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AbstractResting egg production is considered the most common form of dormancy in aquatic invertebrates. Given that many taxa at least partially terminate resting egg state using environmental cues, knowledge on environmental drivers of hatching success is important, particularly within the context of climate change and environmental degradation. Fairy shrimp (anostracans) are temporary wetland specialists that are reliant on resting egg production for population persistence. Temporary wetlands are common in many arid regions projected to experience increases in temperature, and in areas often compromised by human‐mediated activities. In this study, we assessed the combined effects of light and temperature on the hatching success of Streptocephalus cafer (Anostraca) dormant eggs from temporary wetlands in an arid environment. Both temperature and light altered hatching success, with emergent effects evident. Light caused a significant threefold increase in hatching success overall, while temperature effects were non‐linear, with hatching optimised at 27°C, and especially under light conditions. These results are discussed within the context of shifting climates and disturbances to temporary wetland ecosystems.

AbstractClimate change will not only shift environmental means but will also increase the intensity of extreme events, exerting additional stress on ecosystems. While field observations on the ecological consequences of heat waves are emerging, experimental evidence is rare, and lacking at the community level. Using a novel “near‐natural” outdoor mesocosms approach, this study tested whether marine summer heat waves have detrimental consequences for macrofauna of a temperate coastal community, and whether sequential heat waves provoke an increase or decrease of sensitivity to thermal stress. Three treatments were applied, defined and characterized through a statistical analysis of 15 years of temperature records from the experimental site: (1) no heat wave, (2) two heat waves in June and July followed by a summer heat wave in August and (3) the summer heat wave only. Overall, 50% of the species showed positive, negative or positive/negative responses in either abundance and/or biomass. We highlight four possible ways in which single species responded to either three subsequent heat waves or one summer heat wave: (1) absence of a response (tolerance, 50% of species), (2) negative accumulative effects by three subsequent heat waves (tellinid bivalve), (3) buffering by proceeding heat waves due to acclimation and/or shifts in phenology (spionid polychaete) and (4) an accumulative positive effect by subsequent heat waves (amphipod). The differential responses to single or sequential heat waves at the species level entailed shifts at the community level. Community‐level differences between single and triple heat waves were more pronounced than those between regimes with vs. without heat waves. Detritivory was reduced by the single heat wave while suspension feeding was less common in the triple heat wave regime. Critical extreme events occur already today and will occur more frequently in a changing climate, thus, leading to detrimental impacts on coastal marine systems.

Rising seawater temperature and CO2 concentrations (ocean acidification) represent two of the most influential factors impacting marine ecosystems in the face of global climate change. In ecological climate change research, full-factorial experiments performed across seasons in multispecies, cross-trophic-level settings are essential as they permit a more realistic estimation of direct and indirect effects as well as the relative importance of the effects of both major environmental stressors on ecosystems. In benthic mesocosm experiments, we tested the responses of coastal Baltic Sea Fucus vesiculosus communities to elevated seawater temperature and CO2 concentrations across four seasons of one year. While increasing [CO2] levels had only minor effects, warming had strong and persistent effects on grazers, and the resulting effects on the Fucus community were found to be season dependent. In late summer, a temperature-driven collapse of grazers caused a cascading effect from the consumers to the foundation species, resulting in overgrowth of Fucus thalli by epiphytes. In fall/winter (outside the growing season of epiphytes), intensified grazing under warming resulted in a significant reduction in Fucus biomass. Thus, we were able to confirm the prediction that future increases in water temperatures will influence marine food-web processes by altering top-down control, but we were also able to show that specific consequences for food-web structure depend on the season. Since F. vesiculosus is the dominant habitat-forming brown algal system in the Baltic Sea, its potential decline under global warming implies a loss of key functions and services such as provision of nutrient storage, substrate, food, shelter, and nursery grounds for a diverse community of marine invertebrates and fish in Baltic Sea coastal waters.

AbstractSeveral Arctic marine mammal species are predicted to be negatively impacted by rapid sea ice loss associated with ongoing ocean warming. However, consequences for Arctic whales remain uncertain. To investigate how Arctic whales responded to past climatic fluctuations, we analysed 206 mitochondrial genomes from beluga whales (Delphinapterus leucas) sampled across their circumpolar range, and four nuclear genomes, covering both the Atlantic and the Pacific Arctic region. We found four well‐differentiated mitochondrial lineages, which were established before the onset of the last glacial expansion ~110 thousand years ago. Our findings suggested these lineages diverged in allopatry, reflecting isolation of populations during glacial periods when the Arctic sea‐shelf was covered by multiyear sea ice. Subsequent population expansion and secondary contact between the Atlantic and Pacific Oceans shaped the current geographic distribution of lineages, and may have facilitated mitochondrial introgression. Our demographic reconstructions based on both mitochondrial and nuclear genomes showed markedly lower population sizes during the Last Glacial Maximum (LGM) compared to the preceding Eemian and current Holocene interglacial periods. Habitat modelling similarly revealed less suitable habitat during the LGM (glacial) than at present (interglacial). Together, our findings suggested the association between climate, population size, and available habitat in belugas. Forecasts for year 2100 showed that beluga habitat will decrease and shift northwards as oceans continue to warm, putatively leading to population declines in some beluga populations. Finally, we identified vulnerable populations which, if extirpated as a consequence of ocean warming, will lead to a substantial decline of species‐wide haplotype diversity.

It is unclear whether habitat degradation correlates with tolerance of marine invertebrates to abiotic stress. We therefore tested whether resistance to climate change-related stressors differs between populations of the green mussel Perna viridis from a heavily impacted and a mostly pristine site in West Java, Indonesia. In laboratory experiments, we compared their oxygen consumption and mortality under lowered salinity (-13 and -18 units, both responses), hypoxia (0.5 mg/l, mortality only) and thermal stress (+7 °C, mortality only). Mussels from the eutrophied and polluted Jakarta Bay showed a significantly smaller deviation from their normal oxygen consumption and higher survival rates when stressed than their conspecifics from the unaffected Lada Bay. This shows that human induced habitat degradation correlates with mussel tolerance to environmental stress. We discuss possible mechanisms - e.g. the selection of tolerant genotypes or habitat-specific differences in the nutritional status of the mussels - that could explain our observation.

AbstractAlthough reports have documented loss of species diversity and ecological services caused by stressful temperature changes that result from climate change, some species cope through behavioral compensation. As temperatures and magnitudes of temperature extremes increase, animals should compensate to maintain fitness (such as through temporary behavioral shifts in activity times). Appropriate timing of activity helps avoid competition across species. Although coprophagic dung beetles exhibit species‐specific temporal activity times, it is unknown whether temperature drives evolution of these species‐specific temporal activity times. Using nine dung beetle species (three each of diurnal, crepuscular, and nocturnal species), we explored differences in heat stress tolerance measured as critical thermal maxima (CTmax; the highest temperature allowing activity) and heat knockdown time (HKDT; survival time under acute heat stress) across these species, and examined the results using a phylogenetically informed approach. Our results showed that day‐active species had significantly higher CTmax (diurnal > crepuscular = nocturnal species), whereas crepuscular species had higher HKDT (crepuscular > nocturnal > diurnal species). There was no correlation between heat tolerance and body size across species with distinct temporal activity, and no significant phylogenetic constraint for activity. Species with higher CTmax did not necessarily have higher HKDT, which indicates that species may respond differently to diverse heat tolerance metrics. Acute heat tolerance for diurnal beetles indicates that this trait may constrain activity time and, under high acute temperatures with climate change, species may shift activity times in more benign environments. These results contribute to elucidate the evolution of foraging behavior and management of coprophagic beetle ecosystem services under changing environments.

Environmental perturbation, climate change and international commerce are important drivers for biological invasions. Climate anomalies can further increase levels of habitat disturbance and act synergistically to elevate invasion risk. Herein, we use a historical data set from the upper San Francisco Estuary to provide the first empirical evidence for facilitation of invasions by climate extremes. Invasive zooplankton species did not become established in this estuary until the 1970s when increasing propagule pressure from Asia coincided with extended drought periods. Hydrological management exacerbated the effects of post-1960 droughts and reduced freshwater inflow even further, increasing drought severity and allowing unusually extreme salinity intrusions. Native zooplankton experienced unprecedented conditions of high salinity and intensified benthic grazing, and life history attributes of invasive zooplankton were advantageous enough during droughts to outcompete native species and colonise the system. Extreme climatic events can therefore act synergistically with environmental perturbation to facilitate the establishment of invasive species.

AbstractEcosystem functioning is simultaneously affected by changes in community composition and environmental change such as increasing atmospheric carbon dioxide (CO2) and subsequent ocean acidification. However, it largely remains uncertain how the effects of these factors compare to each other. Addressing this question, we experimentally tested the hypothesis that initial community composition and elevatedCO2are equally important to the regulation of phytoplankton biomass. We full‐factorially exposed three compositionally different marine phytoplankton communities to two differentCO2levels and examined the effects and relative importance (ω2) of the two factors and their interaction on phytoplankton biomass at bloom peak. The results showed that initial community composition had a significantly greater impact than elevatedCO2on phytoplankton biomass, which varied largely among communities. We suggest that the different initial ratios between cyanobacteria, diatoms, and dinoflagellates might be the key for the varying competitive and thus functional outcome among communities. Furthermore, the results showed that depending on initial community composition elevatedCO2selected for larger sized diatoms, which led to increased total phytoplankton biomass. This study highlights the relevance of initial community composition, which strongly drives the functional outcome, when assessing impacts of climate change on ecosystem functioning. In particular, the increase in phytoplankton biomass driven by the gain of larger sized diatoms in response to elevatedCO2potentially has strong implications for nutrient cycling and carbon export in future oceans.