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These recent events highlight the urgent need to understand how Southern Ocean organisms respond to MHWs. For example, enhanced ice melt due to heatwaves can provide relief from drought stress for terrestrial plant species, but temperature extremes can simultaneously also enhance thermal stress ( Robinson et al., 2020). Indeed, heatwaves recorded across Antarctica in the summer of 2019–2020 are likely to have significant implications, both negative and positive, for the Antarctic ecosystem. However, between 20 nineteen heatwave events were detected across the Southern Ocean ( Montie et al., 2020) and MHWs are predicted to increase in frequency in the Southern Ocean in coming decades ( Frölicher et al., 2018). The ecological impact of marine heatwaves in the Southern Ocean has not previously received as much attention as MHWs in Arctic, temperate or tropical locations. Furthermore, varied responses among species to MHWs can lead to significant food web alterations in marine habitats ( Ryan et al., 2017 Jones et al., 2018 Peña et al., 2019 von Biela et al., 2019 Piatt et al., 2020), illustrating that the responses of individual species will influence the resilience of entire marine ecosystems under global change. Recent work has uncovered a broad range of organismal response to MHWs, from negative to positive ( Stuhr et al., 2017 Pansch et al., 2018 Bartosiewicz et al., 2019 Saha et al., 2019 Britton et al., 2020). Marine heatwaves (MHWs) are one such example of temperature fluctuations, and are defined as “discrete prolonged anomalous warm water events” ( Hobday et al., 2016) that can result in rapid population declines and reduced ecosystem functioning ( Frölicher and Laufkötter, 2018 Oliver et al., 2019 Smale et al., 2019). Our findings shed light on the potential of Southern Ocean diatoms to tolerate MHWs, which will increase both in frequency and in intensity under future climate change.Įxtreme temperature fluctuations in terrestrial and marine systems have occurred with increasing frequency and duration over the past century, and will increase further with continued anthropogenic climate change ( Frölicher et al., 2018 Lyon et al., 2019 Oliver et al., 2019 Rohini et al., 2019). Finally, there is substantial intraspecific variation in post-heatwave growth rates. Thirdly, hotter and longer heatwaves resulted in more pronounced changes to thermal optima (T opt) immediately following heatwaves. Secondly, growth above the thermal optimum before heatwaves exacerbated heatwave-associated negative effects, leading to increased mortality during heatwaves and slower growth after heatwaves. Firstly, hotter and longer heatwaves increased mortality and decreased post-heatwave growth rates relative to milder, shorter heatwaves.
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The effects of simulated heatwaves on mortality and population growth rates varied with genotype, thermal experience and the cumulative intensity of the heatwave itself. We disentangle the contributions of these factors on population mortality and post-heatwave growth rates by experimentally simulating heatwaves (7.5 or 9.2☌, for up to 9 days) for three genotypes of the Southern Ocean diatom Actinocyclus actinochilus.
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A number of factors can explain variation in responses between populations including their genetic variation, previous thermal experience and the cumulative heatwave intensity (☌ d) of the heatwave itself. However, recent studies have highlighted that population tolerance to MHWs is variable, with some populations even benefitting from MHWs.