Ocean Warming Hotspot Research Articles

Tissue-Specific Tolerance to High-Temperature and Nutrient-Poor Conditions in a Canopy-Forming Macroalga, Surviving at an Ocean Warming Hotspot.

Most canopy-forming macroalgae have disappeared from temperate reefs in southern Japan, one of the ocean warming hotspots, but Sargassum nipponicum is surviving in this region. As this species' annual shoots emerge from holdfasts during summer, both plant components may be highly tolerant to warm and nutrient-poor conditions in this season. The present study examined the effects of temperature and nutrient conditions on holdfast growth, shoot emergence from holdfasts, and shoot growth in S. nipponicum samples collected in Tanegashima Island, southern Japan. The summer temperature in this region (30 °C) allowed holdfast growth and shoot emergence but inhibited shoot growth. Nutrient-poor conditions had limited effects on the first two parameters but suppressed shoot growth. These results suggested that during warm summers and under nutrient-poor conditions in southern Japan, shoots can emerge from S. nipponicum holdfasts but cannot further grow. Additionally, nutrient loading from a nearby river was higher at the only site dominated by S. nipponicum, than at the other sites where this species was absent on Tanegashima Island. This was observed especially between autumn and winter, implying that such a nutrient-rich environment may contribute to shoot growth in S. nipponicum and to the persistence of its population in the area.

Publisher's note to: Multispecies population-scale emergence of climate change signals in an ocean warming hotspot

Publisher's note to: <i>Multispecies population-scale emergence of climate change signals in an ocean warming hotspot</i>

Trends of Satellite-Derived Thermal Fronts in the Southeast and Southwest of Australia Between 1993 and 2019

Oceanic fronts play a significant role in marine ecosystems by enhancing vertical exchange, promoting the aggregation of plankton, and drawdown of organic carbon. Anthropogenic emission of carbon dioxide and other greenhouse gases in the twentieth century has driven global warming, leading to rising ocean temperatures. Specific regions warming faster than the global average—known as ‘ocean warming hotspots’—have been identified, impacting geophysical and biogeochemical dynamics of local ecosystems. Here, we aim to characterize the variability of sea surface temperature (SST) fronts in the southeast and southwest Australia hotspots. Using a histogram frontal detection method, we derived fronts from AVHRR-only and Multi-sensor 6-day SST composites on a 0.02 × 0.02 grid between January 1993 and December 2019. Our results indicate that frontal frequency and frontal density have increased in both regions in the past three decades, by around 0.2–0.3%. In addition, both regions exhibit higher frequency and density of fronts in austral winter and fewer in austral summer. Our calculations show that changes in frontal frequency/density show some relationship to El Niño-Southern Oscillation and Indian Ocean Dipole. Changes in frontal activity could strongly impact local ocean biogeochemistry and marine ecosystems. A better understanding changing fronts in hotspots will help predict and manage future changes in regional oceans to warming.

The endemic kelp Lessonia corrugata is being pushed above its thermal limits in an ocean warming hotspot.

Kelps are in global decline due to climate change, which includes ocean warming. To identify vulnerable species, we need to identify their tolerances to increasing temperatures and determine whether tolerances are altered by co-occurring drivers such as inorganic nutrient levels. This is particularly important for those species with restricted distributions, which may already be experiencing thermal stress. To identify thermal tolerance of the range-restricted kelp Lessonia corrugata, we conducted a laboratory experiment on juvenile sporophytes to measure performance (growth, photosynthesis) across its thermal range (4-22°C). We determined the upper thermal limit for growth and photosynthesis to be ~22-23°C, with a thermal optimum of ~16°C. To determine if elevated inorganic nitrogen availability could enhance thermal tolerance, we compared the performance of juveniles under low (4.5 μmol · d-1) and high (90 μmol · d-1) nitrate conditions at and above the thermal optimum (16-23.5°C). Nitrate enrichment did not enhance thermal performance at temperatures above the optimum but did lead to elevated growth rates at the thermal optimum. Our results indicate L. corrugata is likely to be extremely susceptible to moderate ocean warming and marine heatwaves. Peak sea surface temperatures during summer in eastern and northeastern Tasmania can reach up to 20-21°C, and climate projections suggest that L. corrugata's thermal limit will be regularly exceeded by 2050 as southeastern Australia is a global ocean-warming hotspot. By identifying the upper thermal limit of L. corrugata, we have taken a critical step in predicting the future of the species in a warming climate.

Multispecies population-scale emergence of climate change signals in an ocean warming hotspot

Abstract Ocean waters of the Northeast US continental shelf have warmed rapidly in recent years, with sea surface temperatures rising 2.5 times faster than those of the global oceans. With this strong warming trend, the frequency and duration of marine heatwaves have increased. These temperature changes stood out as a distinct warm temperature regime during the 2010s. During this decade, fish population characteristics also differed from the past. Species distribution shifts were detected for many species, demonstrating one way species could adapt to warming conditions. However, for most species, distribution shifts were insufficient to avoid warmer surface or bottom temperatures. As species occupied warmer habitats, growth patterns aligned with expectations for warming temperatures. Consistent with the temperature-size rule, some species exhibited faster growth at early life stages but plateaued at smaller body sizes; other species, however, experienced reduced growth across all ages, indicating thermal stress. Finally, population productivity indexed by the recruit-to-spawner ratio declined significantly during the 2010s for some populations. Changes in these three processes—distribution, growth, and productivity—indicate the emergence of climate change signals across multiple Northeast US fish populations. These effects create new challenges for fishery managers and industry participants operating in the context of non-stationarity and uncertainty.

Climate-driven invasion and incipient warnings of kelp ecosystem collapse

Climate change is progressively redistributing species towards the Earth’s poles, indicating widespread potential for ecosystem collapse. Detecting early-warning-signals and enacting adaptation measures is therefore a key imperative for humanity. However, detecting early-warning signals has remained elusive and has focused on exceptionally high-frequency and/ or long-term time-series, which are generally unattainable for most ecosystems that are under-sampled and already impacted by warming. Here, we show that a catastrophic phase-shift in kelp ecosystems, caused by range-extension of an overgrazing sea urchin, also propagates poleward. Critically, we show that incipient spatial-pattern-formations of kelp overgrazing are detectable well-in-advance of collapse along temperate reefs in the ocean warming hotspot of south-eastern Australia. Demonstrating poleward progression of collapse over 15 years, these early-warning ‘incipient barrens’ are now widespread along 500 km of coast with projections indicating that half of all kelp beds within this range-extension region will collapse by ~2030. Overgrazing was positively associated with deep boulder-reefs, yet negatively associated with predatory lobsters and subordinate abalone competitors, which have both been intensively fished. Climate-driven collapse of ecosystems is occurring; however, by looking equatorward, space-for-time substitutions can enable practical detection of early-warning spatial-pattern-formations, allowing local climate adaptation measures to be enacted in advance.

Global trends of fronts and chlorophyll in a warming ocean

Ocean fronts affect phytoplankton and higher trophic levels, including commercially important fisheries. As the oceans warm, uncertainty remains around the trends in fronts. Here we examine changes in sea surface temperature fronts (frequency, density, and intensity) and the concentration of chlorophyll, over recent satellite records (2003 – 2020) in ocean warming hotspots - areas that are warming faster than other parts of the ocean. Commonalities exist across hotspots with comparable dynamics. Most equatorial and subtropical gyre hotspots experienced a decline in frontal activity (frequency, density, strength) and chlorophyll concentration, while in high-latitude hotspots, frontal activity and chlorophyll concentration mostly increased. Continued warming may accentuate the impacts, changing both total biomass and the distribution of marine species. Areas with changing fronts and phytoplankton also correspond to areas of important global fish catch, highlighting the potential societal significance of these changes in the context of climate change.

Nutritional quality of eastern school whiting (Sillago flindersi) under contemporary and future environmental conditions

AbstractClimate‐driven environmental change is increasingly impacting global fisheries and marine resource use. Fisheries provide a broad range of economic, social and cultural benefits while delivering essential contributions to nutrient security and human health. Despite this, little is known about how climate change will impact the availability and quality of seafood‐derived nutrients. Here, we quantified spatial and temporal changes in the nutritional quality of the commercially harvested eastern school whiting, Sillago flindersi, sampled throughout the south‐east Australian ocean warming hotspot. Several nutrients measured in S. flindersi, including protein, ash, polyunsaturated fatty acids (PUFA) and the omega‐3 PUFA–docosahexaenoic acid (DHA, 22:6ɷ3), were related to one or more environmental factors (sea bottom temperature, depth and chlorophyll). We also detected seasonal variability in DHA and ash composition throughout the species' commercially harvested distribution. Historical and future spatial modelling predicted a decrease in DHA of up to 6% with increasing ocean temperature under future Representative Concentration Pathway (RCP) 4.5 and 8.5 emission scenarios. Overall, our results identified S. flindersi as a rich source of protein and essential PUFAs for human consumers and supported emerging evidence that reductions in seafood‐derived essential nutrients may occur under future ocean warming, specifically reductions in omega‐3 fatty acids. The development of nutritional quality forecasting tools for seafood holds the potential to inform fishers and managers of locations and times of the year to target species with optimal nutritional quality.

Ocean warming favours a northern Argyrosomus species over its southern congener, whereas preliminary metabolic evidence suggests that hybridization may promote their adaptation.

Anthropogenic-induced climate change is having profound impacts on aquatic ecosystems, and the resilience of fish populations will be determined by their response to these impacts. The northern Namibian coast is an ocean warming hotspot, with temperatures rising faster than the global average. The rapid warming in Namibia has had considerable impacts on marine fauna, such as the southern extension of the distribution of Argyrosomus coronus from southern Angola into northern Namibian waters, where it now overlaps and hybridizes with the closely related Namibian species, A. inodorus. Understanding how these species (and their hybrids) perform at current and future temperatures is vital to optimize adaptive management for Argyrosomus species. Intermittent flow-through respirometry was used to quantify standard and maximum metabolic rates for Argyrosomus individuals across a range of temperatures. The modelled aerobic scope (AS) of A. inodorus was notably higher at cooler temperatures (12, 15, 18 and 21°C) compared with that of A. coronus, whereas the AS was similar at 24°C. Although only five hybrids were detected and three modelled, their AS was in the upper bounds of the models at 15, 18 and 24°C. These findings suggest that the warming conditions in northern Namibia may increasingly favour A. coronus and promote the poleward movement of the leading edge of their southern distribution. In contrast, the poor aerobic performance of both species at cold temperatures (12°C) suggests that the cold water associated with the permanent Lüderitz Upwelling Cell in the south may constrain both species to central Namibia. This is most concerning for A. inodorus because it may be subjected to a considerable coastal squeeze.

Warming signals in temperate reef communities following more than a decade of ecological stability.

Ecosystem structure and function are increasingly threatened by changing climate, with profound effects observed globally in recent decades. Based on standardized visual censuses of reef biodiversity, we describe 27 years of community-level change for fishes, mobile macroinvertebrates and macroalgae in the Tasmanian ocean-warming hotspot. Significant ecological change was observed across 94 reef sites (5-10 m depth range) spanning four coastal regions between three periods (1992-95, 2006-07, 2017-19), which occurred against a background of pronounced sea temperature rise (+0.80°C on average). Overall, fish biomass increased, macroinvertebrate species richness and abundance decreased and macroalgal cover decreased, particularly during the most recent decade. While reef communities were relatively stable and warming was slight between the 1990s and mid-2000s (+0.12°C mean temperature rise), increased abundances of warm affinity fishes and invertebrates accompanied warming during the most recent decade (+0.68°C rise). However, significant rises in the community temperature index (CTI) were only found for fishes, invertebrates and macroalgae in some regions. Coastal warming was associated with increased fish biomass of non-targeted species in fished zones but had little effect on reef communities within marine reserves. Higher abundances of larger fishes and lobsters inside reserves appeared to negate impacts of 'thermophilization'.

Rapid restructuring of the odontocete community in an ocean warming hotspot.

Cetaceans are important consumers in marine ecosystems, but few studies have quantified their climate responses. The rapid, directional warming occurring in the Northeast United States (NEUS) provides a unique opportunity to assess climate impacts on cetaceans. We used stranding data to examine changes to the distribution and relative abundance of odontocetes from 1996 to 2020 in both the NEUS and the Southeast United States (SEUS), which is not warming. We conducted simulations to determine the number of stranding events needed to detect a distributional shift for each species given the speed of the shift and the spatial variability in strandings. We compared observed shifts to climate velocity. Smaller sample sizes were needed to detect more rapid poleward shifts, particularly for species with low spatial variability. Poleward shifts were observed in all species with sufficient sample sizes, and shifts were faster than predicted by climate velocity. For species whose trailing edge of distribution occurred in the NEUS, the center of distribution approached the northern limit of the NEUS and relative abundance declined through time, suggesting shifts north out of US waters. The relative abundance of warm water species in the stranding record increased significantly in the NEUS while that of cool water species declined significantly as their distributions shifted north out of the NEUS. Changes in the odontocete community were less apparent in the SEUS, highlighting the importance of regional warming. Observed poleward shifts and changes in species composition suggest a reorganization of the odontocete community in the NEUS in response to rapid warming. We suggest that strandings provide a key dataset for understanding climate impacts on cetaceans given limitations of survey effort and modeling approaches for predicting distributions under rapidly changing conditions. Our findings portend marked changes to the distribution of highly mobile consumer species across international boundaries under continued warming.

Last snail standing? superior thermal resilience of an alien tropical intertidal gastropod over natives in an ocean-warming hotspot

Last snail standing? superior thermal resilience of an alien tropical intertidal gastropod over natives in an ocean-warming hotspot

Large-scale connectivity of the sandy beach clam Mesodesma mactroides along the Atlantic coast of South America, and climate change implications

The yellow clam Mesodesma mactroides is a cool-water species that typifies sandy beaches of the Southwestern Atlantic Ocean (SAO), which embraces one of the strongest ocean warming hotspots. The region is influenced by the Rio de la Plata (RdlP), which represents a zoogeographic barrier that restricts its larval exchange. We investigated yellow clam larval connectivity patterns using an individual based model (IBM). The IBM combined outputs from a 3D hydrodynamic model with a clam submodel that considered salinity- and temperature-dependent mortality for the planktonic larvae. Connectivity across the RdlP estuary occurred only for larvae released in spring during a strong La Niña event. Mortality due to freshwater precluded larval transport across the RdlP, whereas larval mortality induced by warmer waters reduced connectivity, leading to self-recruitment in most areas. Warming acceleration in this hotspot could further restrict larval connectivity between populations in the SAO, with conservation implications for this threatened species.

Resident lobsters dominate food competition with range-shifting lobsters in an ocean warming hotspot

Species redistributions are one of the most prevalent changes observed in oceans worldwide due to climate change. One of the major challenges is being able to predict temperature-driven changes to species interactions and the outcome of these changes for marine communities due to the complex nature of indirect effects. In the ocean-warming hotspot of south-east Australia, the ranges of many species have shifted poleward. The range of the eastern rock lobster Sagmariasus verreauxi has extended into warming Tasmanian waters inhabited by the resident southern rock lobster Jasus edwardsii, which may lead to increased competitive interactions between the species. Using video monitoring, we investigated how the 2 species compete for food at current (18°C), future (21°C) and future heatwave (24°C) summer temperatures. Behavioural competition occurred in 80% of experiments, during which J. edwardsii won 84% of competitive interactions and showed more aggressive behaviour at all temperatures. This indicates that resident J. edwardsii is not only more dominant in direct food competition than the range-shifting S. verreauxi but, surprisingly, also sustains competitive dominance beyond its physiological thermal optimum under predicted future ocean warming and heatwave scenarios.

Invasive Fucus serratus (Fucaceae, Phaeophyceae) responds to climate change along the Atlantic Coast of Nova Scotia, Canada

Abstract The distribution and ecology of the invasive brown alga Fucus serratus along the 500 km Atlantic coast of Nova Scotia, Canada, has been poorly explored. We observed significant intertidal penetration at four sites in the southwestern part of the province, and then examined numerous sites along the Atlantic coast of Nova Scotia. Surveys of attached algae in intertidal and shallow subtidal zones and wrack show that F. serratus has become a dominant plant in the low to mid-intertidal zone and can be expected on headlands along the South Shore of Nova Scotia where it can occupy up to 40% of the intertidal zone with cover &gt;75% and mean densities of up to 10 kg m−1. In this zone, F. serratus has replaced Chondrus crispus as the major canopy species, although C. crispus and Corallina officinalis remain primary understory species. At slightly higher elevations, F. serratus was common as an understory beneath Ascophyllum nodosum and Fucus vesiculosus. While geographic spread along the Atlantic coast might reflect the natural dispersal capacity of F. serratus, we hypothesize that the ecological extension into the intertidal zone may be facilitated by harvesting of A. nodosum and by climate change in an ocean-warming hotspot.

Cold-Water Coral Reefs in the Langenuen Fjord, Southwestern Norway—A Window into Future Environmental Change

Ocean warming and acidification pose serious threats to cold-water corals (CWCs) and the surrounding habitat. Yet, little is known about the role of natural short-term and seasonal environmental variability, which could be pivotal to determine the resilience of CWCs in a changing environment. Here, we provide continuous observational data of the hydrodynamic regime (recorded using two benthic landers) and point measurements of the carbonate and nutrient systems from five Lophelia pertusa reefs in the Langenuen Fjord, southwestern Norway, from 2016 to 2017. In this fjord setting, we found that over a tidal (&lt;24 h) cycle during winter storms, the variability of measured parameters at CWC depths was comparable to the intra-annual variability, demonstrating that single point measurements are not sufficient for documenting (and monitoring) the biogeochemical conditions at CWC sites. Due to seasonal and diurnal forcing, parts of the reefs experienced temperatures up to 4 °C warmer (i.e., &gt;12 °C) than the mean conditions and high CT concentrations of 20 µmol kg−1 over the suggested threshold for healthy CWC reefs (i.e., &gt;2170 µmol kg−1). Combined with hindcast measurements, our findings indicate that these shallow fjord reefs may act as an early hotspot for ocean warming and acidification. We predict that corals in Langenuen will face seasonally high temperatures (&gt;18 °C) and hypoxic and corrosive conditions within this century. Therefore, these fjord coral communities could forewarn us of the coming consequences of climate change on CWC diversity and function.

Differential tolerance of species alters the seasonal response of marine epifauna to extreme warming

Marine heatwaves are occurring with greater frequency and magnitude worldwide and can significantly alter community structure and ecosystem function. Predicting changes in community structure in extreme temperatures requires an understanding of variation among species in their thermal tolerance, and how potential acclimatization to recent temperatures influences survival. To address this, we determined the tolerance to extreme temperatures in a crustacean epifaunal assemblage that inhabits macroalgae in the southeast Australian ocean warming hotspot. Amphipods were the most abundant group and the thermal tolerance of the most abundant species (two in winter and four in summer) was tested to determine their thermal limits and probability of survival in near-future extreme temperatures. Survival, measured as time to immobilization, was compared across species, sexes, life stage and body size. The greatest variation in tolerance to extreme temperatures was among species (not body sizes or life stages), indicating that heatwaves could shift the composition of the macroalgal associated epifaunal assemblage. Comparison of recent thermal history (between 18 °C to 22 °C) revealed greater thermal tolerance of warm acclimatized individuals. Our results indicate that the impacts of a marine heatwave will depend on local species composition and their timing relative to recent climate conditions.

Seascape topography slows predicted range shifts in fish under climate change

AbstractShifts in marine species distributions are occurring rapidly in response to climate change, yet predicted rates of change are partly dependent on the data used to estimate species distributions. While pelagic fishes are known to respond to dynamic oceanographic variables, static topographic features can also regulate their distributions, yet the effect of topography on rates of range shifts remains poorly understood. For a suite of five pelagic fishes from an ocean warming hotspot, we developed generalized additive mixed models that did and did not incorporate preferences for seascape topographic heterogeneity when estimating species distributions. Monthly spatial predictions over two decades revealed that poleward extensions in species' core environmental habitats were significantly reduced by 5.3–60.4% (7.1–131.4 km per decade) when preferences for seascape topography were incorporated. These findings suggest that ignoring species associations with static environmental features can substantially affect predicted rates of climate‐driven redistributions in fishes.

Climate-Driven Range Shifts Are Rapid Yet Variable Among Recreationally Important Coastal-Pelagic Fishes

Shifts in species distributions are occurring globally in response to climate change, but robust comparisons of redistribution rates among species are often prevented by methodological inconsistencies, challenging the identification of species that are most rapidly undergoing range shifts. In particular, comparable assessments of redistributions among harvested species are essential for identifying climate-driven changes in fishing opportunities and prioritising the development of management strategies. Here we utilise consistent datasets and methodologies to comparably analyse rates of climate-driven range shifts over 21 years for four recreationally important coastal-pelagic fishes (Australian bonito, Australian spotted mackerel, narrow-barred Spanish mackerel, and common dolphinfish) from the eastern Australian ocean warming hotspot. Latitudinal values corresponding to the poleward edge of species’ core oceanographic habitats were extracted from species distribution models (SDMs). Rates of poleward shifts in core oceanographic habitats ranged between 148.7 (i.e., common dolphinfish) and 278.6 (i.e., narrow-barred Spanish mackerel) km per decade over the study period. However, rates of redistribution varied by approximately 130 km per decade among species, demonstrating that subtle differences in species’ environmental responses can manifest in highly variable rates of climate-driven range shifts. These findings highlight the capacity for coastal-pelagic species to undergo rapid, yet variable, poleward range shifts, which have implications for ecosystem structure and the changing availability of key resources to fisheries.

Climate refugia for kelp within an ocean warming hotspot revealed by stacked species distribution modelling

Canopy forming macroalgae are declining globally due to climate change and the identification of refuges for these habitats is crucial for their conservation. This is particularly pertinent in ocean warming hotspots where significant range contractions of kelp have occurred and are projected to continue.We developed a stacked urchin-kelp species distribution model (SDM) to predict climate refugia for kelp (Ecklonia radiata) in an ocean warming hotspot, south-eastern Australia. The optimal stacked-SDM incorporated biotic and abiotic explanatory covariates and was validated using an independent dataset. Density of the urchin Centrostephanus rodgersii, summer bottom temperature and photosynthetically available radiation at the seabed were significant predictors of kelp cover, highlighting the physiological and ecological influence of these variables on the distribution of kelp.Our optimal stacked-SDM predicted three spatially distinct refuge areas, where kelp occurs in deeper waters than surrounding seascapes. The presence of kelp at two of these refuge areas was confirmed using independent data. The identification of these refuge areas is crucial for conservation, as they are likely to facilitate the persistence of ecologically and economically important kelp forests as waters warm in shallow areas and kelp retreat to depth under climate change. Furthermore, identification of refugia will enable proactive spatial planning that prioritises new locations for protection to ensure that key kelp habitats can persist in a future of increasing stress.