Abstract
Extreme environmental fluctuations such as marine heatwaves (MHWs) can have devastating effects on ecosystem health and functioning through rapid population declines and destabilization of trophic interactions. However, recent studies have highlighted that population tolerance to MHWs is variable, with some populations even benefitting from MHWs. A number of factors can explain variation in responses between populations including their genetic variation, previous thermal experience and the cumulative heatwave intensity (°C d) 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°C, for up to 9 days) for three genotypes of the Southern Ocean diatom Actinocyclus actinochilus. The effects of simulated heatwaves on mortality and population growth rates varied with genotype, thermal experience and the cumulative intensity of the heatwave itself. Firstly, hotter and longer heatwaves increased mortality and decreased post-heatwave growth rates relative to milder, shorter 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. Thirdly, hotter and longer heatwaves resulted in more pronounced changes to thermal optima (Topt) immediately following heatwaves. Finally, there is substantial intraspecific variation in post-heatwave growth rates. 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.
Highlights
Extreme 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)
Three genotypes of Actinocyclus actinochilus were used to assess the potential for intraspecific variation in heatwave responses. These genotypes were used in a complex experimental design in which we examined mortality (%) and maximum growth rates in response to differing thermal regimes, which had three phases: (1) growth below (2.5◦C) or above (5.8◦C) the thermal optimum before heatwaves, (2) exposure to heatwaves (7.5 or 9.2◦C) for up to 9 days and (3) growth after heatwaves at seven temperatures spanning the organisms thermal niche to produce thermal performance curves (TPCs)
We examined whether experiencing high temperatures immediately before heatwaves dampens the negative consequences of heatwave exposure by “heat hardening,” or if previous growth at a higher temperature exacerbates these negative effects which is consistent with deteriorating cellular condition rather than heat hardening
Summary
Extreme 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). Enhanced ice melt due to heatwaves can provide relief from drought stress for terrestrial plant species, but temperature extremes can simultaneously enhance thermal stress (Robinson et al, 2020) These recent events highlight the urgent need to understand how Southern Ocean organisms respond to MHWs. Diatoms dominate phytoplankton blooms in the Southern Ocean, are important primary producers that support the Southern Ocean ecosystem and are major exporters of silica and carbon from surface waters to marine sediments (Deppeler and Davidson, 2017). Constant elevated temperature can affect both population dynamics and the nutritional value of Southern Ocean diatoms (Boyd et al, 2016), indicating that diatom responses to future warming could have significant implications for trophic interactions and biogeochemical cycling These findings are echoed by a large body of literature that concludes that environmental change is affecting marine phytoplankton, and will continue to do so in the future (Collins et al, 2020). These studies provide insight into how diatoms can respond to rapid temperature increases, but do not explore how diatoms behave under more complex, thermally variable environments
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