ALBERT

Description: Predicting the implications of ongoing ocean climate warming demands a better understanding of how short-term thermal variability impacts marine ectotherms, particularly at beyond-optimal average conditions during summer heatwaves. Using a globally important model species, the blue mussel Mytilus, in a 5-week-long experiment, we (a) assessed growth performance traits under 12 scenarios, consisting of four thermal averages (18.5, 21, 23.5 and 26℃) imposed as constant or daily fluctuating regimes with amplitudes of 2 or 4℃. Additionally, we conducted a short-term assay using different mussel individuals to (b) test for the species capacity for suppression and recovery of metabolic performance traits (feeding and aerobic respiration) when exposed to a 1-day thermal fluctuation regime (16.8–30.5℃). Using this high-resolution data, we (c) generated short-term thermal metabolic performance curves to predict and explain growth responses observed in the long-term experiment. We found that daily high-amplitude thermal cycles (4℃) improved mussel growth when fluctuations were imposed around an extreme average temperature of 26℃, representing end-of-century heatwaves. In contrast, thermal cycles negatively affected mussel growth at a less extreme average temperature of 23.5℃, resembling current peak summer temperature scenarios. These results suggest that fluctuations ameliorate heat stress impacts only at critically high average temperatures. The short-term assay demonstrated that during the warming phase, animals stopped feeding between 24 and 30℃ while gradually suppressing respiration. In the subsequent cooling phase, feeding and respiration partially and fully recovered to pre-heating rates respectively. Furthermore, nonlinear averaging of short-term feeding responses (upscaling) well-predicted longer term growth responses to fluctuations. Our findings suggest that fluctuations can be beneficial to or detrimental for the long-term performance of ectothermic animals, depending on the fluctuations' average and amplitude. Furthermore, the observed effects can be linked to fluctuation-mediated metabolic suppression and recovery. In a general framework, we propose various hypothetical scenarios of fluctuation impacts on ectotherm performance considering inter- or intra-species variability in heat sensitivity. Our research highlights the need for studying metabolic performance in relation to cyclic abiotic fluctuations to advance the understanding of climate change impacts on aquatic systems. A free Plain Language Summary can be found within the Supporting Information of this article

Description: The shore crab Hemigrapsus takanoi Asakura and Watanabe, 2005, native to the Northwest Pacific, was recorded in European waters about 25 years ago and it was first found in the Baltic Sea in 2014. Information on population structure of invaders and their new niche is needed in order to understand their biological impact. Over one year, we assessed temporal changes in relative abundance, size-class and sex ratio, as well as breeding season of H. takanoi in the Kiel Fjord (Western Baltic Sea). In addition, prey size preference and consumption rates on mussels (Mytilus edulis Linnaeus, 1758) were experimentally assessed in spring, summer and autumn. A total of 596 individuals were collected with highest and lowest abundances in June and February, respectively. Females were dominant over males (sex ratio 1.4:1), but males grew to larger sizes. H. takanoi reproduced between June and August with ovigerous females representing 30% of the entire female abundance registered over the entire year. Males were able to open larger mussels (due to larger claws) and consumed twice as many mussels when compared to females of similar size. Consumption rates for males were 6 and 2 times higher in summer (seawater temperature of 19 °C) compared to spring (8 °C) and autumn (13 °C), respectively. Females consumed 3 times more mussels in autumn than in spring. H. takanoi is an active predator, capable of reproduction in stressful brackish water conditions. Due to large abundances and high feeding pressure, this recently introduced species could play a key role in structuring post-settlement population dynamics of the dominant habitat builder M. edulis.

Description: in a warming ocean, temperature variability imposes intensified peak stress, but offers periods of stress release. While field observations on organismic responses to heatwaves are emerging, experimental evidence is rare and almost lacking for shorter-scale environmental variability. For two major invertebrate predators, we simulated sinusoidal temperature variability (±3 °C) around todays’ warm summer temperatures and around a future warming scenario (+4 °C) over two months, based on high-resolution 15-year temperature data that allowed implementation of realistic seasonal temperature shifts peaking midpoint. Warming decreased sea stars’ (Asterias rubens) energy uptake (Mytilus edulis consumption) and overall growth. Variability around the warming scenario imposed additional stress onto Asterias leading to an earlier collapse in feeding under sinusoidal fluctuations. High-peak temperatures prevented feeding, which was not compensated during phases of stress release (low-temperature peaks). In contrast, increased temperatures increased feeding on Mytilus but not growth rates of the recent invader Hemigrapsus takanoi, irrespective of the scale at which temperature variability was imposed. This study highlights species-specific impacts of warming and identifies temperature variability at the scale of days to weeks/months as important driver of thermal responses. When species’ thermal limits are exceeded, temperature variability represents an additional source of stress as seen from future warming scenarios.

Description: 1. Predicting the implications of ongoing ocean climate warming demands a better understanding of how short-term thermal variability impacts marine ectotherms, particularly at beyond-optimal average conditions during summer heatwaves. 2. Using a globally important model species, the blue mussel Mytilus, in a 5-week-long experiment, we (a) assessed growth performance traits under 12 scenarios, consisting of four thermal averages (18.5, 21, 23.5 and 26℃) imposed as constant or daily fluctuating regimes with amplitudes of 2 or 4℃. Additionally, we conducted a short-term assay using different mussel individuals to (b) test for the species capacity for suppression and recovery of metabolic performance traits (feeding and aerobic respiration) when exposed to a 1-day thermal fluctuation regime (16.8–30.5℃). Using this high-resolution data, we (c) generated short-term thermal metabolic performance curves to predict and explain growth responses observed in the long-term experiment. 3. We found that daily high-amplitude thermal cycles (4℃) improved mussel growth when fluctuations were imposed around an extreme average temperature of 26℃, representing end-of-century heatwaves. In contrast, thermal cycles negatively affected mussel growth at a less extreme average temperature of 23.5℃, resembling current peak summer temperature scenarios. These results suggest that fluctuations ameliorate heat stress impacts only at critically high average temperatures. The short-term assay demonstrated that during the warming phase, animals stopped feeding between 24 and 30℃ while gradually suppressing respiration. In the subsequent cooling phase, feeding and respiration partially and fully recovered to pre-heating rates respectively. Furthermore, nonlinear averaging of short-term feeding responses (upscaling) well-predicted longer term growth responses to fluctuations. 4. Our findings suggest that fluctuations can be beneficial to or detrimental for the long-term performance of ectothermic animals, depending on the fluctuations' average and amplitude. Furthermore, the observed effects can be linked to fluctuation-mediated metabolic suppression and recovery. In a general framework, we propose various hypothetical scenarios of fluctuation impacts on ectotherm performance considering inter- or intra-species variability in heat sensitivity. Our research highlights the need for studying metabolic performance in relation to cyclic abiotic fluctuations to advance the understanding of climate change impacts on aquatic systems.

Description: Highlights: • Effect of sea freshening on keystone predator impact, Asterias rubens, assessed. • Three ecologically relevant salinity treatments (18, 15, 12ppt). • Zero consumption occurred at the lowest, “future” treatment. • Consumption found to mirror larval recruitment results. • Likely implications for the structuring and functioning of ecological communities. Abstract: Predicting the myriad effects of climate change on ecological communities is a major challenge for scientists, and to date relatively few studies have focused on the effects of sea freshening on species interactions. In particular, changes in keystone species predatory effects could be pervasive. Here, we assess the consequences of decreasing salinity on the ecological impact exerted by a keystone predatory sea star, Asterias rubens. We quantified sea star functional responses (FRs; per capita predation as a function of prey density) under decreasing salinity treatments aligned with climate change projections (18ppt, 15ppt, 12ppt). Furthermore, we combined FRs with larval recruitment estimates, i.e. ecological “Impact Potential”, to act as an ecological indicator of predator population-level responses under this environmental change. Attack and maximum feeding rates of sea stars were reduced by decreasing salinities, with no instances of predation found at 12ppt. Given that decreasing salinities also reduced larval sea star recruitment, the overall Impact Potential of this keystone predator species was lessened by decreased salinity. Sea freshening projections by the end of this century could thus drive significant decreases in the effects of this keystone predator, with serious implications for the structuring and functioning of ecological communities.

Description: In a warming ocean, temperature variability imposes intensified peak stress, but offers periods of stress release. While field observations on organismic responses to heatwaves are emerging, experimental evidence is rare and almost lacking for shorter-scale environmental variability. for two major invertebrate predators, we simulated sinusoidal temperature variability (±3 °C) around todays' warm summer temperatures and around a future warming scenario (+4 °C) over two months, based on high- resolution 15-year temperature data that allowed implementation of realistic seasonal temperature shifts peaking midpoint. Warming decreased sea stars' (Asterias rubens) energy uptake (Mytilus edulis consumption) and overall growth. Variability around the warming scenario imposed additional stress onto Asterias leading to an earlier collapse in feeding under sinusoidal fluctuations. High-peak temperatures prevented feeding, which was not compensated during phases of stress release (low-temperature peaks). In contrast, increased temperatures increased feeding on Mytilus but not growth rates of the recent invader Hemigrapsus takanoi, irrespective of the scale at which temperature variability was imposed. This study highlights species-specific impacts of warming and identifies temperature variability at the scale of days to weeks/months as important driver of thermal responses. When species' thermal limits are exceeded, temperature variability represents an additional source of stress as seen from future warming scenarios.

Description: Energy uptake 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. The energy uptake was evaluated at different scales summing the energy uptake for the particular scale of interest: (i) large-scale as the overall sum of the energy uptake, (ii) small-scale (Phase) of 8-day wavelength, for 2-day feeding periods were energy was summed at minimum (Min), maximum (Max) and mean temperatures after minimum (Descending) or maximum (Ascending) during each sinusoidal temperature cycle, and (iii) medium-scale (Period) of 16 days, where energy uptake was summed for 'Pre-heat' (July 19th to August 4th), 'Heat' (August 4th to August 20th) and 'Post-heat' (August 20th to September 5th). To estimate energy uptake, A. rubens and H. takanoi were fed M. edulis mussels every second day. Each individual of A. rubens received five mussels of 25 to 35 mm size (total length). For H. takanoi, 20 mussels of 9 to 12mm were offered. For A. rubens, the shell of every consumed mussel was measured. In the case of H. takanoi, the average size (10.5 mm) of the mussels offered was taken for further estimated on energy uptake. A predictive relationship between soft tissue dry weight of mussels (g) and mussel length (mm) was used to calculate dry mass and energy uptake according to Brey et al., as 18.85 Joules per mg dry mass of mussels.

Description: 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.

Description: 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').