Kara J. Andres

Freshwater biodiversity is declining at a pace that outstrips the capacity of existing monitoring approaches both in temporal and spatial dimensions, highlighting the urgent need for rapid and scalable assessment and attribution of biodiversity states and changes. Here we present a global assessment and unified analysis of riverine fish biodiversity using environmental DNA collected from 1,818 sites across 113 river systems spanning 5 continents. We quantified species richness, functional redundancy, phylogenetic diversity and genetic sequence diversity, and related them to drainage characteristics. Our results showed that environmental DNA effectively captured global patterns of multi-faceted riverine fish biodiversity and disentangled the roles of climate and different aspects of human activities in shaping biodiversity–area relationships. The accumulation of biodiversity with catchment area was consistently enhanced in warmer climates, while human activities weakened this scaling. Further, species richness, functional and genetic sequence diversity exhibited stronger negative responses to human activities in larger catchments. In contrast, phylogenetic diversity was negatively affected by human activities, yet this effect diminished as catchment area increased, highlighting the facet-dependent nature of biodiversity responses to environmental gradients and catchment characteristics. Our findings demonstrate the power of environmental-DNA-based datasets for harmonized, multi-faceted biodiversity assessments, offering a scalable approach for detecting and attributing biodiversity change and informing conservation strategies under accelerating global change.

The analysis of environmental DNA (eDNA) has revolutionized biodiversity assessments in aquatic ecosystems, enabling non‐invasive monitoring of fish communities across diverse regions. However, the global comparability of these eDNA datasets remains ambiguous due to heterogeneous sampling protocols and bioinformatic workflows across studies, making it difficult to assess how robust and comparable the biodiversity patterns inferred from these datasets actually are. Here, we conducted a meta‐analysis of 58 riverine fish eDNA metabarcoding studies, covering 1818 sampling sites worldwide, to evaluate the robustness of eDNA‐derived biodiversity patterns. We found that species richness estimates and metrics of community structure derived under a common bioinformatic workflow were overall consistent with those of original analyses, despite the relatively high variability in bioinformatic analyses in the respective original studies. Contrastingly, congruence of species identity varied more extensively across datasets, mostly reflecting different completeness and regional relevance of reference databases. Restricting taxonomic assignment to basin‐specific species pools improved species identification accuracy, while datasets lacking publicly accessible or well‐curated reference data were more prone to mismatches. Year of sampling had a positive effect on taxonomic congruence, such that more recent studies showed increased robustness, also reflecting improved reference database coverage and enhanced species‐level identification over time and overall method congruence in more recent years. Overall, the suitability and potential of eDNA for global biodiversity monitoring is corroborating overall robust biodiversity estimates, irrespective of the bioinformatic approaches. Our study underlines the effectiveness and need for further harmonization of bioinformatic workflows and strengthened region‐specific reference databases for improved taxonomic resolution and comparability across studies.

Species abundance is a fundamental metric in ecology and conservation. Assessing how populations change across space time enables the identification of population trends and informs management and conservation decisions. However, measuring species abundance can be a challenging task, with logistical constraints, sampling biases, and detection limits inhibiting meaningful abundance estimates. Environmental DNA (eDNA) approaches have improved our ability to monitor species presence and biodiversity and may also serve as a tool for measuring species abundance. However, abundance estimates from eDNA typically rely on the correlation between species abundance and the concentration of target species' DNA in a sample, which may be hindered by complex interactions including variable amounts of DNA being shed by different individuals and environmental factors affecting DNA persistence. In this issue, Ai et al. (2025) present a new framework for estimating species abundance from eDNA that uses the amount of genetic diversity detected in a sample, specifically the number of segregating sites, to predict species abundance. The approach was developed and validated using in silico, in vitro, and in situ experiments, demonstrating improved correlations with species abundance compared to estimates based on eDNA concentration. With further improvements in detecting rare genetic variants, this approach has the potential to enhance our ability to quantify species abundance using eDNA.

Environmental DNA (eDNA) has been established as a noninvasive and efficient approach to sample genetic material from aquatic environments. Although most commonly used to determine species presence and measure biodiversity, eDNA approaches also hold great potential to obtain population-level genetic information from water samples. In this study, we sequenced a panel of multiallelic microsatellite markers from filtered water and fish tissue samples to uncover patterns of intraspecific diversity in the freshwater Round Goby (Neogobius melanostomus) across their invaded range in the Laurentian Great Lakes region. Although we found that the concentration of nuclear eDNA is lower than mitochondrial eDNA, we nonetheless detected over two-thirds of all nuclear alleles identified from genotyped tissues in our eDNA samples, with the greatest recovery of common alleles in the population. Estimates of allele frequencies and genetic variability within and between populations were detected from eDNA in patterns that were consistent with individual tissue-based estimates of genetic diversity and differentiation. The strongest genetic differentiation in both eDNA and tissues exists in an isolation by distance pattern. Our study demonstrates the potential for eDNA-based approaches to characterize key population parameters required to effectively monitor, manage, or sustain aquatic species.

Advancements in environmental DNA (eDNA) approaches have allowed for rapid and efficient species detections in diverse environments. Although most eDNA research is focused on leveraging genetic diversity to identify taxa, some recent studies have explored the potential for these approaches to detect within‐species genetic variation, allowing for population genetic assessments and abundance estimates from environmental samples. However, we currently lack a framework outlining the key considerations specific to generating, analysing and applying eDNA data for these two purposes. Here, we discuss how various genetic markers differ with regard to genetic information and detectability in environmental samples and how analysis of eDNA samples differs from common tissue‐based analyses. We then outline how it may be possible to obtain species absolute abundance estimates from eDNA by detecting intraspecific genetic variation in mixtures of DNA under multiple scenarios. We also identify the major causes contributing to allele detection and frequency errors in eDNA data, discuss their consequences for population‐level analyses and outline bioinformatic approaches to detect and remove erroneous sequences. This review summarizes the key advances required to harness the full potential of eDNA‐based intraspecific genetic variation to inform population‐level questions in ecology, evolutionary biology and conservation management.

Microbiomes play an essential role in promoting host health and fitness, but the factors affecting variation in gut microbiomes among individuals are not fully understood. Investigating the microbiome under different conditions is needed to link gut microbiomes to host physiology and potentially design manipulations to improve rearing success of captive species. In this study, we characterized the gut microbiomes of Atlantic salmon ( Salmo salar ) in individuals at different life stages, vaccination status, and hatchery origin. Microbiomes differed between age-0 sub-adults and adults, with sub-adults exhibiting higher diversity and more similar communities when compared to adults. We also found that vaccines against bacterial kidney disease reduced gut microbial diversity within individual sub-adult salmon, resulting in dissimilar gut microbial communities among individuals. The diversity and structure of microbiomes did not differ between groups of adults that were reared in two different hatcheries and sampled from the wild. Sub-adults, particularly unvaccinated sub-adults, displayed a strong core microbiome present in the majority of individuals. Our results suggest that life stage and vaccination status are essential factors in the gut microbiome development of salmon. Conditions experienced during early life stages appear to have a strong influence on the microbiome, but differences among individuals at early life stages may be lost due to environmental factors experienced later in life. The plasticity of the microbiome throughout the life of individuals may have important implications for understanding host health, with potential applications for improving the rearing and reintroduction success of the ecologically and economically important Atlantic salmon. IMPORTANCE: The Atlantic salmon ( Salmo salar ) is a globally important fisheries and aquaculture species, but the factors affecting gut microbiomes of hatchery-reared fish are not fully understood. Our study explores the influence of life stage, vaccination status, and hatchery origin on the composition and structure of the Atlantic salmon gut microbiome. We found that life stage is an important driver of gut microbiome diversity, likely driven by differences in habitat and diet. Vaccination against bacterial kidney disease led to marked declines in gut microbial diversity within individuals, resulting in highly distinct gut microbial communities among individuals. Hatchery origin did not have a strong influence on adult Atlantic salmon captured from the wild. These findings suggest that life stage and vaccination drive variation in Atlantic salmon microbiomes, but the stability and long-term implications of such variation on host health should be considered in future microbiome research.

Biodiversity surveys may require the use of multiple types of sampling gear to maximize the efficiency of species detections, yet few studies have investigated how to optimally distribute effort among gear. In this study, we conducted eDNA metabarcoding and capture‐based sampling surveys (electrofishing, fyke netting, gillnetting, and seining) to sample fish species richness in a large northern temperate lake. We evaluated the success of the sampling methods individually and in combination to determine the allocation of effort and cost across sampling gear that provides the optimal approach for lake‐wide species inventories. We found that eDNA metabarcoding detected more species than any other sampling method, including 11 species that were not detected with any capture‐based approach. Optimal gear combination analyses revealed that detected species richness is maximized when most of the effort or budget is allocated to eDNA metabarcoding, with smaller allocations to seining and fyke netting. eDNA metabarcoding and capture sampling gear showed similar patterns of spatial heterogeneity in the fish community across habitat types, with pelagic samples forming a group that was distinct from nearshore samples. Our results indicate that eDNA metabarcoding is a rapid and cost‐efficient tool for biodiversity monitoring and that assessing the complementarity of multiple sampling types can inform the development of optimal approaches for measuring fish species richness.

Advances in environmental DNA (eDNA) methodologies have led to improvements in the ability to detect species and communities in aquatic environments, yet the majority of studies emphasize biological diversity at the species level by targeting variable sites within the mitochondrial genome. Here, we demonstrate that eDNA approaches also have the capacity to detect intraspecific diversity in the nuclear genome, allowing for assessments of population‐level allele frequencies and estimates of the number of genetic contributors in an eDNA sample. Using a panel of microsatellite loci developed for the round goby (Neogobius melanostomus), we tested the similarity between eDNA‐based and individual tissue‐based estimates of allele frequencies from experimental mesocosms and in a field‐based trial. Subsequently, we used a likelihood‐based DNA mixture framework to estimate the number of unique genetic contributors in eDNA samples and in simulated mixtures of alleles. In both mesocosm and field samples, allele frequencies from eDNA were highly correlated with allele frequencies from genotyped round goby tissue samples, indicating nuclear markers can be reliably amplified from water samples. DNA mixture analyses were able to estimate the number of genetic contributors from mesocosm eDNA samples and simulated mixtures of DNA from up to 58 individuals, with the degree of positive or negative bias dependent on the filtering scheme of low‐frequency alleles. With this study we document the application of eDNA and multiple amplicon‐based methods to obtain intraspecific nuclear genetic information and estimate the absolute abundance of a species in eDNA samples. With proper validation, this approach has the potential to advance noninvasive survey methods to characterize populations and detect population‐level genetic diversity.

The round goby (Neogobius melanostomus) is among the fastest‐spreading introduced aquatic species in North America and is radiating inland from the Great Lakes into freshwater ecosystems across the landscape. Predicting and managing the impacts of round gobies requires information on the factors influencing their distribution in habitats along the invasion front, yet this information is not available for many recently invaded ecosystems. We evaluated the seasonal habitat use and biomass of round gobies in an inland temperate lake to define the spatiotemporal scope of biological interactions at the leading edge of the round goby invasion. Using novel statistical approaches, we combined hierarchical models that control for imperfect species detection with flexible smooth terms to describe non‐linear relationships between round goby abundance and environmental gradients. Subsequently, we generated accurate detection‐corrected estimates of the standing stock biomass of round gobies. Our results show seasonally differentiated habitat niches, where suitable round goby habitat in summer months is restricted to shallow depths (<18.4 m) with a mixture of vegetative and mussel cover. We found high round goby biomass of 122 kg/ha in occupied habitats during the summer, with a total lake‐wide biomass of 766,000 kg. In winter, round gobies migrate to deep offshore habitats and disperse, dramatically altering their scope for biological interactions with resident aquatic species across summer and winter seasons. The results of this study indicate that the scope of biological interactions in inland lakes may be seasonally variable, with potential for high round goby biomass in shallow lakes or at the periphery of deep lakes in the summer months. Such shallow‐water habitats may therefore present higher risk of ecological impacts from round gobies in invaded lentic ecosystems. As round gobies expand inland, consideration of seasonal habitat use will be an important factor in predicting the impacts of this pervasive invader.