Stress ecology in times of global change – single and combined effects of ocean acidification, temperature and food availability on different life stages of the barnacle Amphibalanus improvisus = Stressökologie in Zeiten des Klimawandels – Einzel- und interaktive Effekte von Ozeanversauerung, Temperatur und Nahrungsverfügbarkeit auf verschiedene Lebensstadien der Seepocke Amphibalanus improvisus

Increasing atmospheric CO2 affects seawater pH and chemistry. This process, commonly known as ocean acidification (OA), has led to a decrease in oceanic seawater pH by 0.1 since the industrial revolution. Oceanic models show that mean pH may fall from the current 8.1 units to 7.8 and 7.5 by 2100 and 2300, corresponding to levels of about 1000 and 2000 µatm pCO2, respectively. Coastal habitats have been described to differ substantially from open ocean conditions. Not only absolute mean values and annual or daily fluctuations but even future predictions differs considerably from open ocean norms. These characteristics evolved over many years and likely formed species or populations, which are more robust to future OA than species or populations from more stable oceanic environments. Calcifying species as well as early life-history stages of marine organisms are considered to be mainly affected by OA. The acorn barnacle Amphibalanus improvisus is a dominant marine calcifier within the western Baltic Sea and has a complex life cycle with various stages such as feeding nauplii and non-feeding cyprids as well as settled calcifying juveniles. Thus, this species is an ideal organism to address a wide range of hypotheses. In this thesis, I investigated the sensitivity of the A. improvisus towards OA stress in combination with additional environmental parameters such as temperature and food availability. The organisms for this study came from the Kiel Fjord, Germany and the Tjärnö Archipelago, Sweden, which, to a certain extent, allows interpretations on the population level. As one of the first cases, this study evaluates the entire life cycle of an invertebrate towards OA in combination with additional stressors. Nauplius larvae of A. improvisus were affected neither by moderate (1500 µatm) OA under summer temperature conditions of 20 °C in the Kiel population or under slightly elevated temperatures (25 °C) in the Tjärnö population. However, in cooler waters (12 °C) severe OA drastically slowed down the larval development of Kiel individuals. Warming generally increased the survival as well as the rate of development in barnacle nauplii but cypris larvae also suffered increased mortality with increasing temperatures. Cyprid size (Tjärnö) and settlement (Kiel) were unaffected by temperature and OA but survival was enhanced under severe OA in the Kiel population. While survival, growth, condition index, reproduction, shell strength as well as development of the F1 generation of juvenile barnacles from Kiel were rather unaffected by OA over 20 weeks, moulting frequency increased with increasing acidification of the seawater. Net-calcification was reduced under increasing acidification with negative impacts on the shell maintenance of adults. This was true for barnacles cohorts collected in summer but when a cohort of juvenile barnacles was collected in autumn and investigated under OA scenarios over the winter, severe OA negatively impacted juvenile growth after 10 weeks. Seawater warming by 4 °C temporary effected the growth of juvenile barnacles. It also decreased the condition index and increased the breaking resistance, i.e. led to an increased investment in shell production compared to body growth. Juvenile barnacles from Tjärnö showed a generally higher sensitivity to OA with reduced growth and survival under moderate and severe OA when combined with food limitation and under severe OA when food availability was increased. Food availability in general, seems to be the major factor driving the performance of juvenile barnacles (increasing growth, condition index, reproduction and shell strength), in both the Kiel and the Tjärnö populations. In the present study, barnacles have been shown to withstand predicted near-future OA well, with if at all, only sub-lethal effects on the different life-history stages. Larval stages of both the Kiel and the Tjärnö populations were tolerant to even the highest OA levels applied (3250 µatm pCO2, pH 7.5). Nevertheless, the sensitivity of juveniles to OA differs remarkably between barnacle populations. Juveniles of A. improvisus from Tjärnö responded more sensitively to OA than juveniles from Kiel. Juveniles of A. improvisus from Tjärnö responded more sensitively to OA than juveniles from Kiel. These population-specific differences might be explained by the high natural variability in pH/pCO2 over the year in the Kiel Fjord and the possibility of barnacles to adapt to these fluctuations over many generations. Assuming that this adaptation is potentially also valid for other barnacle populations, A. improvisus from Tjärnö will likely adapt to anthropogenic OA in the future. In conclusion, barnacles might be pre-selected to withstand strong small-scale as well as large-scale fluctuations in their natural environments. Nevertheless, habitat characteristics and thus population specific differences seem to play a role in determining the resilience of A. improvisus to OA. Even seasonality likely controls the resilience of this species to OA. Since OA is expected to increase more drastically in shallow coastal habitats in the future, the absolute OA tolerance limits of this species are still to be evaluated in more detail. Additionally, the synergistic effects of OA and factors such as warming or desalination need to be considered when future predictions are made. Community structures depend on the responses of various organisms and there will be “losers” and “winners” in the fate of future OA. Thus, although single species such as A. improvisus will be not lethally impacted in a future acidified ocean, community structures are likely to change due to the higher sensitivity of other organisms

A new mesocosm system to study the effects of environmental variability on marine species and communities

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 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

Stressökologie in Zeiten des Klimawandels – Einzel- und interaktive Effekte von Ozeanversauerung, Temperatur und Nahrungsverfügbarkeit auf verschiedene Lebensstadien der Seepocke Amphibalanus improvisus

Increasing atmospheric CO2 affects seawater pH and chemistry. This process, commonly known as ocean acidification (OA), has led to a decrease in oceanic seawater pH by 0.1 since the industrial revolution. Oceanic models show that mean pH may fall from the current 8.1 units to 7.8 and 7.5 by 2100 and 2300, corresponding to levels of about 1000 and 2000 µatm pCO2, respectively. Coastal habitats have been described to differ substantially from open ocean conditions. Not only absolute mean values and annual or daily fluctuations but even future predictions differs considerably from open ocean norms. These characteristics evolved over many years and likely formed species or populations, which are more robust to future OA than species or populations from more stable oceanic environments. Calcifying species as well as early life-history stages of marine organisms are considered to be mainly affected by OA. The acorn barnacle Amphibalanus improvisus is a dominant marine calcifier within the western Baltic Sea and has a complex life cycle with various stages such as feeding nauplii and non-feeding cyprids as well as settled calcifying juveniles. Thus, this species is an ideal organism to address a wide range of hypotheses. In this thesis, I investigated the sensitivity of the A. improvisus towards OA stress in combination with additional environmental parameters such as temperature and food availability. The organisms for this study came from the Kiel Fjord, Germany and the Tjärnö Archipelago, Sweden, which, to a certain extent, allows interpretations on the population level. As one of the first cases, this study evaluates the entire life cycle of an invertebrate towards OA in combination with additional stressors. Nauplius larvae of A. improvisus were affected neither by moderate (1500 µatm) OA under summer temperature conditions of 20 °C in the Kiel population or under slightly elevated temperatures (25 °C) in the Tjärnö population. However, in cooler waters (12 °C) severe OA drastically slowed down the larval development of Kiel individuals. Warming generally increased the survival as well as the rate of development in barnacle nauplii but cypris larvae also suffered increased mortality with increasing temperatures. Cyprid size (Tjärnö) and settlement (Kiel) were unaffected by temperature and OA but survival was enhanced under severe OA in the Kiel population. While survival, growth, condition index, reproduction, shell strength as well as development of the F1 generation of juvenile barnacles from Kiel were rather unaffected by OA over 20 weeks, moulting frequency increased with increasing acidification of the seawater. Net-calcification was reduced under increasing acidification with negative impacts on the shell maintenance of adults. This was true for barnacles cohorts collected in summer but when a cohort of juvenile barnacles was collected in autumn and investigated under OA scenarios over the winter, severe OA negatively impacted juvenile growth after 10 weeks. Seawater warming by 4 °C temporary effected the growth of juvenile barnacles. It also decreased the condition index and increased the breaking resistance, i.e. led to an increased investment in shell production compared to body growth. Juvenile barnacles from Tjärnö showed a generally higher sensitivity to OA with reduced growth and survival under moderate and severe OA when combined with food limitation and under severe OA when food availability was increased. Food availability in general, seems to be the major factor driving the performance of juvenile barnacles (increasing growth, condition index, reproduction and shell strength), in both the Kiel and the Tjärnö populations. In the present study, barnacles have been shown to withstand predicted near-future OA well, with if at all, only sub-lethal effects on the different life-history stages. Larval stages of both the Kiel and the Tjärnö populations were tolerant to even the highest OA levels applied (3250 µatm pCO2, pH 7.5). Nevertheless, the sensitivity of juveniles to OA differs remarkably between barnacle populations. Juveniles of A. improvisus from Tjärnö responded more sensitively to OA than juveniles from Kiel. Juveniles of A. improvisus from Tjärnö responded more sensitively to OA than juveniles from Kiel. These population-specific differences might be explained by the high natural variability in pH/pCO2 over the year in the Kiel Fjord and the possibility of barnacles to adapt to these fluctuations over many generations. Assuming that this adaptation is potentially also valid for other barnacle populations, A. improvisus from Tjärnö will likely adapt to anthropogenic OA in the future. In conclusion, barnacles might be pre-selected to withstand strong small-scale as well as large-scale fluctuations in their natural environments. Nevertheless, habitat characteristics and thus population specific differences seem to play a role in determining the resilience of A. improvisus to OA. Even seasonality likely controls the resilience of this species to OA. Since OA is expected to increase more drastically in shallow coastal habitats in the future, the absolute OA tolerance limits of this species are still to be evaluated in more detail. Additionally, the synergistic effects of OA and factors such as warming or desalination need to be considered when future predictions are made. Community structures depend on the responses of various organisms and there will be “losers” and “winners” in the fate of future OA. Thus, although single species such as A. improvisus will be not lethally impacted in a future acidified ocean, community structures are likely to change due to the higher sensitivity of other organisms.Steigende CO2 Emissionen verändern den pH im Meerwasser sowie auch dessen Karbonatchemie. Dieser Prozess wird im Allgemeinen als Ozeanversauerung (OV) bezeichnet und hat seit der industriellen Revolution zu einer Reduktion des Meerwasser-pHs um 0,1 Einheiten geführt. Zukunftsmodelle für die Ozeane prognostizieren ein weiteres Absinken des pHs von heutigen 8,1 auf Werte um 7,8 im Jahre 2100 und um 7,5 im Jahre 2300. Diese entsprechen einem Anstieg des Meerwasser- pCO2s auf circa 1000 und 2000 µatm. Küstenhabitate unterscheiden sich jedoch drastisch von ozeanischen Bedingungen. Nicht nur absolute Mittelwerte oder Fluktuationen im Jahres- und auch im Tagesgang, sondern auch Prognosen für eine zukünftig zu erwartende Ozeanversauerung weichen hierbei stark von „Ozean“-Modellen ab. Diese Eigenheiten von Küstenhabitaten, welche sich erst über die Jahre herausgebildeten, könnten dazu geführt haben, dass sich lokale Arten oder Populationen mit der Zeit durch Selektion an diese Gegebenheiten angepasst haben. Kalzifizierende Arten sowie frühontogenetische Lebensstadien werden in der Literatur hierbei als sehr empfindlich im Umgang mit einer zukünftigen OV prognostiziert. Die Brackwasser-Seepocke Amphibalanus improvisus ist ein dominanter Kalzifizierer in der westlichen Ostsee und weist einen komplexen Lebenszyklus mit fressenden Nauplius- sowie nicht-fressenden Cyprislarven und einem siedelndem und kalzifizierendem juvenilen Stadium auf. Somit bietet diese Art die Möglichkeit zur Bearbeitung einer Reihe relevanter Fragen zur OV-Forschung. In meiner Arbeit habe ich die Sensibilität der Seepocke A. improvisus auf OV sowie auf OV in Kombination mit zusätzlichen Umweltfaktoren wie Temperaturveränderungen und Futterlimitierung untersucht. Die untersuchten Organismen stammten aus der Kieler Förde sowie aus Gewässern vor Tjärnö in Schweden, wodurch auch Aussagen auf Populationsebene möglich sind. Als eine der ersten Studien, habe ich hierbei die Sensibilität des gesamten Lebenszyklus eines Invertebraten auf OV in Kombination mit zusätzlichen Stressoren untersucht. Naupliuslarven der Art A. improvisus wurden weder von moderater (1500 µatm pCO2) OV beeinträchtigt, wenn sie unter normalen Sommertemperaturen von 20°C (Kiel Population) oder unter leicht erhöhten Temperaturen von 25°C (Tjärnö Population) gehältert wurden. In 12°C kaltem Wasser war die Larvalentwicklung der Nauplien unter OV jedoch stark verlangsamt. Erwärmung förderte das Überleben und beschleunigte die Larvalentwicklung der Nauplien. Cyprislarven hingegen zeigten eine erhöhte Mortalität unter Erwärmung. Die Größe (Tjärnö) sowie der Siedlungserfolg (Kiel) der Cyprislarven waren von der Temperatur sowie der OV unabhängig wobei OV das Überleben der Cyprislarven begünstigte. Das Überleben, Wachstum, der Konditionsindex, Reproduktion und auch die Schalenstabilität von juvenilen Seepocken sowie die Entwicklung derer F1 Generation blieben von der OV über 20 Wochen hinweg unbeeinflusst wobei die Häutungsfrequenz der Tiere unter OV Bedingungen gesteigert war. Die Nettokalzifizierung war mit steigender OV herabgesetzt was sich negativ auf den äußeren Zustand der Schalen adulter Tiere auswirkte. Diese Beobachtungen waren jedoch nur für Sommerkohorten von A. improvisus gegeben. Herbstkohorten von A. improvisus die über den Winter im Experiment groß gezogen wurden wiesen ab der 10. Woche unter starker OV reduzierte Wachstumsraten auf. Erwärmung um 4°C hatte kurzzeitig positive Effekte auf das Wachstum der Seepocken, reduzierte den Konditionsindex und erhöhte die Schalenstabilität und führte somit zu einer gesteigerten Schalenbildung im Vergleich zum Weichkörperwachstum. Juvenile Seepocken aus Tjärnö zeigten im Allgemeinen eine erhöhte Sensibilität gegenüber OV. Das Überleben sowie das Wachstum der Tiere waren bei Futterlimitierung unter moderater sowie starker OV und unter normalen Futterbedingungen nur unter starker OV herabgesetzt. In beiden Populationen war die Futterverfügbargeit jedoch bei Weitem die dominierende Komponente in der Entwicklung der Seepocken. In der vorliegenden Studie wurden Seepocken der Art A. improvisus als sehr resistent gegenüber einer zu erwartenden OV gezeigt. Es konnten in den meisten Fällen, wenn überhaupt, nur subletale Effekte unter moderater OV gezeigt werden. Larven aus Kiel und auch Tjärnö zeigten sich sogar gegenüber extremen pCO2 Behandlungen von 3250 µatm (pH 7,5) resistent, solange die OV nicht in Kombination mit herabgesetzter Temperatur auftrat. Dennoch traten enorme Unterschiede zwischen einzelnen Populationen von A. improvisus auf wobei juvenile Individuen aus Tjärnö viel sensibler auf OV reagierten als Individuen aus Kiel. Diese Populationsspezifischen Unterschiede lassen sich möglicherweise auf Anpassungsprozesse der hiesigen Seepockenpopulation an die starken natürlichen pH/pCO2 Fluktuationen in der Kieler Förde über viele Generationen zurückführen. Unter der Annahme, dass weitere Seepockenpopulationen, wie zum Beispiel die untersuchte Population aus Tjärnö, über ein vergleichbares Potential der Anpassung verfügt wie es für die Kieler Population anzunehmen ist, werden auch weitere Seepockenpopulationen von zukünftiger OV nicht stark beeinflusst sein. Zusammenfassend scheint es, dass Seepocken generell an starke Schwankungen in Ihrem natürlichen Habitat angepasst sind; seien diese kleinskalig oder großskaliger Natur. Dennoch scheinen Habitat-spezifische Komponenten eine wichtige Rolle in der Resistenz von A. improvisus gegenüber OV zu spielen. Es scheint hierbei außerdem auch die saisonale Komponente einen Einfluss auf die Resistenz dieser Art gegenüber OV zu haben. Da die OV für Küstenhabitate in Zukunft jedoch noch gravierender ausfallen wird wie für ozeanische Gewässer, ist es von zentraler Bedeutung die tatsächliche resistenzschwelle von A. improvisus gegenüber OV zu untersuchen. Weitere synergistische Effekte wie Erwärmung und Aussüßung der Meere müssen in Zukunftsprognosen über diese Art ebenfalls mit einbezogen werden. Marine Gemeinschaften sind stark von den Resistenzen einzelner Arten abhängig und es wurde über die letzten Jahre gezeigt, dass es Verlierer und Gewinner geben wird. Auch wenn sich die einzelne Art A. improvisus als relativ tolerant gegenüber der OV zeigte, sind Veränderungen in den marinen Lebensgemeinschaften unter zukünftiger OV dennoch zu erwarten

Consequences of light reduction for anti-herbivore defense and bioactivity against mussels in four seaweed species from northern-central Chile

It is widely assumed that the production of secondary metabolites against grazing and fouling is costly for seaweeds in terms of metabolic energy and should therefore be reduced under conditions of resource limitation. Here we tested the hypothesis that anti-herbivore defenses and bioactivity against mussels in 4 brown seaweeds from northern-central Chile will be reduced when light is limited. In a 2 wk experiment, seaweeds were kept under different low-light conditions (~76 to 99% reduction of ambient sunlight) and grazing situations. Subsequently, we tested their anti-herbivore defense against a common amphipod grazer in feeding assays with living algal tissue and reconstituted food pellets. A standard test employing the production of byssus threads by mussels was furthermore used as an indicator for deterrents in crude algal extracts. All investigated seaweeds showed decreased growth under the stepwise light reduction. Lessonia nigrescens exhibited reduced defense ability under severe low-light conditions when living tissue was offered to the amphipod, probably caused by changes in the tissue structure or in nutritional traits. In Dictyota kunthii, L. trabeculata and Macrocystis integrifolia this effect was absent. None of the investigated seaweeds showed a clear effect of light reduction on chemically mediated defenses against the mesograzer and there was no effect of light limitation on the bioactivity against mussels. Thus, against general assumptions, chemical defense in the investigated seaweeds does not appear to be reduced under severe resource limitation. Results suggest that seaweeds may use different strategies of energy allocation to cope with low-light conditions

Sour times: seawater acidification effects on growth, feeding behaviour and acid–base status of Asterias rubens and Carcinus maenas

The impact of seawater acidification on calcifying organisms varies at the species level. If the impact differs between predator and prey in strength and/or sign, trophic interactions may be altered. In the present study, we investigated the impact of 3 different seawater pCO2 levels (650, 1250 and 3500 µatm) on the acid–base status or the growth of 2 predatory species, the common sea star Asterias rubens and the shore crab Carcinus maenas, and tested whether the quantity or size of prey consumed is affected. We exposed both the predators and their prey, the blue mussel Mytilus edulis, over a time span of 10 wk and subsequently performed feeding experiments. Intermediate acidification levels had no significant effect on growth or consumption in either predator species. The highest acidification level reduced feeding and growth rates in sea stars by 56%, while in crabs a 41% decrease in consumption rates of mussels could be demonstrated over the 10 wk experimental period but not in the subsequent shorter feeding assays. Because only a few crabs moulted in the experiment, acidification effects on crab growth could not be investigated. Active extracellular pH compensation by means of bicarbonate accumulation was observed in C. maenas, whereas the coelomic fluid pH in A. rubens remained uncompensated. Acidification did not provoke a measurable shift in prey size preferred by either predator. Mussels exposed to elevated pCO2 were preferred by previously untreated A. rubens but not by C. maenas. The observed effects on species interactions were weak even at the high acidification levels expected in the future in marginal marine habitats such as the Baltic Sea. Our results indicate that when stress effects are similar (and weak) on interacting species, biotic interactions may remain unaffected

The higher the needs, the lower the tolerance: Extreme events may select ectotherm recruits with lower metabolic demand and heat sensitivity

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

The Role of Recovery Phases in Mitigating the Negative Impacts of Marine Heatwaves on the Sea Star Asterias rubens

During recent years, experimental ecology started to focus on regional to local environmental fluctuations in the context of global climate change. Among these, marine heatwaves can pose significant threats to marine organisms. Yet, experimental studies that include fluctuating thermal stress are rare, and if available often fail to base experimental treatments on available long-term environmental data. We evaluated 22-year high-resolution sea surface temperature data on the occurrence of heatwaves and cold-spells in a temperate coastal marine environment. The absence of a general warming trend in the data may in parts be responsible for a lack of changes in heatwave occurrences (frequency) and their traits (intensity, duration, and rate of change) over time. Yet, the retrieved traits for present-day heatwaves ensured most-natural treatment scenarios, enabling an experimental examination of the impacts of marine heatwaves and phases of recovery on an important temperate predator, the common sea star Asterias rubens. In a 68-days long experiment, we compared a 37- and a 28-days long heatwave with a treatment that consisted of three consecutive 12-days long heatwaves with 4 days of recovery in between. The heatwaves had an intensity of 4.6°C above climatological records, resulting in a maximum temperature of 23.25°C. We demonstrate that heatwaves decrease feeding and activity of A. rubens, with longer heatwaves having a more severe and lasting impact on overall feeding pressure (up to 99.7% decrease in feeding rate) and growth (up to 87% reduction in growth rate). Furthermore, heatwaves of similar overall mean temperature, but interrupted, had a minor impact compared to continuous heatwaves, and the impact diminished with repeated heatwave events. We experimentally demonstrated that mild heatwaves of today’s strength decrease the performance of A. rubens. However, this echinoderm may use naturally occurring short interruptions of thermal stress as recovery to persist in a changing and variable ocean. Thus, our results emphasize the significance of thermal fluctuations and especially, the succession and timing of heat-stress event

Cyclic thermal fluctuations can be burden or relief for an ectotherm depending on fluctuations’ average and amplitude

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

Freshening rather than warming drives trematode transmission from periwinkles to mussels

In the Western Baltic Sea, climate change is happening at much faster rate than in most other seas and organisms are additionally exposed to a steep and variable salinity gradient. Climate change has previously been shown to affect parasite transmission in other marine ecosystems, yet little is known about potential effects of warming and desalination on parasite–host interactions. In laboratory experiments, we determined the combined effects of projected seawater warming and freshening on the emergence, activity, survival, and infectivity of cercariae (free-swimming infectious stage) of the trematode Himasthla elongata (Mehlis 1831), shed from its first intermediate host, the periwinkle Littorina littorea (Linnaeus 1758), in the Baltic Sea. We also assessed the susceptibility of the second intermediate host, the mussel Mytilus edulis Linnaeus, 1758, to cercarial infections. Generally, salinity was the main driver, particularly of cercarial activity, infectivity, and mussel susceptibility to infection. At the lowest salinity (13), cercariae were 50% less active compared to the highest salinity (19). Infection success and host susceptibility followed a similar pattern, with 47% and 43% less metacercariae (encysted stage) present at salinity 13 than at salinity 19, respectively. In contrast, effects of simulated warming were found only for cercarial survival, with cercarial longevity being higher at 19 than at 23 °C. No significant interactions between temperature and salinity were found. In contrast to the literature, the results suggest that a climate change-driven freshening (partly also warming) may lead to a general decline of marine trematodes, with possible beneficial effects for the involved hosts