Daphnia Longispina Research Articles

Spatio-temporal variation in the zooplankton community of the Zahara-El Gastor Reservoir (Cádiz, Spain)

Reservoirs play a crucial role in providing essential social and economic services at different scales. However, in recent years, anthropogenic impacts such as climate change and pollution have increased, affecting this type of ecosystem. Zooplankton communities are fundamental for regulating the biological balance in these environments, so it is essential to understand the processes that affect their dynamics. This study investigated the seasonal and spatial variation in species abundance of the zooplankton community of the Zahara- El Gastor Reservoir, Cádiz, Spain. This reservoir it is located in the Guadalete-Barbate River basin and is included in the Sierra de Grazalema Natural Park, the area with the highest rainfall in the Iberian Peninsula. Water and zooplankton samples were collected monthly over a year at four sampling sites located along the reservoir’s long-axis to determine its physicochemical characteristics and zooplankton species composition. The reservoir presented a mesotrophic state and a monomictic thermal cycle, with the mixing period occurring in winter. Temperature, water depth and volume, phosphates, carbonates, pH, and conductivity were the main environmental variables that affected the seasonal and spatial patterns of the zooplankton community. Rotifers presented the highest number of species and were predominant at the tail of the reservoir, especially in autumn, where Polyarthra spp. reached high densities. Relatively low densities of copepods were found throughout the year. Small branchiopods and cyclopoids were predominant in summer, while Daphnia longispina was linked to the colder seasons. The zooplankton community of the Zahara-El Gastor Reservoir reflected changes in the trophic state of the reservoir both in space and time.

Combined stress of an insecticide and heatwaves or elevated temperature induce community and food web effects in a Mediterranean freshwater ecosystem

Ongoing global climate change will shift nature towards Anthropocene's unprecedented conditions by increasing average temperatures and the frequency and severity of extreme events, such as heatwaves. While such climatic changes pose an increased threat for freshwater ecosystems, other stressors like pesticides may interact with warming and lead to unpredictable effects. Studies that examine the underpinned mechanisms of multiple stressor effects are scarce and often lack environmental realism. Here, we conducted a multiple stressors experiment using outdoor freshwater mesocosms with natural assemblages of macroinvertebrates, zooplankton, phytoplankton, macrophytes, and microbes. The effects of the neonicotinoid insecticide imidacloprid (1 µg/L) were investigated in combination with three temperature scenarios representing ambient, elevated temperatures (+4 °C), and heatwaves (+0 to 8 °C), the latter two having similar energy input. We found similar imidacloprid dissipation patterns for all temperature treatments with lowest average dissipation half-lives under both warming scenarios (DT50: 3 days) and highest under ambient temperatures (DT50: 4 days) throughout the experiment. Amongst all communities, only the zooplankton community was significantly affected by the combined treatments. This community demonstrated low chemical sensitivity with lagged and significant negative imidacloprid effects only for cyclopoids. Heatwaves caused early and long-lasting significant effects on the zooplankton community as compared to elevated temperatures, with Polyarthra, Daphnia longispina, Lecanidae, and cyclopoids being the most negatively affected taxa, whereas Ceriodaphnia and nauplii showed positive responses to temperature. Community recovery from imidacloprid stress was slower under heatwaves, suggesting temperature-enhanced toxicity. Finally, microbial and macrofauna litter degradation were significantly enhanced by temperature, whereas the latter was also negatively affected by imidacloprid. A structural equation model depicted cascading food web effects of both stressors with stronger relationships and significant negative stressor effects at higher than at lower trophic levels. Our study highlights the threat of a series of heatwaves compared to elevated temperatures for imidacloprid-stressed freshwaters.

A preliminary study of subfossil Cladocera from the bottom sediments of Lake Bolshoe Miassovo (Southern Urals, Russia)

An analysis of subfossil cladocerans from the sediment core of Lake Bolshoe Miassovo (Southern Urals) was conducted. In all, 38 cladoceran taxa from 7 families were identified. Most of them fell within the family Chydoridae. Yet, another family, Bosminidae, turned out to be represented by the largest number of individuals. The assemblage was dominated by Bosmina (Eubosmina) longispina. Relatively high concentrations were also characteristic of the Daphnia longispina gr., Chydorus сf. sphaericus, and Alona quadrangularis. The age of the studied core spans the Late glacial and entire Holocene, the maximum age of the sediment core was 13.5 yr BP (Valieva et al., 2023). In the lower layers of the core at the beginning of the Holocene, the discovered Cladocera remains were few and characterized by low species richness. The most significant transformations in the Cladocera community occurred at the end of the Late Dryas - beginning of the Holocene (11400-11950 cal. yr BP), indicating substantial changes in environmental conditions.

Extrapolating Metal (Cu, Ni, Zn) Toxicity from Individuals to Populations Across Daphnia Species Using Mechanistic Models: The Roles of Uncertainty Propagation and Combined Physiological Modes of Action.

Mechanistic effect modeling is a promising tool to improve the ecological realism of environmental risk assessment. An open question for the mechanistic modeling of metal toxicity is whether the same physiological mode of action (PMoA) could be assumed for closely related species. The implications of various modeling choices, such as the use of parameter point estimates and assumption of simplistic toxicodynamic models, are largely unexplored. We conducted life-table experiments with Daphnia longispina, Daphnia magna, and Daphnia pulex exposed to the single metals Cu, Ni, and Zn, and calibrated toxicokinetic-toxicodynamic (TKTD) models based on dynamic energy budget theory. We developed TKTD models with single and combined PMoAs to compare their goodness-of-fit and predicted population-level sensitivity. We identified the PMoA reproduction efficiency as most probable in all species for Ni and Zn, but not for Cu, and found that combined-PMoA models predicted higher population-level sensitivity than single-PMoA models, which was related to the predicted individual-level sensitivity, rather than to mechanistic differences between models. Using point estimates of parameters, instead of sampling from the probability distributions of parameters, could also lead to differences in the predicted population-level sensitivity. According to model predictions, apical chronic endpoints (cumulative reproduction, survival) are conservative for single-metal population effects across metals and species. We conclude that the assumption of an identical PMoA for different species of Daphnia could be justified for Ni and Zn, but not for Cu. Single-PMoA models are more appropriate than combined-PMoA models from a model selection perspective, but propagation of the associated uncertainty should be considered. More accurate predictions of effects at low concentrations may nevertheless motivate the use of combined-PMoA models. Environ Toxicol Chem 2024;43:338-358. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Potential for high toxicity of polystyrene nanoplastics to the European Daphnia longispina

BackgroundCurrent regulatory discussions about microplastics are often questioned based on a lack of data indicating high ecotoxic hazards of these particles within standard and recognized definitions. Moreover, there is scientific debate on what metrics to report the micro-nanoplastics toxicity (i.e. mass or particle counts-based exposure). We present here the high potential sensitivity of three genotypically different clones of the European Daphnia longispina species complex exposed to non-functionalized polystyrene nanobeads of 50 nm and 100 nm in diameter according to adapted OECD 202 test protocol.ResultsEC50s 48 h varied from 0.2 to 8.9 mg L−1 (mean 2.49 mg L−1) for 50 nm beads, and from 32.7 to 90.3 mg L−1 (mean 59.39 mg L−1) for the 100 nm. EC10s 48 h varied from 0.0007 to 7.5 mg L−1 (mean 0.28 mg L−1) for 50 nm beads, and from 25.5 to 69.1 mg L−1 (mean 47.51 mg L−1) for the 100 nm. Inter-clonal variability was about tenfold. Therefore, several 1000 s-fold variations in mass-based ecotoxicity for these polystyrene beads was observed if particle size and Daphnia genotype are considered jointly.ConclusionsSuch ecotoxicity potential is comparable to highly toxic chemicals in global and EU-based regulatory classification and labelling. Ecotoxicity based on particle counts suggested convergence of EC50s, with effects generally observed around 1011 to1015 particles L−1. The present results highlight the potential high hazard of these particles and the relevance of particle size and exposure metrics on hazard conclusion.Graphical

Combined effects of two abiotic stressors (salinity and temperature) on a laboratory-simulated population of Daphnia longispina

Sea level rise is expected to continue apace, with a concomitant rise in temperature on the globe's surface. Natural populations genetic pool compromised by increased salinity would contribute to decrease resilience under future temperature changes. Therefore, this work aimed to understand the long-term effects of salinization on the genetic diversity of a cladoceran-simulated laboratory population under different temperature regimes. For that, six clonal lineages of the cladoceran Daphnia longispina were chosen based on their reported differential lethal sensitivity (LCx) to salinity. The chronic experiment was initiated when each individual clonal lineage reached the population’s carrying capacity, and then were exposed to the LC70,48 h for the most tolerant clonal lineage of D. longispina (corresponding to 5.91 mS/cm) to 17°C, 20°C and 23°C for at least 30 days. Salinity affected D. longispina survival and reproduction, with the disappearance of salt-tolerant earlier than salt-sensitive lineages after chronic exposures. Different sensitivity ranks were observed for clonal lineages when comparing short-term and chronic survival, most probably due to acclimation-driven population recovery. Non-optimal tested temperatures (17°C and 23°C) enhanced negative effects of salinity through loss of the most sensitive clonal lineages, suggesting a potential future synergistic effect between both abiotic factors.

Sulfate sensitivity of aquatic organism in soft freshwaters explored by toxicity tests and species sensitivity distribution

Elevated concentrations of sulfate in waterways are observed due to various anthropogenic activities. Elevated levels of sulfate can have harmful effects on aquatic life in freshwaters: sulfate can cause osmotic stress or specific ion toxicity in aquatic organisms, especially in soft waters where Ca2+ and Mg2+ concentrations are low. Formerly, chronic toxicity test data in soft water have been scarce. The chronic and acute sulfate toxicity tests conducted with aquatic organisms from 10 families across various trophic levels in this study multiplied the number of tests conducted in soft freshwater conditions and enabled derivation of the species sensitivity distribution (SSD) and sulfate hazardous concentrations for soft freshwaters. The cladoceran Daphnia longispina and freshwater snail Lymnaea stagnalis were the most sensitive to sulfate among the studied species. Harmful effects on the reproduction of D. longispina were observed at 49 mg SO4 /L while growth of L. stagnalis was inhibited at 217 mg SO4 /L. Most studied organisms tolerated high sulfate concentrations: the median of chronic effective concentrations (EC10 or LC10) was 1008 mg/L for all the species tested in this study. Based on the species sensitivity distribution of the studied species the hazardous concentration for 5 % of aquatic organism (HC5) in soft waters was 117–194 mg SO4/L. Different data set combinations were used to demonstrate the data variability in SSD-based HC5 estimates. The lowest values were produced from combining biotest results from the present study and earlier literature, while the highest values were calculated from the present study only. The derived chronic no-effect concentrations (PNEC) varied between 39 and 65 mg SO4/L.

Pulling the Plug—Draining an Alpine Lake Failed to Eradicate Alien Minnows and Impacted Lower Trophic Levels

Fish introduction into fishless high-altitude lakes has detrimental effects on biodiversity. Removal of alien fish through intensive fishing is cost-intensive and difficult to achieve in productive lakes. Lake Sulzkarsee is the only lake in the National Park Gesäuse, Austria, and was an important breeding site for amphibians until the lake was stocked with fish in the late 1970s. Salmonids were eradicated in 2005, but the lake remained degraded by the introduced minnows (Phoxinus sp.). In 2018, the lake was drained through a siphon pipe and then by pumping out water with dirt water pumps. The deepest part was treated with slaked lime, but several hundred adult minnows survived in sediment crevices and reproduced in the following season. After drainage, the phytoplankton biomass increased. Indicator species, such as Daphnia longispina and amphibians, showed signs of recovery, but they went back to an impacted state when minnows recovered after the failed eradication attempt. Purse seines proved to be the most efficient gear to catch minnows. These results indicate that deep mountain lakes are difficult to drain efficiently. Sediment treatment is required to eliminate all fish.

Zooplankton of some rivers of the Podkamennaya Tunguska river catchment (Yenisei river catchment)

The article reports the results of studies of zooplankton communities сonducted in 2017–2020 in some rivers of the Podkamennaya Tunguska river catchment. Zooplankton samples were selected and processed according to generally accepted methods. The taxonomic composition was determined for the first time (except for the Kamo River). The taxonomic composition included 48 taxa: Rotifera — 21, Cladocera — 16, Copepoda — 13. The greatest species diversity of zooplankton was recorded in the Tohomo, Tetere and Taiga rivers — 18–26 taxa, the smallest — in the Ter, Levoe Cheremo and Levaya Tokura rivers — one species each. In most rivers there were rotifers Euchlanis dilatata, and crustaceans — Acroperus harpae, Chydorus sphaericus. In general, rotifers and cladocerans played a significant role in the zooplankton of the studied rivers. The average abundance and biomass of zooplankton organisms varied from 22 to 2673 ind./m3 and from 0.001 to 29.63 mg/m3, respectively. Zooplankton was most developed in the Tetere river due to the abundance of large cladoceras of the genus Alona, Daphnia longispina, Eurycercus lamellatus, Simocephalus vetulus. Low the average abundance was in the rivers Maliy Yurubchen, Levaya Tokura, biomass — Verhvnya Madra, Dugungna, Maliy Yurubchen, Yurubchen, Tes, Levoe Cheremo, Levaya Tokura characterized by a relatively high flow rate and limited supply of organic and biogenic substances that determine the development of the invertebrate food supply. According to the quantitative characteristics of zooplankton, the rivers of the Podkamennaya Tunguska river catchment are characterized as poor in feed.

Spatial and Temporal Changes of Planktonic Microcrustaceans (Cladocera, Copepoda) and their Relationship with Physicochemical Parameters in Kırklareli Reservoir (Kırklareli-Türkiye)

The present study focused on monthly densities and abundances of planktonic microcrustaceans (Cladocera and Copepeda) in Kırklareli reservoir, as well as their relationship with some physicochemical parameters at three stations for the period from May 2018 to April 2019. In the present study, a total of 26 planktonic microcrustaceans species, 14 from Cladocera and 12 from Copepoda were identified. The mean annual microcrustacean abundance recorded during the study was 24076 ind/m3, 67% of which was Cladocera (16054 ± 12414 ind/m3) and 33% was Copepoda (8022 ± 5564 ind/m3). The maximum abundance of planktonic microcrustaceans were found in September (51521 ind/m3) and the minimum was found in January (2919 ind/m3). The most common species in the reservoir were Bosmina longirostris, Daphnia pulex, Daphnia cucullata, Daphnia galeata, Daphnia longispina and Diaphanosoma brachyurum from Cladocera and Cyclops abyssorum, Cyclops vicinus, Eudiaptomus vulgaris and Arctodiaptomus wierzejskii from Copepoda. The environmental parameter values measured in the reservoir were acceptable to support aquatic life, especially the zooplankton community. The total density and abundance of zooplanktonic microcrustaceans showed positive correlation with the water temperature and Chlorophyll-a. According to these results, we concluded that Kırklareli reservoir has a mesotrophic character in terms of the microcrustacean fauna and the physicochemical parameters.

Cost-saving population genomic investigation of Daphnia longispina complex resting eggs using whole-genome amplification and pre-sequencing screening.

Resting stages of aquatic organisms that accumulate in the sediment over time are an exceptional resource that allows direct insights into past populations and addressing evolutionary questions. This is of particular interest in taxa that face relatively new environmental challenges, e.g., climate change and eutrophication, such as the Daphnia longispina species complex, a keystone zooplankton group in European freshwater ecosystems. However, genomic analysis might be challenging as DNA yield from many of these resting stages can be low and the material degraded. To reliably allow the resequencing of single Daphnia resting eggs from different sediment layers and characterize genomic changes through time, we performed whole-genome amplification to obtain DNA amounts suitable for genome resequencing and tested multiple protocols involving egg isolation, whole-genome amplification kits, and library preparation. A pre-sequencing contamination screening was developed, consisting of amplifying mitochondrial Daphnia and bacterial markers, to quickly assess and exclude possibly contaminated samples. In total, we successfully amplified and sequenced nine genomes from Daphnia resting eggs that could be identified as Daphnia longispina species. We analyzed the genome coverage and heterozygosity of these samples to optimize this method for future projects involving population genomic investigation of the resting egg bank.

Daphnia japonica sp. nov. (Crustacea: Cladocera) an eastern Palearctic montane species with mitochondrial discordance.

The Daphnia longispina complex (Crustacea: Cladocera) contains several keystone freshwater species such as D. longispina O.F. Müller (D. rosea Sars is a junior synonym), D. galeata Sars, D. cucullata Sars, and D. dentifera Forbes. The complex is common throughout the Holarctic, but there are several geographic regions where local forms have been assigned to European species names based on a superficial morphological resemblance. Here we examine the species status of a form that was previously assigned to D. rosea from a montane bog pond on Honshu, Japan. We used two nuclear non-coding loci (nDNA), mitochondrial sequences (the ND2 protein-coding region) and morphology for evidence. The mitochondrial gene evidence supported the existence of a divergent lineage that is more closely related to D. galeata than to D. dentifera. However, morphology and the nuclear DNA data indicated a lineage that is most closely related to D. dentifera. As our evidence supported the existence of a cohesive divergent lineage, we described a new species, Daphnia japonica sp. nov. Recognition of local and subalpine diversity in this group is critical as ongoing anthropogenic disturbance has been associated with introductions, local extirpations, and hybridization.

Infection by a eukaryotic gut parasite in wild Daphnia sp. associates with a distinct bacterial community.

Host-associated bacterial communities play an important role in host fitness and resistance to diseases. Yet, few studies have investigated tripartite interaction between a host, parasite and host-associated bacterial communities in natural settings. Here, we use 16S rRNA gene amplicon sequencing to compare gut- and body- bacterial communities of wild water fleas belonging to the Daphnia longispina complex, between uninfected hosts and those infected with the common and virulent eukaryotic gut parasite Caullerya mesnili (Family: Ichthyosporea). We report community-level changes in host-associated bacteria with the presence of the parasite infection; namely decreased alpha diversity and increased beta diversity at the site of infection, i.e. host gut (but not host body). We also report decreased abundance of bacterial taxa proposed elsewhere to be beneficial for the host, and an appearance of taxa specifically associated with infected hosts. Our study highlights the host-microbiota-infection link in a natural system and raises questions about the role of host-associated microbiota in natural disease epidemics as well as the functional roles of bacteria specifically associated with infected hosts.

Warming overcomes dispersal-limitation to promote non-native expansion in Lake Baikal

Non-native species and climate change pose serious threats to global biodiversity. However, the roles of climate, dispersal, and competition are difficult to disentangle in heterogeneous landscapes. We combine empirical data and theory to examine how these forces influence the spread of non-native species in Lake Baikal. We analyze the potential for Daphnia longispina to establish in Lake Baikal, potentially threatening an endemic, cryophillic copepod Epischurella baikalensis. We collected field samples to establish current community composition and compared them to model predictions informed by flow rates, present-day temperatures, and temperature projections. Our data and model agree that expansion is currently limited by dispersal. However, projected increases in temperature reverse this effect, allowing D. longispina to establish in Lake Baikal’s main basin. A strong negative impact emerges from the interaction between climate change and dispersal, outweighing their independent effects. Climate, dispersal, and competition have complex, interactive effects on expansion with important implications for global biodiversity.

Automated Counting of Daphnid Neonates, Artemia Nauplii, and Zebrafish Eggs: A Proof of Concept.

In aquatic invertebrate (e.g., daphnids and Artemia sp.) and zebrafish cultures, in ecotoxicological bioassays, or when addressing complex population-level experimental designs, the counting of an organism's progeny is often required. This counting process is laborious, repetitive, and time-consuming, potentially posing health hazards to the operators, and necessarily entailing a higher likelihood of human error. We present an experimental evaluation of a computer-based device for counting neonates (Daphnia magna, Daphnia longispina, and Ceriodaphnia sp.), nauplii (Artemia salina and Artemia franciscana), and zebrafish (Danio rerio) eggs. Manual counts by an experienced technician were compared with the corresponding automated counts achieved by the computer-based counting device. A minimum of 55 counts/species was performed, with the number of counted organisms being up to a maximum of 150 neonates of Ceriodaphnia dubia, 200 neonates of D. magna and D. longispina, 200 nauplii of A. franciscana and A. salina, and 500 zebrafish eggs. Manual and automated counts were both performed in culture medium solutions of 50 ml of volume. Automated counts showed a mean relative acccuracy of 98.9% (97.9%-99.4%) and a relative standard deviation of 1.72%. The results demonstrate that the computer-based device can be used for accurately counting these aquatic organisms. This computer-based counting might be extended to other organisms of similar size, thus facilitating reproduction and life-cycle ecotoxicity tests. Environ Toxicol Chem 2022;41:1451-1458. © 2022 SETAC.

A new microsporidian parasite, Ordospora pajunii sp. nov (Ordosporidae), of Daphnia longispina highlights the value of genomic data for delineating species boundaries.

Speciation is a complex and continuous process that makes the delineation of species boundaries a challenging task in particular in species with little morphological differentiation, such as parasites. In this case, the use of genomic data is often necessary, such as for the intracellular Microsporidian parasites. Here, we characterize the genome of a gut parasite of the cladoceran Daphnia longispina (isolate FI-F-10), which we propose as a new species within the genus Ordospora: Ordospora pajunii sp. nov (Ordosporidae). FI-F-10 closest relative, Ordospora colligata is only found in D. magna. Both microsporidian species share several morphological features. Although it is not possible to estimate divergence times for Microsporidia due to the lack of fossil records and accelerated evolutionary rates, we base our proposal on the phylogenomic and genomic distances between both microsporidian lineages. The phylogenomic reconstruction shows that FI-F-10 forms an early diverging branch basal to the cluster that contains all known O. colligata strains. Whole-genome comparisons show that FI-F-10 presents a greater divergence at the sequence level than observed among O. colligata strains, and its genomic average nucleotide identity (ANI) values against O. colligata are beyond the intraspecific range previously established for yeast and prokaryotes. Our data confirm that the ANI metrics are useful for fine genetic divergence calibration across Microsporidia taxa. In combination with phylogenetic and ecological data, genome-based metrics provide a powerful approach to delimitate species boundaries.

Zooplankton of the Brackish Lakes in Turan-Uyuk Basin (Republic of Tyva)

The study addresses the species composition of zooplankton in five mineralized water bodies located in the Turan-Uyuk basin (Tyva). Lake Beloye is a regional natural landmark. In total, 36 species of 22 genera, 9 families, 4 orders were identified. Cladocera constitute 67 % of the species. The major species are Daphnia magna, Brachionus plicatilis, Daphnia pulex, Daphnia longispina, Megacyclops viridis, Ceriodaphnia reticulata, Chydorus cf. sphaericus, Simocephalus expinosus, Acanthodiaptomus denticornis, Arctodiaptomus baсillifer, Moina macrocopa, Keratella quadrata, Brachionus quadridentatus brevispinus, and Daphnia hyalina. The main abundant species in two lakes, Beloye and Arzhaan, is D. magna, which has been the dominant species in Lake Beloye for decades. The zooplankton of Lakes Beloye and Arzhaan include Macrotrix tabrizensis, a rare species, which is known to occur only in one Russian region – Tyva. Tretocephala ambigua lives in Lake Beloye. In the study period, we observed an increase in the abundance of zooplankton: by a factor of 6 in Lake Beloye and by a factor of 2 in Lakes Kisloe and Arzhaan. We believe that this is a consequence of an increase in organic load on water bodies, resulting from shallowing of the lakes and human impact. The occurrence of the species such as Moina macrocopa and Brachionus quadridentatus brevispinus, which inhabit water bodies rich in organic matter and have not been previously detected in the study lakes, confirms this assumption. Cladocerans, which have a high filtration capacity, remain dominant in Lakes Beloye and Arzhaan, and this is beneficial for the lakes. The increasing abundance of Brachionus plicatilis in Lake Kisloye performs a compensatory function in the processes of natural self-purification. At the same time, the fact that species rare for the fauna of Russia have been constantly found in the study lakes indicates the presence of favorable conditions for their development and life, which further increases the value of these unique natural environments

Comparing recent and preindustrial cladoceran assemblages in 35 metal- and smelting-affected lakes near Sudbury (Ontario, Canada)

Abstract Base metal smelting activity around Sudbury (Ontario, Canada) has persisted for over a century, emitting metals and acidifying contaminants that affected lakes downwind of smelter stacks. Although considerable research has been directed toward the paleolimnological study of diatoms and chrysophytes in the region, relatively little comparable work has been completed on invertebrates. Here, cladoceran (Branchiopoda: Cladocera) assemblages are compared using a paleolimnological “top-bottom approach”, comparing preimpact and recent assemblages. Spatiotemporal differences were likely driven by acidification-related stressors and legacy effects of metal contamination. The replacement of the Daphnia longispina complex with the Daphnia pulex complex is attributable to diminished lake-water calcium concentrations. Increased abundances of early colonist taxa demonstrate that many acidified lakes have not fully recovered. Regional stressors further complicate recovery and the interpretation of temporal changes. Finally, clustering of recent cladoceran assemblages reflects the effects of modern multiple stressors and persistent historical stressors. This study shows that acidification history partially explains differences in cladoceran assemblages, but also that multiple stressors will likely continue to influence assemblage composition and recovery in Sudbury lakes. Defining appropriate reference conditions, considering historical and modern stressors, will be important when assessing the extent and trajectory of ecosystem recovery from widespread anthropogenic impacts.

Morpho-Functional Traits Reveal Differences in Size Fractionated Phytoplankton Communities but Do Not Significantly Affect Zooplankton Grazing.

The recent emergence of approaches based on functional traits allows a more comprehensive evaluation of the role of functions and interactions within communities. As phytoplankton size and shape are the major determinants of its edibility to herbivores, alteration or loss of some morpho-functional phytoplankton traits should affect zooplankton grazing, fitness and population dynamics. Here, we investigated the response of altered phytoplankton morpho-functional trait distribution to grazing by zooplankton with contrasting food size preferences and feeding behaviors. To test this, we performed feeding trials in laboratory microcosms with size-fractionated freshwater phytoplankton (3 size classes, >30 µm; 5–30 µm and <5 µm) and two different consumer types: the cladoceran Daphnia longispina, (generalist unselective filter feeder) and the calanoid copepod Eudiaptomus sp. (selective feeder). We observed no significant changes in traits and composition between the controls and grazed phytoplankton communities. However, community composition and structure varied widely between the small and large size fractions, demonstrating the key role of size in structuring natural phytoplankton communities. Our findings also highlight the necessity to combine taxonomy and trait-based morpho-functional approaches when studying ecological dynamics in phytoplankton-zooplankton interactions.

Hybridization Dynamics and Extensive Introgression in the Daphnia longispina Species Complex: New Insights from a High-Quality Daphnia galeata Reference Genome.

Hybridization and introgression are recognized as an important source of variation that influence adaptive processes; both phenomena are frequent in the genus Daphnia, a keystone zooplankton taxon in freshwater ecosystems that comprises several species complexes. To investigate genome-wide consequences of introgression between species, we provide here the first high-quality genome assembly for a member of the Daphnia longispina species complex, Daphnia galeata. We further resequenced 49 whole genomes of three species of the complex and their interspecific hybrids both from genotypes sampled in the water column and from single resting eggs extracted from sediment cores. Populations from habitats with diverse ecological conditions offered an opportunity to study the dynamics of hybridization linked to ecological changes and revealed a high prevalence of hybrids. Using phylogenetic and population genomic approaches, we provide first insights into the intra- and interspecific genome-wide variability in this species complex and identify regions of high divergence. Finally, we assess the length of ancestry tracts in hybrids to characterize introgression patterns across the genome. Our analyses uncover a complex history of hybridization and introgression reflecting multiple generations of hybridization and backcrossing in the Daphnia longispina species complex. Overall, this study and the new resources presented here pave the way for a better understanding of ancient and contemporary gene flow in the species complex and facilitate future studies on resting egg banks accumulating in lake sediment.