lead to mass mortality events in the benthos, which includes marine algae, seagrasses (e.g. Arias-Ortiz et al., 2018; Thomsen et al., 2019; Wernberg et al., 2013) and invertebrates (e.g. Garrabou et al., 2022; Hughes et al., 2017). One such mass mortality event occurred in southern California when a series of MHWs from 2014 to 2016 reached a peak temperature anomaly of +5.9°C and coincided with nutrient-poor waters; this resulted in a dramatic decline of the kelp *Macrocystis pyrifera* and the loss of over half of the invertebrates and fish (Arafeh-Dalmau et al., 2019). In contrast, opportunistic species can benefit from the detrimental impacts of extreme events by occupying a now vacant niche in the community. For example, the kelp *Undaria pinnatifida* invaded Pile Bay (New Zealand) after the extinction of the bull kelp from a MHW (Thomsen et al., 2019).

The Arctic has experienced a significant increase in the duration and intensity of MHWs in recent years along a backdrop of atmospheric temperatures warming at a rate that is more than four times the global average (Barkhordarian et al., 2024; Rantanen et al., 2022). In the Kara, Laptev and Siberian Seas, the duration of MHWs has increased by 10–12 days over the past 20 years (2000–2020; Golubeva et al., 2021). Hu et al. (2020) reported that the cumulative intensity of MHWs in ice-free areas increased by 0.31°C per year between 1988 and 2017. As the Arctic Ocean is characterized by relatively limited seasonal changes in SST, MHWs may have important impacts on Arctic marine ecosystems (Huang et al., 2021), particularly for shallow benthic communities which are more subjected to MHWs than deeper communities (Arriaga et al., 2024).

In the Arctic, coastal littoral to sublittoral Laminarian kelps (from the order Laminariales) and their associated fauna and flora are one of the most abundant habitats on rocky reefs (Fredriksen & Kile, 2012; Friedlander et al., 2023; Kjellman, 1883; Svendsen, 1959). With the increase of SST in the Arctic, several studies have examined the susceptibility of high latitude kelp to thermal stress, demonstrating a mixed response according to geographical distribution (Diehl et al., 2021; Miller, Gazeau, Lebrun, Gattuso, et al., 2024; Niedzwiedz et al., 2022). Thus, the effects of warming on Laminarian kelp found in the Arctic encompass a spectrum of physiological responses associated with their intrinsic thermal scope along such geographical distributions (Bolton & Lüning, 1982; Krause-Jensen & Duarte, 2014; Müller et al., 2009; Smale et al., 2019). These physiological responses can manifest as hindrances in germination and a decrease in photosynthetic efficiency and storage carbohydrates (Diehl et al., 2020; Fredersdorf et al., 2009). The variation in response to temperature stress is likely due to high phenotypic plasticity, established by ancestral distributions and current local abiotic conditions (King et al., 2018). One such mechanism for this plasticity has been described for *S. latissima* where high levels of N content were critical for heat tolerance (Gerard, 1997). The effect of heatwave stress, however, seasonally, and interannually, has shown that *S. latissima* in temperate regions carries a historical 'memory' of stressful exposure reducing its tolerance to future heat stress (Niedzwiedz et al., 2022). Whether or not this translates to other kelp distributions in Arctic waters remains unclear, as does the response to the magnitude and frequency of MHWs with respect to a historical 'memory'.

Because warming and MHWs are becoming more intense and frequent, it is critical to assess their effects on Arctic and sub-Arctic kelp communities, particularly given that kelp forests are expected to expand poleward due to more favourable temperatures and
increasing substrate availability (Assis et al., 2022; Jayathilake & Costello, 2020). Here, we evaluated the effects of warming and simulated MHWs on the main organisms constituting kelp communities around Tromsø, Norway (69.65°N, 18.95°E) during a 23-day mesocosm experiment. We chose to mostly focus our assessment on the individual physiological performance of kelps exposed to constant warming, and either one or two MHWs. This study was designed to test the species-specific tolerance and physiological responses of kelp community organisms to warming and the specific characteristics of MHWs. The reconstructed communities were composed of three kelp species (S. latissima, L. digitata and A. esculenta), coralline algae, snails, urchins and mussels. The different kelp species were selected based on previous research demonstrating a tolerance by these species in the high-Arctic to future warming and reduced light (Lebrun et al., 2024). For the kelps, changes in net photosynthetic rate, maximum quantum yield, chlorophyll a (Chl a) content, carbon-to-nitrogen (C:N) ratio and growth rate were evaluated as they are physiological metrics that are known to respond quickly to experimental treatments. For all other organisms, growth rates were assessed for the different species in response to each treatment.

## 2 | MATERIALS AND METHODS

### 2.1 | Specimen collection and site description

Kelp communities were replicated in an ex situ mesocosm experiment by collecting 163 mature sporophytes of A. esculenta, L. digitata and S. latissima (shorter than 1 m), 61 coralline algae (an assemblage of rhodoliths dominated by Lithothamnion glaciale, about 5 cm in diameter), 40 mussels (Modiolus modiolus), 68 sea urchins (Strongylocentrotus droebachiensis) and 69 gastropods (Neptunea despecta) by scuba diving between 1 and 7 m depth at Melhomen (69.88°N, 18.86°E), Sommarøy (69.63°N, 17.97°E) and Kvaløyvågen (69.85°N, 18.82°E). All sampling was conducted in June 2022 from sites located less than 50 km from Kårvik, Norway, where the experimental system was installed. All collected organisms were transferred across four holding tanks equipped with flowing seawater within hours of collection until the start of the experiment. Temperature was similar between the collection sites, with a maximum distance of 44 km between the two farthest apart sites (Melhomen and Sommarøy, Figure S1).

The tidal range around the collection sites during this time was 2.5–3 m with sea surface temperatures that ranged from 6.6 to 12.4°C during the month of June, as recorded by NOAA's Long-Term Climate Data Record (OISST; Figure S1). The collection of all organisms was limited to shallow depths as large areas of the benthic environment have been converted into urchin barrens with little regrowth of kelp over the past 30 years (Norderhaug & Christie, 2009). Because of the large barren areas around Tromsø, we chose to work with organismal densities that matched the average biomass reported for Arctic coasts between 1.5 and 7 m depth (Hop et al., 2012).

### 2.2 | Mesocosm experiment

The experiment was carried out from 2022-06-30 to 2022-07-23, in twelve 1 m³ mesocosms (with a 1 m² footprint) that were installed on the outdoor platform of the Havbruksstasjonen i Tromsø (Kårvik) exposed to natural light conditions. Each mesocosm was stocked with kelp and fauna: five to eight individuals of A. esculenta, three to six individuals of S. latissima and two to three individuals of L. digitata per mesocosm. The biomass in each mesocosm was adjusted to about 1000 g fresh-weight (fw) for A. esculenta and L. digitata, and 500 g fw for S. latissima. Such values are representative of the average biomass reported in shallow waters of the coastal Arctic between 1.5 and 7 m depth (Hop et al., 2012). Kelp biomass was distributed evenly inside each mesocosm at approximate equidistant spacing (each kelp's holdfast was tied to a large stone) to ensure that biological effects within the tanks (e.g. modification of pH due to photosynthesis) were homogeneous between experimental units. The three species of kelp were found cohabiting at each collection site, albeit at different densities depending on the sites. Five to seven sea urchins (total biomass between 250 and 280 g fw per mesocosm) were put in 30×30×30 cm wire mesh cages in each mesocosm and fed kelp scraps weighing <10 g in fw from damaged individuals not used for the experiment. Coralline algae (four to six per mesocosm) were also put in cages to facilitate sampling and measurement during the experiment. Mussels and gastropods were directly put in the mesocosms once labelled and weighed. Kelp scraps were checked daily and replenished when needed, and visual inspection of organic detrital material indicated an abundant amount of organic material in each tank for the satiation of the invertebrates. As with the kelp, fauna biomass was distributed evenly inside each mesocosm at the given fw values noted above.

Seawater flowing through the mesocosms was pumped from 30 m depth in front of the Havbruksstasjonen i Tromsø (69.9°N, 18.8°E) to a retention basin from where it was pumped to the mesocosms using a submersible pump (Albatros©, Norsk Pumpservice AS). Previous studies have indicated that temperature in the water column is stable within the initial 50 m this time of year (Rüggeberg et al., 2011; Walker et al., 2022). Temperature was adjusted using a modified version of the experimental set-up described in Miller, Urrutti, Gattuso, Comeau, et al. (2024) by mixing ambient and warmed seawater with a single heat pump for all treatments (15°C). The temperature-adjusted seawater was delivered to each mesocosm at 8 L min⁻¹ corresponding to a complete turnover every 2 h. Each mesocosm was equipped with a 12 W wave pump (Sunsun©, JVP-132, flow rate = 8 m³ h⁻¹) to ensure mixing, and a temperature-conductivity probe (PC4E; Aqualabo) to monitor instantaneous fluctuations (measured every minute). Mesocosms were covered with circular acrylic lids equipped with green (RL244) and neutral light filters (RL211; Lee Filters©, LA-BS) to match the in situ underwater irradiance and spectrum at 5 m depth. The daily integrated irradiance in the mesocosms ranged between 14 and 65 mol photons m⁻² day⁻¹.

The reconstructed kelp communities were exposed to three experimental treatments: high temperature (HT), 1 marine heatwave
(1MH), 2 marine heatwaves (2MH) and a control (Ctrl), where each treatment had three replicates. The control treatment tracked in situ temperature while the 'high temperature' treatment (HT) maintained a consistent offset of ~1.8°C above the control, which corresponds to the 2100 projection if temperature continues to increase in this region at the rate observed over the last 40 years (+0.22°C per decade; Figure 1). This projection is according to NOAA's long-term climate data record (daily optimum interpolation sea surface temperature [OISST]). At the beginning of the experiment, temperature was programmed to increase gradually in the three treatments, reaching a setpoint target at 1.76°C above the control condition within 4 days. We set the 1MH treatment to simulate a long MHW with a programmed maximum offset of +2.8°C compared to the control for a total duration of 13 days. The 2MH treatment simulated two successive, intense, 5-day long MHWs separated by 3 days at the same temperature as the HT treatment. The maximum increase in temperature in the 2MH treatment was set at +3.9°C compared to the control. The temperature increase for the two MHW simulations started on day 6 to allow organisms to first acclimatize to warmer

<figure><img src="image_6.png" /><figcaption>FIGURE 1 (a) Hourly temperature measured during the experiment, (b) offset of temperature between the control and each treatment. 1MH, 1 marine heatwave treatment; 2MH, 2 marine heatwave treatments; Ctrl, control treatment; HT, high temperature treatment.</figcaption></figure>

conditions and was then gradual over 48-72h. The selected treatments were determined by examining the duration and magnitude of previous MHWs in the region over the past 10 years using the marine heat wave tracker data tool (Schlegel, 2020).

## 2.3 | Tissue sampling and measurement timeline

Kelp tissue samples were collected from the meristem to assess Chl *a* and C:N content at the start and the end of the experiment (see Sections 2.5 and 2.6). Tissue sampling was performed on 10 excess individuals directly before the launch of the experiment for each species (*t*₀, 2022-06-30). That is, additional individuals from the same sampling location as those used for the experiment were collected at a given sampling site under the assumption that the group of individuals collected at the site within the sample patch were homogenous. At the end of the experiment (*t*ₚ, 2022-07-23), tissue sampling was performed on all healthy organisms, where we avoided sampling organisms that could have been damaged during the experiment. Organisms considered 'healthy' were complete organisms (frond, stipe and holdfast) that exhibited a firm brown frond without signs of damage. Tissue samples from fronds were cut and stored in aluminium foil at -20°C.

Net photosynthesis and F√Fₘ could not be measured for all the treatments and for all species on the same day at the same interval (Table 1). For this reason, we picked days that would allow us to acquire measurements at the start, in the middle, and at the end of the experiment for all species. Measurements were also made at specific intervals, allowing us to capture physiological responses during the MHW peaks. As an example, net photosynthetic rates of *S. latissima* and *A. esculenta* exposed to the 2MH treatment were measured during the first heat wave (2022-07-10), during the cooling period between two heatwaves (2022-07-14) and again during the second heatwave (2022-07-18). This was done in addition to the measurements made for all treatments at the start and at the end of the experiment. The quantum yield of coralline algae was not measured at the start of the experiment because the measurements made on kelp were prioritized. Because we aimed to do F√Fₘ measurements at similar periods of the day for all species and treatments, it was not technically feasible to conduct these measurements on all the species in every treatment on the same day.

## 2.4 | Rates of net photosynthesis

Photosynthesis normalized to dry-weight (dw) biomass was measured in two kelp species (*S. latissima* and *A. esculenta*) as changes in dissolved O₂. We decided to favour increasing the number of replicated measurements on these two species instead of assessing net photosynthesis of *L. digitata* due to its perceived high thermal tolerance (Lebrun et al., 2024). The same two individuals per species and per mesocosm were used for every incubation over the experimental period. The kelp were incubated for 30 min in 24 or 30 L cylindrical perspex chambers placed in
<table><caption>TABLE 1 Measurement timeline of net photosynthesis, maximum quantum yield, Chl a content, C:N ratio and the growth rate of kelp, coralline algae, and fauna.</caption><thead><tr><th>Date</th><th>Net photosynthesis</th><th>Maximum quantum yield</th><th>Chlorophyll a</th><th>C:N ratio</th><th>Growth</th><th>Sample size (n) (net photosynthesis)</th><th>Sample size (n) (maximum quantum yield)</th></tr></thead><tbody><tr><td>2022-07-01, t<sub>0</sub></td><td></td><td></td><td>AE, SL, and LD: Ctrl, HT, 1MH, 2MH</td><td>AE, SL, and LD: Ctrl, HT, 1MH, 2MH</td><td><ul><li>Bouvant weight of coralline algae, mussels, and snails: Ctrl, HT, 1MH, 2MH</li><li>Weight of sea urchins: Ctrl, HT, 1MH, 2MH</li></ul></td><td></td><td></td></tr><tr><td>2022-07-02, t<sub>0</sub></td><td>AE and SL: Ctrl, HT, 1MH, 2MH</td><td>AE, LD and SL: Ctrl, HT, 1MH, 2MH</td><td></td><td></td><td></td><td>AE: n=8, SL: n=8</td><td>AE: n=24 (4 per condition)</td></tr><tr><td>2022-07-05</td><td>AE and SL: HT, 1MH, 2MH</td><td></td><td></td><td></td><td></td><td></td><td>LD: n=24 (4 per condition)</td></tr><tr><td>2022-07-07</td><td></td><td>AE, LD and SL: Ctrl, HT, 1MH, 2MH</td><td></td><td></td><td></td><td></td><td>SL: n=24 (4 per condition)</td></tr><tr><td>2022-07-10</td><td>AE and SL: 1MH, 2MH</td><td></td><td></td><td></td><td></td><td>AE: n=8, SL: n=6</td><td></td></tr><tr><td>2022-07-11</td><td></td><td>AE, LD and SL: Ctrl, 1MH, 2MH</td><td></td><td></td><td></td><td></td><td>AE: n=24 (4 per condition)</td></tr><tr><td>2022-07-12</td><td>AE and SL: Ctrl</td><td>Coralline algae: Ctrl, HT, 1MH, 2MH</td><td></td><td></td><td></td><td>AE: n=6, SL: n=6</td><td>Ctrl n=14, HT n=14<br>1MH n=12, 2MH n=13</td></tr><tr><td>2022-07-14</td><td>AE and SL: HT, 1MH, 2MH</td><td>AE, LD and SL: HT, 2MH</td><td></td><td></td><td></td><td>AE: HT n=6, 2MH<br>n=6<br>SL: HT n=6, 2MH<br>n=5</td><td>AE: HT n=6, 2MH n=6<br>LD: HT n=5, 2MH n=5<br>SL: HT n=6, 2MH n=6</td></tr></tbody></table>
<table><caption>TABLE 1 (Continued)</caption><thead><tr><th>Date</th><th>Net photosynthesis</th><th>Maximum quantum yield</th><th>Chlorophyll a</th><th>C:N ratio</th><th>Growth</th><th>Sample size (n) (net photosynthesis)</th><th>Sample size (n) (maximum quantum yield)</th></tr></thead><tbody><tr><td>2022-07-18</td><td>AE and SL: 1MH, 2MH</td><td>AE, LD and SL: 1MH, 2MH</td><td></td><td></td><td></td><td>AE: 1MH n=6, 2MH n=6<br>SL: 1MH n=6, 2MH n=5</td><td>AE: 1MH n=5, 2MH n=6<br>LD: 1MH n=5, 2MH n=6<br>SL: 1MH n=5, 2MH n=6</td></tr><tr><td>2022-07-20</td><td></td><td>Coralline algae: Ctrl, HT, 1MH, 2MH</td><td></td><td></td><td></td><td></td><td>Ctrl n=14, HT n=11<br>1MH n=12, 2MH n=12</td></tr><tr><td>2022-07-21</td><td>AE and SL: Ctrl, HT</td><td>AE, LD and SL: Ctrl, HT</td><td></td><td></td><td></td><td>AE: Ctrl n=6, HT n=6<br>LD: Ctrl n=6, HT n=6<br>SL: Ctrl n=6, HT n=6</td><td></td></tr><tr><td>2022-07-22</td><td>AE and SL: 1MH, 2MH</td><td>AE, LD and SL: 1MH, 2MH</td><td></td><td></td><td></td><td>AE: Ctrl n=6, HT n=6<br>1MH n=6, 2MH n=6<br>SL: Ctrl n=4, HT n=5</td><td>AE: 1MH n=6, 2MH n=6<br>LD: 1MH n=6, 2MH n=6<br>SL: 1MH n=6, 2MH n=6</td></tr><tr><td>2022-07-23, t<sub>f</sub></td><td></td><td></td><td>AE, SL, and LD: Ctrl, HT, 1MH, 2MH</td><td>AE, SL, and LD: Ctrl, HT, 1MH, 2MH</td><td></td><td>• Bouyant weight of coralline algae, mussels, and snails: Ctrl, HT, 1MH, 2MH<br>• Weight of sea urchins: Ctrl, HT, 1MH, 2MH</td><td></td></tr></tbody></table>

Abbreviations: 1MH, 1 marine heatwave treatment; 2MH, 2 marine heatwaves treatment; AE, *Alaria esculenta*; Ctrl, control treatment; HT, high temperature treatment; LD, *Laminaria digitata*; SL, *Saccharina latissima*.
a flow-through water bath (194 cm id×200 cm height) at different times (Table 1) during the experiment. O₂ was continuously monitored using an optical oxygen sensor (FireSting®-O₂; PyroScience) with an acquisition rate of 1 measurement min⁻¹. To limit experimental bias, the kelp were suspended in individual transparent chambers, which were wrapped with spectra and light filters (same as the filters placed over top of the mesocosms), and water was constantly recirculated by a 5W water pump (©Decdeal). Each incubation started with fresh seawater originating from the respective treatment mesocosm. The O₂ level during the incubations was maintained between 90% and 125% saturation. Photosynthetically active radiation (PAR) was measured inside one of the four chambers for each incubation using a spherical quantum sensor (US-SQS/L; WALZ). The photosynthetic rate was calculated for each incubation using a linear model and was normalized to (1) the volume inside the incubation chamber and (2) the dry weight of the individual. Dry weights were derived using the linear regression model calculated by Miller, Gazeau, Lebrun, Gattuso, et al. (2024) which correlates fresh weight to dry weight (dw). For S. latissima (dw = 0.1664 × fw + 0.158) and A. esculenta (dw = 0.2614 × fw - 3.199). Net photosynthetic rates are expressed in µmol O₂ g dw⁻¹ h⁻¹. To account for the variability in PAR during the different incubations, net photosynthesis was normalized with the mean PAR during the incubation which was calculated by summing the PAR measurements (1 measurement/5 min, expressed in µmol photons m⁻² s⁻¹) and dividing by the total number of measurements.

## 2.5 | Maximum quantum yield

The maximum quantum yield ($F_v/F_m$) was measured on the three kelp as an indicator of photosynthetic performance (see Table 1 for measurement dates). Spot measurements on individual kelp in each treatment were measured routinely for the control and HT treatments (approximately every 7 days) and every 3–5 days for the 1MH and 2MH treatments corresponding to timepoints of temperature inflection for the 2MH treatment using a DIVING-PAM-II (Walz©) fluorometer (Table 1). The same two individuals per species were measured whenever taking fluorometry readings. Measurements were made on the same three kelp species throughout the whole experiment using leaf clips to darken spots on the frond. These were placed over a small section of the frond for 5 min before the measurement on the meristem. For coralline algae, $F_v/F_m$ was determined using a surface holder on organisms placed in the dark 5 min before the measurement. For the three kelp species, the following settings were used to obtain a signal that was within the recommended values: gain = 4, damping = 3, light intensity = 6 and light frequency = 3. These settings were determined by empirical testing, recommendation from the user manual and personal experience. For coralline algae, the damping was increased to 9 (because of a lower signal). The other parameters remained the same as for the kelp.

## 2.6 | Chlorophyll a content

Chlorophyll a was assessed in the three kelp species at the start and the end of the experiment (Table 1) to determine if the treatments affected the photosystems by modifying the concentration of photosynthetic pigments. Pieces of collected tissue were weighed, hand ground and extracted in 90% aqueous acetone for 24 h in the dark at 4°C. After centrifugation (0°C, 15 min, 3000 rpm), the supernatant was transferred into a glass vial, and $F_0$, the initial fluorescence of Chl a and pheophytin, was measured using a fluorimeter (Turner Design©, 10-AU Fluorimeter). The $F_a$ fluorescence was measured 1 min after the addition of 10 µL of 0.3 N HCl to transform Chl a into pheophytin, followed by subtracting $F_a$ from $F_0$. The Chl a content was calculated as described by Lorenzen (1967) and expressed per gram of fw [µg Chl a (g fw)⁻¹].

## 2.7 | C:N ratio

The C:N ratio in kelp was determined at the start and the end of the experiment (see Table 1 for measurements dates) to assess the effects of the treatments on metabolic pathways such as nitrogen uptake or assimilation processes. Ten milligram of each frozen sample was dried at 60°C for 48 h. Total carbon and nitrogen were measured on a PerkinElmer, Inc. 2400, CHN analyzer. Analyses were based on the oxidative combustion of dried samples at high temperature and analysis of the resulting gas via a thermal conductivity detector. Blanks and standards (Glycine C = 32%; N = 18.66%) were processed between samples for calibration purposes (Miquel et al., 2011). Thawing of the $t_0$ samples of A. esculenta occurred during the initial storage and freezing on site and, unfortunately, could not be used.

## 2.8 | Elongation rate

For kelp species, elongation rate was determined using the hole puncture method (Parke, 1948). Sporophytes were punctured at $t_0$ in the meristem section of each organism, 2 cm from the base of the stipe. The distance from the base of the stipe to the hole was measured at $t_f$. The elongation rate was calculated as follows:

$$ \text{Elongation rate (cm day}^{-1}) = \frac{\text{dist}_f - \text{dist}_0}{t_f - t_0} $$

with dist: distance (in cm) from the base of the stipe to the hole in the meristem at time t (in days).

A log-log relationship was established between the elongation rate and the total length of the organism at the beginning of the experiment. The regression equation (log elongation = 4.28–1.56 × log length, p-value of the slope <0.001) was used to calculate the expected elongation rate as a function of kelp size and to determine its discrepancy with the measured value.

Growth rate for coralline algae, snails and mussels was determined by subtracting their buoyant weight at $t_f$ from that at $t_0$. To
<figure><img src="image_7.png" /></figure>

determine the buoyant weight, the organisms were suspended in seawater by a nylon line that was connected to the attachment point on the underside of the balance. Temperature of the seawater was regularly checked to ensure that it did not deviate by more than 0.5°C. Both weights were converted to dw using the following equation (Spencer Davies, 1989).

$$ \text{Dry weight (g)} = \frac{\text{wet weight}}{1 - \frac{\text{water density}}{\text{calcium carbonate density}}} $$

with wet weight in g, water density in $g cm^{-3}$ and a calcium carbonate density of $2.93 g cm^{-3}$.

Water density was calculated using the thermodynamic equation of seawater (TEOS 10, McDougall et al., 2009). The total wet weight for sea urchins was measured per mesocosm as it was not possible to identify and track individuals.

## 2.9 | Statistics

Rosner's generalized extreme studentized deviation (ESD) test was used to detect the outliers for all parameters using the function rosnerTest of the R package EnvStats (Millard, 2013). For the net photosynthesis rate, one outlier was removed out of 72 measurements for S. latissima; no outliers were detected for A. esculenta (n=79). Regarding the maximum quantum yield, Chl a content and C:N, 3.5%, 4% and 16% of measurements were considered outliers, respectively, and eliminated from the analysis. No outliers were found in the elongation rate of kelp, while 25% of the measurements of buoyant weight of coralline algae, snails and mussel species were removed.

To analyse net photosynthetic rates, maximum quantum yields, Chl a content, C:N ratios and growth, we fit a linear mixed-effects model using the lmer function in R. The model included species, treatment and sampling time (for photosynthetic rates and maximum quantum yields) as fixed effects, with mesocosm included as a random effect to account for variability across experimental units. We used likelihood ratio tests to assess the significance of each factor by comparing the full model to reduced models. Post hoc pairwise comparisons were conducted using the R emmeans package to examine differences between specific levels of significant factors. Model diagnostics were done using residual analysis and QQ plots to assess that the assumptions of normality and homogeneity of variance were met (Figures S4–S8).

# 3 | RESULTS

## 3.1 | Experimental conditions

Temperature was successfully manipulated during the experiment, with a median temperature in each treatment being 8.5,

10.2, 10.9 and 10.8°C in the control, HT, 1MH and 2MH, respectively (Figure 1). Over the duration of the entire experimental period, source seawater temperature increased by ~1.5°C (Figure 1; Figure S1). This was not related to the heatwave, as the only notable temperature increase was observed between 2022-07-06 and 2022-07-13, with a maximum rise to 10.2°C recorded on 2022-07-13. During this MHW, temperature stayed below 11°C, which was inferior to the maximal temperature reached in the mesocosms in the 1MH (12.9°C) and 2MH treatments (13.8°C). Only two individual kelp out of the 163 experimented on suffered mortality during the experiment.

## 3.2 | Rate of net photosynthesis

Net photosynthetic rates were affected by time and species (Figure 2; Table S1). For A. esculenta, net photosynthetic rates decreased between $t_0$ and $t_f$ in all treatments ($p < 0.04$, Table S2). Between $t_0$ and the middle of the experiment, on 2022-07-14, net photosynthetic rates significantly decreased in the HT ($p < 0.01$). Net photosynthetic rates of S. latissima were significantly lower in the 1MH on 2022-07-10 compared to $t_f$ ($p = 0.014$), and in the HT on 2022-07-14 compared to the 2MH on 2022-07-18. Although there was a declining trend of photosynthetic rate over time for S. latissima, it was not statistically significant.

## 3.3 | Maximum quantum yield

The maximum quantum yield ($F_v/F_m$) was affected by species, treatment and sampling time (Table S3). L. digitata was not affected by the different treatments nor by time (Figure 3; Tables S3 and S4). For A. esculenta, the only significant difference was found between the initial increase in temperature and the 1MH on 2022-07-18. However, for S. latissima, $F_v/F_m$ significantly decreased in the 2MH treatment between the initial increase in temperature, and the measurements on 2022-07-11, 2022-07-14, and 2022-07-18 (Table S4). In the 1MH treatment, $F_v/F_m$ significantly decreased between 2022-07-07 and the exposure to the MHW on 2022-07-11 and 2022-07-18 ($p < 0.004$). At the end of the experiment, $F_v/F_m$ was back to the initial values in all treatments.

The maximum quantum yield of coralline algae was measured on 2022-07-12 and 2022-07-20 and was not affected by time or treatment (Figure S2; $p > 0.05$).

## 3.4 | Chlorophyll a content

Chl a content was affected by species and treatment (Table S5, $p = 0.001$). For A. esculenta and S. latissima, the Chl a content significantly decreased between $t_0$ and the 2MH treatment at $t_f$ ($p = 0.044$ and 0.020, Figure 4; Table S6), while it did not vary with time in the
**FIGURE 2** Mean±SD net photosynthetic rates of *A. esculenta* and *S. latissima*. The grey square indicates the beginning of the experiment ($t_0$) when all mesocosms were at ambient conditions. The black symbol (IncreasTemp) corresponds to the rates measured in the C (control), HT (high temperature treatment), 1MH (1 marine heatwave treatment) and 2MH (2 marine heatwaves treatment) when these three treatments were at the same temperature before the start of the marine heatwave simulations.

<figure><img src="image_8.png" /></figure>

other treatments. The Chl *a* content of *L. digitata* was not significantly impacted by time or treatment.

## 3.5 | C:N ratios

C:N ratios were affected by species but not by the treatment (Table S7, *p* < 0.001 and *p* = 0.093). For *S. latissima* and *L. digitata*, C:N ratios at $t_0$ ranged from 16.7 to 30.4 (Figure 5). No statistical difference was found between $t_0$ and $t_f$ for all treatments (Tables S7 and S8). C:N ratios of *A. esculenta* were statistically different between the 1MH and the 2MH treatments at $t_f$ (*p* = 0.009), with lower C:N ratios in the 1MH treatment.

## 3.6 | Elongation and growth rate

The elongation rates of *A. esculenta*, *L. digitata* and *S. latissima* were not significantly impacted by the treatments (Figure 6, *p*-value = 0.189, Tables S9 and S10). Elongation rates varied from 0.06 to 0.46 cm day⁻¹ for *S. latissima*, 0 to 0.04 cm day⁻¹ for *L. digitata* and 0.0 to 0.24 cm day⁻¹ for *A. esculenta*. The elongation rate of *S. latissima* was significantly higher than that of the other two species (*p* < 0.001).

No statistical difference was found between the different treatments for the growth rate of coralline algae, snails, mussels and sea urchins (Figure S3).

# 4 | DISCUSSION

This study highlights distinct responses of *A. esculenta*, *L. digitata* and *S. latissima* to predicted warming and MHW scenarios. While *A. esculenta* and *S. latissima* exhibit some susceptibility to the treatments, *L. digitata* demonstrates greater tolerance. Here, we have demonstrated that kelp community organisms exhibit species-specific tolerance and physiological responses to varying and differing characteristics of MHWs. These findings highlight that certain kelp species have differing optimal thermal niches, and that global change might differentially affect the capacity to maintain homeostasis depending on geographical distribution and acclimation. Evidence for this hypothesis was presented by Diehl et al. (2021) who detailed the response of kelp photo-physiology to thermal stress along a latitudinal gradient of temperature offsets from local ambient conditions.

Overall, our results highlight the resilience and capacity of each species to recover rapidly after a heatwave. The heat tolerance of Laminariales has been well documented, where studies have shown that maximum heat tolerance can increase by 5–10°C depending on whether the kelp were acclimatized to colder (0–5°C) or warmer (mean = 15°C) temperatures (Bartsch et al., 2008 and references therein). Given the dynamic range of temperatures in the Tromsø region (winter ~4 to >12°C in summer, Figure S1), it appears that the maximum temperatures observed during a normative summer warming could provide the necessary temperature exposure for increased tolerance to heatwaves (Figure S1). This could explain the
<figure><img src="image_9.png" /></figure>

FIGURE 3 Mean ± SD maximum quantum yield (Fv/Fm) of *A. esculenta*, *L. digitata* and *S. latissima* during the experiment. The grey square indicates the beginning of the experiment (t0) when all mesocosms were at ambient conditions. The black symbol (IncreasTemp) corresponds to the rates measured in the C (control), HT (high temperature treatment), 1MH (1 marine heatwave treatment) and 2MH (2 marine heatwaves treatment) when these three treatments were at the same temperature before the start of the marine heatwave simulations.

recovery ability and resilience to the heatwaves simulated in this study. Despite the decrease in photosynthetic rate by *S. latissima* and *A. esculenta*, both species demonstrate a recovery and resilience potential, and the depreciation in photosynthetic rate could simply indicate a rapid short-term response to a minor thermal stress. It should be noted that the most extreme natural temperatures observed in the Tromsø region during this study fall within the thermal scope for *S. latissima* and *L. digitata* (~5 to 20°C), at least for particular ecotypes (Bolton & Lüning, 1982; Davison, 1987).

Over the 23-day experiment, net photosynthesis decreased for *A. esculenta* and *S. latissima*. While the net photosynthetic

rate for *L. digitata* was not measured, the ancillary evidence of photo-physiology such as Fv/Fm and Chl a content suggests a non-changing response in both parameters. Given that PAR values did not decrease during the experiment, and nutrient concentrations increased (e.g. PO₄³⁻ increased 0.16 ± 0.03 to 0.28 ± 0.02 between t₀ and t₁; Miller, Gazeau, Lebrun, Alliouane, et al., 2024), the effect on photosynthesis was most likely due to the imposed experimental conditions. However, the decline in net photosynthesis may be linked to an increase in respiration as a response to increased temperature. Thus, net oxygen production decreases if gross photosynthesis does not scale concomitantly (Aamot, 2011). This aligns
<figure><img src="image_10.png" /></figure>

FIGURE 4 Chlorophyll *a* content of *A. esculenta* (AE), *L. digitata* (LD) and *S. latissima* (SL) exposed to the control and the three treatments. Values at *t*₀ correspond to the Chl *a* content at the start of the experiment, while C (control), HT (high temperature treatment), 1MH (1 marine heatwave treatment) and 2MH (2 marine heatwaves treatment) correspond to the final Chl *a* content of each species maintained in the respective treatments for the whole experiment. The horizontal lines in each boxplot represent the median. The whiskers extend to the furthest data points within 1.5 times the interquartile range (the top and bottom of the box). The number in parentheses below each boxplot corresponds to the sample size (excluding outliers). Statistically significant differences are shown with an asterisk (*p* < 0.05). *t*₀ is the beginning of the experiment.

<figure><img src="image_11.png" /></figure>

FIGURE 5 Carbon:nitrogen ratio of *A. esculenta* (AE), *L. digitata* (LD) and *S. latissima* (SL) exposed to the control and the three treatments. Values at *t*₀ (beginning of the experiment) correspond to samples taken at the start of the experiment, while C (control), HT (high temperature treatment), 1MH (1 marine heatwave treatment) and 2MH (2 marine heatwaves treatment) correspond to the final values for kelp maintained in the respective treatments during the whole experiment. The horizontal lines in each boxplot represent the median. The whiskers extend to the furthest data points within 1.5 times the interquartile range (the top and bottom of the box). The number in parentheses below each boxplot corresponds to the sample size (excluding outliers). Statistical differences are marked by an asterisk (*p* < 0.05).
<figure><img src="image_12.png" /><figcaption>FIGURE 6 Elongation rate of *A. esculenta* (AE), *L. digitata* (LD) and *S. latissima* (LD) in the C (control), HT (high temperature treatment), 1MH (1 marine heatwave treatment) and 2MH (2 marine heatwaves treatment) during the whole experiment. The horizontal lines in each boxplot represent the median. The whiskers extend to the furthest data points within 1.5 times the interquartile range (the top and bottom of the box). The number in parentheses above each boxplot corresponds to the sample size (excluding outliers).</figcaption></figure>

with the findings of Umanzor et al. (2021), where a 150% increase in respiration was observed for the giant kelp *Macrocystis pyrifera* when temperature increased from 16 to 22°C. Since respiration was not measured during this experiment due to the Arctic summer day, we can only propose this as a hypothesis. We should note, however, that other studies have observed an enhancement in *S. latissima* physiological performance under increased warming, albeit for high Arctic ecotypes (Li et al., 2019). Another plausible explanation for decreasing net photosynthesis could be the decrease in *Chl a* content, observed for both *A. esculenta* and *S. latissima* in the 2MH treatment at the end of the experiment. These observations are consistent with those by Nepper-Davidsen et al. (2019), who noted a *Chl a* decrease in *S. latissima* at 15°C (location: South Kattegat between Denmark and Sweden), which appeared to be exacerbated when exposed to 24°C. Our previous work also noted this decline in *Chl a* content for both *S. latissima* and *A. esculenta* (see Lebrun et al., 2024 for a broader discussion). Umanzor et al. (2021) also found a decrease in the *Chl a* content of the giant kelp *M. pyrifera* when exposed to warmer temperature (+6°C) for 5 days. These findings highlight the importance of considering temporal changes in the photosynthetic pigments of kelp when assessing their response to MHWs. A final consideration for a decreasing photosynthetic rate as seen across treatments and in the control condition for *A. esculenta* is the natural seasonal decline in productivity. Thus, there may have been a natural decline over the experimental period, but the treatment conditions introduced a volatility to the photosynthetic response. This could have manifested as an initial suppressed rate, followed by a recovery before the natural seasonal reduction in photosynthetic rate.

The increase of the maximum quantum yield of *S. latissima* between the start of the experiment and the in situ temperature increase on 2022-07-07 (i.e. July 3-7) suggests that the exposure to elevated temperature had a mild positive effect on photosystem II of this species. In contrast, exposure to the MHW treatments had a negative effect on $F_v/F_m$. This negative impact quickly reversed, however, as $F_v/F_m$ was back to its original values only 2 days after the end of the MHWs. The decrease of $F_v/F_m$ found here agrees with the study of Niedzwiedz et al. (2022), where $F_v/F_m$ of *S. latissima* from Helgoland (North Sea, Germany) decreased when exposed to MHWs lasting 9-13 days, with temperature increases ranging from 2 to 6°C. Further, in a study conducted by Strasser et al. (2022), the maximum quantum yield of *L. ochroleuca*, an Atlantic kelp species, was at its initial level during the first measurement taken 27 days after a severe MHW (+10°C; maximum temperature of 27°C for 5 days).

Overall, the results suggest that the three kelp species studied here are tolerant to warming and MHWs for most parameters, and resilient with respect to the maximum quantum yield in the case of *S. latissima*. While the warming and MHW treatments induced responses in two of the three species, these responses were either reversible, such as the decrease of the maximum quantum yield, or an enhancement of the natural decline rate of net photosynthesis and *Chl a* over time, a process typically observed at the end of summer. Overall, this result is not surprising since these species are also distributed in warmer regions. Thus, many physiological experiments testing the photosynthetic response to increasing temperature demonstrate a large thermal optimum (up to 20°C) for *S. latissima*, *A. esculenta* and *L. digitata* (Bischoff & Wiencke, 1993; Bolton & Lüning, 1982; Davison, 1987; Müller et al., 2009; Park et al., 2017).
<figure><img src="image_13.png" /></figure>

For example, Fredersdorf et al. (2009) found that the photosynthesis thermal optimum for populations of *A. esculenta*, living in colder waters than in Tromsø (Kongsfjorden, Svalbard), was between 13 and 17°C. In this study, temperature levels >13°C were only reached for a few hours in the 2MH treatment over the entire experiment. However, the three species may have different sensitivities and tolerance levels to temperature stress. *L. digitata* seems to acclimatize rapidly to MHWs, evidenced by the absence of effects on the parameters measured. In contrast, the physiology of *S. latissima* and *A. esculenta* exhibited treatment-induced effects, albeit differences in relaxation times. That is, evidence defining differences in these sensitivities across species was expressed by *S. latissima*, which displayed remarkable resilience to the MHWs, as seen by its maximum quantum yield returning to its initial level. Thus, it appears that the stress of MHWs forces individuals to quickly acclimatise to new conditions. In particular, individuals can adjust pigment concentration and location to maintain an elevated net photosynthetic rate, although it might not be possible over a long period of time (Niedzwiedz et al., 2022). This could elucidate the absence of impact observed from the decrease in maximum quantum yield in the MHW treatments on the photosynthesis of *S. latissima*.

*A. esculenta* was the only species to display a significant difference with respect to its C:N ratio and elongation rate. Regarding the C:N ratio and the elongation rate, no significant difference was observed at *t*<sub>f</sub> between the different treatments apart from *A. esculenta*, which exhibited higher C:N ratios in the 2MH treatment compared to the 1MH at the end of the experiment. Benthic marine macroalgae and seagrasses from temperate and tropical regions have a mean C:N ratio of 22 (Atkinson & Smith, 1983), which is consistent with our measurements (mean ratios of 21.5 and 22.7 for *S. latissima* and *L. digitata* at *t*<sub>0</sub> and 18.8, 17.3 and 22 in the control treatment at *t*<sub>f</sub> for *A. esculenta*, *S. latissima* and *L. digitata*, respectively). Under heat stress, organisms often prioritize resource allocation toward stress tolerance mechanisms, which can affect nitrogen uptake or assimilation processes, thereby reducing nitrogen availability and increasing the C:N ratio (Diehl et al., 2020). However, for *A. esculenta*, the opposite effect was observed for the 1MH treatment, where C:N was depressed compared to any other treatment, suggesting a possible change in metabolism when exposed to a long duration MHW. Our results are consistent with Kinnby et al. (2021) who found no effect of a 5°C warming of 7 weeks on the C:N ratio and elongation rate of *Fucus vesiculosus* from Sweden.

With respect to the coralline algae *L. glaciale*, an important component of these benthic communities, neither their growth rate nor quantum yield varied significantly across the treatments. This suggests that *L. glaciale* may exhibit either a tolerance or a delayed response to short-term MHWs. That is, the relatively short duration of the experiment may not have fully captured the long-term effects on slow-growing organisms such as *L. glaciale*. Given the species' slow generation time, it is possible the perceived tolerance to the simulated MHW(s) may not translate to a longer term exposure. Such longer term experiments could provide insights into the potential acclimatization or adaptation to future temperature anomalies.

However, while our experimental design was not optimal to detect such changes in growth and long-term response of this species, it must be noted that we did not note any signs of necrosis or bleaching in response to the treatments, which are early signs of stress in coralline algae. Furthermore, Cornwall et al. (2020) highlighted the importance of studying coralline algae over multiple generations to characterize potential gains of tolerance to stressors. In particular, species with short generation times (8 weeks) may be more resilient and more capable of acclimatizing to future warming (Cornwall et al., 2019). However, such acclimation might be compromised for long-lived species such as polar species with annual reproduction cycles (Adey, 1973). The growth rate of these organisms in the Arctic and sub-Arctic is very low (average = 0.2 mm month⁻¹; Adey & Vassar, 1975); therefore, a longer experiment would be necessary to confirm our observations.

Similarly, the growth rate of snails, mussels and sea urchins also showed no significant variations with the treatments. However, it is important to note that while only growth was studied (with the exception of *F*<sub>v</sub>*F*<sub>m</sub> for coralline algae), we did not note any mortality or changes in behaviour (reduced grazing) associated with the treatments. Further investigations such as lipid content analyses or reproduction are necessary to draw more comprehensive conclusions regarding the tolerance of these organisms to warming and MHWs. For example, Axenov-Gribanov et al. (2015) found that exposure to 20°C (compared to 15°C) induced a higher synthesis of HSP70, a heat-shock protein produced when the organism is exposed to a stressful environment, and caused changes in the activities of antioxidative enzymes and glycogenic metabolism of the gastropod *Lymnaea stagnalis*. Also, Hiscock et al. (2004) recorded a negative impact of warming on the horse mussel, *Modiolus mo-diolus*. This warming could contribute to its noted decline (ongoing since 1990) off the coast of Northern Ireland. In Tromsø, except in isolated areas with strong currents, kelp populations have been decimated by sea urchins (Sivertsen, 2006). Already in the 1980s, Hart and Scheibling (1988) noted the positive effect of warming and MHW on sea urchins, especially on larval growth, and the potential indirect impact on kelp forests through increased grazing. Furthermore, sea urchins might become more susceptible to the outbreak of diseases with warming temperatures, as demonstrated for *Strongylocentrotus droebachiensis* in Nova Scotia (Scheibling & Hennigar, 1997). In the present experiment, we did not see any signs of disease, but the duration of the experiments might have been too short to detect them. Due to the strong top-down control of sea urchins on kelp, changes in sea urchin abundances might have a higher impact on kelp abundance than climate change in the Arctic (Krause-Jensen et al., 2020). Therefore, while we did not measure any direct effects of warming and MHW on the sea urchins, it will be important for future work to focus on longer multifactorial experiments that incorporate both abiotic and biotic factors controlling kelp abundances.

While the experimental period was rather short (20 days) for testing long-term effects of warming, the time of exposure to the simulated MHW was representative of past MHW events, despite
the short duration of each temperature peak in the 2MHW treat-
ment. Between 2000 and 2016, MHWs globally lasted an average
of 10-27 days (during El-Nino years), with intensities between 1.8
and 2.3°C (Oliver et al., 2018). It should be considered, however,
that the intensity and duration of MHWs are likely to increase in
the future (Oliver et al., 2021). Thus, the duration of this experi-
ment may not capture the future long-term effects of warming and
MHWs on kelp communities. Kelp communities are dynamic and
complex, and their responses to environmental stressors can vary
over different time scales. Niedzwiedz et al. (2022) found a bet-
ter survival of *S. latissima* from Helgoland (North Sea, Germany)
to heatwave treatments in June than in August 2018 (18-24°C).
A longer experimental period would provide greater insights into
potential acclimation mechanisms that species could use to cope
with prolonged heat stress. It would also allow for the examina-
tion of potential shifts in species composition, competitive in-
teractions and ecological dynamics that can occur as a result of
sustained MHW events. In any case, the duration of perturbation
experiments cannot likely reproduce the expected future decadal
changes. Thus, it will be essential to carry out in situ studies to ob-
serve the multitude of environmental changes such as the arrival of
new opportunistic species, with affinities for warmer temperature
and intensification of MHWs (Arafeh-Dalmau et al., 2019).

AUTHOR CONTRIBUTIONS

Anaïs Lebrun, Cale A. Miller, Frédéric Gazeau, Jean-Pierre Gattuso and Steeve Comeau designed the study. Frédéric Gazeau, Pierre Urrutti and Steeve Comeau designed the experimental system. Jean-Pierre Gattuso, Samir Alliouane and Steeve Comeau collected the samples by scuba diving. Anaïs Lebrun and Cale A. Miller performed the experiment with the assistance of Jean-Pierre Gattuso, Samir Alliouane and Steeve Comeau. Anaïs Lebrun analysed the data and wrote the first draft of the manuscript, which was then finalized by all the co-authors.

ACKNOWLEDGEMENTS

We are extremely grateful to Haakon Hop for making his boat avail-
able, his invaluable assistance for organism collection and for the
generous loan of a Diving-PAM. This work could not have been
performed without his help. We also acknowledge the efforts of
Havbruksstasjonen personnel and warmly thank Maxime Pistone
for his invaluable assistance, Nathalie Leblond for her help with
the CHN analysis, Robert Schlegel for his contribution to the ex-
perimental design, Catriona Hurd for her valuable advice and the
Alfred Wegener Institute (AWI) and Institut polaire français Paul
Emile Victor (IPEV) for assisting with the shipping of our equip-
ment. This study was conducted in the frame of the FACE-IT project
(The Future of Arctic Coastal Ecosystems—Identifying Transitions in
Fjord Systems and Adjacent Coastal Areas) which received funding
from the European Union's Horizon 2020 research and innovation
programme under grant agreement No. 869154. This study was sup-
ported by the Prince Albert II of Monaco Foundation (FPA2, project
ORCA No. 3051).

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.

PEER REVIEW

The peer review history for this article is available at https://www.
webofscience.com/api/gateway/wos/peer-review/10.1111/1365-
2745.70074.

DATA AVAILABILITY STATEMENT

All data are available in PANGAEA: https://doi.org/10.1594/PANGA
EA.975971 (Lebrun et al., 2025).

ORCID

Anaïs Lebrun https://orcid.org/0000-0002-9183-8524

Cale A. Miller https://orcid.org/0000-0002-0049-9361

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## SUPPORTING INFORMATION

Additional supporting information can be found online in the Supporting Information section at the end of this article.

**Figure S1.** Daily mean sea surface temperature (°C) in Tromsø region (69.5° to 70°N, 18.5° to 19°E).

**Figure S2.** Maximum quantum yield ($F_v/F_m$) of coralline algae during the experiment.

**Figure S3.** Difference between the initial and final dry weight of (a) coralline algae, (b) snails, (c) mussels and wet weight of (d) urchins exposed to the control and the three treatments.

**Figure S4.** Diagnostic plots of the model used to infer the effects of species and treatment on photosynthesis.

**Figure S5.** Diagnostic plots of the model used to infer the effects of species and treatment on the $F_v/F_m$.

**Figure S6.** Diagnostic plots of the model used to infer the effects of species and treatment on Chl *a*.

**Figure S7.** Diagnostic plots of the model used to infer the effects of species and treatment on C:N.

**Figure S8.** Diagnostic plots of the model used to infer the effects of species and treatment on growth.

**Table S1.** Results of the likelihood ratio tests used to assess the effects of species, treatment and date on the photosynthesis of *Alaria esculenta* and *Saccharina latissima*.

**Table S2.** Pairwise comparisons of the net photosynthesis rate.

**Table S3.** Results of the likelihood ratio tests used to assess the effects of species, treatment and date on the $F_v/F_m$ values of the three kelp species.

**Table S4.** Pairwise comparisons of the maximum quantum yield.

**Table S5.** Results of the likelihood ratio test used to infer the effects of species and treatment, on Chl *a*.

**Table S6.** Pairwise comparisons of the chlorophyll *a* content.

**Table S7.** Results of the likelihood ratio test used to infer the effects of species and treatment on C:N.

**Table S8.** Pairwise comparisons of the C:N ratio.

**Table S9.** Results of the likelihood ratio test used to infer the effects of species and treatment on the log transformed difference between the expected and the actual growth rates.

**Table S10.** Pairwise comparisons of the difference between the expected and the actual growth rates.

**How to cite this article:** Lebrun, A., Miller, C. A., Gazeau, F., Urrutti, P., Alliouane, S., Gattuso, J.-P., & Comeau, S. (2025). Tolerance of organisms composing an Arctic kelp community to ocean warming and marine heatwaves. *Journal of Ecology*, 00, 1–17. https://doi.org/10.1111/1365-2745.70074