Rühmkorff S et al. (2023), Marine heatwaves and upwelling shape stress res...

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Proc Biol Sci 2023 Jan 25;2901991:20222262. doi: 10.1098/rspb.2022.2262.

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Marine heatwaves and upwelling shape stress responses in a keystone predator.

Rühmkorff S , Wolf F , Vajedsamiei J , Barboza FR , Hiebenthal C , Pansch C .

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Climate change increases the frequency and intensifies the magnitude and duration of extreme events in the sea, particularly so in coastal habitats. However, the interplay of multiple extremes and the consequences for species and ecosystems remain unknown. We experimentally tested the impacts of summer heatwaves of differing intensities and durations, and a subsequent upwelling event on a temperate keystone predator, the starfish Asterias rubens. We recorded mussel consumption throughout the experiment and assessed activity and growth at strategically chosen time points. The upwelling event overall impaired starfish feeding and activity, likely driven by the acidification and low oxygen concentrations in the upwelled seawater. Prior exposure to a present-day heatwave (+5°C above climatology) alleviated upwelling-induced stress, indicating cross-stress tolerance. Heatwaves of present-day intensity decreased starfish feeding and growth. While the imposed heatwaves of limited duration (9 days) caused slight impacts but allowed for recovery, the prolonged (13 days) heatwave impaired overall growth. Projected future heatwaves (+8°C above climatology) caused 100% mortality of starfish. Our findings indicate a positive ecological memory imposed by successive stress events. Yet, starfish populations may still suffer extensive mortality during intensified end-of-century heatwave conditions.

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	Figure 1. . Schematic representation of the treatments experienced by individuals of Asterias rubens throughout the duration of the experiment. No heatwave: followed a smoothed natural mean seasonal temperature profile (blue line in (a); see methods for further information). Present-day: experienced a short heatwave with the intensity and duration of present-day events (9 days above the seasonal profile depicted in (a) with a maximum +5°C, green polygon in (b)). Extended: a heatwave of extended duration in comparison to Present-day (13 days above seasonal profile with a maximum +5°C, yellow polygon in (c)). Amplified: a heatwave of increased intensity in comparison to Present-day (9 days above seasonal profile with a maximum +8°C, pink polygon in (d)). Future: a heatwave with the combined characteristics of those described in (c) and (d) (13 days above seasonal profile with a maximum +8°C, red polygon in (e)). All treatments received an upwelling event (blue polygon) towards the end of the experiment, which was characterized by a drop in temperature (−4.4°C), oxygen concentration (−6.3 mg l−1) and pHNBS (−0.5 units) as well as an increase in salinity (+2.2 units; details in electronic supplementary material, figure S1). Black dots represent measuring events of wet weight, while grey triangles represent assessments of righting responses of A. rubens.
	Figure 2. . Feeding rate (milligrams of mussel dry weight per day, (a)), wet weight (g, (b)) and righting time (minutes, (c)) of Asterias rubens throughout 63 days of our experiment, under No (blue), Present-day (green) and Extended (yellow) heatwave treatments (see figure 1 for treatment descriptions). All treatments received an upwelling event towards the end of the experiment. The red dashed lines represent the periods of heatwaves (Present-day and Extended) and the blue dashed lines the period of upwelling. Data are represented as means (dots) of n = 12 experimental units. Trends in (a) and (b) were modelled using GAMM (explained deviance = 37.6% and 40.5%, respectively). Solid lines show the mean fitted trends and the shaded areas the associated 95% confidence intervals (a,b). Whiskers in (c) represent 95% confidence intervals. Differences between No and Present-day, between No and Extended and between Present-day and Extended are represented by solid, dashed and dotted black lines placed at the bottom of the plots, respectively (see electronic supplementary material S3: figure S8 for further details). See also electronic supplementary material S3: figures S9, S10, and S11 for related bar plots and 95% confidence intervals. Detailed statistical outcomes are presented in the electronic supplementary material S3: tables S2–S4. All starfish died after 24 days in the Amplified treatments and were therefore excluded from the plots.
	Figure 3. . Mean feeding rate (milligrams of mussel dry weight per day, (a)) and wet weight (g, (b)) of Asterias rubens during 63 days of incubation, under No (blue), Present-day (green) and Extended (yellow) heatwave treatments (see figure 1 for treatment descriptions). All treatments received an upwelling event towards the end of the experiment. Data are presented as means and 95% confidence intervals (n = 12). Lower case letters represent significant differences between treatments based on LMM (see electronic supplementary material S3: tables S2 and S3). All starfish died after 24 days in the Amplified treatments and were therefore excluded from the plots.

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