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Abstract

Marine heatwaves (MHWs) have been documented around the world, causing widespread mortality of numerous benthic species on shallow reefs (less than 15 m depth). Deeper habitats are hypothesized to be a potential refuge from environmental extremes, though we have little understanding of the response of deeper benthic communities to MHWs. Here, we show how increasing depth moderates the response of seaweed- and coral-dominated benthic communities to an extreme MHW across a subtropical–temperate biogeographical transition zone. Benthic community composition and key habitat-building species were characterized across three depths (15, 25 and 40 m) before and several times after the 2011 Western Australian MHW to assess resistance during and recovery after the heatwave. We found high natural variability in benthic community composition along the biogeographic transition zone and across depths with a clear shift in the composition after the MHW in shallow (15 m) sites but a lot less in deeper communities (40 m). Most importantly, key habitat-building seaweeds such as Ecklonia radiata and Syctothalia dorycarpa which had catastrophic losses on shallow reefs, remained and were less affected in deeper communities. Evidently, deep reefs have the potential to act as a refuge during MHWs for the foundation species of shallow reefs in this region.

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During the 1997-1998 El Nino, we examined seasonally a giant kelp population in deep water (25-40 m) off the coast of Northern Baja California. Though most populations in the region completely disappeared, large fertile adults survived the entire warming event at depth. At 25 m, there was no significant change in density or number of fronds per individual during the warming period from spring 1997 to spring 1998, though the surface canopy sloughed off (died) down to 15 m depth. By summer 1998, recruitment occurred at all depths at the site. Adult survival at depth was most likely important in post-disturbance recovery in surrounding populations by occupying substratum, providing vegetative growth, and producing spores. Survival in deep water during this extreme El Nino may have been due to local hydrography, such as internal waves bringing cool nitrate-rich water into the deeper regions of the shelf from below the thermocline, providing a refugium against the warm temperatures, low nutrients, and heavy wave action associated with warming events. Deep-water populations may regularly survive El Nino warming in this region due to internal wave activity, and go undetected due to the depth at which they occur and the sloughing of the shallow (<15 m) biomass.
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Species distributions have shifted in response to global warming in all major ecosystems on the Earth. Despite cogent evidence for these changes, the underlying mechanisms are poorly understood and currently imply gradual shifts. Yet there is an increasing appreciation of the role of discrete events in driving ecological change. We show how a marine heat wave (HW) eliminated a prominent habitat-forming seaweed, Scytothalia dorycarpa, at its warm distribution limit, causing a range contraction of approximately 100 km (approx. 5% of its global distribution). Seawater temperatures during the HW exceeded the seaweed's physiological threshold and caused extirpation of marginal populations, which are unlikely to recover owing to life-history traits and oceanographic processes. Scytothalia dorycarpa is an important canopy-forming seaweed in temperate Australia, and loss of the species at its range edge has caused structural changes at the community level and is likely to have ecosystem-level implications. We show that extreme warming events, which are increasing in magnitude and frequency, can force step-wise changes in species distributions in marine ecosystems. As such, return times of these events have major implications for projections of species distributions and ecosystem structure, which have typically been based on gradual warming trends.
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Background: Coral reefs face increasing pressures particularly when on the edge of their distributions. The Houtman Abrolhos Islands (Abrolhos) are the southernmost coral reef system in the Indian Ocean, and one of the highest latitude reefs in the world. These reefs have a unique mix of tropical and temperate marine fauna and flora and support 184 species of coral, dominated by Acropora species. A significant La Niña event during 2011 produced anomalous conditions of increased temperature along the whole Western Australian coastline, producing the first-recorded widespread bleaching of corals at the Abrolhos. Methodology/ principal findings: We examined long term trends in the marine climate at the Abrolhos using historical sea surface temperature data (HadISST data set) from 1900-2011. In addition in situ water temperature data for the Abrolhos (from data loggers installed in 2008, across four island groups) were used to determine temperature exposure profiles. Coupled with the results of coral cover surveys conducted annually since 2007; we calculated bleaching thresholds for monitoring sites across the four Abrolhos groups. Conclusions/ significance: In situ temperature data revealed maximum daily water temperatures reached 29.54°C in March 2011 which is 4.2°C above mean maximum daily temperatures (2008-2010). The level of bleaching varied across sites with an average of ∼12% of corals bleached. Mortality was high, with a mean ∼50% following the 2011 bleaching event. Prior to 2011, summer temperatures reached a mean (across all monitoring sites) of 25.1°C for 2.5 days. However, in 2011 temperatures reached a mean of 28.1°C for 3.3 days. Longer term trends (1900-2011) showed mean annual sea surface temperatures increase by 0.01°C per annum. Long-term temperature data along with short-term peaks in 2011, outline the potential for corals to be exposed to more frequent bleaching risk with consequences for this high latitude coral reef system at the edge of its distribution.
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Smale, D. A., Kendrick, G. A., Harvey, E. S., Langlois, T. J., Hovey, R. K., Van Niel, K. P., Waddington, K. I., Bellchambers, L. M., Pember, M. B., Babcock, R. C., Vanderklift, M. A., Thomson, D. P., Jakuba, M. V., Pizarro, O., and Williams, S. B. 2012. Regional-scale benthic monitoring for ecosystem-based fisheries management (EBFM) using an autonomous underwater vehicle (AUV). – ICES Journal of Marine Science, 69: 1108–1118. Monitoring marine habitats and biodiversity is critical for understanding ecological processes, conserving natural resources, and achieving ecosystem-based fisheries management (EBFM). Here, we describe the application of autonomous underwater vehicle (AUV) technology to conduct ongoing monitoring of benthic habitats at two key locations in Western Australia. Benthic assemblages on rocky reefs were sampled with an AUV, which captured >200 000 geo-referenced images. Surveys were designed to obtain 100% coverage of 25 × 25 m patches of benthic habitat. In 2010, multiple patches were surveyed at 15–40-m depths at three reference sites at the Houtman Abrolhos Islands and at six reference sites at Rottnest Island. The following year, repeat surveys of the same geo-referenced patches were conducted. Benthic assemblages at the Houtman Abrolhos Islands were varied in that one reference site was dominated by hard corals, whereas the other two were macroalgae dominated. Conversely, assemblages at Rottnest Island were dominated by the kelp Ecklonia radiata. The AUV resurveyed each patch with high precision and demonstrated adequate power to detect change. Repeated observations at the reference sites will track natural variability in benthic habitat structure, which in turn will facilitate the detection of ecological change and ultimately feed back into EBFM processes.
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Extreme climatic events, such as heat waves, are predicted to increase in frequency and magnitude as a consequence of global warming but their ecological effects are poorly understood, particularly in marine ecosystems1, 2, 3. In early 2011, the marine ecosystems along the west coast of Australia—a global hotspot of biodiversity and endemism4, 5—experienced the highest-magnitude warming event on record. Sea temperatures soared to unprecedented levels and warming anomalies of 2–4 °C persisted for more than ten weeks along >2,000 km of coastline. We show that biodiversity patterns of temperate seaweeds, sessile invertebrates and demersal fish were significantly different after the warming event, which led to a reduction in the abundance of habitat-forming seaweeds and a subsequent shift in community structure towards a depauperate state and a tropicalization of fish communities. We conclude that extreme climatic events are key drivers of biodiversity patterns and that the frequency and intensity of such episodes have major implications for predictive models of species distribution and ecosystem structure, which are largely based on gradual warming trends.
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Aim Identifying and protecting refugia is a priority for conservation under projected anthropogenic climate change, because of their demonstrated ability to facilitate the survival of biota under adverse conditions. Refugia are habitats that components of biodiversity retreat to, persist in and can potentially expand from under changing environmental conditions. However, the study and discussion of refugia has often been ad hoc and descriptive in nature. We therefore: (1) provide a habitat-based concept of refugia, and (2) evaluate methods for the identification of refugia.
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Late in summer 2003, extensive mass mortality of at least 25 rocky benthic macro-invertebrate species (mainly gorgonians and sponges) was observed in the entire Northwestern (NW) Mediterranean region, affecting several thousand kilometers of coastline. We were able to characterize the mortality event by studying six areas covering the main regions of the NW Mediterranean basin. The degree of impact on each study area was quantified at 49 sites by estimating the proportion of colonies affected in populations of several gorgonian species compared with reference data obtained in years without mortality signs. According to these data, the western areas (Catalan coast and Balearic Islands) were the least affected, while the central areas (Provence coast and Corsica-Sardinia) showed a moderate impact. The northernmost and eastern areas (Gulf of Genoa and Gulf of Naples) displayed the highest impact, with almost 80% of gorgonian colonies affected. The heat wave of 2003 in Europe caused an anomalous warming of seawater, which reached the highest temperatures ever recorded in the studied regions, between 1 and 3 °C above the climatic values (mean and maximum). Because this exceptional warming was observed in the depth ranges most affected by the mortality, it seems likely that the 2003 anomalous temperature played a key role in the observed mortality event. A correlation analysis between temperature conditions and degree of impact seems to support this hypothesis. Under the present climate warming trend, new mass mortality events may occur in the near future, possibly driving a major biodiversity crisis in the Mediterranean Sea.
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In this article, we review evidence of how climate change has already resulted in clearly discernable changes in marine Arctic ecosystems. After defining the term ‘footprint’ and evaluating the availability of reliable baseline information we review the published literature to synthesize the footprints of climate change impacts in marine Arctic ecosystems reported as of mid-2009. We found a total of 51 reports of documented changes in Arctic marine biota in response to climate change. Among the responses evaluated were range shifts and changes in abundance, growth/condition, behaviour/phenology and community/regime shifts. Most reports concerned marine mammals, particularly polar bears, and fish. The number of well-documented changes in planktonic and benthic systems was surprisingly low. Evident losses of endemic species in the Arctic Ocean, and in ice algae production and associated community remained difficult to evaluate due to the lack of quantitative reports of its abundance and distribution. Very few footprints of climate change were reported in the literature from regions such as the wide Siberian shelf and the central Arctic Ocean due to the limited research effort made in these ecosystems. Despite the alarming nature of warming and its strong potential effects in the Arctic Ocean the research effort evaluating the impacts of climate change in this region is rather limited.
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Given the global degradation of shallow-water coral reef ecosystems resulting from anthropogenic activities, mesophotic coral reef ecosystems (MCEs) are gaining attention because they are generally considered a de facto refuge for shallow-water species. Despite their inferred importance, MCEs remain one of the most understudied reef habitats, and basic information on the taxonomic composition, depth range, habitat preferences, and abundance and distribution of MCE taxa is scarce. The processes that structure these communities are virtually unknown. Here, we provide a review of what is known about MCEs community ecology and outline essential gaps in our knowledge of these deeper water coral reef ecosystems. The primary findings of this review are as follows: (1) many dominant shallow-water species are absent from MCEs; (2) compared to shallow reefs, herbivores are relatively scarce, perhaps due to limited habitat complexity at depth; (3) changes in the dominant photosynthetic taxa with depth suggest adaptation and specialization to depth; (4) evidence regarding the importance of heterotrophy for zooxanthellate corals at depth is conflicting and inconclusive; and (5) decreased light with depth, but not temperature, appears to be the primary factor limiting the depth of MCEs. The majority of research done to date has been performed in the Caribbean, where some generalization can be made about the community structure and distribution of MCEs. The larger and more diverse Indo-Pacific remains largely unexplored with no apparent generalizations from the few sites that have been comparatively well studied. For MCEs, large gaps in knowledge remain on fundamental aspects of ecology. Advanced technologies must be harnessed and logistical challenges overcome to close this knowledge gap and empower resource managers to make informed decisions on conserving shallow-water and mesophotic coral reef ecosystems.
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The increased occurrence of extreme climate events, such as marine heatwaves (MHWs), has resulted in substantial ecological impacts worldwide. To date metrics of thermal stress within marine systems have focussed on coral communities, and less is known about measuring stress relevant to other primary producers, such as seagrasses. An extreme MHW occurred across the Western Australian coastline in the austral summer of 2010/2011, exposing marine communities to summer seawater temperatures 2‐5 °C warmer than average. Using a combination of satellite imagery and in situ assessments, we provide detailed maps of seagrass coverage across the entire Shark Bay World Heritage Area (ca. 13,000 km2) before (2002, 2010) and after the MHW (2014, 2016). Our temporal analysis of these maps documents the single largest loss in dense seagrass extent globally (1,310 km2) following an acute disturbance. Total change in seagrass extent was spatially heterogenous, with the most extensive declines occurring in the Western Gulf, Wooramel Bank and Faure Sill. Spatial variation in seagrass loss was best explained by a model that included an interaction between two heat stress metrics, the most substantial loss occurring when degree heat weeks (DHWm) was ≥ 10 and the number of days exposed to extreme sea surface temperature during the MHW (DaysOver) was ≥ 94. Ground‐truthing at 622 points indicated that change in seagrass cover was predominantly due to loss of Amphibolis antarctica rather than Posidonia australis, the other prominent seagrass at Shark Bay. As seawater temperatures continue to rise and the incidence of MHWs increase globally, this work will provide a basis for identifying areas of meadow degradation, or stability and recovery; and potential areas of resilience.
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The influence of the Leeuwin Current on the marine flora of the Houtman Abrolhos is examined. The marine plants (seaweeds and seagrasses) are assessed by comparisons of the Houtman Abrolhos species diversity and composition relative to a nearby coastal region (Jurien Bay), using the tropical and temperate floras of Western Australia as benchmarks. Our results demonstrate that, in terms of assemblage structure and taxonomic distinctness, the marine flora of the Houtman Abrolhos clearly represent a transitional zone between tropical and temperate regions, with the strong tropical influence a direct result of the Leeuwin Current. In contrast, the nearby inshore flora of the Jurien Bay region exhibits a much lower, almost negligible tropical Influence.
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The accelerating rate of global change has focused attention on the cumulative impacts of novel and extreme environmental changes (i.e., stressors), especially in marine ecosystems. As integrators of local catchment and regional processes, freshwater ecosystems are also ranked highly sensitive to the net effects of multiple stressors, yet there has not been a large-scale quantitative synthesis. We analysed data from 88 papers including 286 responses of freshwater ecosystems to paired stressors, and discovered that overall, their cumulative mean effect size was less than the sum of their single effects (i.e., an antagonistic interaction). Net effects of dual stressors on diversity and functional performance response metrics were additive and antagonistic, respectively. Across individual studies, a simple vote-counting method revealed that the net effects of stressor pairs were frequently more antagonistic (41%) than synergistic (28%), additive (16%) or reversed (15%). Here, we define a reversal as occurring when the net impact of two stressors is in the opposite direction (negative or positive) from that of the sum of their single effects. While warming paired with nutrification resulted in additive net effects, the overall mean net effect of warming combined with a second stressor was antagonistic. Most importantly, the mean net effects across all stressor pairs and response metrics were consistently antagonistic or additive, contrasting the greater prevalence of reported synergies in marine systems. Here, a possible explanation for more antagonistic responses by freshwater biota to stressors is that the inherent greater environmental variability of smaller aquatic ecosystems fosters greater potential for acclimation and co-adaptation to multiple stressors. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.
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It is shown from historical data and from modeling experiments that a proximate cause of the cold winter in North America in 2013-14 was the pattern of sea surface temperature (SST) in the Pacific Ocean. Each of the three dominant modes of SST variability in the Pacific is connected to the Tropics and has a strong expression in extratropical SST and weather patterns. Beginning in the middle of 2013 the third mode of SST variability was two standard deviations positive and has remained so through January of 2015. This pattern is associated with high pressure in the northeast Pacific and low pressure and low surface temperatures over central North America. A large ensemble of model experiments with observed SSTs confirms that SST anomalies contributed to the anomalous winter of 2014.
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Temperature is a major determinant of the performance and geographical ranges of marine species. Changes in temperature can therefore result in localised mortality and shifts in species distributions, but the phenology and temperature sensitivity of many important habitatforming seaweed species has not yet been investigated. Through field observations and culture growth experiments, the present study investigated the temperature sensitivity of reproductive timing, early post-settlement growth and survival, and recruitment success of the widespread foundation seaweed Scytothalia dorycarpa in Australia. In culture, the highest settlement densities and lowest mortality rates were achieved at 15°C, and optimal temperatures for germling fertilisation occurred at 18°C, whereas temperatures greater than 20°C delayed germling settlement and significantly increased mortality rates, with no germlings surviving at temperatures greater than 23°C. Experimental findings were consistent with field observations that found adult reproductive development and gamete release to occur in synchronous pulses throughout the winter months, when seawater temperatures were ~18°C. Surveys of the latitudinal distribution of S. dorycarpa recruits showed significantly lower recruit densities in warmer, low-latitude reefs where summer maximum temperatures often exceed 23°C compared to much higher recruit densities in cooler pole-ward reefs where average summer maximum temperatures are 21 to 22°C. The timing of reproduction, latitudinal distribution of recruits and culture temperature optima all indicate high temperature sensitivity among the early life stages of S. dorycarpa. These findings help to explain the rapid range contraction of this species following a 2011 marine heatwave off the southwest coast of Australia
Article
In austral summer 2011, an unprecedented Ningaloo Nin ̃o event occurred off the west coast of Australia, with sea surface temperature anomalies reaching 5◦C and significant impacts on marine ecosystems. In this study, a high resolution (∼2 km) hydrodynamic model (Regional Ocean Modeling System) is used to simulate the variations in the near-surface temperature in the region from 2009 through mid-2011. Model results indicate that the peak temperatures in the broad mid-west coast of Australia during the event are predominantly due to poleward advection of warmer, tropical water (≈ 2/3 contribution). In addition, positive air-sea heat flux into the ocean also contributes (≈ 1/3 contribution) to the rise in temperature. The anomalous advection of warm water is caused by changes in the poleward flowing Leeuwin Current due to both local and remote wind forcing. In early 2011, the Leeuwin Current intensified owing to remote forcing by the equatorial easterly wind anomalies in the Pacific Ocean associated with the 2010-2011 La Nin ̃a. In addition, the southerly winds off the west coast of Australia weakened, allowing the Leeuwin Current to further intensify in speed at the peak of the event. Concurrently, the inshore, equatorward Capes Current was suppressed and reversed direction. The poleward flow over the shelf contributed to near-shore warming in contrast to cooling by upwelling and equatorward advection from the Capes Current in previous years. http://audioslides.elsevier.com//ViewerLarge.aspx?source=1&doi=10.1016/j.csr.2014.09.014
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Seagrasses have substantial capacity to survive long periods of light reduction, but how acclimation to chronic low light environments may influence their ability to cope with additional stress is poorly understood. This study examines the effect of temporal light reduction by adding two levels of shading to Halophila ovalis plants in two meadows with different light histories, one characterized by a low light (turbid) environment and the other by a relatively high light (clear) environment. Additional shading resulted in complete mortality for both shading treatments at the turbid site while the clear site showed a pattern of decreased shoot density and increased photochemical efficiency (Fv/Fm) with increased shading. These contrasting results for the same species in two different locations indicate that acclimation to chronic low light regimes can affect seagrass resilience and highlights the importance of light history in determining the outcome of exposure to further (short-term) stress.
Article
Record high ocean temperatures were experienced along the Western Australian coast during the austral summer of 2010/2011. Satellite-derived sea surface temperature (SST) anomalies in February 2011 peaked at 3 °C above the long-term monthly means over a wide area from Ningaloo (22°S) to Cape Leeuwin (34°S) along the coast and out to > 200 km offshore. Hourly temperature measurements at a number of mooring sites along the coast revealed that the temperature anomalies were mostly trapped in the surface mixed layer, with peak nearshore temperatures rising to ~ 5 °C above average in the central west coastal region over a week encompassing the end of February and early March, resulting in some devastating fish kills as well as temporary southward range extensions of tropical fish species and megafauna such as whale sharks and manta rays. The elevated temperatures were a result of a combination of a record strength Leeuwin Current, a near-record La Niña event, and anomalously high air–sea heat flux into the ocean even though the SST was high. This heat wave was an unprecedented thermal event in Western Australian waters, superimposed on an underlying long-term temperature rise.
Article
A decrease in salinity and temperature over the past 3000 years has presented the marine algae of the Baltic Sea with very considerable problems in adaptation. The effects of salinity upon a number of Baltic algae have been measured. The results showed cell mortality to be severe in 0, 68 and 102‰, and minimal in 6 and 11‰: there was most variation in tolerance to 34 and 51‰. The salt tolerances of Baltic marine algae have proved more hyposaline than those of British intertidal algae. Water uptake and loss in tissues of Chorda filum and Fucus vesiculosus from Baltic and British populations have been measured in response to salinity changes. The results revealed significant population differences in both live and killed tissues. Receptacle development and oogonial maturation have been observed in Baltic and British F. vesiculosus, and found to differ in seasonality. Some observations were associated with local sea temperatures but differences in the timing of receptacle initiation and in oogonial size were not. Th depauperate thallus, commonly ascribed to the effects of low salinity, was found to be a complicated phenomenon, comprising numerous attributes which are combined differently in different taxa. The morphological differences between Baltic and British marine algae were usually striking.The marine algae of the Baltic Sea have therefore diverged in a number of ways from their N. Atlantic counterparts. The naturally high variability of these taxa has enabled them to survive the period of increasingly strong selection pressure which followed the Littorina Sea episode. Divergence seems not to have advanced to the point where speciation may be said to have occurred. The Baltic may therefore be contrasted with the much older Mediterranean Sea, which contains a large number of endemic species. Nevertheless, the Baltic is a site of very considerable evolutionary importance.
Article
Thermal acclimation and photoacclimation of photosynthesis were compared in Laminaria saccharina sporophytes grown at temperatures of 5 and 17 °C and irradiances of 15 and 150μmol photons m−2 s−1. When measured at a standard temperature (17°C), rates of light-saturated photosynthesis (Pmax) were higher in 5 °C-grown algae (c. 3.0 μmol O2 m−2 s−1) than in 17 °C-grown algae (c. 0.9 μmol O2 m-2 s-1). Concentrations of Rubisco were also 3-fold higher (per unit protein) in 5 °C-grown algae than in algae grown at 17 °C. Light-limited photosynthesis responded similarly to high temperature and low light Photon yields () were higher in algae grown at high temperature (regardless of light), and at 5 °C in low light, than in algae grown at 5 °C in high light Differences in a were correlated with light absorption; both groups of 17 °C algae and 5 °C low-light algae absorbed c. 75% of incident light, whereas 5 °C high-light algae absorbed c. 55%. Increased absorption was correlated with increases in pigment content PSII reaction centre densities and the fucoxanthin-Chl ale protein complex (FCP). Changes in a were also attributed, in part, to changes in the maximum photon yield of photosynthesis (0max). PSI reaction centre densities were unaffected by growth temperature, but the areal concentration of PSI in low-light-grown algae was twice that of high-light-grown algae (c. 160.0 versus 80.0 nmol m−2). We suggest that complex metabolic regulation allows L, saccharina to optimize photosynthesis over the wide range of temperatures and light levels encountered in nature.
Article
Climate change will increase the recurrence of extreme weather events such as drought and heavy rainfall. Evidence suggests that modifications in extreme weather events pose stronger threats to ecosystem functioning than global trends and shifts in average conditions. As ecosystem functioning is connected with ecological services, this has far-reaching effects on societies in the 21st century. Here, we: (i) present the rationale for the increasing frequency and magnitude of extreme weather events in the near future; (ii) discuss recent findings on meteorological extremes and summarize their effects on ecosystems and (iii) identify gaps in current ecological climate change research.