Freshwater Biology

Freshwater salinization is of increasing concern for integrity and functioning of freshwater habitats worldwide. Experiments so far often have studied drastic salt additions, while gradient designs have been performed less commonly. We tested the effect of freshwater salinization in a mesocosm exposing the plankton community of the oligotrophic Lake Lunz, Austria, to a four-fold salinization gradient (control, 0.2, 1, a 5 ppt salt). Salinity was manipulated in a factorial design with enrichment, with 10 g L-1 and 30 g L-1 phosphorus, resulting in 8 treatments with 3 replicates each. We followed the effects of salinization on diversity, community composition and resource use over 36 days. Community composition was assessed by amplicon sequencing, Diversity loss and community turnover followed upon salt addition. All levels of salinization caused pronounced changes in community composition, with 5 ppt causing the most drastic changes. Salinization caused trophic downgrading by kicking out especially protistan consumers and rotifers, while some green algae and chrysophytes were especially tolerant, resulting in reduced phylogenetic and functional diversity with increasing salinization. In line with reduced top down control, salinization affected temporal variability in chlorophyll-a (chl-a) and resource use (RUE), with higher salinity causing more extreme fluctuations in chl-a and RUE. Enrichment overall aggravated salinization, enhancing temporal turnover and temporal fluctuations in resource use.

Quagga mussels (Dreissena rostriformis bugensis) are ecosystem engineers that can alter nutrient cycling, benthic-pelagic coupling, and food-web structure in deep lakes. Although their invasion trajectories are well documented in the Laurentian Great Lakes in North America, depth-specific population dynamics remain poorly resolved in recently invaded European perialpine lakes. We analyzed five annual lake-wide surveys (2021-2025) from 54 stations spanning 2.4-253 m depth in Lake Constance to quantify changes in quagga mussel density, biomass, and shell-length distribution. Contrary to expectations of lake-wide exponential growth, shallow-water populations (< 20 m) showed no significant increase during the study period and appear to have reached carrying capacity before monitoring began. In contrast, densities increased monotonically at intermediate depths (40-125 m), indicating ongoing expansion into deeper strata. Mean shell length declined with depth, and size distributions in shallow waters shifted toward larger individuals, consistent with a transition from active recruitment to somatic growth of established mussels. Compared with the Laurentian Great Lakes, Lake Constance already has substantially higher shallow-water biomass, whereas deeper invasion trajectories are broadly similar. These results show that quagga mussel invasion in deep European lakes can combine rapid littoral saturation with slower profundal expansion, complicating direct transfer of predictions from the Great Lakes. Continued depth-stratified monitoring will be essential for anticipating future ecosystem effects in perialpine lakes.

Inland waters in the Northern Hemisphere are experiencing increased annual runoff due to higher overall precipitation as well as intensified short-term events such as heavy rainfall, floods and storms. These events affect the total loading and variability of inputs of allochthonous, coloured dissolved organic matter (cDOM) and inorganic nutrients into lakes. Previous studies have shown that increased total cDOM and inorganic nutrient loads affect phytoplankton biomass and metabolic rates, but it is unknown how the effects of different cDOM and nutrient pulse scenarios are modified by spatial and seasonal differences in lake characteristics. Here, we conducted a coordinated, standardized mesocosm experiment across three lakes with different ambient cDOM and nutrient concentrations. In two of these lakes, the experiment was implemented in two seasons. The same total amounts of cDOM, nitrate and phosphate were added to all mesocosms, but in pulses that differed in intensity and frequency. We found that pulse intensity and frequency affected chlorophyll a and phycocyanin concentrations and metabolic rates, i.e. gross primary production and respiration, differently. Specifically, more pronounced effects were found in response to the extreme pulse scenario compared to those with more frequent, smaller pulse additions. Furthermore, the effects were mainly temporary and varied more among lakes than between seasons. The clearest differences between the extreme and more gradual runoff scenarios were found in the lake with the lowest background cDOM and nitrate concentrations, likely because lower light limitation and possibly stronger initial N-limitation caused a faster response to the nutrient addition. Our results highlight that both antecedent lake conditions and characteristics of runoff events can affect phytoplankton biomass and metabolic rates and that comparative experimental approaches are needed to reveal the complexity of the responses.

The prevalence of nitrogen limitation and nitrogen-phosphorus co-limitation (henceforth referred to as nitrogen-related limitation) in Norwegian lakes and their relationships with atmospheric nitrogen deposition, climate, dissolved organic matter (DOM), and catchment characteristics were assessed across space and time. Routine monitoring data from 1,529 lakes in the national Vannmiljo database were analyzed for two multi-year periods (1995-2009 and 2010-2025). Limitation was inferred using the molar NO--N/TP ratio as an indicator of dissolved inorganic nitrogen availability. Nitrogen-related limitation was widespread in both periods and exhibited strong regional structure, with highest prevalence in northern regions and lowest prevalence in southwestern Norway. Overall prevalence increased from 31% to 38% between periods, with significant increases in western regions. Regional-scale models identified climate, forest cover, DOM, agriculture, and atmospheric nitrogen deposition as predictors of limitation probability, whereas study period per se and bog/peatland cover were not significant. At the local scale, atmospheric nitrogen deposition and DOM were the only consistent predictors, with substantially lower explanatory power than at the regional scale. These results indicate that large-scale environmental gradients play a major role in shaping nutrient stoichiometry in Norwegian lakes. Because the monitoring dataset primarily represents lakes affected by human activities, the findings are particularly relevant for water management. The widespread occurrence of nitrogen-related limitation suggests that nitrogen availability may influence phytoplankton growth in many systems and that dual-nutrient management strategies addressing both nitrogen and phosphorus may be required in many regions.

Lake Constance is a pre-Alpine, monomictic, oligotrophic lake situated at the southern end of Germany composed of two main water bodies: deep, oligotrophic Upper Lake Constance (ULC) and the shallow, more mesotrophic Lower Lake Constance (LLC). To date, no sequencing-based study exists of the seasonal succession of the microbial plankton in Lake Constance. Over one-year, microbial plankton communities were sampled biweekly from the top 20 m of the water column in both sites and separated into nanoplankton (NP) and picoplankton (PP). Communities were analysed using rDNA amplicon sequencing: NP samples were analysed by 18S rDNA, and PP samples by 18S and 16S rDNA sequencing. Temporal community diversity was compared between sites and the effect of two major environmental perturbations, winter vertical mixing in ULC and oxygen depletion of the bottom-water layer in LLC, on the community was examined. Despite strong environmental contrasts, microbial plankton communities exhibited conserved seasonal temporal dynamics across basins. In contrast, pronounced compositional shifts occurred during mixing and oxygen depletion events. Approximately 20% of detected taxa were positively associated with these events, with log fold changes reaching 9.82, reflecting rare or undetectable taxa outside these periods. Taxa favoured by these perturbations commonly exhibited high metabolic flexibility, including mixotrophy, fermentation, or anaerobic respiration, or possessed functional traits conferring tolerance to altered redox and mixing regimes. Our results suggest that the temporal dynamics of freshwater microbial plankton communities are driven by deterministic processes and highlight the profound impact of large, and less known, environmental changes on these communities.

The global study of freshwater clams in the genus Corbicula is frequently confounded by invasive androgenetic lineages that experience mitochondrial DNA capture and clonal propagation. In contrast, the endemic Corbicula of Sulawesi's ancient lakes reproduce sexually, offering a uniquely reliable system for mitochondrial population genetics. This study provides the first population-genetic framework for two endemic species, Corbicula possoensis (Lake Poso) and C. linduensis (Lake Lindu), using the cytochrome c oxidase subunit I (COI) marker. We analysed 90 newly generated COI sequences from C. possoensis (six stations) and C. linduensis (three stations), integrated with reference sequences from GenBank, to assess genetic diversity, population structure, and phylogeographic patterns. Hierarchical AMOVA revealed deep divergence between the two lakes ({Phi}_CT = 0.607), consistent with prolonged independent isolation rather than a single shared vicariance event, as the two species do not form a sister pair in the phylogeny. Within Lake Poso, C. possoensis exhibited exceptionally high genetic diversity (24 haplotypes; h = 0.876; {pi} = 0.016) and pronounced micro-geographic structuring into three phylogeographic zones (North: Tentena and Siuri; East: Tando Nceppo and Busogo Beach; Southwest: Bancea and Pendolo), each characterised by distinct haplogroups. Remarkably, the maximum divergence between zones (K2P = 2.33%) approached the interspecific distance between C. possoensis and C. linduensis (K2P = 2.42%), indicating that within-lake mitochondrial divergence has reached near-interspecific levels. Conversely, C. linduensis displayed near-panmixia and extreme genetic depauperation (3 haplotypes; h = 0.246; {pi} = 0.0004), indicating long-term demographic stasis within a restricted habitat. The deep phylogeographic zonation in C. possoensis suggests that its discrete populations should be treated as separate Management Units (MUs) in conservation planning to preserve locally adapted gene complexes, whereas the severely depauperate gene pool of C. linduensis renders it critically vulnerable to environmental disturbance and invasive species, warranting urgent IUCN Red List assessment. To validate these mitochondrial boundaries and inform future conservation strategies, multi-marker and genome-wide reassessments are strongly recommended.

Coastal wetlands of the Laurentian Great Lakes support abundant populations of fish, invertebrates, and vegetation, though the trophic linkages connecting primary production and lower consumers is not well understood in these systems. We implemented a multiple-tracer approach to evaluate trophic pathways, pairing traditional food web isotope tracers like carbon ({delta}13C) and nitrogen ({delta}15N) with total mercury concentrations (THg). We predicted that filamentous algae would be the dominant energy resource in the diet of lower trophic-level invertebrates in the Grand River Estuary, a network of riverine coastal wetlands adjacent to Lake Michigan. In addition, we predicted that adding THg as a tracer would improve the resolution of our food web models by clarifying trophic levels and relationships between wetland species. Four basal energy sources were sampled, including filamentous algae, emergent macrophytes, submersed macrophytes, and phytoplankton, along with organic detritus. Aquatic invertebrates were sampled across multiple functional guilds to represent primary and secondary consumers and included amphipods and odonates. Our findings suggest that organic detritus is the dominant resource responsible for energetically supporting these lower trophic levels in the Grand River estuary, although submersed macrophytes were important alternative energy sources for secondary consumers. THg concentrations enhanced the resolution of dietary contribution estimates in MixSIAR models applied to consumer and source data. Isotope biplots revealed that THg concentrations were a more reliable predictor of trophic position than {delta}15N in Grand River Estuary (GRE) sites. This methodology has important implications for future food web studies in complex ecosystems such as coastal wetlands and demonstrates the novel use of mercury as an ecological tracer in a Bayesian mixing model approach.

Nutrient availability, ecosystem productivity, and consumer assemblages are intricately linked through complex interactions and feedbacks. Nutrients influence the diversity and functional roles of consumers via shifts in resource quality and quantity, and consumers can alter ecosystem production and nutrient availability. However, our understanding of how characteristics of consumers respond to and influence concomitant shifts in nutrient availability and production is limited. We quantified the response of well-studied consumer assemblages (benthic invertebrates and zooplankton) to realistic nutrient loads that altered gross primary production (GPP) and ecosystem respiration (ER). We fertilized 14 outdoor experimental ponds for 2 months and monitored total water column carbon (TC), nitrogen (TN), and phosphorus (TP), GPP, ER, and net ecosystem production (NEP) weekly. Then, we evaluated how fertilization and the variation in nutrients and metabolism caused by fertilization were related to shifts in consumer assemblages. Fertilization increased water column TN and TP and reduced TC:TP ratios, TN:TP ratios, and rates of GPP and ER. However, consumer assemblages were more tightly linked to variation in nutrient availability and production across ponds than to fertilization. Greater declines in benthic diversity occurred in ponds with higher average TN:TP ratios during the experiment. Consistent with predicted effects of cladocerans on nutrient availability, shifts in cladoceran abundances were positively associated with average water column TN:TP ratios during the experiment. Finally, elevated GPP and ER were associated with greater increases in the abundance of benthic invertebrate predators, suggesting the possibility of top-down control. Our study highlights the critical role of consumer-mediated processes in the interaction between nutrient availability and production. Manuscript HighlightsO_LIFertilization reduced pond gross primary production and ecosystem respiration rates. C_LIO_LIInvertebrate predator abundance was inversely related to gross primary production. C_LIO_LIShifts in consumer assemblages were tightly linked to nutrients and production. C_LI

O_LIEnvironmental DNA (eDNA) metabarcoding is an emerging tool for biodiversity assessment in freshwater systems, offering high-resolution insights into community composition. Here, we apply eDNA metabarcoding to evaluate the ecological impacts of Eurasian beaver (Castor fiber) activity within a seminatural enclosure in the Scottish Highlands. C_LIO_LIWe collected seasonal water samples from nine sites, six influenced by beaver dams and three control sites with no evidence of beaver engineering, across a 40-hectare enclosure. Samples were analysed for vertebrate and macroinvertebrate diversity using established 12S and COI markers. C_LIO_LIVertebrate alpha diversity did not differ significantly between beaver and control sites, likely reflecting the small spatial scale and low species richness of upland Scottish streams. However, community composition differed significantly between treatments, especially for fish (PERMANOVA, R2 = 0.55, P < 0.001), with beaver-influenced sites dominated by three-spined stickleback and control sites by brown trout. Macroinvertebrate communities showed a 78% increase in gamma diversity in beaver-modified habitats relative to controls. Species composition varied strongly with beaver presence (PERMANOVA, R2 = 0.29, P < 0.001), likely due to the creation of lentic-lotic mosaics and associated microhabitat diversity. Seasonal variation was significant in both taxonomic groups, with the lowest species richness and highest community dispersion observed in summer, probably reflecting hydrological and temperature-driven dynamics in eDNA production and transport. C_LIO_LIOur findings reinforce previous evidence that beaver dam-building activity enhances beta diversity in headwater systems. Additionally, we demonstrate that eDNA metabarcoding is a sensitive method for detecting spatial patterns in freshwater biodiversity associated with these activities at scales ranging from tens to hundreds of meters. These approaches could inform future monitoring strategies aligned with landscape-scale beaver management and reintroductions. C_LI

Understanding the genetic structure of keystone species within river networks is essential for effective conservation and management. While population differentiation of anadromous species often occurs between river systems, less research has been conducted on differentiation within rivers with smaller catchment areas. In this study, we investigated the population genetic structure of wild Atlantic salmon (Salmo salar) across the small-scale river Ronne [a] system in southernmost Sweden using Restriction-site Associated DNA sequencing (RADseq). Although the Admixture analysis did not detect clearly defined genetic clusters, significant pairwise FST values and DAPC revealed emerging population differentiation among the Ronne [a] tributaries. The observed patterns are consistent with a system characterized by connectivity, where genetic flow is present but can be reduced by behavioral and ecological factors such as spawning homing behavior and selective movements. These findings suggest that, despite overall connectivity, Atlantic salmon populations in the Ronne [a] catchment area may function as partially independent sub-populations. This highlights the importance of conservation and management strategies in fragmented river systems to consider population genetic structure to support resilient salmon populations under ongoing anthropogenic pressures.

Capelin (Mallotus villosus), a key forage fish in the Northwest Atlantic, links zooplankton to predators including commercial fishes, seabirds, and marine mammals, yet its life-cycle movements in the Gulf of St. Lawrence (GSL) remain poorly understood. Between 2022 and 2024, otoliths from 927 individuals collected during and after spawning were analysed by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Building on previous work on regional structuring, seven trace elements (Li, B, Mg, K, Zn, Sr, Ba) were used to discriminate three regions. Early-life regional signatures were inferred through an edge-to-core approach, assigning otolith core chemistry to one of these regions using quadratic discriminant analysis. The core was treated as an integrated early-life signal (late-larval to early juvenile period) rather than a strictly natal signature. Spatial variation in core chemistry was consistent across cohorts, mirroring the stability documented on the otolith edge. Results revealed widespread dispersal alongside partial regional residency: individuals with northeastern early-life signatures showed the strongest correspondence between early-life and capture regions, whereas other regions were more connected. Fish sampled during spawning were more often reassigned to their inferred early-life region than post-spawning fish, a regional-scale homing-like pattern consistent with regional spawning fidelity. This coexistence of dispersive and resident strategies likely generates a portfolio effect buffering the population against environmental variability and localised reproductive failures.

Japanese chum salmon (Oncorhynchus keta) populations supported by one of the worlds largest hatchery programs have experienced severe declines in marine survival and abundance. To evaluate long-term changes in reproductive traits, we analyzed temporal and geographic variation in female fork length (FL) and egg size from 13 hatchery-enhanced rivers in Japan during 1999-2019. Females from northern rivers tended to have smaller FL but produced larger eggs, indicating persistent latitudinal differentiation in reproductive traits. Despite this geographic structure, both FL and egg size declined synchronously among rivers over time. Using nationwide annual means, exponential decay analyses showed that egg size declined by approximately 2% per generation, whereas marine survival and population abundance declined by approximately 22% per generation. Mean egg size was positively correlated with subsequent marine survival, explaining 45% of the variation in return rates. Mean female FL explained 64% of the variation in mean egg size. Exponential decay rates estimated from standardized variables were nearly identical among egg size, marine survival, and chum salmon abundance, suggesting common underlying drivers of long-term decline.

The maintenance of endosymbiosis in cnidarians depends on the tight regulation of host immunity, cell cycle, and nutrient exchange, yet how these processes are impacted by interacting environmental stressors remains largely unknown. To address this, we employed physiological metrics, gene expression analysis, microbiome characterization, imaging (NF-{kappa}B localization, endoplasmic reticulum ultrastructure, EdU labeling), and stable isotope tracing in the model sea anemone Exaiptasia diaphana to examine the effects of heat and nitrate on these regulatory processes, individually and in combination. Heat treatment led to NF-{kappa}B activation, proteostatic stress, suppression of nutrient exchange, decreased cell-cycle progression, and microbiome restructuring, with all effects more pronounced in symbiotic than aposymbiotic anemones. In symbiotic anemones, nitrate partially offset these heat-induced responses through sustained carbon translocation, suggesting that the presence of symbionts, in conjunction with elevated nitrate, can temporarily buffer host thermal stress. However, prolonged combined exposure resulted in holobiont failure. These findings reveal that while nitrate enrichment can transiently delay the onset of bleaching, it does not preserve the regulatory networks required for symbiotic stability -- underscoring the vulnerability of cnidarian holobionts to the compounding effects of warming and nitrate pollution.

Ballast water (BW) is an important vector for the global translocation of bacterial taxa, including pathogens. Legal frameworks establishing limits on the discharge of live organisms into recipient environments have been designed to reduce risks of microbial invasions, and understanding the impacts of BW management on bacterial communities is critical for assessing the effectiveness of these practices. Here we evaluate changes in bacterial communities associated with both BW treatment (BWT) and a combined management approach of BWT plus BW exchange (BWT+E). Samples were collected on two experimental voyages designed specifically to compare BW-associated biota before and after management. Microbial community structure and inferred function were assessed based on high throughput sequencing of 16S rRNA amplicons, and bacterial indicator taxa E. coli and enterococci were analyzed using targeted qPCR. As expected, both BWT alone and BWT+E dramatically changed bacterial communities, with the latter resulting in the largest overall decreases in bacterial diversity. Increases in Gammaproteobacteria, especially in the genus Pseudomonas, were particularly notable, with concomitant decreases in Alphaproteobacteria and Bacteroidia. Shifts in predicted bacterial function associated with BWT were similar for both voyages, despite significant differences in community structure, and may represent selection for r-strategists capable of active regrowth after BWT. qPCR estimates of indicator taxa were similar to those obtained through standard culture methods but may offer increased sensitivity for detecting changes associated with management. Our results indicate that BW management is effective at reducing bacterial communities but suggest that further research is needed to understand risks associated with taxa that may survive BWT.

Allopatric isolation under contrasting environments can drive rapid phenotypic divergence, even over contemporary timescales. Rapid changes in morphology or physiology can allow organisms to adapt to biotic and abiotic characteristics of their habitats. While studying metabolism, growth and resources needs may allow to understand adaptation to several selective pressures, these traits are rarely jointly considered. We investigated morphological, growth, and metabolic divergence in two allopatric populations of Arctic charr (Salvelinus alpinus) sharing a common evolutionary origin but inhabiting contrasting environments. We combined field observations, common garden and quantitative genetic approaches to disentangle contributions of genetic divergence and plasticity to phenotypic variability. Wild adults differed in body shape and growth trajectories, potentially reflecting plasticity related to resource availability and temperature variations. Under common garden conditions, juveniles displayed inter-population differences in routine metabolic rate, its allometric scaling with body mass. These patterns suggest divergent selection on physiological traits. Despite low neutral genetic differentiation, phenotypic divergence unfolded in fewer than 100 years, suggesting that plasticity and selection can promote rapid multi-trait changes. These findings highlight that considering changes in physiological, growth and morphological traits can reveal the adaptive potential of small, isolated populations facing rapid environmental change.

Gorgonopsian therapsids represent a transitional condition in the evolution of synapsid locomotion and postcranial structure. Most descriptions of gorgonopsians have focused on cranial material, however, limiting their usefulness for informing patterns of postcranial evolution on the mammal stem. While some recent work has begun to focus on postcrania, especially the pectoral girdle and forelimbs, comparatively little data are available on the pelvic girdle, hind limbs and tail. We report a new specimen of the late Permian gorgonopsian Aelurognathus tigriceps comprising a partial skull and well-preserved postcranial skeleton, including the near-complete series of dorsal vertebrae and ribs, complete pelvic girdle, hind limbs, feet, and a nearly complete tail. The tail is longer than any other published gorgonopsian. The new material presented here provides an opportunity to better establish broader patterns of morphology in the gorgonopsian postcranial skeleton.

Downstream migration and sea entry are periods of high mortality for sea trout smolts and migration timing is a critical aspect of survival. We aimed to investigate migration timing of PIT-tagged sea trout smolts across a latitudinal gradient in five freshwater systems across Europe: Norway, Sweden, Denmark, France and Portugal. In two systems, smolt migration was further examined to assess a) differences in sex, body size and condition between autumn and spring migrants; b) the influence of spatial origin within the stream; and c) relationships between individual body size and migration date. Tagged smolts were not detected migrating in the Portuguese watershed. Spring migration timing differed significantly between watersheds in Norway, Sweden, Denmark and France. Generally, there was a trend of earlier migration at lower latitudes. Autumn vs spring migration was examined in Gudso-Denmark. Autumn migrants were larger in both length and mass, with no differences in sex ratios or body condition. Fish originating from an upstream site were more likely to migrate in the autumn compared to the spring and vice versa. Size dependent migration was found in the Swedish system, Haga [a]-Sweden, with larger individuals migrating earlier in the spring than smaller individuals. Outward-migrating smolts were also more likely to originate from a downstream site than an upstream site. Overall, these results show both large-scale geographic and fine-scale individual influences on migration timing. Given that climate change may have large impacts on migration patterns in sea trout, understanding variability in migratory patterns across a latitudinal gradient is an important tool for predicting responses to environmental changes.

Antimicrobial resistance (AMR) genes are increasingly recognized as an emerging environmental contaminant. Yet, the ecological mechanisms shaping their distribution across natural landscapes remain poorly understood. Here, we quantified AMR gene abundances in microbial communities sampled from wild fish from eight freshwater lakes on Vancouver Island and paired these gene-level measurements with fine-scale limnological and land-use data. Using droplet digital PCR, field surveys, and an iterative spatial forecasting framework that integrates Random Forest models with regression kriging, we explored how watershed-scale processes relate to variation in AMR genes across lakes. Our analyses reveal potential associations between elevated AMR gene levels, changes in water quality, deforestation, and geographic proximity to salmon aquaculture. By integrating data across biological and spatial scales, from genes within microbial communities to lake-level conditions and landscape patterns, this study illustrates the value of combining quantitative molecular measurements with geospatial modeling to identify environmental factors that may promote antimicrobial resistance in natural systems. Our approach provides a proof-of-concept and a general predictive framework for generating hypotheses and informing future monitoring efforts aimed at understanding, managing, and forecasting environmental reservoirs of resistance. SignificanceAntimicrobial resistance (AMR) genes are ancient components of environmental microbiomes. Yet, the mechanisms that generate modern hotspots of resistance across natural landscapes remain unclear. Here, we reveal how watershed-scale environmental change, including water quality metrics linked with deforestation and proximity to salmon aquaculture, predicts elevated AMR gene levels in the microbiomes of wild fish populations. By combining quantitative droplet digital PCR with ecological data and geospatial modeling, we move beyond isolated surveillance data to identify ecological mechanisms that promote antimicrobial resistance in freshwater ecosystems. This integrative approach provides mechanistic insight into why certain habitats, and the organisms within them, become reservoirs of resistance while others do not. Our findings highlight the importance of ecological context in understanding resistance evolution and offer a predictive tool for informing proactive monitoring and management strategies.

Philippine freshwater ecosystems are considered one of the most diverse ecosystems harboring numerous fish species. However, in the Philippines, these ecosystems are threatened by invasive species that potentially disrupt ecological balance. In this study, we focused on the vermiculated sailfin catfish Pterygoplichthys disjunctivus, an invasive aquarium species reported in several Philippine aquatic ecosystems. Despite its documented spread, its potential range under a rapidly changing climate remains poorly understood for the country. Hence, in this study, we utilized the MaxEnt model to predict its near-current and future habitat suitability in the Philippines. Using 11 reported occurrences, our model showed high predictive accuracy (AUC = 0.882{+/-} .034, TSS = 0.7394 {+/-} 0.154, SEDI = 0.971 {+/-} 0.019). Across the current and future scenarios, slope was the primary contributor (78.7% - 81.3%), followed by BIO 10 or the mean temperature of the warmest quarter(18% - 27.8%), and flow accumulation (0% - 5.2%). However, for the SSP126 scenario, BIO10 is projected to triple by 2050 (18 - 48%). Current projections identify high-risk regions, particularly central Luzon (Laguna de Bay and Lake Taal), the Cagayan River Valley, and portions of eastern Mindanao (Agusan Marsh and Lake Mainit). Sankey transition analysis confirms a high habitat stability rate (>73%) for high-suitability pixels in both SSPs, indicating persistent invasion risk. Overall, our study provides a framework for invasive species management and contributes to the conservation of Philippine aquatic ecosystems.

Environmental DNA (eDNA) methods offer a powerful tool for monitoring aquatic species, yet field applications remain challenged by uncertainty in DNA transport, mixing, and detection, particularly in flowing or tidally influenced systems. One approach to improve confidence in eDNA surveys is the use of controlled DNA sources (positive controls), but questions remain regarding how the biological condition of the source influences eDNA release and detectability. This study evaluated differences in eDNA concentrations emitted from live versus dead fish in a controlled, shallow, well-mixed channel. Using a fixed point-sampling design, we measured eDNA concentrations over time and modeled the effects of treatment, sampling time, temperature, and water velocity. Dead fish consistently released significantly higher concentrations of eDNA than live fish, while eDNA concentrations declined over time in both treatments. Water temperature and velocity did not significantly influence detection, and the rate of eDNA decline was similar between live and dead treatments. These findings highlight the importance of source condition and site-specific mixing dynamics when interpreting positive control experiments and underscore the value of site characterization when designing eDNA sampling protocols.