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Urbanization transforms natural river channels, and surface water of some rivers disappeared over time. How and whether the subsurface domains of the original waterways and aquifers connecting them (a phantom of historical landscape) are functional is not known. This study examined the effects of tributary groundwater inflow on the response of river–riparian organisms in an alluvial mainstem river in northern Japan, where the tributary disappeared over the course of urban landscape transformation.A 2.8‐km lowland segment of the mainstem gravel‐bed river was examined for water properties and the river–riparian food web. In addition, watershed‐wide water sampling was conducted to isotopically distinguish several types of groundwater that contributed to the hyporheic water in the study segment. Altitude had a clear effect on the hydrogen/oxygen stable isotope ratios in the river water collected across the watershed.Groundwater unique both in chemical and isotopic signatures occurred in several spots within the study segment, and its properties resembled and its upwelling locations matched groundwater from a tributary river whose surface channel disappeared 60 years ago. Positive numerical increases in abundance and/or a sign of nitrogen transfer in river–riparian communities (algae, invertebrates and riparian trees) originating from groundwater high in nitrate with elevated nitrogen stable isotope ratios were found.We demonstrated that tributary groundwater with unique chemical properties manifested by an urban watershed river network continued to have trophic effects on biota across the river–riparian boundary in the mainstem river, even after urbanization transformed the tributary into a phantom river. We highlighted the legacy effects of landscape transformation in the subsurface domain and the significance of scrutinizing the past landscape and hydrological connectivity at the watershed scale in urban environments. [ABSTRACT FROM AUTHOR]

Although the transfer of methane‐derived carbon (MDC) to benthic macroinvertebrates such as chironomid larvae and oligochaetes has been well documented, knowledge of the transfer of MDC to fish is limited.We investigated the MDC contribution to 28 fish species through their growth stages and examined the relationship between fish traits including body size, and MDC contribution to fish within species in Lake Kasumigaura, Japan. We used carbon stable isotope ratios and a two‐source mixing model (methane‐oxidation bacteria [MOB] and particulate organic matter) to estimate the MDC contribution to fish.The MDC contribution to individual fish ranged from 0% to 21.5%. Anguilla japonica showed the highest MDC contribution (mean 3.8%). Fish species were classified into four groups according to two criteria: (a) whether the correlation between the MDC contribution and fish size (within species) was significantly negative (negative vs. not significant; ns); and (b) whether the 90th percentile of MDC contribution was 0% or not. The mean MDC contribution was highest (2.18% ± 2.93%, 12 species) for the group with (a)ns/(b)>0%, including the high contribution species such as Abbottina rivularis (mean 5.5%) and Biwia zezera (mean 9.8%). This group (12 species) included relatively high numbers of benthic habitat use (eight species) and omnivores (nine species). The mean MDC contributions were almost zero for the group with (a)ns/(b)=0% (seven species), which included only two species of benthic habitat use and one species of omnivores. The mean MDC contributions were intermediate for the other groups with (a)negative/(b)>0% (1.30% ± 1.17%, seven species) and (a)negative/(b)=0% (0.10%, two species). The negative correlation between fish size within species and MDC contribution in groups with (a)negative/(b)>0% and (a)negative/(b)=0% indicated that ontogenetic habitat and dietary shifts induced changes in MDC transfer.Our results suggest that 21 out of 28 fish species were involved in the MDC transfer in a shallow lake, but variation in MDC transfer could be explained by fish traits (trophic guild and habitat use). Although the mean MDC contribution to all fish species was low (0.61% ± 1.97%), the roughly estimated annual consumption rate of fish on MOB corresponded to approximately 10% of the MOB production in the sediment, suggesting that fish contribute to methane dynamics from the perspective of biogeochemical cycles. [ABSTRACT FROM AUTHOR]

Identifying potential links between food web structure and animal body condition is fundamental for predicting the long‐term persistence of populations under rapidly changing environments. Northern lakes, and the cold‐water adapted species that inhabit them, are particularly vulnerable to a warming climate.We explored relationships among body condition and energy channel use in lake trout (Salvelinus namaycush) and seasonal variation in food‐chain length in the Lake of Two Rivers, Ontario, Canada. Bayesian mixing models using carbon isotope values (δ13C) of essential amino acids showed that individuals utilising multiple energy channels had higher gonad mass, suggesting that dietary diversity has reproductive fitness benefits.Amino acid nitrogen isotope (δ15N) analysis revealed seasonal variation in food chain length, with extension of the food web during winter and spring and truncation of the food web during summer. Although these findings illustrate increased omnivory of lake trout during summer, there was no clear evidence that seasonal changes in food chain length had an impact on trout condition.These findings provide new insight into potential relationships between food web structure, energy flow and consumer condition in freshwater ecosystems, thereby revealing potential responses of species to shifting ecosystem states under intensifying climate change at high latitudes. [ABSTRACT FROM AUTHOR]

The extent and magnitude of parasitism often vary among closely related host species and across populations within species. Determining the ecological basis for this species and population‐level variation in parasitism is critical for understanding infection dynamics in multi‐host–parasite systems. To investigate such ecological underpinnings of variation in parasitism, we studied Enallagma damselfly host species and their water mite (Arrenurus spp.) ectoparasites in lakes.We first evaluated how host identity and density could shape parasitism. To test the effects of con‐ and heterospecific host density on parasitism, we used a field experiment with Enallagma basidens and E. signatum. We found that parasitism did not vary with con‐ or heterospecific density and was determined by host identity alone, with no spillover effects.We also evaluated the potential role of local adaptation and resource availability in shaping parasitism. To do so, we used E. signatum in a reciprocal transplant experiment crossed with a prey resource‐level manipulation. This experiment revealed that parasitism declined sharply for one host population in its non‐local lake, but not the other source population, with no effects of prey levels. This asymmetry implies that damselflies express enhanced defences against parasitism that are neither population‐specific nor dependent on resource abundance, or that mites developed heightened local host specificity.The results of multivariate modeling from an observational study generally supported these experimental findings: neither host density nor resource abundance strongly explained among‐population variation in parasitism. Instead, local abiotic conditions (pH) had the strongest relationship with parasitism, with minimal associations with predator density, temperature and a measure of immune function.Collectively, our findings suggest a crucial role for the local environment in shaping host–parasite interactions within multi‐host–parasite systems. More generally, these results show that research at the intersection of community ecology and disease ecology is critical for understanding host–parasite dynamics within natural communities. [ABSTRACT FROM AUTHOR]

Cyanobacterial dominance or blooms can influence ecosystem structure in reservoirs, yet there are only few studies of its effect on the resource use and trophic structure of zooplankton. We hypothesised that zooplankton traits would exhibit a strong response to the increase of invasive and toxic cyanobacteria.We investigated the effect of an invasive bloom‐forming cyanobacterium Raphidiopsis raciborskii (formerly Cylindrospermopsis raciborskii) on zooplankton functional traits (e.g., growth, reproduction, allochthonous resource use and realised niche) through laboratory experiments and 3‐year field investigations in two subtropical reservoirs. The realised niche (i.e., resource and habitat use) was quantified using stable isotopes (termed isotopic niches).We fed the cladoceran zooplankton Moina sp. with Scenedesmus obliquus (chlorophyte), R. raciborskii, and a mixture of S. obliquus and R. raciborskii in laboratory experiments. A diet of R. raciborskii alone depressed the growth and reproduction of Moina sp. compared to a diet of S. obliquus alone, but a mixture of S. obliquus and R. raciborskii greatly alleviated the inhibition effects caused by cyanobacteria.Under natural field conditions, we did not find a strong inhibition effect of R. raciborskii dominance on the zooplankton, but a high biomass of R. raciborskii was associated with an increase (17%) in the relative allochthonous resource (terrestrially derived organic matter) use by cladocerans (but not for copepods), even though the chlorophyll‐a concentration increased from 17 μg/L in the non‐bloom period to 28 μg/L during periods of R. raciborskii dominance or blooms. Furthermore, a high biomass of R. raciborskii increased the contribution of inter‐taxa variation (taxa sorting) to the expanded isotopic niches of the zooplankton community.Our study helps to clarify cyanobacteria‐zooplankton interactions and highlights the importance of cyanobacterial dominance or blooms in regulating cross‐ecosystem resource use and biogeochemical cycling mediated by zooplankton in aquatic ecosystems. [ABSTRACT FROM AUTHOR]

Wetlands are especially vulnerable to invasive plants because seasonal movements of sediments, water, nutrients, and debris from adjacent terrestrial habitats create ecological conditions suitable for invasion. Lonicera maackii is a relatively abundant and broadly distributed invasive shrub in the eastern U.S.A., yet few studies have simultaneously tested for effects of its leaf litter on multiple trophic levels within wetland food webs.Hatching success, hatchling survival, and hatchling size of tadpoles (Acris crepitans) were assessed using a laboratory mesocosm experiment with no‐leaf, native‐leaf, and L. maackii‐leaf treatments. In the field, short‐term (c. 3 week) and long‐term (c. 2 years) mesocosm experiments assessed the initial and persistent impacts, respectively, of L. maackii on measures of primary productivity, invertebrate abundance and community structure, and oviposition preference and larval survival of Cope's gray treefrog (Hyla chrysoscelis).Exposure of eggs to L. maackii leaves reduced body size at hatching and larval survival of newly hatched A. crepitans. In short‐term experiments, L. maackii leaves reduced dissolved oxygen levels, filamentous algal biomass, and macroinvertebrate abundance, and altered macroinvertebrate community structure. In long‐term experiments, duckweed cover was 15× times greater in mesocosms with L. maackii leaf additions. Oviposition by a local population of H. chrysoscelis was 10× lower in both short‐ and long‐term mesocosms with L. maackii present.Although ecological impacts of L. maackii have been reported for terrestrial systems, our study is the first to demonstrate the simultaneous effects of its leaves on multiple trophic levels within replicated experimental wetlands. The relatively rapid decomposition of L. maackii leaves, and associated pulse of phenolic compounds, is the most likely proximate mechanism of the bottom‐up effects we observed. The mechanisms, timing, and significance of effects is predicted to vary among natural wetlands.Our study demonstrates multiple ecological impacts of a terrestrial invasive shrub within experimental wetlands. Detailed studies on the specific mechanisms, and their spatial and temporal variability in natural systems, will elucidate management strategies and improve the efficiency of wetland conservation efforts. [ABSTRACT FROM AUTHOR]

Accurately accounting for flows of energy through food webs is challenging because of the spatial and temporal variability associated with energy production and consumption. Wet–dry tropical rivers have a highly seasonal discharge regime where wet season flows allow access to energy sources (inundated wetlands) that are not available during the dry season when aquatic consumers are confined to disconnected waterholes.We combined measures of fish community biomass with previously published feeding guild specific stable isotope analyses to explore how opposing wet‐ and dry‐season habitat templates influence spatial and temporal trends in the sources of energy supporting fish biomass throughout a river network in the wet–dry tropics of northern Australia.Accounting for the relative contribution of each feeding guild to fish community biomass was a critical component of our analyses, as a single feeding guild (invertivore/piscivore) influenced spatial and temporal patterns in the sources of energy supporting overall fish biomass. During the early dry season, the reliance of fish communities on autochthonous sources of energy (periphyton) decreased from the upper to lower reaches of the river network, which correlates with increasing floodplain area and wet season inundation times. These patterns disappeared by the late dry season as fish in both upper and lower reaches became increasingly reliant on autochthonous sources produced within waterholes over the course of the dry season, indicating that the large wet‐season gains in fish biomass are maintained through the dry season by energy produced within waterhole refuges.Collectively these results indicate that a combination of autochthonous and allochthonous sources of energy work in unison to support fish community biomass throughout the Mitchell River catchment and that access to these sources of energy is dictated by seasonal patterns in discharge interacting with spatial variability in river geomorphology (channel geometry and floodplain area).Many rivers are experiencing decreased flows due to water resource development and more frequent and severe droughts. Thus, we suggest our study provides insight into how changes in discharge regime could influence food web energetics throughout river networks. [ABSTRACT FROM AUTHOR]

Temperate headwater streams traditionally have been considered heterotrophic and brown food web dominated with little primary production. Recent work, however, suggests algae on leaves in these streams may play a greater role than previously thought through interactions with microbial decomposers like fungi. Algae also may be important for macroinvertebrates colonizing leaves in streams. Algae are a more nutritious food resource for shredders than fungi and bacteria and provide a food resource for non‐shredder macroinvertebrates.In a field experiment, we manipulated light in three low‐nutrient and three high‐nutrient streams using leaf bags filled with red maple leaves in winter and spring. After four weeks we measured algal and fungal biomass, leaf stoichiometry, and macroinvertebrate abundance and biomass associated with the leaf bags. We also identified the macroinvertebrate community and examined differences in functional feeding guilds and taxa under ambient‐ and shaded‐light treatments and low‐ and high‐nutrient concentrations in relation to measured leaf characteristics.Algal biomass on leaves was greatest in high‐nutrient streams and ambient‐light treatments in both seasons. Fungal biomass on leaves was greatest in high‐nutrient streams and showed a moderate marginally significant positive correlation with algae during the winter. Leaf C:N was negatively correlated to algae in winter and fungi in both seasons, while leaf N:P and C:P were negatively correlated to fungi in winter and algae in spring. Interactions between fungi and algae on leaves and the nutritional importance of each for macroinvertebrates likely change across seasons, potentially impacting macroinvertebrate community composition.Macroinvertebrate diversity did not differ, but biomass was significantly greater in shaded‐light treatments during spring. Abundance was highest in the high‐nutrient ambient‐light conditions in both seasons, corresponding to greatest algal biomass. Functional feeding guild biomass and abundance were related to different leaf characteristics by season and guild. Higher algal biomass was an important factor for colonization of certain macroinvertebrates (e.g., Ephemerella (Ephemeroptera: Ephemerellidae) and Stenonema (Ephemeroptera: Heptageniidae)), while others were more abundant under shaded treatments with lower algal biomass (e.g., Tipula (Diptera: Tipulidae)), indicating taxa‐specific responses.Leaf‐associated algae may be an important factor mediating macroinvertebrate communities associated with leaves in temperate headwater streams. Our results demonstrate that green and brown food webs intersect within leaf packs, and they cannot be easily disentangled. We therefore should consider both autochthonous and allochthonous resources within headwater streams when examining their communities or developing water management strategies. [ABSTRACT FROM AUTHOR]

Daphnia are key organisms in pelagic food webs, acting as a food resource for fish and predatory zooplankton and regulating phytoplankton through grazing. Its population dynamic follows regular seasonal patterns, with spring peaks followed by summer population declines (midsummer declines, MSDs). Midsummer declines show high inter‐annual variation, which has been attributed to different causes. However, the mechanisms controlling the MSD remain poorly understood, especially in deep stratified lakes.We tried to disentangle the factors causing Daphnia MSDs in Lake Lugano and Lake Iseo (in Switzerland and Italy), two deep peri‐alpine lakes with similar trophic status and vertical mixing dynamics, characterised by phosphorus accumulation in the hypolimnion and variable mixing during late‐winter turnovers.Specifically, we assessed the effects of three different hypothetical pathways according to which: (1) winter air temperature controls MSDs by influencing mixing depth during turnovers and epilimnetic phosphorus replenishment; (2) vernal air temperature influences MSD by accelerating the timing of spring population peak; and (3) summer temperature influences MSDs by increasing fish predation. We assessed the relative strength of these pathways using structural equation modelling on long‐term datasets for the two lakes (29 years for Lake Lugano and 19 years for Lake Iseo).Between the hypothesised pathways, the one driven by winter air temperature (through P replenishment) influenced Daphnia abundance in spring in both lakes, but the effects propagated to summer Daphnia abundance only in Lake Lugano. Additionally, summer Daphnia abundance was influenced by the summer air temperature through a positive (although weak) effect. By comparison, vernal air temperature had no detectable effects on summer Daphnia abundance.The results revealed marked differences between the meromictic study lakes and the shallow hypertrophic water bodies that were the focus of previous research on Daphnia MSD, and also between the two study lakes. The influence of epilimnetic P replenishment on the summer Daphnia abundance in Lake Lugano, which was recovering from past eutrophication, may have reflected the greater susceptibility of deep, stratified lakes to P depletion after spring compared to shallow hypertrophic lakes or reservoirs. This effect might not have been detected in Lake Iseo because P was more consistently depleted during the study period (i.e. variance in the predictor was too low to detect an effect).This study highlighted the complexity of the effects of climate variability on Daphnia MSD in deep lakes, showing that the responses can differ even between two neighbouring lakes with similar vertical mixing dynamics and trophic status. At the same time, the results suggest that future increases in winter air temperature, caused by global warming, may cause critically low densities of Daphnia during spring and summer and compromise the ability of zooplankton to control phytoplankton biomass. [ABSTRACT FROM AUTHOR]

Ecology of the glacial relict macrocrustacean Gammaracanthus lacustris, a rare inhabitant of deep Fennoscandian lakes, is poorly understood. We studied the life cycle and trophic position of this cold‐stenothermic amphipod in Lake Paasivesi, eastern Finland. The study is based on intensive sampling and analyses of fatty acid composition as well as stable carbon (δ13C) and nitrogen (δ15N) isotope ratios.Both day and night, the G. lacustris population occurred at depths below 25 m at temperatures <8°C, and the density increased towards the bottom of the lake, where it was 0.4–0.6 individuals/m3.Gammaracanthus lacustris was observed to reach a length of >40 mm and live up to 4 years. The oldest and the largest individuals and females seemed to favour the deepest zones.In October, almost 100% of females with length at least 25 mm (i.e. females presumably at least 1‐year‐old) had an embryo sack with 20–200 eggs or embryos—the larger the female, the more young it had.The developing eggs of G. lacustris had a very high fatty acid content, indicating that the females invest heavily on provisioning their young. Furthermore, the fatty acid composition differed among life stages, and in particular the eggs had a higher proportion of eicosapentaenoic acid than juveniles or adults.The stable δ13C and δ15N isotope ratios and fatty acid biomarkers of the food web, together with visual observations, indicate that G. lacustris is a carnivore that feeds mainly on zooplankton and other relict macrocrustaceans.Due to its high content of essential polyunsaturated fatty acids, G. lacustris is a valuable prey for fish and other predators in the food web. However, as a glacial relict with strict habitat requirements the species is vulnerable to global warming and local environmental changes. These facts should be considered in the management of lakes and their catchments. [ABSTRACT FROM AUTHOR]

Understanding how habitat heterogeneity is linked to biodiversity patterns within flood–pulse catchments is needed for their effective conservation. To study those communities, researchers have begun to explore how local environmental factors influence species composition patterns at relatively fine scales within complex habitat mosaics. However, a robust description of the link between communities' composition and their floodplain habitat characteristics remains poorly articulated.In the Tonle Sap Lake (TSL) of Cambodia, we evaluated whether mesohabitat‐scale (1–5 km) factors throughout the floodplain structured the fish assemblage temporally and spatially. First, we determined whether TSL fish assemblage changed seasonally and, if so, what assemblage components were driving that change. Second, we determined whether environmental factors structured fish composition across floodplain mesohabitat patches during the wet season. Third, we evaluated whether dominant species displayed affinities for specific areas within the floodplain during the wet season, potentially identifying critical mesohabitat patches for the fish assemblage.Fish were collected from nine sites along an elevational gradient traversing the floodplain every 3 months from March 2014 through January 2015. Correspondence analysis and analysis of variance were used to assess seasonal variation in the species composition. Canonical correspondence analysis and a mantel test were then used to identify the significance of environmental factors structuring the fish assemblage.Fish assemblages changed across seasons; in particular, migratory species occurrence decreased during the dry season. During the wet season, the fish assemblage was structured spatially by mesohabitat‐scale factors such as vegetation and water quality. Furthermore, the flooded forest mesohabitat directly adjacent to the permanent lake displayed particularly high species richness and abundance. However, a majority of TSL species appeared to be highly mobile, occurring across multiple floodplain mesohabitats.We concluded that fish assemblage structure and floodplain mesohabitat use within TSL may depend strongly upon the maintenance of the natural flow regime. Since the flow regime will probably be modified by upstream dam development, trade‐offs will emerge between the cycle of fishing on which local residents depend, and the hydropower and agricultural benefits of damming the Mekong River system. [ABSTRACT FROM AUTHOR]

Rice fields serve as alternative habitats for wetland species, and their recent reduction in area due to land conversion is considered one of the main human drivers of wetland biodiversity loss. Even for existing rice fields, physico‐chemical disturbances, such as farmland consolidation and agrochemical application, also have negative effects on wetland biodiversity. For fish that inhabit rice fields for at least part of their life history, differences in water level between rice fields and irrigation ditches caused by irrigation system improvements are critical to their abundance by preventing their access.The ecological impact of differences in water level may extend to the foraging habits and trophic positions of the top predators in rice fields, resulting in the alteration of wetland food webs. Here, we investigated how physical disruptions to habitat networks affect food‐web properties in rice‐field ecosystems.We conducted stable nitrogen and carbon isotope analysis on a threatened giant water bug, Kirkaldyia deyrolli, to estimate its trophic position as an indicator of food‐chain length. Based on field observations, we found that fish that were at a high trophic position were only available to K. deyrolli in rice fields where there were no physical barriers blocking access to the network of habitats, and the diets of K. deyrolli included fish prey (18%, based on visual observation). The stable isotope analysis revealed that the trophic position of K. deyrolli was significantly lower in rice fields with physical barriers (no fish/10 dips by a dip net) than in those without physical barriers (5.6 fish) due to the absence of fish in locations with barriers.In conclusion, our study demonstrates that physical alterations are important to community composition and food‐web properties in rice‐field ecosystems because they can act as barriers to fish. [ABSTRACT FROM AUTHOR]

Mercury (Hg) is a widespread environmental pollutant anthropogenically released into aquatic ecosystems where it is tightly retained and cycled through abiotic and biotic processes. Understanding the transport and fate of Hg, including the role of organisms in these cycles, is important. However, few studies, if any, have evaluated the role of macroconsumers in Hg cycling. This requires accounting for organismal regulation of body methylmercury (MeHg) via biodynamics and then scaling individual MeHg concentrations to the population level.Recent evidence suggests that MeHg concentrations will become decrease within an organism as it displays higher growth efficiency (i.e. somatic growth dilution [SGD] concept). However, the implications of individual concentrations, SGD, population size, and individual growth on ecosystem‐level mass‐storage and accumulation of MeHg are uncertain.We examined the influence of population size and individual growth on total MeHg standing stock and accumulation in stoneroller (Campostoma oligolepis) populations in a stream contaminated by Hg in east Tennessee, U.S.A. We assessed patterns in periphyton and stoneroller MeHg concentrations in relation to relative growth and population density to determine evidence of SGD and density‐dependent growth, respectively. We simulated the internal regulation of MeHg concentration in stonerollers using a biodynamic model, which was then scaled to the population level to analyse the effects of population density and growth on MeHg standing stock (μg/m2) in stoneroller populations and accumulation rates (i.e. flux, μg m−2 day−1) from periphyton communities to stonerollers. MeHg storage and flux in stoneroller populations were compared to standing stocks and methylation rates in periphyton communities.We found evidence of both density‐dependent and density‐independent growth patterns for stonerollers in East Fork Poplar Creek. Stoneroller MeHg concentrations were positively related to periphyton MeHg concentrations; however, evidence of SGD was equivocal. Total MeHg standing stock and accumulation in stoneroller populations was positively related to population density, irrespective of growth. Additionally, higher relative growth was positively related to a higher MeHg accumulation, regardless of density dependence. We found that the standing stock of MeHg in stoneroller populations could be as high as 32× the standing stock of MeHg in periphyton. Finally, the rate of accumulation of MeHg in stonerollers ranged from 5% to over 100% of previously documented methylation rates in periphyton.Our results suggest that abundant macroconsumers may play significant roles in contaminant cycles by mobilising considerable quantities of Hg, storing significant reservoirs of Hg and MeHg, and transferring MeHg to higher trophic levels. Furthermore, the implications of population size and growth of macroconsumers on the storage and flux of MeHg in streams are complicated and dependent upon age structure, density‐dependence, and MeHg concentrations. Our findings suggest that, even though rapid and efficient growth may dilute individual concentrations of contaminants, this may have little influence on ecosystem‐level contaminant fluxes. [ABSTRACT FROM AUTHOR]

How energy is transformed and distributed within ecosystems is a fundamental question in ecology. Parasites have been shown to play an essential role in these processes. In particular, the larval stages of trematodes, that is, cercariae, appear to contribute significantly to biomass and productivity in aquatic systems.Overall, ecosystem‐wide studies on parasite productivity remain scarce and have typically investigated systems with large hosts and high parasite infection rates. These studies may thus represent isolated cases of exceptionally high parasite contribution to ecosystem energetics, potentially overestimating the importance of parasite biomass.Here, we quantified the productivity of trematode cercariae from a small but hyper‐abundant snail intermediate host with only moderate trematode prevalence (i.e., proportion of infected individuals) in an entire lake ecosystem. We assessed individual larval output from snails and calculated the overall trematode productivity in the ecosystem.Average output of individual trematode species ranged from 3 to 62 cercariae per snail per day and correlated negatively with individual cercarial size. Cercarial productivity was not uniformly distributed across trematode taxa, but dominated by the most common species that accounted for more than 80% of the productivity.Total cercarial productivity amounted to 1.85 g m−2, which falls within the ranges of previous studies from freshwater systems. Small but abundant snail populations may thus support a considerable productivity of parasites. However, total annual cercarial productivity in the study system amounted to 5.9 kg, which constituted just 1.2% of the standing stock snail biomass, suggesting that intermediate host populations are potentially underexploited by their parasites. Moreover, comparisons with previous studies revealed contrasting patterns of parasite productivity and biomass contribution across different habitats, showing that impacts of parasites on ecosystem energetics can vary widely.Overall, we are still far away from having a complete picture of the dynamics of parasite productivity and biomass in many ecosystems. It therefore remains critical to quantify the contribution of parasites to the flow and distribution of energy and nutrients within and across habitats, to better understand their impacts on fundamental ecological principles, such as food‐web structure and ecosystem energetics. [ABSTRACT FROM AUTHOR]

Abstract: Non‐native freshwater fish species can have adverse ecological impacts on native populations. However, the mechanisms determining the success or otherwise of their invasion and their role in invaded communities remain largely unknown. This is particularly true for the Mediterranean region, where endemic species characterised by restricted natural ranges may be at high risk of extinction. The largemouth bass (Micropterus salmoides) is native to North America but is invasive in the Italian Peninsula. The aim of this study was to explore the trophic position of M. salmoides, its diet and niche overlap with native fish species in the littoral areas of a Mediterranean lake. Our study was supported by analysis of stable C and N isotopes in the tissues of fish and their potential food sources, twenty years after the introduction of the largemouth bass to Lake Bracciano (Italy). Samples were collected in locations varying in terms of physical structure and resource availability at lower trophic levels in the food web, which was greater in the southern (hereafter: South) than in the northern (hereafter: North) area of the lake. These differences made it possible to explore the mechanisms linking environmental conditions and the role of alien predators in the invaded food web. The abundance of M. salmoides was higher, and the diversity of native fish species was lower, in North than South. In North, M. salmoides had a piscivorous diet and occupied a higher trophic position in the food web than in South, where invertebrates constituted an important part of its diet. As a consequence, trophic niche interference with other fish species at intermediate trophic levels was higher in South. In contrast, in North, M. salmoides showed stronger trophic interference with the percid Perca fluviatilis, a native top predator in the food web, but weaker interference with remaining fish species. Our results help to understand the role of alien species in the food webs of Lake Bracciano, which primarily depends on the habitat and the availability of prey across trophic levels. Physical and ecological variations in the habitat were associated with differences in predatory interactions among native and alien fish species. This suggests that a reduction in productivity and biodiversity at lower trophic levels in lake food webs may favour the success of opportunistic invasive fish species, given the ability of the invaders to maintain some of their characteristics silent, and to fully express their genotype under favourable environmental conditions. [ABSTRACT FROM AUTHOR]

Abstract: Ecological networks are a cornerstone of ecological theory, offering an integrated approach to understanding food webs and ecosystem dynamics required for restoration and conservation ecology. We investigated ecological network dynamics in a large floodplain undergoing extreme variation in water availability, with drought and subsequent flooding representing a resource pulse. We used structural equation models to quantify ecological network dynamics for the Lowbidgee floodplain (Australia), based on surveys over 5 years while the floodplain transitioned from extremely dry (2009, 2010) to wet (2011) and post‐wet (2013, 2014) conditions. We identified significant associations of species and trophic guilds with inundation at the site and floodplain scale, which allowed us to quantify the strength of biotic interactions within the network and the stability of interactions under differing patterns of resource availability. At the floodplain scale, most taxa responded strongly in distribution and abundance to the 2011 resource pulse, a widespread flood, but this response did not persist during subsequent years of moderate floods. In contrast, fish species, both native and exotic, responded strongly only in the post‐wet period. At the fine spatial scale (i.e., sites), complex responses were observed, with only waterbirds, frogs and tadpoles positively associated with inundation, while fish species showed a range of associations with fine‐scale inundation. Biotic interactions within sites, across all trophic guilds, were predominately overridden by inundation and water temperature, mediated by strong associations with aquatic vegetation. Stratifying the ecological network to dry, wet and post‐wet periods highlighted varying associations of taxa with fine‐scale inundation, generally responding synchronously to resource pulses, with relatively weak biotic interactions. Associations with site‐scale inundation were strongest during the post‐wet period for fishes and frogs. Only Litoria spp. (Hylidae) tadpoles, waterbirds and aquatic vegetation had positive associations with site‐scale inundation during the dry period. We conclude that responses of trophic guilds are largely dependent on the way they interact with their environment at particular spatial and temporal scales. Our investigation of this ecological network reinforced the importance of hydrological drivers over biotic interactions, with clear implications for the management of environmental flows, particularly in systems recovering from long‐term flow alteration. Management efforts should focus environmental flows to promote specialist species (e.g., southern bell frog, Litoria raniformis) and Murray hardyhead (Craterocephalus fluviatilis: Atherinidae), waterbirds and aquatic vegetation over the more generalist fish species that have established because of the loss of the natural flow regime. [ABSTRACT FROM AUTHOR]

Tadpoles can be abundant consumers in stream ecosystems, and may influence the structure and function of streams through their feeding activities and interactions with other organisms. To understand the contribution of tadpoles to stream functioning, and the potential impact of their loss, it is necessary to determine their diets and how they might influence food-web structure., Using gut-content analysis and stable-isotope analysis of N and C, we determined the main food sources and trophic positions of tadpoles of five native frog species, invertebrates, and fish in upland and lowland Australian Wet Tropics streams., Omnivory was prevalent among the tadpoles and invertebrates. Tadpoles consumed different food according to availability and nutrient quality, but assimilated mainly biofilm and algae. Most tadpoles and invertebrates assimilated the same high-quality foods., Food webs in upland riffles were simplified by local extinction of tadpoles, and were probably simplified in pools in the cooler months by seasonal decline in tadpole abundance. Food-web complexity was increased in some pools by the presence of predatory fish and a greater number of basal sources., As tadpoles are important seasonal components in stream food webs, their local extinction can greatly alter food-web structure and complexity and, possibly, processes such as leaf litter breakdown and sediment accumulation. [ABSTRACT FROM AUTHOR]

The trophic ecology of invasive species has important implications for their impacts on recipient ecosystems, with omnivorous invaders potentially affecting native species and processes over multiple trophic levels. The trophic ecology of invaders might be affected by both their body size and the characteristics of their habitat due to variation in energy requirements and resource availability., Here, using stable-isotope analysis, we investigated the trophic ecology of the invasive crayfish Procambarus clarkii in 15 populations in southwest France over a gradient of individual (crayfish body size), population (crayfish abundance) and ecosystem (lake size, productivity and predation pressure) characteristics. We predicted that population niche width, level of omnivory and trophic position of individuals would change with abiotic and biotic conditions, but that these relationships would vary with lake size., The trophic position of individual crayfish increased with body size in lakes with low productivity, but decreased with body size in more productive lakes. As crayfish abundance increased (and therefore potential intraspecific competition), individual trophic position and population niche width decreased. This was most apparent in smaller lakes, suggesting it related to an increase in encounter rates with conspecifics., Body size, population abundance, lake size and lake productivity influenced the trophic ecology of invasive crayfish, which can affect their interactions with native species. Our results demonstrated that the trophic ecology of invasive species can be variable across invaded landscapes, with implications for their ecological impacts on native communities. This emphasizes the importance of characterising the diet of invasive species across their non-native range and environmental gradients to better predict and manage their impacts. [ABSTRACT FROM AUTHOR]

The upstream migration of juvenile amphidromous shrimps has been proposed as a source of marine or estuarine-derived nutrients into fresh water. Little is known about the size and ecological importance of any such subsidy as there have been few observational or empirical studies on the topic., We investigated the upstream migration of the amphidromous shrimp, Macrobrachium spinipes (Palaemonidae) in the Daly River, of tropical northern Australia, to determine migration phenology, estimate migration biomass and determine whether migrating shrimps transport marine-derived energy and nutrients upstream., Field observations over 2 years revealed that juvenile M. spinipes migrate upstream en masse during extended periods of declining discharge over a period of 4-6 weeks during the wet season (March-May). In addition, juvenile atyid shrimps from the genus Caridina were also observed migrating upstream during the same period., Fine-scale sampling using fyke nets over 2 years (2013 and 2014) consistently found discharge to be the strongest predictor of M. spinipes and Caridina spp. biomass, while moon illumination and cloud cover were also important predictors. An estimated 10-20 million shrimps migrated upstream during each wet season, transporting c. 100 kg of carbon and c. 28 kg of nitrogen per year., Muscle sulphur stable isotopes (δ34S) and exoskeleton strontium isotope ratios (87Sr/86Sr) were used to establish if marine carbon was transported upstream by the juvenile M. spinipes. Isotope data from migratory M. spinipes were compared to the non-migratory freshwater Macrobrachium bullatum. No evidence of a marine signature in body tissue or exoskeleton was found using either technique, suggesting very rapid turnover of body tissues, This study provides key insights into the migration phenology of amphidromous shrimps and, importantly, suggests that migrating M. spinipes do not transport significant amounts of marine-derived energy and nutrients across the marine/freshwater ecotone. [ABSTRACT FROM AUTHOR]

1. We examined the effects of wind-induced mixing and particle resuspension on the pelagic food web in eutrophic shallow lakes. These processes are known to have a major impact on a variety of biological, physical or chemical parameters such as the underwater light climate, the nutrient availability in the water column and the abundance and composition of the phytoplankton community. However, little is known about the effects of these processes on other compartments of the freshwater food web. 2. We conducted a 9 week experiment comprising a manipulation of mixing intensity in 15 m3 mesocosms equipped with wavemakers in order to explore the impact of two mixing regimes on water chemistry as well as viral, bacterial, phytoplankton and zooplankton communities. 3. The turbidity level in mixed mesocosms (compared to calm conditions) was higher on average, especially at the bottom, indicating a successful resuspension of the sediment bed. Mixing increased chlorophyll a concentration without any clear increase in algal abundance, measured as cell counts by flow cytometry, which pointed to a change in species composition or a physiological adaptation to mixing. pH increased strongly in mixed mesocosms, suggesting enhanced primary productivity in perturbed conditions. Zooplankton responses to mixing were neutral for cladocerans and negative for copepods, which potentially mediated top down controls on the rotifer population. 4. Bacterial and viral abundances were not significantly changed by the mixing regimes; however, peaks of viral lysis of heterotrophic bacteria were seen in each mixed mesocosm, while none were observed in calm mesocosms. These results suggest that viral lysis is enhanced by the water column mixing. 5. Our experiment demonstrates that mixing is likely to influence shallow lake functioning through a complex combination of direct and indirect effects on the underwater light climate and water chemistry, phytoplankton physiology and productivity, zooplankton growth and possibly virus host interactions. These complex effects could play a major role in structuring pelagic and benthic communities in shallow lakes. [ABSTRACT FROM AUTHOR]

Pesticides can have strong deleterious impacts in fresh waters, but understanding how these effects cascade through natural ecosystems, from microbes to apex predators, is limited because research that spans multiple levels of biological organisation is rare., We report how an accidental insecticide spill altered the structure and functioning of a river across levels ranging from genes to ecosystems. We quantified the impacts on assemblages of microbes, diatoms, macroinvertebrates and fish and measured leaf-litter decomposition rates and microbial functional potential at upstream control and downstream impacted sites 2 months after the spill., Both direct and indirect impacts were evident across multiple levels of organisation and taxa, from the base of the food web to higher trophic levels. At the molecular level, differences in functional gene abundance within the impacted sites reflected a combination of direct and indirect effects of the pesticide, via elevated abundances of microbial populations capable of using chlorpyrifos as a resource (i.e. direct effect) and oxidising ammonia released by decaying macroinvertebrate carcasses (i.e. indirect effect)., At the base of the food chains, diatom taxa found only in the impacted sites were an order-of-magnitude larger in cell size than the largest comparable taxa in control communities, following the near extirpation of their consumers. Population biomass of the key detritivore Gammarus pulex was markedly lower, as was the rate of litter decomposition in the impacted sites. This was partially compensated for, however, by elevated microbial breakdown, suggesting another indirect food-web effect of the toxic spill., Although many species exhibited population crashes or local extirpation, total macroinvertebrate biomass and abundance were largely unaffected due to a compensatory elevation in small tolerant taxa such as oligochaetes, and/or taxa which were in their adult aerial life stage at the time of the spill (e.g. chironomids) and thus avoided contact with the polluted waters and were able to repopulate the river quickly. Mass-abundance scaling of trophic links between consumers and resources revealed extensive restructuring within the food web., This case study shows that pesticides can affect food-web structure and ecosystem functioning, both directly and indirectly across levels of biological organisation. It also demonstrates how an integrated assessment approach, as adopted here, can elucidate links between microbiota, macroinvertebrates and fish, for instance, thus improving our understanding of the range of biological consequences of chemical contamination in natural ecosystems. [ABSTRACT FROM AUTHOR]

1. Anthropogenic stressors may influence hosts and their pathogens directly or may alter host-pathogen dynamics indirectly through interactions with other species. For example, in aquatic ecosystems, eutrophication may be associated with increased or decreased disease risk. Conversely, pathogens can influence community structure and function and are increasingly recognised as important members of the ecological communities in which they exist. 2. In outdoor mesocosms, we experimentally manipulated nutrients (nitrogen and phosphorus) and the presence of a fungal pathogen, Batrachochytrium dendrobatidis (Bd), and examined the effects on Bd abundance on larval amphibian hosts (Pseudacris regilla: Hylidae), amphibian traits and community dynamics. We predicted that resource supplementation would mitigate negative effects of Bd on tadpole growth and development and that indirect effects of treatments would propagate through the community. 3. Nutrient additions caused changes in algal growth, which benefitted tadpoles through increased mass, development and survival. Bd-exposed tadpoles metamorphosed sooner than unexposed individuals, but their mass at metamorphosis was not affected by Bd exposure. We detected additive rather than interactive effects of nutrient supplementation and Bd in this experiment. 4. Nutrient supplementation was not a significant predictor of infection load of larval amphibians. However, a structural equation model revealed that resource supplementation and exposure of amphibians to Bd altered the structure of the aquatic community. This is the first demonstration that sublethal effects of Bd on amphibians can alter aquatic community dynamics. [ABSTRACT FROM AUTHOR]

Interspecific overlap in resource use can determine the degree to which different presumed guild members have distinct or similar effects on ecosystem processes or responses to environmental change. Many headwater streams of North America support multiple species of larval salamanders commonly defined as a single guild that can influence macroinvertebrate communities and nutrient dynamics and are sensitive to stream alteration., We explored macroinvertebrate distributions in conjunction with stable isotope and gut content analyses of salamanders to examine similarities in diet between two sympatric larval salamander species ( Desmognathus quadramaculatus and Eurycea cirrigera) in four headwater streams. We determined the degree to which larval salamanders used similar prey functional feeding groups (FFGs) and taxa and determined the primary source habitat (pools versus riffles) of prey., Stable isotopes of carbon (δ13C) and nitrogen (δ15N) suggest the two salamander species occupied similar trophic positions and individual-based stable isotope mixing models indicated similar use of macroinvertebrate predators and filterers by both species. Diet analyses were generally consistent with stable isotope results in identifying prey FFGs that composed the largest biomass of salamander diets. However, despite similarities in diet at the resolution of FFGs, there was little overlap in the specific taxa consumed by the two salamander species: 52 prey taxa were consumed over all samples, with only 16 taxa in common. Further, only five prey taxa were common in dominating diet biomass of both species; there was more overlap in taxa in terms of diet abundance., We assessed patterns in benthic macroinvertebrate biomass and compared the biomass of taxa that were in pools versus riffles to the biomass and abundance of taxa in salamander diets. Total macroinvertebrate biomass was generally higher in pools; however, the majority of salamander prey biomass was from riffle habitats, a trend that was stronger for D. quadramaculatus than for E. cirrigera. There was greater similarity in taxa comprising diet by abundance with the majority of prey items originating from pools., The larval salamander species used similar prey FFGs but differed significantly in specific prey taxa. We hypothesise that species differences in diets were most likely a function of differences in larval size and microhabitat use. Consequently, the treatment of larval salamanders as a guild is probably inadequate for predicting the effect of larval salamander diversity on some stream processes, and species may differ in how they respond to factors affecting prey assemblages. [ABSTRACT FROM AUTHOR]

Multi-indicator palaeoecological studies have become increasingly popular over the last decade as the need for a more complete understanding of lake ecological histories has increased. However, the true potential of the full biological record for assessing the potential drivers of observed ecological shifts in lake sediment records has rarely been demonstrated., Here, we examine the remains of a range of food-web components including algae (diatoms), macrophytes (plant macrofossils), zooplankton (chitinous and ephippial Cladocera remains), invertebrates (including chironomids, bryozoans, Mollusca) and fish (fish scales and fish leech egg cocoons) in multiple sediment cores from Groby Pool, an enriched English shallow lake, to assess whole-ecosystem response to eutrophication over the last two centuries. We focus on three striking changes in the palaeorecord, namely the post-1900 increase in Daphnia spp., the post-1840 decline in Cristatella mucedo and the post-1940 increase in Cocconeis placentula, and utilise the multi-indicator palaeoecological data to evaluate possible explanations for these patterns., Principal curves analysis revealed marked and broadly simultaneous changes in the plant macrofossils, cladocerans, diatoms and chironomids (as well as in other animal remains such as bryozoans and Mollusca), indicating an early period of enrichment most likely associated with land-use change in the late 18th century, followed by a more recent eutrophication phase coincident with the discharge of sewage effluent to the lake from 1935., Ecological change, resulting from eutrophication, was shown to have progressed slowly and steadily and to have occurred at all trophic levels with a shift from a relatively diverse 'mesotrophic' macrophyte assemblage, dominance by benthic diatoms and plant-associated Chydoridae and chironomids towards a relatively species-poor, 'eutrophic' macrophyte community with dominance by planktonic algae (e.g. Cyclostephanoid diatom taxa), planktonic Cladocera ( Bosmina, Daphnia) and a chironomid fauna dominated by mud-associated taxa. The inferred shift in the macrophyte community from charophyte to fine-leaved pondweed and Callitriche truncata suggests a reduction in the seasonal duration of plant dominance., The multi-indicator analysis indicates that a combination of increased phytoplankton biomass and low zooplanktivorous fish predation is likely to explain the recent increases in Daphnia spp., while loss of plant habitat and increased competition for food appear to be the most likely causes of the observed decline in C. mucedo, and resistance to increased grazing pressure from invertebrates is the most probable driver of the C. placentula increase., Our study illustrates the potential of using the full array of fossil groups preserved in lake sediment cores to infer ecosystem dynamics over centennial timescales and to contribute to our understanding of the mechanisms that may link biological assemblages to a range of forcing factors. Further, this paper provides methodological guidance by demonstrating the ability of amalgamated records from three cores to reveal a strong sequence of events and coherent patterns. [ABSTRACT FROM AUTHOR]

In large lakes around the world, depth-based changes in the abundance and distribution of invertebrate and fish species suggest that there may be concomitant changes in patterns of resource allocation. Using Lake Superior of the Laurentian Great Lakes as an example, we explored this idea through stable isotope analyses of 13 major fish taxa., Patterns in carbon and nitrogen isotope ratios revealed use of both littoral and profundal benthos among populations of most taxa analysed regardless of the depth of their habitat, providing evidence of nearshore-offshore trophic linkages in the largest freshwater lake by area in the world., Isotope-mixing model results indicated that the overall importance of benthic food-web pathways to fish was highest in nearshore species, whereas the importance of planktonic pathways increased in offshore species. These characteristics, shared with the Great Lakes of Africa, Russia and Japan, appear to be governed by two key processes: high benthic production in nearshore waters and the prevalence of diel vertical migration ( DVM) among offshore invertebrate and fish taxa. DVM facilitates use of pelagic food resources by deep-water biota and represents an important process of trophic linkage among habitats in large lakes., Support of whole-lake food webs through trophic linkages among pelagic, profundal and littoral habitats appears to be integral to the functioning of large lakes. These linkages can be disrupted though ecosystem disturbance such as eutrophication or the effects of invasive species and should be considered in native species restoration efforts. [ABSTRACT FROM AUTHOR]

In the Alps, as in other mountainous regions across the world, nitrogen ( N) enrichment of ecosystems via atmospheric deposition is a major environmental concern. Nitrogen enrichment can cause acidification in poorly buffered lakes and streams, but here we assessed the less well-known effects of eutrophication on the phytobenthos of lakes and its consequences for grazing., To simulate the effects of N deposition, we conducted experimental nutrient additions ( N and N+ P) in two lakes in the French Alps, in summer 2012. In each lake, we deployed nutrient-diffusing substrata for 22 days. The substrata were placed within cages that either allowed or prevented access by macrograzers. We tested the hypotheses that N enrichment: (i) increases phytobenthic biomass, (ii) alters phytobenthic taxonomic composition and (iii) reduces benthic grazing., Supporting the first hypothesis, at the end of the experiments N-enriched substrata had a greater phytobenthic biomass than unenriched substrata, indicating N limitation. Supporting the second hypothesis, the taxonomic composition of the phytobenthos differed between N-enriched and control substrata. Green algae, including filamentous forms, were favoured by N enrichment to a greater extent than diatoms and cyanobacteria. By contrast, the third hypothesis was not supported. Grazing had the same effect on the biomass of the phytobenthos regardless of nutrient treatment., Our results highlight the sensitivity of Alpine lakes to deposition of atmospheric N. Its ecological effects are not limited to acidification, but also include increased biomass (eutrophication) and altered taxonomic composition of the phytobenthos. Because filamentous green algae favoured by N enrichment are usually unpalatable to macrograzers, inputs of atmospheric N could reduce the efficiency of transfer of energy from the phytobenthos up through the food web. [ABSTRACT FROM AUTHOR]