The endemic kelp Lessonia corrugata is being pushed above its thermal limits in an ocean warming hotspot

Kelps are in global decline due to climate change, which includes ocean warming. To identify vulnerable species, we need to identify their tolerances to increasing temperatures and determine whether tolerances are altered by co‐occurring drivers such as inorganic nutrient levels. This is particularly important for those species with restricted distributions, which may already be experiencing thermal stress. To identify thermal tolerance of the range‐restricted kelp Lessonia corrugata, we conducted a laboratory experiment on juvenile sporophytes to measure performance (growth, photosynthesis) across its thermal range (4–22°C). We determined the upper thermal limit for growth and photosynthesis to be ~22–23°C, with a thermal optimum of ~16°C. To determine if elevated inorganic nitrogen availability could enhance thermal tolerance, we compared the performance of juveniles under low (4.5 μmol · d−1) and high (90 μmol · d−1) nitrate conditions at and above the thermal optimum (16–23.5°C). Nitrate enrichment did not enhance thermal performance at temperatures above the optimum but did lead to elevated growth rates at the thermal optimum. Our results indicate L. corrugata is likely to be extremely susceptible to moderate ocean warming and marine heatwaves. Peak sea surface temperatures during summer in eastern and northeastern Tasmania can reach up to 20–21°C, and climate projections suggest that L. corrugata's thermal limit will be regularly exceeded by 2050 as southeastern Australia is a global ocean‐warming hotspot. By identifying the upper thermal limit of L. corrugata, we have taken a critical step in predicting the future of the species in a warming climate.

disproportionately important in marine environments and support hundreds of associated species and a wide array of ecosystem services (Jones et al., 1994;Teagle et al., 2017;Woodhead et al., 2019).However, rising ocean temperatures are contributing to declines of these important species around the world (Leggat et al., 2019;Nguyen et al., 2021;Smale et al., 2019), and there is a pressing need to identify susceptible species early while management interventions may still be possible (Layton et al., 2022;Saunders et al., 2020;Van Oppen et al., 2015).This is particularly important for species with restricted ranges that are unable to shift their distributions further poleward due to geographic barriers.Accordingly, we need to identify species' thermal tolerances by understanding their thermal ranges and how their performance varies over these ranges.Moreover, we also need to understand whether other co-occurring drivers such as nutrient availability, irradiance, and CO 2 can alter their thermal performance (Boyd et al., 2018;Britton et al., 2020;Endo et al., 2017).All this information then becomes essential for improving our ability to accurately predict population changes in abundance and distribution and to identify vulnerable species and communities in our rapidly warming oceans.
Kelps (orders Fucales and Laminariales) are the dominant habitat forming species across temperate and subpolar rocky reef systems globally.They form highly productive and structurally complex ecosystems that support high biodiversity by providing food and habitat for various marine organisms (Hurd et al., 2014a(Hurd et al., , 2014b;;Steneck et al., 2002;Teagle et al., 2017).Globally, kelp forests are under threat from a variety of stressors, including rising ocean temperatures, with declines in abundance and local extinctions occurring due to ocean warming and marine heatwaves (Butler et al., 2020;Krumhansl et al., 2016;Smale et al., 2019).Although kelps can experience fluctuations of temperature on a daily and seasonal basis, exposure to temperatures outside their thermal ranges (especially at the upper limits) is considered thermal stress (Eggert, 2012;Hurd et al., 2014aHurd et al., , 2014b)).Such increases in temperature can cause modifications at physiological and biochemical levels and affect kelp by impairing their respiration, growth, photosynthesis, and basic cellular maintenance, ultimately causing mortality (Flukes et al., 2015;Supratya et al., 2020;Wernberg et al., 2016).The effects of temperature on kelps can also vary due to geographic location, genetic differences, and life stage (Paine, Schmid, Gaitán-Espitia et al., 2021;Vranken et al., 2021;Wernberg et al., 2016).
Generally, growth and photosynthetic rates will increase with temperature until a maximum level before rapidly declining as temperatures exceed the maximum (Eggert, 2012).These patterns can be described by thermal performance curves (TPCs), which illustrate how changes in temperature translate into performance (e.g., growth) and are tools to understand how a species/population responds to changing temperatures.Thermal performance curves also provide estimates of critical parameters and thresholds such as thermal optimum (T opt ), thermal minimum (CT min ), and thermal maximum (CT max , Angilletta, 2006).These parameters can then be used to design targeted experiments at a specific temperature to explore the interactive effects of co-occurring drivers (Boyd et al., 2018;Britton et al., 2024) such as increasing concentrations of CO 2 or nutrients.As climate change persists, it is vital that experiments focus on a species's response to multiple environmental drivers (Collins et al., 2022).
Environmental changes at global and regional scales, such as ocean warming, can interact with local environmental changes, such as nitrogen enrichment of coastal waters.Inorganic nitrogen is essential for photosynthesis and growth in kelp, with nitrate (NO 3 ¯) and ammonium (NH 4

+
) being the two main sources of dissolved inorganic nitrogen available in seawater (Bristow et al., 2017;Hurd et al., 2014aHurd et al., , 2014b)).Nitrogen plays a crucial role in regulating enzymatic activities and seaweed productivity via direct effects on cellular membrane fluidity, protein production, photosynthesis, and thermal plasticity (Huppe & Turpin, 1994;Turpin et al., 1988).In the marine environment, nitrogen can be enriched through run off from sewage outlets and agriculture as well as land-based aquaculture farming.For the kelps Macrocystis pyrifera and Saccharina latissima, nitrogen enrichment can enhance thermal tolerance, resulting in higher growth and photosynthetic rates at elevated temperatures when more nutrients are available (Fernández et al., 2020;Gerard, 1998).The potential of elevated nitrate to enhance thermal tolerance may be particularly important for species with restricted thermal ranges by providing a mechanism by which these species can persist under elevated ocean temperatures.As such, it is critical to identify not only the upper thermal limits of species with narrow thermal ranges but also how this is influenced by nutrient enrichment.Doing so will allow us to identify species and/or populations that are vulnerable to future ocean warming before they become threatened.
Southeastern Australia, including Tasmania where this study was conducted, is a global ocean warming hot spot (Frusher et al., 2014;Hobday & Pecl, 2014), with temperatures in the region increasing at a rate 3 to 4 times the global average.Currently, ocean temperatures surrounding Tasmania range seasonally between 10 and 19°C (Oliver et al., 2017).The kelp Lessonia corrugata is endemic to Tasmania, with a narrow latitudinal and geographic distribution (approx. 40.6-43.6° S;Nardelli, Visch, Wright, et al., 2023, Hurd et al., 2023).It is one of the few dominant species of kelp that supports diverse communities in shallow rocky reef systems along Tasmania's coasts.The gametophyte life stage has a narrow optimal temperature range for growth and sexual development, between ~16 and 18°C, and the kelp is highly sensitive to temperatures above this optimum (Paine, Schmid, Gaitán-Espitia et al., 2021).However, the thermal tolerance of the adult, sporophyte life stage is unknown, yet this information is critical, since kelp gametophytes often have different thermal ranges than conspecific sporophytes (Bartsch et al., 2013;Veenhof et al., 2022).Since the species is unable to migrate further poleward due to there being no suitable habitat available south of Tasmania, it will be highly susceptible to ocean warming (Martínez et al., 2018).As such, the aims of this study were to (1) to identify the thermal optima of the sporophyte stage of L. corrugata and the upper thermal limit for growth and photosynthesis by constructing TPCs and (2) to examine if its thermal tolerance would be enhanced under inorganic nitrogen enrichment as has been observed for other kelps (e.g., Fernández et al., 2020).We hypothesized that L. corrugata's thermal optima for growth and photosynthesis would be 15-17°C, in line with their gametophyte life stage, and that declines in growth and photosynthetic performance above the optimum temperatures would be partly ameliorated by nitrogen enrichment, therefore enhancing their thermal tolerance under high nutrient conditions.

Seaweed collection and pre-experimental treatment
Experiment 1: Thermal performance curves Forty-nine juvenile Lessonia corrugata individuals (average length: 20 ± 7.16 cm) were collected by divers using SCUBA at 1-5 m depths in May 2022 at Coal Point, Bruny Island, Tasmania (43.3353° S, 147.3247°E).Individuals were transported in seawater in an insulated container to the laboratory, where they were placed in a single 40-L container with UV-sterilized and filtered seawater (0.22 μm pore size) with constant aeration in a 14°C temperature-controlled room (seawater temperature at the site during collections).Controllable LED lights (Zeus-70, Ledzeal, Hong Kong) that simulated the color spectrum at ~5 m water depth were set to a 12:12 light:dark cycle and a simulated daily cycle of increasing then decreasing irradiance (mean daily irradiance: 75 μmol photons • m −2 • s −1 , maximum light level: 130 μmol photons • m −2 • s −1 ).Light levels were set at the saturating irradiance determined by rapid light curves conducted with a Pulse Amplitude Modulation Fluorometer (Junior PAM, Walz, Germany) on a subset of individuals prior to the experiment (see Figure S1 and Table S1 in the Supporting Information; mean ± standard deviation = 85.74 ± 13.02).Light levels were measured at the height of the culture chambers using a LI-COR Light Meter (LI-193 Spherical Quantum Sensor, LI-COR Biosciences, USA) and a flat (2 π) quantum sensor.The seaweeds were maintained in these conditions for 72 h for acclimation prior to beginning the experiment.
Experiment 2: Temperature & nutrient interactions Eighty Lessonia corrugata juveniles (average length: 16.65 ± 1.78 cm) were collected at the same site as experiment 1 in July 2022 using the same protocols as experiment 1.In the laboratory, juveniles were haphazardly allocated to six 15-L containers containing UV-sterilized and filtered seawater (0.22 μm pore size) with constant aeration in a 12°C temperature-controlled room (seawater temperature at the site at time of collection).Light levels were as per experiment 1, and individuals were maintained under these conditions for 72 h for acclimation prior to beginning the experiment.

Experimental treatments
Experiment 1: Thermal performance curves On day 1 of experiment 1, 49 individuals were each secured to a small rock (~40 mm ɸ) by their holdfast using a wide elastic band and then haphazardly transferred to an individual experimental chamber filled with 1.8 L of filtered seawater.Seven experimental chambers (n = 7) were then haphazardly assigned to one of seven temperature treatments: 4, 7, 10, 14, 17, 20, or 22°C.Treatments were applied by placing chambers in randomly assigned temperature-controlled water baths, which were maintained at experimental temperatures using either aquarium heaters (Jager 3612, Eheim, Germany), heated circulators (GRANT Optima T100, England), or chiller units (Ratek RC1 Immersion cooler, Australia), all coupled with temperature controllers (Inkbird ITC-308S, China).Temperatures in each bath were recorded every 30 min using temperature loggers placed in the baths (HOBO Pendant MX temperature logger, Vietnam).The temperature of each water bath was initially set to the acclimatization temperature (14°C) but was subsequently increased/ decreased by 2°C every 24 h to avoid heat shock.This was done until the desired experimental temperature was achieved.Once each bath reached its experimental temperature, the treatment was maintained for 14 days.Throughout this period, ~90% of the seawater in each chamber was exchanged with fresh filtered seawater (0.22 μm) three times each week (Monday, Wednesday, and Friday).Following each water change, 9 μmol of nitrate was added to each chamber on Monday's and Wednesday's, and 13.5 μmol was added on Friday (thus overall ensuring on average of 4.5 μmol per day, close to ambient winter seawater nutrient levels for the region).Chambers were haphazardly rearranged within each bath following water changes to minimize any potential effects of bath position on the kelp.Light regime was the same as in the pre-experimental treatment.
Experiment 2: Temperature & nutrient interactions For experiment 2, 80 individuals were haphazardly assigned to individual experimental chambers filled with 1.8 L of filtered seawater as per experiment 1.Each chamber was randomly assigned to one of eight treatments.Treatments consisted of four temperature levels (16, 18.5, 21, and 23.5°C); temperatures were derived from experiment (1) crossed with two levels of nitrate concentrations: low NO 3 − (4.5 μmol per day) or high NO 3 − (90 μmol per day), with 10 individuals for each combination of temperature and nitrate concentration (n = 10).Temperatures were maintained as per experiment 1, with two water baths for each temperature.As with experiment 1, the temperature of each bath was initially set to the acclimation temperature (here, 12°C) and was subsequently increased/decreased by 2°C every 24 h until the experimental temperature was achieved.Once each bath reached its experimental temperature, the treatment was maintained for 19 days.Throughout this period, ~90% of the seawater in each chamber was exchanged with fresh filtered seawater (0.22 μm) three times each week (Monday, Wednesday, and Friday).Following each water change, 9 μmol of nitrate was added to each chamber for the low treatments and 180 μmol was added to each chamber for the high treatments on Mondays and Wednesdays.On Fridays, 13.5 μmol was added to each chamber for the low nitrate treatments and 270 μmol was added to each chamber for the high nitrate treatments (thus overall ensuring on average of 4.5 μmol of nitrate per day for the low treatments and 90 μmol of nitrate per day for the high treatments).Chambers were haphazardly rearranged within each bath following water changes to minimize any potential effects of bath position on the kelp.Light levels were set on the same 12:12 light:dark cycle as used previously in experiment 1.

Seawater nutrient concentrations
To determine the realized nitrate concentrations of the high and low nitrate treatments in experiment 2, seawater samples for nutrient analysis were taken from each chamber immediately following nutrient additions on day 11 (low nitrate treatment: n = 36; high nitrate treatment: n = 40).Samples were filtered through a 0.7 μm GF/F filter and frozen at −20°C in 12-mL polyethylene nutrient tubes.Ammonium and nitrate concentrations were determined using a QuikChem® 8500 Nutrient Analyzer (LaChat Instruments, Australia).

Biotic responses
The response measurements and protocols were the same for both experiments, except for respiration and the biochemical responses of percent tissue carbon, percent tissue nitrogen, C:N ratios, and soluble tissue nitrate, which were only measured in experiment 2.

Growth (linear extension), erosion (tissue loss), and change in wet weight
Growth as linear extension of the kelp lamina was measured using the holepunch method (Mann & Kirkman, 1981) for which a 5-mm diameter hole was made in the center of the lamina above the meristem.Photos were taken of each sample on day 1 and again at the end of each experiment.Using the software Fiji (Schindelin et al., 2012), the change in distance of the hole from the base of the blade was calculated to quantify growth.The entire length of the blade was also measured to quantify total blade length and erosion (see Table S2 in the Supporting Information for the equations used).Tissue erosion is a well-known function of kelp growth, and at times it can surpass growth rates, leading to negative changes in length despite lamina growth still occurring (Fairhead & Cheshire, 2004;Mann & Kirkman, 1981).
Prior to weighing for biomass measurements, individuals were cleaned of any epiphytes and their surfaces dried using absorbent paper towel.Individuals were weighed on day 1 and again at the end of each experiment.For both growth and change in wet weight, linear regressions showed no significant relationship between either the initial length and growth or initial weight and final weight.There was a significant relationship between initial weight and final weight in experiment 1; however, initial weight was not significantly different within each temperature treatment; therefore, no adjustments were made (see Table S3 in the Supporting Information).

Net photosynthesis and respiration rates
Net photosynthesis (oxygen production) was measured on the final day of experiment 1 and both respiration (oxygen consumption) and net photosynthesis was measured on the final day of experiment 2. Each blade was placed in a separate biochemical oxygen demand bottle, filled with 250 mL of filtered seawater (0.22 μm), and kept in the dark for 15 min to ensure all photosynthetic activity had ceased prior to taking the initial respiration measurements.After 15 min, dissolved oxygen measurements were taken using a portable oxygen meter (Fibox 4, PreSens and dissolved oxygen sensor, oxygen dipping probe, PreSens, Germany).The bottles were then sealed and placed on an orbital shaker table (OM7 Large Orbital Shaker, Ratek Instruments, Australia) set at 60 rpm to provide water motion and left in the dark for 3 h.After the 3-h incubation time, dissolved oxygen measurements were taken again.This was repeated for net photosynthesis under the experimental level irradiance.Measurements were conducted at experimental temperature and treatment nutrient concentrations.For experiment 1, four blanks containing no seaweed per temperature treatment were used, while three chambers containing no seaweed per nutrient and temperature combination were used in experiment 2. Respiration and net photosynthetic rates have been expressed as μmol O 2 • g −1 • h −1 wet weight consumed or produced, respectively.

Biochemical responses
Percent tissue carbon, percent tissue nitrogen, and C:N ratios For all samples in experiment 2, the blade was destructively sampled and immediately frozen at −20°C for determination of tissue carbon, nitrogen, and C:N ratios.All samples were freeze-dried within a week (FreeZone 4.5, Labconco, USA) and kept at −20°C until later analysis.Freeze-dried samples were ground to a fine powder by hand, using a mortar and pestle.Carbon and nitrogen content were determined using an elemental analyzer (FlashSmart, Thermo Scientific, USA).

Soluble tissue nitrate
Soluble tissue nitrate was analyzed using a boiling water extraction method (Paine, Schmid, Revill, & Hurd, 2021).Fresh tissue samples were placed in individual 50-mL boiling tubes containing 20 mL of distilled water and placed in a 100°C water bath for 40 min.Samples were cooled and then 10 mL of the liquid was filtered through a 0.7 μm GF/F filter and frozen at −20°C in 12-mL polyethylene nutrient tubes.This process was repeated three times for each sample to ensure complete extraction.From the liquid samples, nitrate was determined using a QuikChem® 8500 Nutrient Analyzer (LaChat Instruments, Australia).One soluble tissue sample was lost from the low nutrient treatment at 16°C due to equipment malfunction.

Data analysis
Experiment 1: Thermal performance curves All analyses were conducted in the statistical software R v. 4.2.0.(R Core Team, 2022).For each biotic response variable (i.e., growth, change in wet weight, and net photosynthesis), we used nonlinear least squares and the package rTPC (Padfield et al., 2021) or stats (R Core Team, 2022) to fit nine models that are commonly used to assess thermal performance (see Table S4 in the Supporting Information for full list of models used).The most appropriate model was chosen by first excluding all models that had a root mean square error (RMSE) > 2% of the best fitting model; second, those model fits were then visually inspected to ensure they made sense (e.g., they were without unnecessary curvilinearity and overfitting throughout the central portion of the TPC), and if they did not, then they were excluded.Last, if more than one model was deemed appropriate, then the model with the most useful biological parameters was chosen (Adams et al., 2017).Model outputs and RMSE can be seen in Table S4.From the best fit model, estimates of the thermal performance parameters (T opt , R max , and T max ) were extracted using the rTPC package (Padfield et al., 2021), and 95% confidence intervals around model fits and parameter estimates were derived by bootstrapping (case resampling) using the car package (Fox & Weisberg, 2019).
Experiment 2: Temperature & nutrient interactions Two-way ANOVAs were used to assess differences between treatments for all response variables with the fixed factors "Temp" (four levels: 16, 18.5, 21, 23.5) and "Nitrate" (two levels: high and low), and with the Temp × Nitrate interaction.When the factor Temp was significant (p ≤ 0.05), Tukey's honestly significant different (THSD) post hoc tests were used to determine specific differences between each temperature.No post hoc tests were conducted when the main effect of nitrate was significant, as there were only two levels of this factor.When a significant interaction was detected (p ≤ 0.05), pairwise comparisons were made between the nutrient levels for each level of temperature using the package emmeans (Lenth, 2023).Since the aim of experiment 2 was to identify whether nutrient levels had effects at different temperatures, we compared nutrient levels within each level of temperature rather than comparing temperatures within each level of nutrients when a significant interaction was detected.The assumptions of the ANOVA model (homogeneity of variances and normality of residuals) were assessed using residual versus fitted plots and normal Q-Q plots.When model assumptions were not met, the Box-Cox method was used to determine an appropriate transformation (Box & Cox, 1964).Transformations were required for growth (Y 0.6 ), respiration (Y 0.4 ), and erosion (natural log).
Three individuals died from the 23.5°C treatment in the last week of the experiment: two from the high nutrient level group and one from the low nutrient level group.These individuals were excluded from the analysis as they were too badly degraded for biotic and biochemical measurements.

Experiment 1: Thermal performance curves
Growth (linear extension), change in wet weight, net photosynthesis, and erosion (loss of tissue) Growth (linear extension), change in wet weight, and net photosynthesis all followed patterns typical of a thermal performance curves, with rates increasing from low temperatures toward an optimum, followed by a decline at temperatures beyond the optimum (Figure 1a-c).Thermal optima were similar for the three response variables: growth (linear extension) was 16.02°C, change in wet weight was 16.71°C, and net photosynthesis was 14.97°C (Table 1).The maximum growth rate at the thermal optimum was 2.45 mm • d −1 (Table 1).The model predictions for changes in wet weight also projected the thermal maximum (23.05°C); however, because this is beyond the range of our measured data (which stopped at 22°C), this estimate should be treated with caution.Tissue erosion (% of total length), varied between 4 and 17°C (range of means = 0.49-3.26);however, erosion peaked at 20-22°C (range of means = 6.27-7.64, Figure 1d).

Erosion (loss of tissue), growth (linear extension), and change in wet weight
There was no effect of nitrate concentrations on growth (linear extension) in the 18.5, 21, and 23.5°C treatments, while nitrate enrichment significantly increased growth rates at 16°C (25% increase, Figure 2a).This led to a significant interaction between temperature and nutrient level (Table 2).Overall, growth rates declined as temperatures increased beyond the optimum.There was minimal effect of nitrate concentrations on change in wet weight across all temperatures, which led to no significant effect of nitrate treatment or the interaction between temperature and nutrient level in the ANOVA.However, temperature significantly affected change in wet weight (Table 2).Similar values were observed at 16 and 18.5°C (means: 16°C = 0.14 g • d −1 ; 18.5°C = 0.16 g • d −1 ) and significantly declined beyond these temperatures (mean decrease: 16-21°C = 48%; 16-23.5°C= 135%, indicating negative net tissue loss at 23.5°C, Figure 2b).There was a significant effect of temperature on erosion rates, with erosion being significantly lower at 16°C relative to all other temperatures (Figure 3, Table 2).Erosion was also significantly higher at 23.5°C relative to 21°C (Figure 3, Table 2).There was no significant effect of nutrient level or the interaction on erosion rates (Table 2).Despite there being no significant interaction between temperature and nutrient level at 23.5°C, all physiological responses were worsened when nutrients were enriched.

Net photosynthesis and respiration rates
There was no effect of nitrate concentrations on net photosynthesis at 16, 18.5, and 21°C treatments (Figure 4a); however, nitrate enrichment significantly decreased photosynthetic rates at 23.5°C (130% decrease, indicating negative net photosynthetic rates at 23.5°C in the high nutrient treatment).This led to a significant interaction between temperature and nutrient level (Table 2).Nitrate enrichment had no effect on respiration rates across all temperature treatments (Figure 4b).Temperature, however, did significantly affect respiration (Table 2).THSD tests indicated that respiration rates at 21°C were significantly lower than at 16 and 23.5°C but not significantly different from 18.5°C (Figure 4b).There was a consistent trend at 23.5°C, with responses diminishing at greater rates in the high nutrient treatment.

responses
Percent tissue nitrogen and soluble tissue nitrate Percent tissue nitrogen was significantly higher in the high nitrate treatments than the low nitrate treatments across all temperatures (32% higher), and as temperatures increased, percent tissue nitrogen also increased.This led to both main effects being significant (Table 2).The THSD tests indicated that percent tissue nitrogen at 18.5°C was significantly lower than at 23.5°C.There was minimal effect of nitrate concentrations on soluble tissue nitrate at 23.5°C, while nitrate enrichment significantly increased soluble tissue nitrate at 16, 18.5, and 21°C, leading to a significant interaction between  Note: Where parameters were not explicitly included as a parameter in the chosen model, they are denoted as N/A (T max in Gaussian model).

Response
Abbreviations: T opt , thermal optimum; R max , maximum rate at thermal optimum; T max , maximum temperature at which performance is positive.
temperature and level (Table 2).Overall, soluble tissue nitrate decreased as temperatures increased beyond the optimum (see Table S6 in the Supporting Information for soluble tissue nitrate analysis).

D ISCUSSION
We have shown that growth and net photosynthetic rates of juvenile Lessonia corrugata in southeastern Tasmania have a thermal optimum of ~16°C, similar to that of its gametophyte life stage (Paine, Schmid, Gaitán-Espitia et al., 2021), and have a critical thermal maximum of 22-23°C.Currently, ocean temperatures surrounding Tasmania range seasonally between 10 and 19°C (Oliver et al., 2017).We determined nitrate enrichment enhanced the growth rates at the thermal optimum, but this effect was not observed at any higher temperatures, contrary to our hypothesis.Likewise, there were minimal effects of nitrate enrichment on net photosynthesis except at 23.5°C, when we saw a 130% decrease in net photosynthesis when nitrate was enriched; this trend was consistent with all physiological responses deteriorating at 23.5°C in high nutrient conditions.Based on current and forecast ocean conditions in Tasmania, these results suggest that L. corrugata may very soon be pushed beyond its thermal limits due to ocean warming and marine heatwaves.This is particularly concerning, as there is no suitable habitat available south of Tasmania for L. corrugata to retreat poleward, suggesting extinction is a real risk for this endemic species.
Temperatures beyond 16°C caused Lessonia corrugata performance to decline, and the decline was not mitigated under elevated nitrate concentrations.Our findings suggest that L. corrugata will be negatively impacted by the predicted 2-3°C increase in ocean temperatures predicted to occur by the end of the century (IPCC, 2023) regardless of nitrate availability, unless they are able to adapt and adjust their thermal sensitivity, which has been a process identified in Ecklonia radiata gametophytes (Mohring et al., 2014).Juvenile kelp sporophytes, such as we tested here, may also be more resilient to thermal stress than adult sporophytes, potentially due to greater adaptive potential and plasticity (Hoos & Harley, 2021;Mabin et al., 2019;Umanzor et al., 2021).However, distinct differences in stress responses between juvenile and adult conspecifics remain unclear and are likely complex and mediated by a suite of biological and physical factors (Hoos & Harley, 2021;Layton et al., 2020;Schiel & Foster, 2015).
Notably, Lessonia corrugata is already experiencing temperatures close to their critical thermal maximum, as peak sea surface temperatures during summer in eastern and northeastern Tasmania can reach to 20-21°C (Banzon et al., 2016).This raises immediate concern for L. corrugata, as the species is likely to be pushed beyond thermal limits during summer if there is just a 2°C increase in warming or if a 3°C marine heatwave were to occur during summer.The narrow thermal optimum and habitat range for L. corrugata are in strong contrast with those of Ecklonia radiata, another dominant kelp species in F I G U R E 2 Responses of juvenile Lessonia corrugata to different inorganic nitrogen concentrations and temperature exposure after 19 days in culture in experiment 2. Black box and whisker plots refer to treatments where NO 3 − was high, and yellow box and whisker plots refer to treatments where NO 3 − was low.The box shows the interquartile range with the bounds of the box showing the 25th and 75th percentile of the range of observations.The median is displayed by the solid horizontal line within each box.Error bars display the range of values, and the raw data is plotted as points.Response variables and significant differences are described: (a) growth (linear extension, mm • d −1 ); there was a significant interaction between temperature and nutrient level.Pairwise comparisons between nutrient level for each level of temperature showed there was no effect of nutrients at all temperatures except at 16°C (p = 0.003), at which individuals in the high nutrient treatment had significantly higher growth rates.* indicates a significant difference between nutrient treatments within that Ecklonia radiata has a thermal optimum ~16 to 18°C (Britton et al., 2024) and a distribution along 8000 km of coastline, which includes warmtemperature populations in Western Australia that can survive temperatures as high as 28°C (Wernberg et al., 2016).The distributions of many kelp species are shifting southward due to ocean warming (Wernberg et al., 2011), with projections of major declines and poleward shifts in almost all kelp species in Australia by 2100 (Martínez et al., 2018).It is possible that L. corrugata could become extinct or severely limited in distribution by 2100, particularly as it is at its geographic limit in Tasmania, as there is no suitable habitat (i.e., shallow rocky reef) further south.Indeed, largescale declines in L. corrugata populations have already occurred along Tasmania's coasts with >30% declines between 2008 and 2018 (Mellin et al., 2021); however, due to the lack of historical data and monitoring, the full extent of their decline is unknown (Hurd et al., 2023).Certainly, ocean warming in Tasmania has already led to extreme declines of another kelp, Macrocystis pyrifera, which has a similar thermal range to L. corrugata (Butler et al., 2020;Layton & Johnson, 2021;Schiel & Foster, 2015).Biotic interactions such as competition and predation can also be expected to change with ocean warming and climate stress, while also interacting with abiotic pressures to influence species distributions and population stability (Bonaviri et al., 2017;Vergés et al., 2019).Future monitoring is therefore critical for understanding the current state and distribution of L. corrugata and Tasmania's multispecies kelp forests as well as for understanding how their populations will change with increasing temperatures.
We observed that Lessonia corrugata's thermal performance was not enhanced under increased nitrate conditions.Similar responses have been observed in the kelp Phyllospora comosa, also abundant in Tasmania (Flukes et al., 2015).However, the opposite was observed for Macrocystis pyrifera, which showed increased growth and photosynthetic rates at high temperatures when cultured in high nitrate conditions (Fernández et al., 2020), and kelps from the northern hemisphere such as Saccharina latissima Note: Significant effects (p < 0.05) have p-values displayed in bold.When main effects were significant, Tukey's honestly significant different (THSD) tests were used to identify specific differences (see Figures 2b and 4b).For significant interactions, pairwise comparisons were made between nutrient level for each level of temperature using the package emmeans (Lenth, 2023).n.s, no significant differences (p > 0.05).
and Fucus have shown similar responses (Colvard & Helmuth, 1998).Additionally, L. corrugata juveniles took up and stored inorganic nitrogen at greater rates in the high nitrate treatment than in the low nitrate treatment, evidenced by the elevated % tissue N and soluble tissue nitrate content.At the optimum temperature (16°C), increased nitrogen resulted in growth, but this was not apparent at higher temperatures, suggesting that above the optimum temperature, the nitrogen was used for another physiological process such as the production of heat shock proteins and antioxidants (Collén et al., 2007;Hammann et al., 2016;Wahid et al., 2007).Further research on such potential cellular mechanisms will provide a better understanding of kelps adaptive capabilities and responses to ocean warming.
Nitrogen enrichment had varying effects on net photosynthesis.At the upper temperature limit (23.5°C),net photosynthesis was low in the low nitrate conditions, but high nitrate conditions resulted in negative net photosynthetic rates.It is possible that the microbiome associated with the kelps benefited from the excess nutrients, which caused a large increase in bacterial respiration rates and the decrease in net photosynthesis.A healthy and functioning microbiome can have positive effects on kelps by producing micronutrients as well as antibiotics that aid in kelp immune health by preventing the colonization of pathogenic bacteria (Vadillo Gonzalez et al., 2023;  , 2015;& Reddy, 2016).However, recent studies that associations between kelp and their microbial symbionts can be disrupted by increased temperatures (Minich et al., 2018;Qiu et al., 2019), and microbial communities shift when hosts are stressed (Marzinelli et al., 2015).Studies have also suggested that bleaching may be a result of temperature mediated bacterial infection (Campbell et al., 2011;Case et al., 2011).The kelp that experienced thermal stress beyond the optimum in our experiments may have experienced a shift from a healthy microbiome to an increase in pathogenic bacteria.This interaction may have driven the high respiration rates at the upper temperature limit, degrading the kelps and resulting in negative photosynthetic rates.Furthermore, this may be associated with the trend of increased erosion rates in the high nitrate treatment at 23.5°C (although this was not significant at α = 0.05); bacteria may have been benefiting from the excess nutrients, further degrading and eroding the kelp at a faster rate in the high nitrate treatment than in the low nitrate treatment.
Our results show that Lessonia corrugata has a much narrower thermal range compared with those of other kelp species that grow in Tasmania.Temperature was the critical factor regulating growth and photosynthesis beyond the species optimum, with no consistent or interactive effects detected between nitrate levels and high temperatures.This suggests that high nitrate conditions will not enhance thermal tolerance and therefore will not offset the negative effects in L. corrugata of high temperatures.This raises concerns for the future of this unique, endemic species, as they appear to be highly sensitive to relatively small increases in temperature.It is possible that the species will be at an extinction risk if ocean warming continues at the current rate, with subsequent effects on entire ecosystems and the communities the kelp supports, which themselves have been almost completely unstudied (Hurd et al., 2023;Nardelli, Visch, Wright, et al., 2023).Moreover, recent studies have shown 100% mortality rates of farmed L. corrugata during summer (Nardelli, Visch, Farrington, et al., 2023); the narrow thermal range identified in our study further suggests that L. corrugata may not be suitable for aquaculture in a future, warmer ocean, particularly as temperatures are already reaching close to their thermal limits.As such, Australian seaweed aquaculture may be better served by focusing on species with wider temperature tolerances that are able to cope with elevated temperatures under ocean warming (Visch et al., 2023).By identifying the physiological performance of L. corrugata in warming under nitrate enrichment, we have taken a critical first step in predicting the future of the species in a warming ocean.This study highlights the importance of identifying thermal tolerances of species with narrow thermal ranges, which have minimal scope to extend their ranges poleward.

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Thermal response curves (a-c) and rates of erosion (d) of Lessonia corrugata juvenile individuals cultured between 4.15 and 21.73°C in experiment 1. Gray bands refer to 95% confidence intervals around model fits.Black dots refer to mean values for each temperature treatment, and error bars represent standard error.n = 7 for each temperature.(a) growth (linear extension, mm • d −1 ), (b) change in wet weight (g • d −1 ), (c) net photosynthesis (μmol O 2 • g −1 • h −1 ) and (d) erosion (% of total length).TA B L E 1 Extracted model parameters for the fitted models in Experiment 1 for growth (linear extension), net photosynthesis, and change in wet weight.
temperature, as indicated by pairwise comparison tests.(b) change in wet weight (g • d −1 ); THSD post hoc tests represented as letters revealed a significant effect of temperature, such that temperature treatments that share a letter are not significantly different (p < 0.05).n = 10 for each treatment, excluding 23.5: high, n = 8 and 23.5: low, n = 9. [Color figure can be viewed at wileyonlinelibrary.com]

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I G U R E 3 Erosion (% of total length) of juvenile Lessonia corrugata to different inorganic nitrogen concentrations and temperature exposure after 19 days in culture in experiment 2. Black box and whisker plots refer to treatments where NO 3 − was high, and yellow box and whisker plots refer to treatments where NO 3 − was low.The box shows the interquartile range with the bounds of the box showing the 25th and 75th percentile of the range of observations.The median is displayed by the solid horizontal line within each box.Error bars display the range of values, and the raw data are plotted as points.THSD post hoc tests represented as letters revealed a significant effect of temperature such that temperature treatments that share a letter are not significantly different (p < 0.05).n = 10 for each treatment, excluding 23.5: high, n = 8 and 23.5: low, n = 9. [Color figure can be viewed at wileyonlinelibrary.com]F I G U R E 4 Responses of juvenile Lessonia corrugata to different inorganic nitrogen concentrations and temperature exposure after 19 days in culture in experiment 2. Black box and whisker plots refer to treatments where NO 3 − was high, and yellow box and whisker plots refer to treatments where NO 3 − was low.The box shows the interquartile range with the bounds of the box showing the 25th and 75th percentile of the range of observations.The median is displayed by the solid horizontal line within each box.Error bars display the range of values, and the raw data are plotted as points.Response variables and significant differences are described: (a) net photosynthesis (μmol O 2 • g −1 • h −1 ); there was a significant interaction between temperature and nutrient level.Pairwise comparisons between nutrient level for each level of temperature showed there was no effect of nutrients at all temperatures except at 23.5°C (p < 0.001), at which individuals in the low nutrient treatment had significantly higher net photosynthetic rates.* indicates a significant difference between nutrient treatments within that temperature, as indicated by pairwise comparison tests.(b) respiration (μmol O 2 • g −1 • h −1 ); THSD post hoc tests represented as letters revealed a significant effect of temperature, such that temperature treatments that share a letter are not significantly different (p < 0.05).n = 10 for each treatment, excluding 23.5: high, n = 8 and 23.5: low, n = 9. [Color figure can be viewed at wileyonlinelibrary.com] et al.
Analysis of variance (ANOVA) table displaying F-values, degrees of freedom (df) and p-values for all response variables in Experiment 2.