Cold-water Species Research Articles

Spatial changes in the summer diatom community of the northern Bering Sea in 2017 and 2018

In recent years, the northern Bering Sea has experienced changes in the timing of sea-ice retreat and in hydrographic conditions during the summer. The influence of these environmental changes on the diatom community has not been examined. In this study, we investigated the spatial changes in the diatom community of the northern Bering Sea during the summers of 2017 and 2018, and evaluated the effects of environmental variability on these communities. We found that the diatom cell density and diversity varied with water masses. A cluster analysis based on cell density revealed that the diatom communities were separated into four groups, and that the distributions of three of these groups were different spatially between 2017 and 2018. In the Bering Strait and the Chirikov Basin regions, the diatom communities differed between 2017 and 2018. In 2017, these diatom communities were dominated by cold-water species such as Chaetoceros gelidus and Chaetoceros spp. (subgenus Hyalochaetae), while in 2018, the community was dominated by cosmopolitan species such as Thalassionema nitzschioides and Chaetoceros spp. (subgenus Phaeoceros). NMDS and multiple regression analysis indicated that the timing of the sea-ice retreat was the most important contributor to the differences in the diatom community. In contrast, there was no year-to-year difference south of St. Lawrence Island, possibly because nutrients were depleted and phytoplankton types other than diatoms were dominant.

The impact of climate change induced alterations of streamflow and stream temperature on the distribution of riparian species.

Distributions of riparian species will likely shift due to climate change induced alterations in temperature and rainfall patterns, which alter stream habitat. Spatial forecasting of suitable habitat in projected climatic conditions will inform management interventions that support wildlife. Challenges in developing forecasts include the need to consider the large number of riparian species that might respond differently to changing conditions and the need to evaluate the many different characteristics of streamflow and stream temperature that drive species-specific habitat suitability. In particular, in dynamic environments like streams, the short-term temporal resolution of species occurrence and streamflow need to be considered to identify the types of conditions that support various species. To address these challenges, we cluster species based on habitat characteristics to select habitat representatives and we evaluate regional changes in habitat suitability using short-term, temporally explicit metrics that describe the streamflow and stream temperature regime. We use stream-specific environmental predictors rather than climatic variables. Unlike other studies, the stream-specific environmental predictors are generated from the time that species were observed in a particular reach, in addition to long term trends, to evaluate habitat preferences. With species occurrence data from local monitoring surveys and streamflow and stream temperature modeled from downscaled Coupled Model Intercomparison Project - Phase 5 (CMIP5) climate projections, we predict change in habitat suitability at the end-of-century. The relative importance of hydrology and stream temperature varied by cluster. High altitudinal, cold water species' distributions contracted, while lower elevation, warm water species distributions expanded. Modeling with short-term temporally explicit environmental metrics did produce different end-of-century projections than using long-term averages for some of the representative species. These findings can help wildlife managers prioritize conservation efforts, manage streamflow, initiate monitoring of species in vulnerable clusters, and address stressors, such as passage barriers, in areas projected to be suitable in future climate conditions.

The breeding seabird community reveals that recent sea ice loss in the Pacific Arctic does not benefit piscivores and is detrimental to planktivores

Recent dramatic reductions of winter sea ice in the northern Bering Sea have raised the possibility that a rapid ecological transformation is underway. It has been hypothesized that with sufficient sea ice loss the cold pool thermal barrier separating the northern and southern Bering Sea would be breached, potentially benefiting piscivorous seabirds with an influx of abundant forage fish, but maybe causing food limitations for planktivorous seabirds that rely on cold-water associated species of zooplankton. During 2016–2019, we examined responses of the seabird community breeding on St. Lawrence Island to variable winter sea ice extent (low in 2018–2019). The seabird community includes piscivorous black-legged kittiwakes, thick-billed and common murres, and planktivorous crested and least auklets. We used a combination of stable isotope analysis and nutritional stress status analyses to examine if the region's ecosystem has shifted to a pelagic prey base, and whether either foraging guild benefitted from such a shift. To interpret bird responses, we used stable isotope analyses of prey and trawl survey-derived abundance of forage size fish. Stable isotope values in blood tissues revealed no change in the prey base as reflected in the isotopic space used by the seabird community across the four study years. Sea-ice loss was, however, associated with increased nutritional stress in all seabird species and diverging foraging niches within guilds. Benthic forage-sized fish remained a key food source, with their abundance stable since 2010, and benthic-foraging thick-billed murres experienced relatively low nutritional stress across the study period. A significant increase in common murre nutritional stress levels between 2016 and 2017, and a shift in their isotopic niche indicative of higher reliance on benthic prey coincided with a decline in the abundance of pelagic forage fish in the region. Surface-foraging black-legged kittiwakes experienced steady increases in nutritional stress as the abundance of pelagic forage fish declined. The spring sea-ice loss was detrimental to planktivorous least and crested auklets that rely on zooplankton advected from the Bering Sea basin. During the study period both auklets have experienced severe nutritional stress (2018) and colony-wide reproductive failures (2018 and 2019). In conclusion, we found that increasingly warm conditions during 2016–2019 have challenged the adaptive ability of seabirds relying on pelagic fish and zooplankton advected into the region each summer, but have not yet resulted in a reorganization of the northern Bering Sea's benthic-dominated ecosystem.

The Risky Decrease of Fishing Reference Points Under Climate Change

In Europe, implementation of sustainable fisheries management has been reinforced in the latest common fisheries policy, and presently marine fish stocks are mostly managed through assessment of their exploitation and ecological status compared to reference points such as Maximum Sustainable Yield (MSY). However, MSY and its associated fishing mortality rate F-MSY are sensitive to both stock characteristics and environment conditions. In parallel, climate change impacts are increasingly affecting fish stocks directly and indirectly but might also change the exploitation reference points and the associated level of catch. Here we explored the variability of MSY reference points under climate change by using a multi-species model applied to the Eastern English Channel, a highly exploited semi-continental sea. The spatial individual-based OSMOSE explicitly represents the entire fish life cycle of 14 species interacting through size-based opportunistic predation. The model was first parameterized and run to fit the historical situation (2000-2009) and then used to assess the ecosystem state for the 2050-2059 period, using two contrasting climate change scenarios (RCP 4.5 and RCP 8.5). For each condition, a monospecific MSY estimation routine was performed by varying species fishing mortality independently and allowed estimation of reference points for each species. The F-MSY estimated with OSMOSE were mostly in accordance with available values derived from stock assessment and used for fishing advice. Evolution of reference points with climate change was compared across species and highlighted that overexploited cold-water species are likely to have both MSY and F-MSY declining with climate warming. Considering all species together, MSY under RCP scenarios was expected to be higher than historical MSY for half of them, with no clear link with species temperature preferences, exploitation status or trophic level, but in relation with expected change of species biomass under climate change. By contrast, for 80% of cases F-MSY projections showed consistent decreasing pattern as climate conditions changed from historical to RCP scenarios in the Eastern English Channel. This result constitutes a risk for fisheries management, and anticipation of climate change impacts on fish community would require targeting a smaller fishing mortality than F-MSY to ensure sustainable exploitation of marine stocks.

Biogeography of intertidal and subtidal native and invasive barnacles in Korea in relation to oceanographic current ecoregions and global climatic changes

AbstractThe coastline of the Korean Peninsula is influenced by three major oceanographic ecoregions, including the estuarine Yellow Sea ecoregion on the west coast, the warmer and saline East China Sea ecoregion on the south coast, and the cold East Sea ecoregion on the east coast. The influence of these marine ecoregions on the distribution of intertidal barnacles has not been extensively studied. The present study examines the biogeography of thoracican barnacles from intertidal and shallow subtidal zones, along the coasts of Korea. Twenty-one species in seven families were identified, including three species of coral-associated barnacles. Species composition varied significantly in the three marine ecoregions. Multivariate analysis showed barnacle assemblages were significant among the three ecoregions, although there are large overlaps of clusters between the Yellow Sea and East China Sea ecoregions. The estuarine species,Fistulobalanus albicostatus, occurred mainly in the Yellow Sea ecoregion; warm-water species,Tetraclita japonica, and sponge inhabiting barnaclesEuacasta dofleiniwere observed in the East China Sea ecoregion; and cold-water species,Balanus rostratusandPerforatus perforatus, were found in the East Sea ecoregion. Four invasive barnacle species were recorded and the European barnaclePerforatus perforatusexpanded its range northward from its recorded distribution nine years earlier. The cold-water species,Chthamalus dalliandSemibalanus cariosus, previously recorded in the East Sea ecoregion, were absent in the present survey. A trend of increasing seawater temperatures in Korean waters may have a significant impact on the distribution of cold-water species and enhance the northward invasion ofP.perforatus.

Global Warming Impacts Micro-Phytoplankton at a Long-Term Pacific Ocean Coastal Station

Understanding impacts of global warming on phytoplankton – the foundation of marine ecosystems – is critical to predicting changes in future biodiversity, ocean productivity, and ultimately fisheries production. Using phytoplankton community abundance and environmental data that span ~90 years (1931-2019) from a long-term Pacific Ocean coastal station off Sydney, Australia, we examined the response of the phytoplankton community to long-term ocean warming using the Community Temperature Index, an index of the preferred temperature of a community. With warming of ~1.8℃ at the site since 1931, we found a significant increase in the community temperature index from 1931-1932 to 2009-2019, suggesting that the relative proportion of warm-water to cold-water species has increased. The Community Temperature Index also showed a clear seasonal cycle, with highest values at the end of austral summer (Feb/Mar) and lowest at the end of winter (Aug/Sept), a pattern well supported by other studies at this location. The shift in CTI was a consequence of the decline in the relative abundance of the cool-affinity (optimal temperature = 18.7℃), chain-forming diatom Asterionellopsis glacialis (40% in 1931-1932 to 13% in 2009 onwards), and a substantial increase in the warm-affinity (21.5℃), also chain-forming diatom Leptocylindrus danicus (20% in 1931-1932 to 57% in 2009 onwards). L. danicus reproduces rapidly, forms resting spores under nutrient depletion, and displays a wide thermal range. Species such as L. danicus may provide a glimpse of the functional traits necessary to be a ‘winner’ under climate change.

Juvenile Greenland halibut (Reinhardtius hippoglossoides) growth in the context of rising temperature in the Estuary and Gulf of St. Lawrence

In a context of climate change, understanding the influence of temperature on fish species growth is important for the management of fisheries. The effect of increasing temperature on the growth of juvenile Greenland halibut (Reinhardtius hippoglossoides), a cold-water species, circum-continental in the Arctic, was investigated on juveniles that had been captured in the wild and kept in captivity. Mortality rate increased with higher temperature, from 4.5 % at 4.0 °C to 15.2 % at 7.5 °C. Relative growth (normalized in degree-days) was lower at 7.5 °C than at the two other temperatures tested. Food conversion efficiency, muscle energy content, and the Fulton condition index were not influenced by temperature, but food intake significantly increased over time at 5.5 °C. No clear difference in growth trajectories between sexes was highlighted. Overall, the results suggest that optimal temperature conditions were exceeded at 7.5 °C and that any further increase in temperature would significantly decrease survival and impair juvenile growth. With the current deep-water warming trends in the Estuary and Gulf of St. Lawrence, recruitment and commercial fishing of Greenland halibut may be impaired in the near future.

Vertical distribution of bivalves fauna in the northwestern Tatar Strait (Japan Sea)

General patterns of bivalves distribution by depth in the northwestern Tatar Strait (within Khabarovsk region) are analyzed on the data of 384 trawl, 573 drag, and 1177 diving stations during research surveys in 2003–2016 where the samples were collected with commonly accepted methods. Depths from 0 to 600 m were surveyed. Besides, scientific publications and archival materials related to this area were taken into account. The species richness (y) decreases with depth (x) exponentially from 51 species at 1–20 m to 3 species at 400–600 m that could be approximated satisfactory by the equation y = 31.799. e–0.0502x (r2 = 0.89). Sublittoral and bathial faunas can be separated by cluster analysis of special composition in the depth range 0–150 m and 150–600 m, respectively, with similarity of 0.11 between them. Within these boundaries, 5 local faunas are distinguished: I (< 2 m, the surf zone at the upper boundary of the sublittoral zone), II (2–30 m, the upper sublittoral zone), III (30–150 m, the lower sublittoral zone), IV (150–400 m, the transitional zone) and V (400–600 m, the upper bathyal zone), with similarity between them from 0.14 to 0.36. The upper sublittoral zone has the maximum species richness — 64 species and is the habitat for a «core» of Bivalve fauna with almost ⅔ of its species, preserving the ratio of the main biogeographic groups typical for the researched area. Commercial fishery of scallop Mizuhopecten yessoensis exploited this zone mainly and now is banned to prevent reduction of its stock. Other commercial bivalves, as Callista brevisiphonata, Serripes laperousii, Keenocardium californiense, and Mercenaria stimpsoni, which commercial stocks are estimated in order of 105 t, are also concentrated in this zone but are not landed currently. Portion of moderately cold-water species (wide-boreal and low-boreal) increases and portion of warm-water species (subtropical-boreal and subtropical-low-boreal) decreases with depth, with the slope coefficients of the regressions α = 9.2 ± 4.1 (p = 0.11) and α = –9.6 ± 2.3 (p = 0.03), respectively. The coldwater species are absent in the surf and upper bathyal zones but their portion in other zones is 20–26 %. Rather high portion of boreal-arctic species on shallow depths reflects relative severity of the northwestern Tatar Strait that is the most cold-water area of the Japan Sea. The warm-water species are completely absent in the upper bathyal zone, i.e. at the depths > 400 m. On the other hand, portion of banal species increases and portion of specific species decreases with depth, also portion of rare species increases and portion of mass species decreases with depth, with the slope coefficients α = 9.10 ± 0.49 (p = 0.0003) and α = –4.5 ± 2.5 (p = 0.01), respectively. Vertical distribution of frequent species is rather uniform: 33–57 %. These patterns of the species distribution by zones almost do not change spatially: distribution of different biogeographic groups of species in three coastal areas (47–49о N, 49–51о N, and > 51о N) has no statistically significant differences. Distribution of species richness and composition by depth ranges relates to ecotopic variation (74 % of diversity), to the degree of exploration (22 %), and to the influence of such complex factor as a depth (4 %). Further faunistic studies are recommended in the most diverse areas, as bays, harbours, and capes vicinities with variable grounds and submarine vegetation, in all available depth ranges. Such surveys can provide faunistically representative information on the species wealth. The list of Bivalve mollusk species for the northwestern Tatar Strait can be enlarged possibly in 1/5 if detailed studies of their fauna will be conducted. The fauna on great depths is the most underexplored. The total expected number of bivalve species in this area is at least 120.

Adaptation of Fish Metabolism to Thermal Pollution of Subarctic Lake Imandra

Kola nuclear power plant, which discharges warm water into one of the bays of sub-arctic Lake Imandra, significantly changes the fish habitat, creating temperature gradient from 2 to 8°C compared to water temperature in the other part of the lake. The permeation, metabolism and bioaccumulation of elements in the stenothermal cold-water species (Coregonus lavaretus L.) in warm water impact zone are characterized. One of the main adaptation mechanisms of ionic regulation is the intense metabolism of Na due to the high respiratory activity of whitefish in warmer water. This feature is associated with elevated accumulation of Rb and loss of Se, Mo, and Si. At elevated temperatures, the main metabolic need is caused by Se deficiency. An extensive metabolism of selenoproteins involves risks of toxic effects and bioaccumulation of Hg, As and Cu with an increase of existing stressor or appearance of new ones.

Oxygen-dependence of upper thermal limits in crustaceans from different thermal habitats.

The critical thermal maximum (CTMAX) is the temperature at which animals exhibit loss of motor response because of a temperature-induced collapse of vital physiological systems. A central mechanism hypothesised to underlie the CTMAX of water-breathing ectotherms is insufficient tissue oxygen supply for vital maintenance functions because of a temperature-induced collapse of the cardiorespiratory system. The CTMAX of species conforming to this hypothesis should decrease with declining water oxygen tension (PO2) because they have oxygen-dependent upper thermal limits. However, recent studies have identified a number of fishes and crustaceans with oxygen-independent upper thermal limits, their CTMAX unchanged in progressive aquatic hypoxia. The previous studies, which were performed separately on cold-water, temperate and tropical species, suggest the oxygen-dependence of upper thermal limits and the acute thermal sensitivity of the cardiorespiratory system increases with decreasing habitat temperature. Here we directly test this hypothesis by assessing the oxygen-dependence of CTMAX in the polar Antarctic krill (Euphausia superba), as well as the temperate Baltic prawn (Palaemon adspersus) and brown shrimp (Crangon crangon). We found that P. adspersus and C. crangon maintain CTMAX in progressive hypoxia down to 40mmHg, and that only E. superba have oxygen-dependent upper thermal limits at normoxia. In E. superba, the observed decline in CTMAX with water PO2 is further supported by heart-rate measurements showing a plateauing, and subsequent decline and collapse of heart performance at CTMAX. Our results support the hypothesis that the oxygen-dependence of upper thermal limits in water-breathing ectotherms and the acute thermal sensitivity of their cardiorespiratory system increases with decreasing habitat temperature.

Is there always space at the top? Ensemble modeling reveals climate-driven high-altitude squeeze for the vulnerable snow trout Schizothorax richardsonii in Himalaya

Mountain systems throughout the globe are conspicuously sensitive to on-going climate alterations. This condition is much more detrimental in the Himalaya, where the rate of warming and thus the glacier meltdown is much higher than elsewhere. The Himalayan coldwater species are concerningly most vulnerable to these changes because of their limited thermal range. Whilst climate studies strongly prognosticate the altered distribution of plants and mammals in this region, the impact on coldwater fishes still remains unknown. We used snow trout (Schizothorax richardsonii), a Himalayan coldwater specialist as a model organism to predict the current suitability and climate-driven potential range shift in an ensemble-based modeling framework. We collated occurrence data from a long term, extensive field-based sampling with additional records derived from an in-depth literature survey. A comprehensive input data set including topographic, hydrogeomorphic and climatic variables were used to build correlative species distribution models for the current and future time-periods (2050 and 2070) under three-carbon emission scenarios (Representative Concentration Pathways) by integrating five General Circulation Models. Furthermore, we predicted the elevation-specific range shifts of the snow trout in response to climate change. We predict that a wide-ranging mid-elevation river network is currently suitable for the snow trout in Himalaya, however a significant part of its current distributional range would be lost over time. Our results highlight that snow trout would expand their range upwards into the high-altitude streams with a concurrent predominant range contraction in most of their lagging edges, ultimately creating a high-altitude squeeze. The net habitat loss under three RCP scenarios (RCPs 2.6, 4.5 and 8.5) was estimated to range from 7.41% to 16.29% for the year 2050 which would further increase in the year 2070 ranging from 9.46% to 26.56%. These results provide a strategic information on prioritizing climate-adaptive actions to target the currently suitable habitats and future refugia identified. Our modeling framework also provides a foremost basis to conserve not only the snow trout but also to evaluate the climate impact on several other coldwater species, which are equally vulnerable and ecologically important in the Himalaya.

Identification and characterization of long noncoding RNAs provide insight into the regulation of gene expression in response to heat stress in rainbow trout (Oncorhynchus mykiss)

Rainbow trout are typical cold-water fish species. However, with the intensification of global warming, high temperatures have severely restricted the development of aquaculture during the summer. Understanding the molecular regulatory mechanisms of rainbow trout responses to heat stress will be beneficial for alleviating heat stress-related damage. In this study, we performed RNA-seq of liver tissues from rainbow trout under heat stress (24 °C) and control conditions (18 °C) to identify lncRNAs and target genes by strand-specific library. Changes in nonspecific immune parameters revealed that a strong stress response occurred in rainbow trout at 24 °C. More than 658 million filtered reads and 5916 lncRNAs were identified from six libraries. A total of 927 novel lncRNAs were identified, and 428 differentially expressed lncRNAs were screened with stringent thresholds. The RNA-seq results were verified by RT-qPCR. In addition, a regulatory network of lncRNA-mRNA functional interactions was constructed, and the potential antisense, cis and trans targets of lncRNAs were predicted. GO and KEGG enrichment analyses showed that many target genes involved in maintenance of homeostasis or adaptation to stress and stimuli were highly induced under heat stress. Several regulatory pathways were also found to be involved in heat stress, including the thyroid hormone signaling pathway, the PI3K-Akt signaling pathway, and the estrogen signaling pathway, among others. These results broaden our understanding of lncRNAs associated with heat stress and provide new insights into the lncRNA mediated regulation of the rainbow trout heat stress response.

Mitochondrial genome diversity and population mitogenomics of polar cod (Boreogadus saida) and Arctic dwelling gadoids

High-latitude fish typically exhibit a narrow thermal tolerance window, which may pose challenges when coping with temperatures that shift outside of a species’ range of tolerance. Due to its role in aerobic metabolism and energy balance, the mitochondrial genome is likely critical for the acclimation and adaptation to differing temperature regimes in marine ectotherms. As oceans continue to warm, there is growing need to understand the ability of organisms to respond to changing environmental conditions given evidence that some species, in particular cold-water species, may already be experiencing difficulties. To assess how Arctic gadids in Alaska have responded to differential thermal preferences in the past and how regions are interconnected, we sequenced complete mitochondrial genomes for four Arctic gadids to determine the distribution of mitochondrial diversity and population-level structure as well as to detect signatures of selection acting on the mitochondrial genome. We found little population-level structure within all four species with the clear exception of Gulf of Alaska saffron cod (Eleginus gracilis). Northern localities exhibited higher levels of genetic diversity and primarily northern lineages were observed within polar cod (Boreogadus saida) and saffron cod, likely reflecting asymmetrical dispersal and potentially admixture of distinct lineages via ocean currents. The main evolutionary force shaping the evolution of the mitogenome appears to be purifying selection, but we also identified potential positive selection of candidate amino acid replacements primarily in complex I (ND genes) in polar cod. The high levels of mitochondrial diversity observed in our study and large population size may provide this species with the ability to respond evolutionarily (i.e. long-term) to a changing environment.

Influence of Thermal Pollution on the Physiological Conditions and Bioaccumulation of Metals, Metalloids, and Trace Metals in Whitefish (Coregonus lavaretus L.).

The Kola nuclear power plant, which discharges warm water into one of the bays of subarctic Lake Imandra, significantly changes fish habitats. The temperature gradient of the lake is between 2 and 8 °C, which makes it significantly different from the natural temperature of the lake water. The stenothermal cold-water native species (lake whitefish (Coregonus lavaretus L.)), living for more than 40 years under conditions of thermal pollution, has adapted to this stressor. Moreover, this population differs favorably from the population in the natural-temperature environment in terms of its physiological state. Firstly, the hemoglobin concentrations in the fish blood are in the range of the ecological optimum, and secondly, it has a higher somatic growth, as estimated by Fulton’s condition factor. One of its main adaptive mechanisms of ion regulation is an intense metabolism of Na due to the high respiratory activity of the whitefish in warmer water. An increased accumulation of Rb and excretion of Se, Mo, and Si are associated more or less with that feature. Under conditions of an increased water temperature, the main metabolic need is due to a deficiency of Se in fish. The intensive metabolism of selenoproteins may involve risks of toxic effects and the bioaccumulation of Hg, As, and Cu in cases of increased existing stressors or the appearance of new ones.

Direct and Indirect Climate Change Impacts on Brown Trout in Central Europe: How Thermal Regimes Reinforce Physiological Stress and Support the Emergence of Diseases

Water temperature is one of the most important abiotic parameters in rivers having direct and indirect effects on fish. Especially cold-water species like the brown trout (Salmo trutta) are limited by high temperatures. Beside direct physiological stress, higher water temperatures also reinforce the emergence of diseases. In this study we investigate thermal regimes of rivers based on a large-scale dataset covering Austria (approximately 70,000 km²). The analyses aim to clarify to what extent water temperatures support the emergence of proliferative kidney disease (PKD) and assemble physiological stress for brown trout under current and future climate conditions. Data from 274 gauging stations at 184 rivers were used to calibrate a water temperature model and to investigate critical water temperature thresholds. Therefore, we developed a risk assessment scheme to identify river reaches that already have or will develop critical thermal regimes for brown trout in respect of PKD emergence and thermal physiological stress. The results revealed severe changes in the thermal regimes of the investigated rivers under climate change. Furthermore, the variable characterising riparian vegetation played a vital role to explain cooling of the water in downstream direction. In respect of PKD, the amount of river reaches having unlikely outbreaks of PKD decreased from 72.6 % under current conditions to 37.7 % in the far future RCP8.5 scenario. Within small rivers that currently showed optimal thermal regimes over large extents (10,244 km), the habitat suitability will be reduced by combined effects of PKD and physiological stress to 6,554 km. In general, suitable habitats of S. trutta will shift upstream, thus to higher altitudes, and to smaller, alpine rivers in Austria. The warming leads to physiological stress that induces a diminished growth due to the non-positive transition of caloric values to growth as well as cardiac dysfunction in brown trout. These factors will further restrict the distribution of brown trout. However, the results also underline the enormous importance of the alpine region as a future refuge for brown trout in Central Europe. Thus, this study will help to inform managers to timely develop robust conservation strategies.

An annual cycle of diatom succession in two contrasting Greenlandic fjords: from simple sea-ice indicators to varied seasonal strategists

Abstract We recorded diatom species succession over one full year (May 2017–May 2018) using automated sediment traps installed in two contrasting Greenlandic fjords: the seasonally ice-covered Young Sound in high-arctic Northeast Greenland and the nearly sea-ice free Godthabsfjord in subarctic Southwest Greenland. The traps were positioned at differing water depths (37 m in Young Sound vs. 300 m in Godthabsfjord). Distinct differences between the study sites were observed in both sediment and diatom fluxes. In Young Sound, total diatom flux was extremely seasonal and as high as 880 × 106 valves m−2 d−1 in the spring. In Godthabsfjord, total diatom flux was more stable throughout the year, with a maximum of 320 × 106 valves m−2 d−1 in the summer. The diatom assemblage in Young Sound was dominated by the sea-ice species Fragilariopsis oceanica, Fragilariopsis reginae-jahniae and Fossula arctica, which exhibited pulse-like deposition in the trap during and after the ice melt. In Godthabsfjord, the fluxes were dominated by Chaetoceros (resting spores), while the remaining assemblage was characterised by the cold-water indicator species Detonula confervacea (resting spores) and Thalassiosira antarctica var. borealis (resting spores) together with Fragilariopsis cylindrus. Our data show that, F. oceanica, F. reginae-jahniae and F. arctica exhibit similar seasonal behaviour and a clear linkage to sea ice. Fragilariopsis cylindrus seems to have a more flexible niche, and based on our study, cannot be considered an unequivocal ice indicator. Taking into account these ecological and seasonal preferences of individual diatom species is crucial when reconstructing past sea-ice conditions both qualitatively and quantitatively.

Marine macroinvertebrate species-area relationships, assemblage structure and their environmental drivers on submarine banks

Modern extensions of the theory of island biogeography (TIB) posit that the slope of the species-area relationship (SAR) reflects the insularity of ecological communities and is strongly influenced by species’ motility. We explore the relative insularity of crustacean, echinoderm and mollusk/Cirripedia assemblages in terms of both alpha diversity (species richness) and assemblage structure (relative biomass of species). These taxa/groups differ in adult motility and larval dispersal capacity. The habitats of interest were 10 offshore banks on the Scotian Shelf, northwest Atlantic Ocean, a region dominated by the NE- to SW-flowing Nova Scotia Current (NSC). Banks in the NE tended to be larger, more heterogeneous, cooler, less saline, more retentive and more productive (higher chlorophyll a) than those in the SW. Only mollusks/Cirripedia, the least motile and dispersive group, had a significant SAR slope, supporting TIB. For crustaceans and echinoderms, temperature/salinity properties and habitat heterogeneity, respectively, were important predictors of alpha diversity. Inter-bank variation in crustacean assemblage structure was accounted for largely by bank location relative to the NSC; the leading variables accounting for echinoderm and mollusk/Cirripedia assemblage structure were retention time and mean annual chlorophyll concentration, respectively. Along the NE to SW axis of the NSC, there was a substantial loss of species (7 crustacean, 9 echinoderm and 13 mollusk/Cirripedia species) and decreases in the biomass of common cold-water species. A complex interplay of species motility/dispersal capacity, local oceanography and habitat properties determine the extent to which (1) TIB applies to submarine macroinvertebrate assemblages and (2) upstream and downstream assemblages are interconnected.

Palynological evidence for Early to Mid-Holocene sea-level fluctuations over the present-day Ningshao Coastal Plain in eastern China

Sea-level fluctuation in a coastal plain setting was crucial for Neolithic human occupations of the Ningshao Coastal Plain in eastern China. In this study, we carry out analyses using detailed palynological records of dinoflagellate cysts (dinocysts), freshwater algae and foraminiferal organic linings from two sediment cores (HMD1501 and HMD1502) at the Tianluoshan archaeological site, Zhejiang Province, eastern China. Brief sea-level fluctuations and environmental changes on the Ningshao Coastal Plain during the Early to Mid-Holocene were recovered by using radiocarbon chronology. The results show the Tianluoshan site underwent marine transgression processes, and experienced warm and humid climatic conditions during the ca. 10, 708-5850 cal. yr BP. After that, there was a relatively cold and dry climate, marine regression processes occurred at the Tianluoshan site during the ca. 5850-5655 cal. yr BP, indicated by an abrupt increase in the cold-water species of dinoflagellates (especially O. centrocarpum), freshwater algae concentration and the ratio of pollen to dinoflagellates (P/D), with an abrupt decrease appeared in foraminifer linings concentration. Marine regression processes provided an opportunity for Hemudu ancestors to enter the Tianluoshan site, which helped create a brilliant Hemudu-Tianluoshan Culture on the Ningshao Coastal Plain.

Campanian to Danian dinoflagellate cyst assemblages from the southwestern Tethyan margin (Tattofte section, western External Rif, Morocco): Biostratigraphic and paleobiogeographic interpretations

A palynological study of the Upper Cretaceous–lower Paleocene outcrop section at Tattofte (westernmost External Rif, Morocco) has produced a well-preserved palynological assemblage rich in dinoflagellate cysts (dinocysts). For the first time, these assemblages have allowed us to determine the age of this section, to highlight a new Cretaceous–Paleogene boundary, and to make paleobiogeographic interpretations. The samples analyzed yielded 209 dinocyst species and 81 biostratigraphic dinocyst events (first occurrence, or FO, and last occurrence, or LO). We recognized the upper Campanian based on the FOs of Areoligera coronata, Areoligera reticulata, Areoligera senonensis, Cerodinium diebelii and Andalusiella mauthei subsp. mauthei, as well as the LOs of Batiacasphaera solida, Odontochitina costata, Odontochitina porifera, Sentusidinium eisenackii, Trichodinium castanea and Trithyrodinium suspectum. The lower Maastrichtian is determined by the FOs of Isabelidinium bujackii, Palaeocystodinium golzowense and Trithyrodinium evittii, and by the LO of Alterbidinium acutulum, and the upper Maastrichtian is recognized primarily by the FOs of Cerodinium speciosum subsp. speciosum, Disphaerogena carposphaeropsis, Glaphyrocysta perforata and Manumiella seelandica and the LO of Dinogymnium spp. The Danian is recognized on the basis of the FOs of several taxa, including Carpatella cornuta, Danea californica and Senoniasphaera inornata. The succession of the dinocyst events in the uppermost part of the studied interval has revealed a relatively complete record across the Cretaceous–Paleogene boundary. The Cretaceous–Paleogene boundary coincides with a 8 cm-thick reddish layer, which resembles those observed previously in the Cretaceous–Paleogene boundary reference sections in the Mediterranean area, most notably the Cretaceous–Paleogene boundary stratotype at El Kef (Tunisia). The upper Campanian dinocyst assemblage indicates a subtropical to temperate setting, while the Maastrichtian and lower Danian assemblages indicate a subtropical to warm temperate setting. The incursion of a few higher latitude cold-water dinocyst species in these provinces during the latest Campanian and around the Cretaceous–Paleogene boundary may suggest a southward migration of these species, probably related to short cooling episodes and ocean circulation.

Thermal acclimation of rainbow trout myotomal muscle, can trout acclimate to a warming environment?

Climate change is a looming threat to the planet. Cold-water aquatic species will face significant physiological challenges due to elevated summer temperatures. Salmonids, such as rainbow trout (Oncorhynchus mykiss) maintain fidelity to native streams, limiting their ability to mitigate the impact of climate change through migration. We examined how rainbow trout swimming performance and muscle function were shaped by the thermal environment. We hypothesized that trout would show slower muscle contractile properties and slower swimming performance with long-term exposure to warmer water. For fish held at either 10 °C or 20 °C, maximum steady swimming speed (Ucrit) was determined, and contractile properties of both fast-twitch (white) and slow-twitch (red) myotomal muscle were examined. In addition, immunohistochemistry and quantitative PCR were used to assess changes in myosin content of the myotomal muscle in response to holding temperature. Rainbow trout exposed to warm water for six weeks displayed relatively limited thermal acclimation response. When tested at a common temperature (10 °C), 20 °C acclimated fish had modestly slower muscle performance compared to 10 °C acclimated fish. Significant differences in swimming performance and muscle contractile properties were primarily at colder test temperatures (e.g. 2 °C for muscle mechanics). Shifts in myosin heavy chain protein composition and myosin heavy chain gene expression in the swimming muscle were observed in white but not red muscle. Our results suggest that rainbow trout will have a limited ability to mitigate elevated environmental temperature through thermal acclimation of their myotomal or swimming muscle.