Using a metabolomics approach to investigate the sensitivity of a potential Arctic-invader and its Arctic sister-species to marine heatwaves and traditional harvesting disturbances.

Coastal species are threatened by fishing practices and changing environmental conditions, such as marine heatwaves (MHW). The mechanisms that confer tolerance to such stressors in marine invertebrates are poorly understood. However, differences in tolerance among different species may be attributed to their geographical distribution. To test the tolerance of species occupying different thermal ranges, we used two closely related bivalves the softshell clam Mya arenaria (Linnaeus, 1758), a cold-temperate invader with demonstrated potential for establishment in the Arctic, and the blunt gaper Mya truncata (Linnaeus, 1758), a native polar species. Clams were subjected to a thermal stress, mimicking a MHW, and harvesting stress in a controlled environment. Seven acute temperature changes (2, 7, 12, 17, 22, 27, and 32 °C) were tested at two harvesting disturbance intensities (with, without). Survival was measured after 12 days and three tissues (gills, mantle, and posterior adductor muscle) collected from surviving individuals for targeted metabolomic profiling. MHW tolerance differed significantly between species: 26.9 °C for M. arenaria and 17.8 °C for M. truncata, with a negligeable effect of harvesting. At the upper thermal limit, M. arenaria displayed a more profound metabolomic remodelling when compared to M. truncata, and this varied greatly between tissue types. Network analysis revealed differences in pathway utilization at the upper MHW limit, with M. arenaria displaying a greater reliance on multiple DNA repair and expression and cell signalling pathways, while M. truncata was limited to fewer pathways. This suggests that M. truncata is ill equipped to cope with warming environments. MHW patterning in the Northwest Atlantic may be a strong predictor of population survival and future range shifts in these two clam species. As polar environments undergo faster rates of warming compared to the global average, M. truncata may be outcompeted by M. arenaria expanding into its native range.

Arctic marine forest distribution models showcase potentially severe habitat losses for cryophilic species under climate change.

The Arctic is among the fastest-warming areas of the globe. Understanding the impact of climate change on foundational Arctic marine species is needed to provide insight on ecological resilience at high latitudes. Marine forests, the underwater seascapes formed by seaweeds, are predicted to expand their ranges further north in the Arctic in a warmer climate. Here, we investigated whether northern habitat gains will compensate for losses at the southern range edge by modelling marine forest distributions according to three distribution categories: cryophilic (species restricted to the Arctic environment), cryotolerant (species with broad environmental preferences inclusive but not limited to the Arctic environment), and cryophobic (species restricted to temperate conditions) marine forests. Using stacked MaxEnt models, we predicted the current extent of suitable habitat for contemporary and future marine forests under Representative Concentration Pathway Scenarios of increasing emissions (2.6, 4.5, 6.0, and 8.5). Our analyses indicate that cryophilic marine forests are already ubiquitous in the north, and thus cannot expand their range under climate change, resulting in an overall loss of habitat due to severe southern range contractions. The extent of marine forests within the Arctic basin, however, is predicted to remain largely stable under climate change with notable exceptions in some areas, particularly in the Canadian Archipelago. Succession may occur where cryophilic and cryotolerant species are extirpated at their southern range edge, resulting in ecosystem shifts towards temperate regimes at mid to high latitudes, though many aspects of these shifts, such as total biomass and depth range, remain to be field validated. Our results provide the first global synthesis of predicted changes to pan-Arctic coastal marine forest ecosystems under climate change and suggest ecosystem transitions are unavoidable now for some areas.

Freshwater early life growth influences partial migration in populations of Dolly Varden (Salvelinus malma malma).

Populations of northern Dolly Varden (Salvelinus malma malma) exhibit partial seaward migration, yet little is known about this phenomenon in Dolly Varden populations. Our study analyzed data from three different Dolly Varden populations in the western Canadian Arctic in order to determine if: (1) differences in size-at-first seaward migration exist between fish that migrate at early and late ages among populations inhabiting different river systems, and (2) annual growth influences anadromous or resident life history choice. Otolith strontium analysis and back-calculation were used to determine age- and size-at-first seaward migration, respectively. Differences in age- and size-at-first seaward migration were determined across river system and migration age. Back-calculated fish lengths were compared using a mixed effect model to determine how early growth influences migratory tactics (early or late aged smolt, or resident). Our results indicate that fish exhibiting faster early growth migrated in earlier years and at smaller sizes than slower growing fish, however size- and age-at first seaward migration varied by river system. Faster growing Dolly Varden tended to become either residents or early smolts, while slower growth was associated with smolting later in life. This is contrary to life history theory where the fastest growing fish in a population should mature as a resident. Our results indicate factors other than growth may be influencing life history 'decisions' in Dolly Varden. Future work on growth efficiencies and metabolic rates is needed to assess how they affect migratory behaviours.

Three decades of Canadian marine harmful algal events: Phytoplankton and phycotoxins of concern to human and ecosystem health.

Spatial and temporal trends of marine harmful algal events in Canada over the last three decades were examined using data from the Harmful Algal Event Database (HAEDAT). This database contains the most complete record of algal blooms, phycotoxins and shellfish harvesting area closures in Canada since 1987. This 30-year review of 593 Canadian HAEDAT records from 1988 to 2017, together with other Canadian data and publications, shows that recurring harmful algal events have been widespread throughout both the Atlantic and Pacific coastal regions. The 367 paralytic shellfish toxin (PST) reports revealed annual and frequent recurrence throughout both the Atlantic and Pacific regions, including multi-year PST events in the Bay of Fundy, the Estuary and Gulf of St. Lawrence and the Strait of Georgia. The 70 amnesic shellfish toxin (AST) records revealed no recognizable trend, as these events were usually area specific and did not recur annually. The increasing frequency of diarrhetic shellfish toxin (DST) events over the period of this review, in total 59 records, can be at least partially explained by increased sampling effort. Marine species mortalities caused by harmful algae (including diatoms, dictyochophytes, dinoflagellates, and raphidophytes), were a common occurrence in the Pacific region (87 reports), but have been reported much less frequently in the Atlantic region (10 reports). Notable Canadian records contained in HAEDAT include the first detection worldwide of amnesic shellfish poisoning (ASP), attributed to the production of domoic acid (an AST) by a diatom (Pseudo-nitzschia multiseries) in Prince Edward Island in 1987. The first proven case of diarrhetic shellfish poisoning (DSP) in Canada and North America was recorded in 1990, and the first closures of shellfish harvesting due to DST (associated with the presence of Dinophysis norvegica) occurred in Nova Scotia in 1992, followed by closures in Newfoundland and Labrador in 1993. In 2008, mass mortalities of fishes, birds and mammals in the St. Lawrence Estuary were caused by Alexandrium catenella and high levels of PST. During 2015, the Pacific coast experienced a large algal bloom that extended from California to Alaska. It resulted in the closure of several shellfish harvesting areas in British Columbia due to AST, produced by Pseudo-nitzschia australis. Data from the Canadian Arctic coast is not included in HAEDAT. However, because of the emerging importance of climate change and increased vessel traffic in the Arctic, information on the occurrence of harmful algal species (pelagic and sympagic = sea ice-associated) in that region was compiled from relevant literature and data. The results suggest that these taxa may be more widespread than previously thought in the Canadian Arctic. Information in HAEDAT was not always robust or complete enough to provide conclusions about temporal trends. Compilation of spatial and temporal information from HAEDAT and other records is nevertheless important for evaluating the potential role of harmful algae as a stressor on Canadian marine ecosystems, and will support the next step: developing a knowledge gap analysis that will establish research priorities for determining their consequences on human and ecosystem health.

Among-individual diet variation within a lake trout ecotype: Lack of stability of niche use.

In a polyphenic species, differences in resource use are expected among ecotypes, and homogeneity in resource use is expected within an ecotype. Yet, using a broad resource spectrum has been identified as a strategy for fishes living in unproductive northern environments, where food is patchily distributed and ephemeral. We investigated whether specialization of trophic resources by individuals occurred within the generalist piscivore ecotype of lake trout from Great Bear Lake, Canada, reflective of a form of diversity. Four distinct dietary patterns of resource use within this lake trout ecotype were detected from fatty acid composition, with some variation linked to spatial patterns within Great Bear Lake. Feeding habits of different groups within the ecotype were not associated with detectable morphological or genetic differentiation, suggesting that behavioral plasticity caused the trophic differences. A low level of genetic differentiation was detected between exceptionally large-sized individuals and other piscivore individuals. We demonstrated that individual trophic specialization can occur within an ecotype inhabiting a geologically young system (8,000-10,000 yr BP), a lake that sustains high levels of phenotypic diversity of lake trout overall. The characterization of niche use among individuals, as done in this study, is necessary to understand the role that individual variation can play at the beginning of differentiation processes.

What and where? Predicting invasion hotspots in the Arctic marine realm.

The risk of aquatic invasions in the Arctic is expected to increase with climate warming, greater shipping activity and resource exploitation in the region. Planktonic and benthic marine aquatic invasive species (AIS) with the greatest potential for invasion and impact in the Canadian Arctic were identified and the 23 riskiest species were modelled to predict their potential spatial distributions at pan-Arctic and global scales. Modelling was conducted under present environmental conditions and two intermediate future (2050 and 2100) global warming scenarios. Invasion hotspots-regions of the Arctic where habitat is predicted to be suitable for a high number of potential AIS-were located in Hudson Bay, Northern Grand Banks/Labrador, Chukchi/Eastern Bering seas and Barents/White seas, suggesting that these regions could be more vulnerable to invasions. Globally, both benthic and planktonic organisms showed a future poleward shift in suitable habitat. At a pan-Arctic scale, all organisms showed suitable habitat gains under future conditions. However, at the global scale, habitat loss was predicted in more tropical regions for some taxa, particularly most planktonic species. Results from the present study can help prioritize management efforts in the face of climate change in the Arctic marine ecosystem. Moreover, this particular approach provides information to identify present and future high-risk areas for AIS in response to global warming.

Author Correction: Effects of sampling effort on biodiversity patterns estimated from environmental DNA metabarcoding surveys.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Ecological risk assessment of predicted marine invasions in the Canadian Arctic.

Climate change is impacting environmental conditions, especially with respect to temperature and ice cover in high latitude regions. Predictive models and risk assessment are key tools for understanding potential changes associated with such impacts on coastal regions. In this study relative ecological risk assessment was done for future potential introductions of three species in the Canadian Arctic: periwinkle Littorina littorea, soft shell clam Mya arenaria and red king crab Paralithodes camtschaticus. These species occur in locations connected to Canadian Arctic ports through shipping and have the potential to be introduced via ballast water discharge. The methodology proposed in this study is unique in the sense that it considers not only ballast water origin, but also the distribution of the species being assessed and the sensitivity of the receiving habitat. It combines detailed information (ballast water source of each tank, transit time, time of the year when the water is released, environmental suitability of receiving habitat, impact, and habitat sensitivity) in order to assess ecological risk. Through the use of this approach it is highlighted that domestic discharge events pose a higher relative overall risk on a vessel-specific and cumulative annual bases than international discharges. The main ports of Deception Bay and Churchill were classified as being at moderate to high relative risk for L. littorea and M. arenaria, especially from domestic vessels, while relative overall risk for P. camtschaticus was low for international vessels and null for domestic vessels due to few ships transiting from its range of distribution to Canadian Arctic ports. This work can serve as an approach to help build a list of potential high risk species-a "grey" watch list-for the Canadian Arctic, and provides useful information for consideration in future decision making actions such as the identification of high risk pathways, species and ports.

Effects of sampling effort on biodiversity patterns estimated from environmental DNA metabarcoding surveys.

Environmental DNA (eDNA) metabarcoding can greatly enhance our understanding of global biodiversity and our ability to detect rare or cryptic species. However, sampling effort must be considered when interpreting results from these surveys. We explored how sampling effort influenced biodiversity patterns and nonindigenous species (NIS) detection in an eDNA metabarcoding survey of four commercial ports. Overall, we captured sequences from 18 metazoan phyla with minimal differences in taxonomic coverage between 18 S and COI primer sets. While community dissimilarity patterns were consistent across primers and sampling effort, richness patterns were not, suggesting that richness estimates are extremely sensitive to primer choice and sampling effort. The survey detected 64 potential NIS, with COI identifying more known NIS from port checklists but 18 S identifying more operational taxonomic units shared between three or more ports that represent un-recorded potential NIS. Overall, we conclude that eDNA metabarcoding surveys can reveal global similarity patterns among ports across a broad array of taxa and can also detect potential NIS in these key habitats. However, richness estimates and species assignments require caution. Based on results of this study, we make several recommendations for port eDNA sampling design and suggest several areas for future research.

From top to bottom: Do Lake Trout diversify along a depth gradient in Great Bear Lake, NT, Canada?

Depth is usually considered the main driver of Lake Trout intraspecific diversity across lakes in North America. Given that Great Bear Lake is one of the largest and deepest freshwater systems in North America, we predicted that Lake Trout intraspecific diversity to be organized along a depth axis within this system. Thus, we investigated whether a deep-water morph of Lake Trout co-existed with four shallow-water morphs previously described in Great Bear Lake. Morphology, neutral genetic variation, isotopic niches, and life-history traits of Lake Trout across depths (0-150 m) were compared among morphs. Due to the propensity of Lake Trout with high levels of morphological diversity to occupy multiple habitat niches, a novel multivariate grouping method using a suite of composite variables was applied in addition to two other commonly used grouping methods to classify individuals. Depth alone did not explain Lake Trout diversity in Great Bear Lake; a distinct fifth deep-water morph was not found. Rather, Lake Trout diversity followed an ecological continuum, with some evidence for adaptation to local conditions in deep-water habitat. Overall, trout caught from deep-water showed low levels of genetic and phenotypic differentiation from shallow-water trout, and displayed higher lipid content (C:N ratio) and occupied a higher trophic level that suggested an potential increase of piscivory (including cannibalism) than the previously described four morphs. Why phenotypic divergence between shallow- and deep-water Lake Trout was low is unknown, especially when the potential for phenotypic variation should be high in deep and large Great Bear Lake. Given that variation in complexity of freshwater environments has dramatic consequences for divergence, variation in the complexity in Great Bear Lake (i.e., shallow being more complex than deep), may explain the observed dichotomy in the expression of intraspecific phenotypic diversity between shallow- vs. deep-water habitats. The ambiguity surrounding mechanisms driving divergence of Lake Trout in Great Bear Lake should be seen as reflective of the highly variable nature of ecological opportunity and divergent natural selection itself.

Migration tactics affect spawning frequency in an iteroparous salmonid (Salvelinus malma) from the Arctic.

Otolith strontium and multi-year mark-recapture information were used to characterize associations between migration patterns and spawning frequencies in an anadromous Dolly Varden (Salvelinus malma) population (Rat River, Northwest Territories, Canada) that undertakes a long migration between freshwater spawning/overwintering (Fish Creek; a tributary to Rat River) and marine feeding habitats (Beaufort Sea) (~800 km round trip). Reconstructions of lifetime annual migration histories among otolith annuli was matched to information on reproductive status (current-year 'spawner' or 'non-spawner') that was known in two different, sometimes successive, years for each fish. Two migratory life histories were observed: fish either migrated annually after smoltification or periodically skipped an annual ocean migration to remain in freshwater and spawn. Different spawning frequencies were detected where fish not migrating annually tended to spawn in alternate years (84.6%) more often than those migrating annually (50%). Additionally, annually migrating fish had lower longevity (≤9 years vs. ≤13 years). The evaluation of differences in spawning frequency between sexes, independent of migration tactic, revealed males (84.6%) skipped spawning more often than females (51.2%) suggesting fitness trade-offs between life histories differ between sexes. Further, some fish returned from the sea considerably earlier than the majority of other current-year migrants. Our findings demonstrate intrapopulation diversity in migration behaviour and reproductive frequency.

Multiple generalist morphs of Lake Trout: Avoiding constraints on the evolution of intraspecific divergence?

A generalist strategy, as an adaptation to environmental heterogeneity, is common in Arctic freshwater systems, often accompanied, however, by intraspecific divergence that promotes specialization in niche use. To better understand how resources may be partitioned in a northern system that supports intraspecific diversity of Lake Trout, trophic niches were compared among four shallow-water morphotypes in Great Bear Lake (N65° 56' 39″, W120° 50' 59″). Bayesian mixing model analyses of stable isotopes of carbon and nitrogen were conducted on adult Lake Trout. Major niche overlap in resource use among four Lake Trout morphotypes was found within littoral and pelagic zones, which raises the question of how such polymorphism can be sustained among opportunistic generalist morphotypes. Covariances of our morphological datasets were tested against δ13C and δ15N values. Patterns among morphotypes were mainly observed for δ15N. This link between ecological and morphological differentiation suggested that selection pressure(s) operate at the trophic level (δ15N), independent of habitat, rather than along the habitat-foraging opportunity axis (δ13C). The spatial and temporal variability of resources in Arctic lakes, such as Great Bear Lake, may have favored the presence of multiple generalists showing different degrees of omnivory along a weak benthic-pelagic gradient. Morphs 1-3 had more generalist feeding habits using both benthic and pelagic habitats than Morph 4, which was a top-predator specialist in the pelagic habitat. Evidence for frequent cannibalism in Great Bear Lake was found across all four morphotypes and may also contribute to polymorphism. We suggest that the multiple generalist morphs described here from Great Bear Lake are a unique expression of diversity due to the presumed constraints on the evolution of generalists and contrast with the development of multiple specialists, the standard response to intraspecific divergence.

Microsatellite and mtDNA analysis of lake trout, Salvelinus namaycush, from Great Bear Lake, Northwest Territories: impacts of historical and contemporary evolutionary forces on Arctic ecosystems.

Resolving the genetic population structure of species inhabiting pristine, high latitude ecosystems can provide novel insights into the post-glacial, evolutionary processes shaping the distribution of contemporary genetic variation. In this study, we assayed genetic variation in lake trout (Salvelinus namaycush) from Great Bear Lake (GBL), NT and one population outside of this lake (Sandy Lake, NT) at 11 microsatellite loci and the mtDNA control region (d-loop). Overall, population subdivision was low, but significant (global F(ST) θ = 0.025), and pairwise comparisons indicated that significance was heavily influenced by comparisons between GBL localities and Sandy Lake. Our data indicate that there is no obvious genetic structure among the various basins within GBL (global F(ST) = 0.002) despite the large geographic distances between sampling areas. We found evidence of low levels of contemporary gene flow among arms within GBL, but not between Sandy Lake and GBL. Coalescent analyses suggested that some historical gene flow occurred among arms within GBL and between GBL and Sandy Lake. It appears, therefore, that contemporary (ongoing dispersal and gene flow) and historical (historical gene flow and large founding and present-day effective population sizes) factors contribute to the lack of neutral genetic structure in GBL. Overall, our results illustrate the importance of history (e.g., post-glacial colonization) and contemporary dispersal ecology in shaping genetic population structure of Arctic faunas and provide a better understanding of the evolutionary ecology of long-lived salmonids in pristine, interconnected habitats.