Freshwater Biology

AimA detailed, comparative DNA-barcoding and morphospecies based overview of the vertical and horizontal distribution of Lake Ohrid endemic Gammarus species-flock. Re-evaluation of the origin of the species-flock dating, identification of events that putatively influenced diversification patterns in the species-flock. LocationLake Ohrid: a deep and ancient lake of tectonic origin, biosphere reserve, UNESCO World Heritage Site, located on the Macedonia/Albania border. TaxonGammarus species-flock (Amphipoda, Crustacea) MethodsExtensive sampling and DNA barcoding of 600 individuals were carried out. DNA sequences were analysed using species delimitation methods, haplotype network reconstructions, Bayesian molecular dating and demographic analysis. The COI-based delimitation results were validated with nuclear 28S RNA data. ResultsThe species flock distribution has weak horizontal but clear vertical structure. The diversity across bathymetric gradients correlates with temperature and salinity; and the highest diversity with sublittoral and springs of lakes shore. Two new MOTUs representing putatively new species are revealed and supported also by the nuclear marker. The time of flock radiation overlaps with the time of lake formation. The COI gene shows signs of positive selection and an acceleration in substitution rate. The demographic changes of the flock happened during the last ky. Main conclusionsDistribution of the Gammarus species-flock is vertically structured, reflecting habitat zonation. Parapatric speciation as one of the mechanisms in flocks diversification is suggested. Detection of new MOTU suggests that the flocks diversity is still not fully revealed. Nevertheless, failure to recover three other MOTUs suggests the loss of gammarid diversity in the lake. This represents,together with the current threats to the lake ecosystem (i.e. climate changes, development of tourism), a clear call for conservation efforts. The speciation events and demographic changes within the flock relate presumably to glacial and postglacial water level changes and to colonisation of new depth ranges and the associated springs.

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.

O_LIMicrobial communities in the hyporheic zone (HZ) are important in self-purification as the riverbed is metabolically active and responsible for the retention, storage and mineralisation of organic matter transported to the surface water. However, studies exploring HZ microbial community responses to disturbances (e.g. floods) remain scarce. C_LIO_LIHere, we characterised the microbial community structure among the three (downwelling, upwelling and intermediate) HZ points within and among gravel bars at high and low discharge levels in a dam-regulated river using 16S rRNA metabarcoding. C_LIO_LIWe observed significant dissimilarity in the microbial community at low discharge exhibiting local adaptation due to gravel bar spatial environmental heterogeneity. Moreover, the homogenisation effect resulted in similar microbial community structures among the three points within the gravel bars at high discharge. Microbial communities across adjacent gravel bars were dissimilar, potentially attributing to different bar morphologies. C_LIO_LIOur study highlights the role of spatial environmental heterogeneity in the biological processes that govern microbial community structure at three hyporheic points in gravel bars at two discharge levels. C_LIO_LIOur results are essential to understand the HZ microbial communities response to the river discharge levels. C_LI

Quagga mussels (Dreissena rostriformis bugensis) are among the most impactful invaders in freshwaters of the Northern Hemisphere. As filter-feeders, they can reduce harmful algal blooms (HABs), but their effects are expected to be dependent on cyanobacteria species and water temperature. However, conclusive studies on these traits and their combination are lacking. Here, we combined laboratory experiments with an analysis of long-term data from a temperate shallow lake 10 years before and after quagga mussel invasion, respectively. We tested the hypotheses that quagga mussel filtration rates in the laboratory would 1) vary among common cyanobacteria species and 2) decrease above a critical temperature. Regarding the field data, we expected that 3) quagga mussels can reduce the summer biovolume of palatable cyanobacteria, but that 4) this effect disappears above a critical temperature. Our results support all four hypotheses. In laboratory experiments, Dolichospermum flos-aquae was classified as palatable to quagga mussels, while Aphanizomenon flos-aquae, Anabaenopsis elenkinii and Microcystis aeruginosa were less-palatable cyanobacteria. Filtration rates decreased above 28.9{degrees}C (CI: 27.6-30.2{degrees}C) with mussels dying at 32{degrees}C. Our long-term lake data show that cyanobacteria biovolumes were lower after quagga mussel invasion, but only below 27.7{degrees}C (CI: 26.9-28.4{degrees}C), confirming a critical thermal window for quagga mussel filtration. Global warming will therefore facilitate HABs by increasing the growth rates of cyanobacteria and reducing the filtration rates of quagga mussels above critical summer water temperatures, which are increasingly being reached in invaded lakes. This critical thermal window must be considered when making HAB predictions. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=93 SRC="FIGDIR/small/707163v1_ufig1.gif" ALT="Figure 1"> View larger version (19K): org.highwire.dtl.DTLVardef@175851eorg.highwire.dtl.DTLVardef@76a481org.highwire.dtl.DTLVardef@12a3965org.highwire.dtl.DTLVardef@11e3e7d_HPS_FORMAT_FIGEXP M_FIG C_FIG

Despite its widespread distribution in North America, many populations of walleye (Sander vitreus) declined to the point where restoration measures, including restocking, are necessary. In this study, we compared population structure and dietary composition of walleye populations in two degraded lakes and two non-degraded lakes in the Abitibi-Temiscamingue region of Quebec, Canada. Food resources were assessed for walleye larvae, young-of-the-year, juveniles, and adults. Growth and relative abundance were also quantified for young-of-the-year, juvenile, and adult walleye. Young-of-the-year were more abundant and grew faster in degraded lakes compared to non-degraded controls, benefiting from high populations of spring zooplankton, which are a critical larval resource. The simplified food webs in degraded lakes lacked pollution-sensitive macroinvertebrates, which resulted in walleye diets being even more dominated by fish. Although juveniles and adults were equally or more abundant in degraded lakes compared to control lakes, premature adult mortality compromised population stability. We recommend focusing on improving adult fish habitat, manage for prey species, and review fishing regulations to enhance the survival of mature walleye and ensure sustainable populations.

Dissolved organic carbon (DOC) concentrations in many freshwater ecosystems of the northern hemisphere have increased in recent decades due to additional terrestrial inputs. This phenomenon, known as brownification, can strongly alter the physical, chemical, and biological traits of aquatic ecosystems. Extreme rainfall can also cause sudden brownification, known as blackwater events in rivers, while longer term effects on lakes are unknown. Here, we investigated the resilience of a small, temperate, shallow lake to a strong natural flooding-induced brownification event in 2011-2012. From initial DOC and total phosphorus (TP) concentrations of ~12 and 0.04 mg L-1, respectively, the lake rapidly reached peak DOC and TP concentrations of 60 and 0.35 mg L-1, respectively. By the following year, water levels had returned close to initial values, yet two additional years of monitoring (until summer 2015) and a more recent sample in spring 2019 showed that the lake did not fully return to its pre-brownification state. Instead, DOC and TP concentrations plateaued at concentrations respectively 1.5-fold and twofold greater than pre-brownification values within less than two years and remained at these concentrations in spring 2019. During this initial recovery period the lake exhibited a decline of phytoplankton and a partial recovery of summer periphyton biomass and production, albeit a full return to pre-brownification values was not recorded in either case. DOC and TP concentrations were positively correlated to phytoplankton biomass and negatively to periphyton. As increases in phytoplankton production outpaced decreasing periphyton production, the net result of this brownification event has been an increase in whole-lake areal summertime primary production. This incomplete resilience to a flooding-induced brownification event implies consequences for several ecological and biogeochemical functions of shallow lakes that warrant further investigation and might contribute to the gradual increase of freshwater DOC concentrations in the northern hemisphere.

Trophic rewilding restores top-down interactions and may, in turn, drive novel patterns of evolutionary change. Predatory Northern pike invaded lakes in Alaska and eliminated native fish fauna. In 2018, pike were extirpated to reset these ecosystems. Contrasting trophic ecomorphs (benthic vs. limnetic) of three-spine stickleback were then re-introduced as part of whole-lake rewilding experiments. We examined the top-down effects of stickleback on phenotypic change in Daphnia over three years in lakes that naturally contained sticklebacks and compared such trends with the experimental lakes. We observed consistent phenotypic differences between lakes with naturally occurring populations of benthic vs. limnetic stickleback. The patterns of phenotypic divergence indicate that predatory selection is stronger on Daphnia in lakes with benthic stickleback. The trends in the experimental lakes were temporally variable and opposed the trends observed in natural lakes. Rewilding provides insights into the inconsistencies that can manifest between short- and long-term influences of natural selection.

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.

O_LIEnvironmental conditions bottlenecking species population demographics are less known in cold regions with harsh winters despite of global concerns of declining freshwater mussels (Unionidae). Few studies examined both cold and summer environments in attempts to promote habitat conservation of Unionidae species. C_LIO_LIWe identified the taxonomically confused Unionidae species using phylogenetic analysis with mtDNA (COI region) and tested the hypothesis that winter mortality is the main cause of contrasting population demographics and structures in floodplain lakes in northern Japan. Demographic surveys were conducted in two lakes, which contrasted in recruitment rates, over one year including 4-month ice-covered periods C_LIO_LIAlthough previous studies have identified freshwater mussels as introduced Anemina arcaeformis (Heude 1877) based on morphology, this study confirmed the focal species as potentially native Buldowskia iwakawai (Suzuki, 1939). High winter mortality (30-40%) of adult mussels was found, although the mortality did not significantly differ between the populations with contrasting recruitment. C_LIO_LISurprisingly, the annual mortality was much lower in juveniles (10%) than in gravid and nongravid adult individuals (40-75%). The main inter-population difference was attributed to the higher summer mortality of gravid females, but not juveniles and non-gravid individuals, in the population with low recruitment. C_LIO_LIThese results collectively suggest that summer hypoxia combined with physiological stresses on females in winter is a likely population growth-limiting mechanism. To prevent a chain of adult abundance decreases in winter and high mortality of gravid mussels and newly born juveniles in summer, improvements in summer habitat conditions are necessary, while winter conditions need to be considered simultaneously. Increases in water circulation rates and alleviations of hypoxic conditions is an option for short-term habitat improvement approach. The current study sheds light on the winter-mediated mortality of freshwater mussels in shallow eutrophic floodplain lakes and contributes to improved management strategies for degraded floodplain waterbodies. C_LI

O_LIGlacial relict mysid and amphipod crustaceans are characterised by their affinity for cold and well-oxygenated waters and inability to disperse upstream or by external agents. These crustaceans occur in large and deep lakes of Northern and Central Europe and North America, their distributions shaped by glaciation events. In Europe, along the southern edge of their distribution (Germany, Poland, Lithuania, Belarus), glacial relict crustaceans are threatened by eutrophication and global warming. C_LIO_LIThis study assesses the status of three glacial relict malacostracan species in Lithuania; the amphipods Monoporeia affinis and Pallaseopsis quadrispinosa, and mysid Mysis relicta, and models their abundance as a function of environmental variables and the presence of invasive Ponto-Caspian mysids and amphipods. C_LIO_LIOur results revealed that M. affinis is likely extinct in the country, whereas M. relicta was found in only 9 out of 16 locations from which it was previously recorded. The distribution of P. quadrispinosa appears to be shrinking. C_LIO_LILake depth and water flowthrough intensity were significantly and positively associated with the relative abundance of relict mysids and amphipods, but no association was found with lake size or the presence of invasive Ponto-Caspian crustaceans. C_LIO_LIWe conclude that urgent action to mitigate the effects of nutrient run-off is needed to improve the status of glacial relict and other species that require good water quality. We also propose the re-introduction of glacial relict species in Lake Dr[u]k[s]iai, where they went extinct during the operation of the Ignalina nuclear power plant that heated the lake, but where deep-water environmental conditions have improved following the powerplant closure in 2010. C_LI

(1) Picophytoplankton are important primary producers, but not always adequately recognized, e.g. due to methodological limitations. (2) In this study, we combined flow cytometry and metabarcoding to investigate seasonal and spatial patterns of picophytoplankton abundance and community composition in the Elbe estuary. (3) Picophytoplankton (mostly picoeukaryotes such as Mychonastes and Minidiscus) contributed on average 70 % (SD = 14 %) to the total phytoplankton counts. In the summer picocyanobacteria (e.g. Synechococcus) played a more significant role. The contributions of picophytoplankton to the total phytoplankton were particularly high from summer to winter as well as in the mid estuary. However, at salinities of around 10 PSU in the mixing area the proportion of picophytoplankton was comparably low (average 49 %, SD = 13 %). (4) Our results indicate that picophytoplankton prevail in the Elbe estuary year-round with respect to cell counts. Picophytoplankton could occupy important niche positions to maintain primary production under extreme conditions where larger phytoplankton might struggle (e.g. at high or low temperature and high turbidity), and also benefit from high nutrient availability here. However, we did not find evidence that they played a particularly significant role at the salinity interface. Our study highlights the importance of including picophytoplankton when assessing estuarine phytoplankton as has been suggested for other ecosystems such as oceans.

Phenotypic plasticity is an important adaptation to spatial and temporal environmental variations. For submerged macrophytes, adaptation to water depth and light variation is particularly important. To determine the morphological and physiological adaptive strategies of Vallisneria natans at different water depths and light conditions, we combined field investigation, light control experiment and in situ physiological response experiment. In the field investigation and the light control experiment, both water depth and light intensity had prominent effects on the morphological of V. natans, especially in fresh weight and leaf length. The leaf length elongated more rapidly at intermediate water depth sites with lower light intensity. In the in situ experiment, the survival boundary of V. natans is 5.5 m in Lake Erhai. Below this depth, the chlorophyll-a content increased gradually with increasing water depth. Our results demonstrated that V. natans can adapt to water depth and light availability by changing morphological, physiological and resource allocation. At low light condition, V. natans invested more resource for light acquisition, simultaneously, changing the photosynthetic pigment content to compensate for light attenuation; conversely, more resource was directed towards reproduction. These results will provide new insight for species selection when conducting aquatic plants restoration in freshwater ecosystem. HIGHLIGHTSO_LIWater depth and light availability affect the morphology, physiology, and resource allocation of V. natans. C_LIO_LIAn alternative resource allocation pattern of V. natans could shift between light acquisition and reproduction. C_LI

The Laurentian Great Lakes (LGL) formed following the retreat of the Wisconsin glaciation 15-9 ka and provided a vast volume of freshwater habitat for coregonines, which included a diverse endemic complex of ciscoes. However, anthropogenic impacts in the 19th and 20th centuries resulted in declines and losses of ciscoes, leaving only Lake Superior with a relatively intact complex that still dominates its prey fish community. In the 1920s, W. Koelz described six putative cisco species from Lake Superior: Coregonus artedi, C. hoyi, C. kiyi, C. zenithicus, C. reighardi, and C. nigripinnis. Of these, artedi, hoyi and kiyi remain extant and abundant in Lake Superior, zenithicus is considered extirpated throughout the LGL except in Lake Superior, and reighardi and nigripinnis are considered extinct and had been synonymized as zenithicus in Lake Superior. Using an integration of morphological and genetic approaches, we present results reaffirming the presence of artedi, hoyi, and kiyi, and new evidence for the presence of reighardi, a species thought to be extinct. Our analysis did not elucidate the presence of zenithicus and nigripinnis, leaving their status unresolved. The principal morphological characters that discriminate artedi, hoyi, kiyi, and reighardi are related to habitat and trophic (ecological) specialization and foster niche segregation: eye size, gill raker length, and gill raker spacing, the same as those that foster niche segregation in subarctic European whitefishes. The discovery of reighardi is expected to stimulate new interest in the development of conservation and restoration plans for rare and imperiled cisco species in the LGL and other freshwater lakes throughout the Northern Hemisphere, and our integrated morphological and genetic approach can benefit these efforts by providing insights for the evolution, diversity, and ecology of ciscoes.

Acute exposure to ionizing radiation has well-documented, immediate negative consequences for individuals. However, the evolutionary consequences for populations exposed to ionizing radiation is unclear. For example, a meta-analysis of taxa exposed to Chernobyl fallout found some evidence for elevated mutation rates in animal and plant taxa; however, in people, de novo mutation rates in offspring of parents exposed to radiation during and after the Chernobyl accident were no higher than controls. Furthermore, whether irradiation and increased mutation rates drive adaptation to radiation also has mixed support. Ambiguity in both cases likely arises from the difficulty of studying mutation rates and adaptation after rare nuclear events whose ionizing radiation is distributed heterogeneously in time and space. Here, we report an attempt to better address this difficulty with a "resurrection ecology" study of Daphnia spp. in Utah lakes that experienced nuclear fallout from US Department of Energy weapons testing in the 1950s and 1960s. The idea was to recover dormant Daphnia eggs from sediment cores that spanned the nuclear testing era in the American West. We predicted that survival and fecundity of eggs hatched in the lab would show fitness declines correlated with ionizing radiation fallout and a potential recovery once nuclear testing stopped. We successfully obtained multiple cores from three lakes that dated back to the 1800s. We isolated >4700 dormant eggs from those cores, spanning the nuclear era, but were only able to hatch a single egg in the lab. Thus, we could not conduct life history experiments to test our prediction. The purpose of this manuscript, therefore, is to describe the study and make our radioisotope core dating and sedimentation data available to other paleolimnological researchers. We also report a side study of stable isotope change through time measured from dormant eggs and the sediment.

Interannual fluctuations in the abundance of young-of-the-year (YOY) European perch (Perca fluviatilis) were studied in two large French peri-alpine lakes. Using a 12-year dataset of late-summer hydroacoustic surveys, we quantified YOY perch densities and explored their relationship with environmental drivers through Generalized Additive Models (GAMs). Our analysis revealed a strong link between surface temperature during the embryonic phase and YOY recruitment success. Embryonic thermal conditions alone explained 44 to 88% of the variability in YOY density across lakes, confirming temperature as a key predictor of perch recruitment. In contrast, trophic factors and adult stock showed no consistent trends. These findings validate in natura the central role of spring thermal regimes in shaping recruitment and provide a framework to better understand recruitment variability under ongoing climate change.

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.

O_LIUnderwater sediments are a natural archive of biological information. Reconstruction of past fauna has been conducted for various taxonomic groups using morphological remains and DNA derived from these remains. However, information on past occurrences of fish species, the top predator of lake ecosystems, could have been reproduced only in exceptional environments, and past quantitative information on fish, particularly in lake ecosystems, has been a knowledge gap in reconstructing past fauna. Tracking the quantitative fluctuations of fish is essential for reconstructing multiple trophic levels of organisms in lake ecosystems. C_LIO_LITo acquire past quantitative fish information from lake sediments, we collected approximately 30 cm-length of underwater sediments in Lake Biwa. We extracted sedimentary environmental DNA (eDNA) and measured temporal fluctuations in the eDNA concentration of the native and fishery target species Plecoglossus altivelis and Gymnogobius isaza. For P. altivelis, we examined the possibility of tracking quantitative fluctuations by comparing sedimentary eDNA with recorded catch per unit effort (CPUE). C_LIO_LIThe chronology of the sediments allowed us to obtain information on sediments collected in Lake Biwa over the past 100 years. The deepest depths at which sedimentary eDNA was detected were 30 and 13 cm for P. altivelis and G. isaza from the surface, corresponding to approximately 100 and 30 years ago, respectively. In the comparison of sedimentary eDNA concentrations and biomass, we found a significant correlation between the CPUE of P. altivelis and its sedimentary eDNA concentration adjusted to compensate for DNA degradation. Sedimentary eDNA fluctuations were observed in P. altivelis, possibly reflecting the abundance fluctuation due to variations in the main food resources of zooplankton. C_LIO_LIOur findings provide essential pieces for the reconstruction of past fauna of lake ecosystems. The addition of quantitative information on fish species will reach a new phase, for instance, by investigating population shifts or biological interactions in the reconstruction of past fauna in lake ecosystems. C_LI

Salinity is an environmental factor that strongly characterizes the habitat use patterns of aquatic organisms. However, knowledge is biased toward the effect of differences in osmotic pressure among salinity habitats; how ambient specific gravity (SG) differences determine species distribution is scarcely understood. Diadromous fish, which migrate between marine and freshwater habitats, may encounter this SG problem when they are unexpectedly landlocked in or colonize freshwater areas with low environmental SG. This is particularly serious for planktonic larval fish, which must maintain neutral buoyancy for foraging and passive locomotion, although their swimbladders are generally underdeveloped. Then, we hypothesized that the SG problem limits the establishment of freshwater resident populations in marine-originated diadromous fishes. To test this hypothesis, the SG modulation ability of newly hatched larvae was compared among three closely related diadromous goby species in Gymnogobius, one of which has freshwater resident populations. The aquarium experimental results did not support that only the species deriving freshwater residents can maintain neutral buoyancy even in freshwater conditions; that is, all three species could modulate their body SG almost equally to those of both fresh and sea waters. This suggests that the ability to maintain neutral buoyancy in freshwater had been pre-adaptively acquired prior to larval freshwater colonization. On the other hand, it is highly noteworthy that the early larvae of the target group maintained neutral buoyancy in various SG environments using swimbladders, which is the first such evidence in teleosts.

Pacific salmon are keystone species to North Pacific freshwater, coastal, and oceanic ecosystems, but many populations have declined or become more variable in recent decades due to anthropogenic impacts and climate change. Long-term records are needed to understand past changes, identify ecosystem stressors, and guide restoration. We used sedimentary DNA (sedDNA), an emerging paleoecological approach offering broader taxonomic information than traditional methods, to reconstruct ecosystem changes across five Pacific salmon nursery lakes in British Columbia (Canada). DNA metabarcoding targeting the 18S ribosomal RNA gene V7 region was used to track shifts in eukaryotic communities including algae and invertebrates over centuries to millennia. Most lakes showed notable algal community shifts over the past two centuries, with declining green algae and rising diatom relative abundances. Chrysophytes and dinoflagellates also increased over the past century in most lakes, likely driven by stronger thermal stratification, which favored these motile and mixotrophic algae that are capable of vertical migration and flexible nutrient acquisition. We contextualized the trajectories of each core through an ordination analysis based on 98 lakes distributed across British Columbia, which identified land-use changes and longer growing seasons as potential drivers. Network analyses of the sedDNA time series revealed decreasing modularity and increasing connection across lakes, suggesting a shift in resilience mechanisms from between-module buffering by compartmentalized specialists to within-guild insurance via functional overlap among generalists. Our findings demonstrate that sedDNA provides taxonomically rich, long-term insights into aquatic ecological dynamics, which are foundational for understanding and protecting Pacific salmon nursery habitats.

Trophic diversification is one of the main mechanisms driving the adaptive radiation. The polyploid lineage of the cyprinid genus Labeobarbus represent an excellent model for studying the trophically-based adaptive radiation in either lacustrine or riverine environments. Recently discovered four diversifications in rivers of the Ethiopian Highlands (East Africa) demonstrate independently evolved repeated mouth polymorphisms each represented by four core mouth phenotypes: (i) generalized, (ii) thick-lipped, (iii) scraping, and iv) large-mouthed. Mouth phenotypes in some radiations can be further divided to subtypes representing from four to eight sympatric ecomorphs. Using the stable isotope and gut content analyses we tested hypothesis on trophic resource partitioning within each radiation, revealed disparity in degree of diversification between radiations and tried to reconstruct the process of trophic diversification. Three of four radiations demonstrated partitioning of trophic resources within five trophic niches: i) detritophagy, ii) macrophytophagy, iii) invertivorous benthophagy, iv) periphyton feeding, and v) piscivory. The studied riverine radiations were likely at the different stages of the diversification. One radiation having a similar set of mouth phenotypes was not trophically divergent displaying a remarkable decouple of form and function. A unique case of ecologically non-functional mouth polymorphism at an incipient stage of trophic diversification supports a concept of the plasticity-first evolution. This phenomenon stems from the pre-existing genomic templates of mouth polymorphism ancestrally inherited upon the allopolyploid origin of the Labeobarbus lineage. The predetermined and preadaptive mouth polymorphism can be considered a key innovation of the Labeobarbus that promoted to resource-based diversification via adaptive radiation.