• Energy Research

Ongoing climate warming demands a better understanding of whether or how the ectotherms that evolved in response to fluctuating stress regimes may acquire increased heat tolerance. Using blue mussels, Mytilus spp., a globally important and well-studied species, we provide empirical evidence supporting that (i) extremely warm (future) summer conditions may select rare recruits that are more capable of expressing metabolic (feeding and respiration) suppression and recovery in response to daily thermal fluctuations in mild to critical temperature range, (ii) this higher heat tolerance can be mediated by lower baseline metabolic demand, possibly decreasing the risks of heat-induced supply and demand mismatch and its associated stress during thermal fluctuations, and (iii) the capacity to acquire such heat tolerance through acclimation is minor. We discuss our results, methodological limitations and offer a perspective for future research. Further evaluation of mechanistic hypotheses such as the one tested here (based on the role of metabolic demand) is needed to generalize the significance of drivers of fast warm adaptation in ectothermic metazoan populations.

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.

AbstractClimate change will shift mean environmental conditions and also increase the frequency and intensity of extreme events, exerting additional stress on ecosystems. While field observations on extremes are emerging, experimental evidence of their biological consequences is rare. Here, we introduce a mesocosm system that was developed to study the effects of environmental variability of multiple drivers (temperature, salinity, pH, light) on single species and communities at various temporal scales (diurnal ‐ seasonal): the Kiel Indoor Benthocosms (KIBs). Both, real‐time offsets from field measurements or various dynamic regimes of environmental scenarios, can be implemented, including sinusoidal curve functions at any chosen amplitude or frequency, stochastic regimes matching in situ dynamics of previous years and modeled extreme events. With temperature as the driver in focus, we highlight the strengths and discuss limitations of the system. In addition, we examined the effects of different sinusoidal temperature fluctuation frequencies on mytilid mussel performance. High‐frequency fluctuations around a warming mean (+2°C warming, ± 2°C fluctuations, wavelength = 1.5 d) increased mussel growth as did a constant warming of 2°C. Fluctuations at a lower frequency (+2 and ± 2°C, wavelength = 4.5 d), however, reduced the mussels’ growth. This shows that environmental fluctuations, and importantly their associated characteristics (such as frequency), can mediate the strength of global change impacts on a key marine species. The here presented mesocosm system can help to overcome a major short‐coming of marine experimental ecology and will provide more robust data for the prediction of shifts in ecosystem structure and services in a changing and fluctuating world.

The experiment consisted of three treatments: 0HW = no heat wave; 1HW = one single summer heat wave; and 3HW = three subsequent heat waves. In all three treatments, working with continuous flow-through of fjord water maintained the natural small-scale variability of most environmental parameters except temperature, which was adjusted to the logged thermal regime of the year 2009.

Size and weight of H. takanoi and A. rubens during the experimental period (72 and 64 days, respectively) measured under 4 temperature conditions: ambient ('constant'), warm ('constant'), ambient sinusoidal and warm sinusoidal. The experiments were conducted from July to September 2017 in the Kiel Indoor Benthocosms (KIBs), at GEOMAR Kiel, Germany. Wet and dry weight (g) were quantified at the start (July 12th) and at the end of the experiment (September 21st (day 72) for A. rubens and on September 14th (day 64) for H. takanoi). For this, individuals were weighed after gently blotting dry with a tissue. At the end of the experimental period, all individuals of both species were frozen (6 hours at -20°C), and dried (48hours, 80°C), and weighted to estimate dry weight.

Temperature regimes (ºC) implemented in the experiment for the four temperature treatments: ambient ('constant'), warm ('constant'), ambient sinusoidal and warm sinusoidal. The treatment regimes were based on 15 years (2000–2014) temperature data measured by GEOMAR weather station (Data source: GEOMAR, Ocean Circulation and Climate Dynamics - Marine Meteorology (http://www.geomar.de/en/service/weather; published in: doi:10.1594/PANGAEA.888599). Data gathered during the summer months June to September (experimental period) were taken into consideration only. The eight warmest years (reaching maximum values above 20°C in summer) were chosen and fitted to a polynomial (4th degree), representing today's average warm summer conditions (Ambient ('constant')). The 'Warm' treatment represented the identical polynomial function but with the offset of +4°C (projected future warming the Baltic Sea). Around these two 'constant' but seasonally fluctuating treatments, sinusoidal temperature fluctuations of 3°C amplitude were modelled ('Ambient sinusoidal' and 'Warm sinusoidal').

Climate change will shift mean environmental conditions and also increase the frequency and intensity of extreme events, exerting additional stress on ecosystems. While field observations on extremes are emerging, experimental evidence of their biological consequences is rare. Here, we introduce a mesocosm system that was developed to study the effects of environmental variability of multiple drivers (temperature, salinity, pH, light) on single species and communities at various temporal scales (diurnal - seasonal): the Kiel Indoor Benthocosms (KIBs). Both, real- time offsets from field measurements or various dynamic regimes of environmental scenarios, can be implemented, including sinusoidal curve functions at any chosen amplitude or frequency, stochastic regimes matching in situ dynamics of previous years and modelled extreme events. With temperature as the driver in focus, we highlight the strengths and discuss limitations of the system. In addition, we examined the effects of different sinusoidal temperature fluctuation frequencies on mytilid mussel performance. High-frequency fluctuations around a warming mean (+2°C warming, ±2°C fluctuations, wavelength=1.5 days) increased mussel growth as did a constant warming of 2°C. Fluctuations at a lower frequency (+2 and ±2°C, wavelength=4.5 days), however, reduced the mussels' growth. This shows that environmental fluctuations, and importantly their associated characteristics (such as frequency), can mediate the strength of global change impacts on a key marine species. The here presented mesocosm system can help to overcome a major short-coming of marine experimental ecology and will provide more robust data for the prediction of shifts in ecosystem structure and services in a changing and fluctuating world. Supplement to: Pansch, Christian; Hiebenthal, Claas (2019): A new mesocosm system to study the effects of environmental variability on marine species and communities. Limnology and Oceanography-Methods, 17(2), 145-162