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Aim To reconstruct the phylogeographic history of the Holarctic carnivorous genus Leptodora (Crustacea: Cladocera: Haplopoda). Location We studied the DNA of between one and five specimens each from 28 populations distributed across the Holarctic, but with emphasis on Eurasia. Methods We sequenced a mitochondrial (cytochrome c oxidase subunit I) and a nuclear (elongation factor-1α) gene, and combined this molecular information with geological and palaeoclimatological data. Haplotype networks and phylogenetic trees were constructed using a Bayesian and maximum likelihood approach. A molecular clock was applied. Results Leptodora consists of three clades (Leptodora kindtii in Europe, Leptodora richardi in China and Japan, and Leptodora sp. in North America), with insular subclades in Japan and in the eastern Mediterranean. The North American clade was not studied in detail. Leptodora richardi is the more thermophilic of the three. It extends from the Tropic of Cancer in the south to the Heilong Basin in the north. The western European L. kindtii is more cold-water adapted than the eastern Mediterranean subclade. ‘West European' and ‘Chinese' clades are broadly separated by a hybrid zone in Siberia and European Russia as far west as the Volga. These hybrids have the mitochondrial DNA of L. kindtii, the nuclear DNA of L. richardi and the low-temperature preference of L. kindtii, and may have formed as recently as the Holocene hypsithermal. A pure L. kindtii population in the Upper Irtysh catchment, east of the Dzungarian Gates, has been sequestered in endorheic Lake Wulungu, Xinjiang, since the mid-Pleistocene. Main conclusions Application of a molecular clock places the most recent common ancestor of the North American, East Asian and European populations in the mid-Miocene. The North American taxon is still living in isolation, while the Eurasian taxa, separated by the Alpine folding, made contact again in the Pleistocene, when the cold-stenothermic L. kindtii repeatedly moved eastwards across Siberia and back. The population in Xinjiang is a relict of an early wave coming from western Europe: it crossed the Dzungarian Gates during a humid mid-Pleistocene event, probably corresponding to the Apsheron transgression in the Caspian Basin. Later aridity isolated it there, and it started accumulating private haplotypes. The Holocene Euro-Siberian hybrid zone may eventually engulf all European populations.

While numerous reports exist on the results of lake restoration initiatives in temperate regions, only a few exist from subtropical lakes. We present results of the lake restoration of shallow, subtropical Lake Wuli, China, conducted between 1999 and 2010. After restoration, annual average concentrations of total nitrogen, total phosphorus (TP), and chlorophyll a and the chemical oxygen demand declined significantly, though summer TP remained high. Suspended solids increased significantly over the years, whereas transparency decreased, though not significantly so. The contribution of cryptophytes to total phytoplankton biomass decreased, while the proportion of cyanobacteria, especially potentially N 2 -fixing species, increased. Rotifers were superseded by crustaceans as the dominant taxon of the zooplankton community. Enhanced abundance of Daphnia spp., appearance of Leptodora kindti , and increased biomass ratios of zooplankton to phytoplankton, calanoids to cyclopoids, and nauplii to copepods in the post-restoration period indicate reduced fish predation and stronger top-down control of phytoplankton. However, the increase in non-algal turbidity, probably caused by the higher biomass of benthivorous fish, apparently prevented the re-establishment of submerged macrophyte communities. We conclude that removal of fish, particularly benthivorous species, will further improve water quality in this and other subtropical shallow lakes.

In this study we investigate the extent to which successive range fragmentations, postglacial expansions, and recolonizations influenced intraspecific genetic diversity and patterns of diversification in freshwater zooplankton. Specifically, we explore the global phylogeography of the Holarctic predatory cladoceran Leptodora kindtii Focke, 1844. Phylogenetic analyses based on the mitochondrial cytochrome c oxidase subunit I recover seven deeply divergent lineages (11.8–20.0%) comprising two sister clades within the Nearctic and east Palearctic and three related clades within the west Palearctic. Strong continental endemism was also supported by the nuclear 18S ribosomal gene. Intra-continental divergence levels (< 6%) suggest that many speciation events in Leptodora were initiated by glacial isolation. Demographic and network analyses indicate that in the Nearctic and west Palearctic, regions heavily affected by the Pleistocene glaciations, Leptodora persisted in both classic (e.g., Cascadia, Mississippi, southern Europe) and cryptic refugia (e.g., northeastern Europe, Carpathian basin). Strong signatures of late Pleistocene range expansions and secondary contact were observed in most clades. Deeper intraspecific phylogeographic structure occurs across much finer geographic scales in unglaciated regions of the east Palearctic than across glaciated Nearctic and west Palearctic regions. Leptodora’s ecological and biological characteristics, such as a reliance on temperate, permanent lakes, weak dispersal ability, and specific geological settings likely shaped the complex pattern of diversification in this important planktonic predator. Our data suggest the Pleistocene glaciations were important in the diversification of Holarctic freshwater zooplankton.

Many fish species exhibit female-biased size dimorphism that may lead to spatial segregation of sexes. We selected two common European percids (Percidae, European perch Perca fluviatilis and ruffe Gymnocephalus cernua) differing in total body size, reproduction mode, habitat use and diurnal activity, to test whether they display size dimorphism and its effect on habitat use and diet. Females were significantly larger than equally old males (by 76% in perch, 23% in ruffe). No differences in habitat use by sexes were found along depth and longitudinal gradients of reservoir or between inshore and offshore habitats. Perch females had fuller guts, but both sexes were equally likely to consume same prey items (Leptodora kindtii, Daphnia spp., Chironomidae larvae, fish). Both sexes of ruffe had similar stomachs fullness, but females preferred L. kindtii and males Asellus aquaticus. In summary, perch and ruffe show strong female-biased size dimorphism, but sexes do not segregate spatially. Their unequal sex-specific dietary demands are satisfied by higher feeding rate of female perch and by preference for different prey items in ruffe. The magnitude of dimorphism was related to species body size and reproduction mode being larger in larger species and in total spawner (perch).

To assess top-down effects of planktivorous fish and Leptodora in the freshwaters of southern China, a mesocosm experiment was conducted by manipulating bighead carp (Aristichthys nobilis) and Leptodora richardi within in situ enclosures installed in an oligo-mesotrophic reservoir. During the winter–spring transition, a low biomass (≈1 g wet weight m⁻³) of fish significantly reduced Daphnia biomass and zooplankton clearance rates and markedly increased the biomass of total phytoplankton, small phytoplankton (GALD <30 μm), and large phytoplankton (GALD ≥30 μm). However, there was no significant effect of Leptodora and no interactive effect from fish and Leptodora on herbivorous zooplankton and phytoplankton. By contrast, exclusion of fish from the enclosures triggered the outbreak of Daphnia and thus resulted in higher zooplankton clearance rates. Algal biomass decreased to a low level in the absence of fish relative to in their presence, particularly during the last 10 days (mean biomass ratio, 1:7–1:36). Our results indicate that fish play a more important role in top-down effects than Leptodora. This study, together with previous research, suggests that fish may prey heavily on large-bodied herbivores, especially Daphnia, in southern China and reduce the chances for top-down control of phytoplankton.

Prey communities in natural environments face a diverse array of predators with distinct hunting techniques. However, most studies have focused only on the interactions between a single prey species and one or more predators and typically only one of many induced defense traits, which limits our understanding of the broader effects of predators on prey communities. In this study, we conducted a common garden experiment using five clones each of three Daphnia species (D. cucullata, D. galeata, and D. longispina) from the D. longispina species complex to investigate the plasticity of predator‐induced defenses in response to two predators in a community ecology setting. Five clones from each species were subjected to predator kairomones from two closely related invertebrate predators that are common in several European lakes, Bythotrephes longimanus or Leptodora kindtii for a duration of 10 days, and the morphological traits of body size, head size, spina size, and the presence of spinules on the ventral and dorsal carapace margins were measured. We show that among the species within this species complex there are different antipredator reactions to the invertebrate predators. The induced responses exhibited were species, trait, and predator‐specific. Notably, D. galeata and D. cucullata developed distinctive helmets as defensive mechanisms, while microdefenses were induced in D. galeata and D. longispina, but not in D. cucullata. This demonstrates that the expression of micro‐ and macrodefenses across species was unrelated, highlighting the possible independent evolution of microstructures as defensive modules in Daphnia's antipredator strategies. This study is the first to document both micro‐ and macrodefensive phenotypic plasticity in three co‐occurring Daphnia species within the D. longispina species complex. The differences in inducible defenses may have a substantial impact on how these three species cohabit with Bythotrephes and Leptodora. This study addresses the gap in predator–prey research by investigating the plasticity of predator‐induced defenses within coexisting Daphnia species. Using a common garden experiment, responses to two invertebrate predators were examined, revealing species‐specific and trait‐specific antipredator reactions. Unique defensive adaptations and the independent evolution of microstructures as defensive modules were observed, highlighting the complexity of predator‐induced strategies across species.

The zooplankton community of Lake Champlain has been altered over the past decade due to the introduction of two predatory zooplankton species. Bythotrephes longimanus Leydig, 1860 was first detected in Lake Champlain in August 2014, and Cercopagis pengoi (Ostroumov, 1891) was detected in August 2018. Monitoring for both invasive species at 15 lake sites using whole water tow sampling has been ongoing since 2010 with no detection of either species until 2014. Utilizing data from the Lake Champlain long-term monitoring program, we assessed pre- and post-invasion population dynamics of both invasive species on the native zooplankton community. Our results showed shifts in community structure following invasion, including a reduction in Diacyclops thomasi (Forbes, 1882) populations by both invaders and a reduction in Daphnia retrocurva Forbes, 1882 following the introduction of B. longimanus. Other cyclopoids, bosminids, and rotifers were also reduced, corresponding to both introduced species. The native large-bodied plankton predator Leptodora kindtii (Focke, 1844) appears to be unable to coexist with either invasive predatory cladoceran, displaying seasonal partitioning between L. kindtii and the Cercopagidae. These findings suggest that the invasion of both large predators in Lake Champlain have impacted pelagic zooplankton community structure.

The recognition of long-term patterns in the seasonal dynamics of Daphnia longispina, Leptodora kindtii and cyanobacteria is dependent upon their interactions, the water temperature and the hydrological conditions, which were all investigated between 1999 and 2008 in the lowland Sulejow Reservoir. The biomass of cyanobacteria, densities of D. longispina and L. kindtii, concentration of chlorophyll a and water temperature were assessed weekly from April to October at three sampling stations along the longitudinal reservoir axis. The retention time was calculated using data on the actual water inflow and reservoir volume. A self-organising map (SOM) was used due to high interannual variability in the studied parameters and their often non-linear relationships. Classification of the SOM output neurons into three clusters that grouped the sampling terms with similar biotic states allowed identification of the crucial abiotic factors responsible for the seasonal sequence of events: cluster CL-ExSp (extreme/spring) corresponded to hydrologically unstable cold periods (mostly spring) with extreme values and highly variable abiotic factors, which made abiotic control of the biota dominant; cluster CL-StSm (stable/summer) was associated with ordinary late spring and summer and was characterised by stable non-extreme abiotic conditions, which made biotic interactions more important; and the cluster CL-ExSm (extreme/summer), was associated with late spring/summer and characterised by thermal or hydrological extremes, which weakened the role of biotic factors. The significance of the differences between the SOM sub-clusters was verified by Kruskal-Wallis and post-hoc Dunn tests. The importance of the temperature and hydrological regimes as the key plankton-regulating factors in the dam reservoir, as shown by the SOM, was confirmed by the results of canonical correlation analyses (CCA) of each cluster. The demonstrated significance of hydrology in seasonal plankton dynamics complements the widely accepted pattern proposed by the plankton succession model for lakes, the PEG (Plankton Ecology Group), and may be useful for the formulation of management decisions in dam reservoirs.

We explore the effects of the invasive predatory cladoceran Bythotrephes longimanus on the abundances and seasonal zooplankton consumption by the native predatory invertebrates Leptodora kindtii, Chaoborus spp., and Mysis relicta in inland lakes of Ontario. In lakes with Bythotrephes, the seasonal consumption by all invertebrate predators combined ranged from 2.39 to 13.50 g m⁻² and was 300% higher than in lakes without the invader. This was due to Bythotrephes because there was no invasion effect on Chaoborus or Mysis consumption, while it actually decreased Leptodora consumption. Leptodora and Chaoborus abundances were lower in invaded lakes, but only Leptodora abundance was negatively correlated with Bythotrephes abundance. There was no effect of Bythotrephes on Mysis abundance. Bythotrephes consume more zooplankton than most other predatory invertebrates, including copepods, and often consume more zooplankton than planktivorous fish. The large increase in predatory invertebrate abundance and consumption due to Bythotrephes means that substantial portions of zooplankton production are probably being diverted from other consumers, such as juvenile and planktivorous fish, and that the role of predatory invertebrates in the pelagia of inland lakes has been intensified by the arrival of Bythotrephes.

Total mercury (THg) and methylmercury (MeHg) concentrations were analyzed in zooplankton (≥450 and ≥850 µm size fractions) collected seasonally over 6 years in Lake Maggiore (Northern Italy), characterized by a legacy mercury contamination. Analysis of δ 15N and δ13C stable isotopes was carried out to trace how taxa with different trophic levels and carbon sources contributed to mercury concentrations and trends. THg ranged between 44–213 µg kg−1 d.w. and MeHg 15–93 µg kg−1 d.w., representing 24–61% of THg. Values showed strong seasonal variations, with peaks in winter, due to the high biomass of predator taxa (Bythotrephes longimanus, Leptodora kindtii) and of Daphnia longispina-galeata gr. A positive correlation between THg and MeHg and δ15N signature was observed. D. longispina-galeata gr. prevailed in both size fractions, substantially contributing to THg and MeHg concentrations. Δ13C signature was strictly bound to lake thermal circulation dynamics. Mercury stock in the zooplankton compartment ranged between 19–140 ng THg m−2 and 6–44 ng MeHg m−2 for the ≥450 µm size fraction and between 2–66 ng THg m−2 and 1–7 ng MeHg m−2 for the ≥850 µm fraction, with the highest values in spring when zooplanktivorous fish actively prey in the pelagic zone. The results highlighted the crucial role of zooplankton as a repository of mercury, easily available to higher trophic levels.