Invertebrate Grazers Research Articles

Invertebrate herbivores influence seagrass wasting disease dynamics.

Although invertebrate herbivores commonly impact terrestrial plant diseases by facilitating transmission of plant pathogens and increasing host susceptibility to infection via wounding, less is known about the role of herbivores in marine plant disease dynamics. Importantly, transmission via herbivores may not be required in the ocean since saline ocean waters support pathogen survival and transmission. Through laboratory experiments with eelgrass (Zostera marina), we showed that isopods (Pentidotea wosnesenskii) and snails (Lacuna spp.) created grazing scars that increased disease severity and thus indirectly facilitated transmission of Labyrinthula zosterae (Lz), a protist that causes seagrass wasting disease. Experiments also quantified different feeding preferences among herbivores: Amphipods (Ampithoe lacertosa) selectively consumed diseased eelgrass, while isopods and snails selectively grazed asymptomatic leaves, suggesting different herbivore taxa may have contrasting impacts on disease dynamics. Our experiments show no sign that herbivores directly vector Lz from diseased to asymptomatic eelgrass. However, we isolated live Lz from isopod, amphipod, and snail feces and detected Lz with quantitative polymerase chain reactionin amphipods and snails, suggesting that herbivores eating diseased eelgrass could pass the live pathogen. Finally, field surveys demonstrated a close association between seagrass wasting disease and invertebrate grazing scars; disease prevalence was 29 ± 4.7% (95% CI) higher on eelgrass leaves with herbivore scars. Collectively, these findings show that some herbivores can increase eelgrass disease risk by facilitating the spread of an important pathogen via wounding, but not via direct transmission. Thus, herbivores may play different roles in plant disease dynamics in terrestrial versus marine ecosystems depending on the pathogen's ability to survive and transmit without a vector.

The effects of diatom polyunsaturated aldehydes on embryonic and larval zebrafish (Danio rerio).

Marine diatoms are pervasive in many planktonic and benthic environments and represent an important food source for a wide range of species. Some diatoms produce polyunsaturated aldehydes (PUAs) as defensive toxins. PUA exposure is known to reduce the fecundity of invertebrate grazers like copepods and echinoderm larvae, but little is known about the effects of PUAs on vertebrates. Many fish species are likely to come into close contact with diatoms. Many may deposit eggs on diatom-coated substrates, consume diatoms as larvae, and/or feed heavily on zooplankters that may be gut-loaded with diatoms. The purpose of this study was to test whether dissolved diatom PUAs affect the early life stages of a model fish, Danio rerio (zebrafish). To test this, zebrafish embryos and larvae were exposed to proportionally increasing mixtures of the dissolved diatom PUAs 2E,4E-decadienal, 2E,4E-octadienal and 2E,4E-heptadienal. Under PUA exposure, three metrics of fitness were assessed: embryo heart rate, larval size at hatch, and pre-feeding mortality rate. Zebrafish embryos exposed at 24 h post fertilization (hpf) experienced decreased average heart rate after 2 days of PUA exposure. Embryos 24 hpf exposed to PUA mixtures for 6 days showed a reduction in size in comparison to embryos from controls. Embryos exposed to PUAs from 2 hpf until death showed lower survivorship compared to larvae in controls. The results of this study suggest that larval fish that are sympatric with PUA producing diatoms during their embryonic and larval stages may be susceptible to detrimental effects from PUA exposure.

Loss of amphibian species alters periphyton communities in montane ponds

Amphibian larvae can affect the structure and functioning of freshwater ecosystems, but their effects have been little explored although amphibian biodiversity is rapidly declining. Given that larvae of different amphibian species belong to different trophic levels, their effects on freshwater communities and processes can be expected to differ, with herbivores likely having direct effects on algae and predators having indirect effects through trophic cascades. We explored this question through a mesocosm experiment conducted in montane ponds, using an anuran and a urodele species affected by emergent diseases. We used different scenarios of reduction and loss of one or both species, and compared them to a control scenario representing a typical amphibian community in the study area composed of four species, with total larval density held constant. Loss of the anuran resulted in lower chlorophyll concentration and algal density, likely due to replacement by more efficient grazers. Loss of the urodele produced similar trends but weaker, possibly due to an increase of invertebrate grazing activity in the absence of this predator. Our study shows how the loss of amphibian species can alter the structure of montane ponds, but also how the mechanisms involved and the intensity of effects differ for different species.

Fine sediment and the insecticide chlorantraniliprole inhibit organic‐matter decomposition in streams through different pathways

Abstract Intensive agriculture drives an ongoing deterioration of stream biodiversity and ecosystem functioning across the planet. Key agricultural stressors include increased deposited fine sediment and insecticides flushed from adjacent land into streams. The individual and combined effects on aquatic biota are increasingly studied, but the functional consequences of biodiversity loss associated with agricultural stressors remain poorly understood. We addressed this knowledge gap by examining the effects of fine sediment and different concentrations of the insecticide chlorantraniliprole on organic‐matter decomposition. We conducted an outdoor stream mesocosm experiment. Mesocosms contained a standardised organic‐matter assay (the cotton‐strip assay), which was used to assess organic‐matter decomposition rates (as tensile‐strength loss of the fabric) and microbial respiration of the cotton strips. The decomposition rate of strips buried under fine sediment was inhibited, a result we attribute to the limited accessibility for invertebrate feeding and microbial activities, as well as the limited nutrient and dissolved oxygen exchange. The insecticide also inhibited decay rates, a finding we relate to reduced invertebrate grazing and associated excessive algal growth. In contrast to decomposition rates, we did not observe stressor effects on microbial respiration. An interaction between fine sediment and chlorantraniliprole was not identified. Our results suggest that stressors induced by agriculture affect functions of stream ecosystems through a variety of pathways and operate by modifying habitats and biotic interactions. By examining a combination of stressors and responses that have not been addressed before, this study gives important insights into the effects of agricultural practices on streams. Understanding the effects of chlorantraniliprole is especially important since it is likely to become more widely used in future agricultural practice due to the increasing ban on neonicotinoid insecticides. Furthermore, most experimental studies address multiple stressor effects on biota. For a comprehensive understanding of complex stressor effects on ecosystems, ecosystem functions also need to be studied, such as the organic‐matter decomposition within streams.

A domesticated photoautotrophic microbial community as a biofilm model system for analyzing the influence of plastic surfaces on invertebrate grazers in limnic environments.

The environmental fate of plastic particles in water bodies is influenced by microbial biofilm formation. Invertebrate grazers may be affected when foraging biofilms on plastics compared to biofilms on natural substrata but the mechanistic basis for these effects is unknown. For analyzing these effects in ecotoxicological assays stable and reproducible biofilm communities are required that are related to the environmental site of interest. Here, a defined biofilm community was established and used to perform grazing experiments with a freshwater snail. For this, snippets of different plastic materials were incubated in the photic zone of three different freshwater sites. Amplicon sequencing of biofilms formed on these snippets showed that the site of incubation and not the plastic material dominated the microbial community composition. From these biofilms, individual microbial strains as well as photoautotrophic consortia were isolated; these consortia consisted of heterotrophic bacteria that were apparently nourished by microalga. While biofilms formed by defined dual cultures of a microalga and an Alphaproteobacterium were not accepted by the snail P. fontinalis, a photoautotrophic consortium (Co_3) sustained growth and metabolism of this grazer. Amplicon sequencing revealed that consortium Co_3, which could be stably maintained on solid medium under photoautotrophic conditions, reproducibly formed biofilms of a defined composition on three different plastic materials and on glass surfaces. In conclusion, our study shows that the generation of domesticated photoautotrophic microbial communities is a valid novel approach for establishing laboratory ecotoxicological assays with higher environmental relevance than those based on defined microbiota.

Short-term exposure to independent and combined acidification and warming elicits differential responses from two tropical seagrass-associated invertebrate grazers

Short-term exposure to independent and combined acidification and warming elicits differential responses from two tropical seagrass-associated invertebrate grazers

Physiological responses of cooccurring intertidal limpets (Cellana spp.) to acute and repeated heat stress

Limpets help maintain bare space and act as sentinel species of thermal stress in intertidal reef ecosystems. However, little is known about the physiological responses of New Zealand's widely distributed Cellana spp. limpets to thermal stress. We tested if there were differences in thermal tolerance in summer and winter and heat shock protein concentrations among four Cellana species that occupy different shore elevations. For one highly abundant species (C. denticulata), we tested whether thermal tolerance was affected by limpet size in summer and winter and whether thermal tolerance and oxygen consumption in air and water were affected by repeated heat stress. Limpet species from higher shore elevations did not always have higher breakpoint temperatures than those from lower elevations, potentially due to differences in homing behaviour. Similarly, there was no effect of size, season, or repeated heat stress on thermal tolerance. HSP70 concentrations were highest in C. denticulata, but were not affected by temperature (19–34 °C), indicating that limpets maintain high concentrations of constitutive HSP70. Limpets exposed to repeated heat stress in air had higher heart rates and oxygen consumption than those maintained at ambient temperature (19 °C). However, when both treatments were flooded with 18 °C seawater, heart rates were similar, but oxygen consumption was greater in the ambient treatment, indicating that non-stressed limpets may incur an oxygen debt during aerial exposure. This study provides new understanding about the effect of acute and repeated heat stress on the physiology of these conspicuous and abundant grazing invertebrates, which will help better predict their distributions in response to climate change.

Compound‐specific stable isotopes resolve sources and fate of polyunsaturated fatty acids in biota of headwater streams

Abstract Organisms at the base of stream food webs are typically poor in long‐chain polyunsaturated fatty acids (LC‐PUFA), especially in docosahexaenoic acid (DHA), whereas consumers at higher trophic levels are often rich in LC‐PUFA. For example, fish tissues, especially the brain, are DHA‐rich. This obvious mismatch between consumer LC‐PUFA and their basal dietary supply may result from selective retention and/or endogenous conversion of dietary precursors to LC‐PUFA. To determine which is more likely, we investigated compound‐specific carbon stable isotopes in PUFA (δ13CPUFA) of potential basal resources (stream epilithon, leaf litter) and consumers (invertebrates, European bullhead, and two salmonid species and their brain, eye, liver, and muscle tissues). We predicted that consumer‐PUFA, depleted in 13C values relative to their dietary sources, would indicate internal de novo PUFA synthesis. Alternatively, higher consumer‐δ13CPUFA values would imply selective retention of aquatic (epilithon and conditioned leaves) rather than terrestrial resources or internal production, irrespective of trophic levels. Invertebrate grazers and predators resembled δ13C values of essential fatty acids (δ13CEFA) of benthic algae, while shredder‐δ13CEFA values reflected those of conditioned rather than fresh leaves. Lower eye‐δ13CEFA values of salmonids than in livers indicated high retention of dietary PUFA sources (invertebrates and epilithon). Stable isotope values of eicosapentaenoic acid suggest that all consumers retained algal EPA, while insectivorous fish produced DHA in their liver. There is no further evidence from carbon stable isotopes for local PUFA conversion within neural tissues. Our study demonstrates that δ13CPUFA can be used to track sources of these highly functional molecules in aquatic consumers and highlights the importance of algal derived‐PUFA for these consumers in oligotrophic headwater streams.

Environmental risks of a commonly used pyrethroid: Insights from temporary pond species of the Lake Manyara Basin, Tanzania

Environmental risks posed by widespread pesticide application have attracted global attention. Currently, chemical risk assessments in aquatic environments rely on extrapolation of toxicity data from classic model species. However, similar assessments based on local species could be complementary, particularly for unusual living environments such as temporary ponds. Here, we carried out an environmental risk assessment (ERA) of a pyrethroid model compound, cypermethrin, based on local temporary pond species. First, we measured cypermethrin residue concentrations in rivers, irrigation canals and temporary ponds in the Lake Manyara Basin (LMB). Then, we estimated the environmental risks of cypermethrin by combining these data with acute toxicity data of three resident species across three trophic levels: primary producers (Arthrospira platensis), invertebrate grazers (Streptocephalus lamellifer) and fish (Nothobranchius neumanni). Furthermore, we compared the derived ERA to that obtained using toxicity data from literature of classic model species. Cypermethrin residue concentrations in contaminated systems of the LMB ranged from 0.01 to 57.9 ng/L. For temporary pond species, S. lamellifer was the most sensitive one with a 96 h-LC50 of 0.14 ng/L. Regardless of the assumed exposure concentration (0.01 and 57.9 ng/L), the estimated risks were low for primary producers and high for invertebrate grazers, both for local species as well as for classic model species. The highest detected cypermethrin concentration resulted in a moderate risk estimation for local fish species, while the estimated risk was high when considering classic fish models. Our results confirm that, at least for pyrethroids, ERAs with classic model species are useful to estimate chemical risks in temporary pond ecosystems, and suggest that complementary ERAs based on local species could help to fine-tune environmental regulations to specific local conditions and conservation targets.

Periphyton and benthivorous fish affect charophyte abundance and indicate hidden nutrient loading in oligo‐ and mesotrophic temperate hardwater lakes

Abstract Charophytes are a key group of submerged macrophytes in temperate hardwater lakes, but have declined or been replaced by angiosperms as a result of eutrophication for more than 100 years. At present, this process is continuing in many European lakes despite an overall reduced nutrient loading. In eutrophic lakes, light attenuation by periphyton is a major factor responsible for declining angiosperm abundance. For charophyte declines, the impact of benthivorous and herbivorous fish has been discussed often, yet periphyton shading has been largely neglected. In our study, we investigated 11 temperate oligo‐mesotrophic (total phosphorus [P] concentrations 9–22 μg/L) hardwater lakes along a gradient of charophyte abundances. Charophyte abundance was mapped using underwater cameras or scuba diving, and coverage was estimated in five classes. Periphyton biomass, nutrient stoichiometry and invertebrate grazer abundance were measured twice on artificial substrates exposed inside and outside fish exclosures for 4 weeks in July and August. Fish biomass was sampled once using multi‐mesh gill netting. We analysed the relationships between water chemistry (concentrations of nutrients and dissolved organic carbon), periphyton biomass and nutrient stoichiometry, periphytic invertebrate grazers and fish biomass to test (a) whether periphyton biomass was controlled bottom‐up by nutrient availability or top‐down by an omnivorous fish–invertebrate–grazer cascade and (b) whether charophyte abundance was affected by periphyton biomass, benthivorous and herbivorous fish. Multiple linear regressions revealed that charophyte abundance was significantly predicted by the biomasses of periphyton and benthivorous bream (Abramis brama L.), whereas herbivorous rudd (Scardinius erythrophthalmus L.) had no significant effect in the investigated lakes. Bream biomass and its proportion in the total fish biomass were linearly related to lake total P concentrations. Periphyton biomass in the tested lakes reached 0.1–21 g dry weight/m2 and primarily was controlled bottom‐up by P availability as indicated from stoichiometry and positive correlations with grazer abundance. Top‐down control was found in only three lakes, which had significantly lower periphyton biomass inside fish exclosures as compared to controls. Our data imply that periphyton shading and negative fish impacts on charophytes increase with P loading. Low charophyte abundances at moderate P concentrations in the lake water indicate a high sensitivity of charophytes and/or unnoticed nutrient loading due to nutrient retention by charophyte stands. Management measures thus must focus on reducing nutrient loading and/or benthivorous fish biomass at early stages of impact to preserve charophyte stands in oligo‐mesotrophic hardwater lakes.

Marine Achnanthales (Bacillariophyceae) from New Caledonia (Melanesia): assemblage specificities, ultramafic environment

The marine Achnanthales (Bacillariophyceae), with a particular focus on Cocconeidaceae, were investigated in a variety of habitats from the South Province of New Caledonia (Melanesia, Pacific Ocean). Species diversity ranged from species-poor and scarce assemblages on mangrove mud flats to highly diverse and abundant assemblages on short turfs and scrapings of aerial pneumatophores of Avicennia marina. Macroalgae (in place or as sea leash), as well as feces of Acanthopleura gemmata (invertebrate grazer, living on intertidal rocky environments), contained few diatoms, with low diversity. The New Caledonia assemblages show some taxa previously found in the tropical Indian Ocean. Several noteworthy taxa, listed as rare from Polynesia, are not uncommon in New Caledonia, such as Vikingea gibbocalyx and Cocconeis pseudodiruptoides. The notion of endemism concerning marine eukaryotes is approached.

Impacts of piscicide-induced fish removal on resource use and trophic diversity of lake invertebrates

Chemical eradication of non-native species has become a widely used method to mitigate the potential negative impacts of altered competitive or predatory dynamics on biodiversity and natural ecosystem processes. However, the responses of non-target species can vary from rapid full recovery to delayed or absent recolonization, and little is known about the potential shifts in resource use and trophic diversity of native species following chemical treatments. We used a before-after-control-impact approach to study the effects of rotenone piscicide treatment on abundance and trophic niche of benthic invertebrates in three untreated and three treated lakes in central Norway, the latter group hosting non-native roach (Rutilus rutilus) and pike (Esox lucius) prior to rotenone treatment. Based on community composition data, the relative abundance of invertebrate grazers and collectors decreased while that of predators increased following fish removal in the treated lakes. The stable isotope data indicated minor shifts in resource use of, and trophic diversity among, benthic invertebrate communities. While the predatory dragonfly larvae (Odonata) and grazer snails (Lymnaeidae) showed increased δ13C values indicating increased reliance on littoral benthic algae, the collector mayfly larvae (Leptophlebia) showed decreased δ13C values following fish removal in treated lakes. Grazer snails also showed a shift to a lower trophic position, while the predatory dragonflies and collector mayflies showed no changes in δ15N values following fish removal. The community-level isotopic niches of benthic invertebrates showed no consistent changes, although the sample-size corrected and Bayesian estimates of standard ellipse areas (SEAC and SEAB) slightly increased in two of the three treated lakes due to an increased range in δ15N. In conclusion, our study findings indicate some changes in species assemblages but minor shifts in the resource use and trophic diversity of benthic invertebrate communities following fish removal in rotenone treated lakes.

Grazing Effects of Soil Fauna on White-Rot Fungi: Biomass, Enzyme Production and Litter Decomposition Ability.

Soil invertebrates and microorganisms are two major drivers of litter decomposition. Even though the importance of invertebrates and microorganisms in biogeochemical soil cycles and soil food webs has been studied, the effects of invertebrates on fungi are not well understood compared to other organisms. In this work, we investigated the effects of soil invertebrates on fungi as a factor that cannot be ignored in the study of nutrient cycling. The result showed the grazing of isopods on white-rot fungi was transitive and persistent. The grazed fungi appeared “compensatory” growing. The biomass of fungi increased after grazing. The activities of enzymes associated with nutrient cycling were increased under grazing. The zymography images showed the enzyme hotspots and activities also increased significantly in the grazing area. The results suggest that invertebrate grazing can significantly increase the fungal biomass and enzyme activity, accelerating litter decomposition in the unreached grazer area. The grazing effects of invertebrate plays an important role in promoting the nutrient cycling of the forest ecosystem. We believe that this study will be a good reference related to showing the relationship between soil invertebrates, fungi and soil biogeochemical cycles.

A reciprocal transplant experiment sheds new light on a classic marine seagrass-algal symbiosis and suggests influence of epiphytic symbiont on seagrass microbiota

In aquatic foundation species, composition and abundance of associated epibionts can change substantially over small spatial distances. Such spatial variation can reflect top-down control by consumers, bottom-up control by abiotic factors or facilitation, or a combination of processes. We used visual and molecular surveys to describe spatial patterns in the abundance and distribution of the epiphytic red macroalga Smithora naiadum in a meadow of the seagrass Zostera marina on the Central Coast of British Columbia. We detected Smithora using 18 S ribosomal RNA molecular marker throughout the seagrass meadow at both interior and edge sites, even in the absence of macroscopic Smithora. We used a reciprocal transplant experiment to test two hypotheses: that patterns in Smithora abundance reflect local environmental conditions, or alternately, that patterns reflect spatial variation in the host plant attributes, microbiota and grazers. Zostera shoots hosted more Smithora at meadow edges relative to meadow interior sites, and shoots with Smithora were associated with distinct invertebrate grazer and bacterial communities relative to shoots 5 m in from the meadow edge without Smithora. Macroscopic Smithora grew on shoots experimentally transplanted from the interior to the meadow edge and shoots hosting Smithora that were transplanted to the interior did not lose Smithora. Our survey and experimental results suggest that presence of macroscopic Smithora blades on Zostera shoots changes the Zostera microbiota. Altogether, we conclude that environmental variation, not host plant attributes or dispersal limitation, affects Smithora colonization on Zostera, and once established, Smithora alters the microbiota on Zostera.

Invertebrate Grazing on Live Turtlegrass (Thalassia testudinum): A Common Interaction That May Facilitate Fungal Growth

In coastal wetlands and tropical reefs, snails can regulate foundation species by feeding on marsh grasses and hard corals. In many cases, their impacts are amplified because they facilitate microbial infection in grazer-induced wounds. Whether snails commonly graze live plants and facilitate microbial growth on plants in tropical seagrass systems is less explored. On a Belizean Caye, we examined patterns in snail-generated grazer scars on the abundant turtlegrass (Thalassia testudinum). Our initial survey showed the occurrence of snail-induced scarring on live turtlegrass blades was common, with 57% of live leaves scarred. Feeding trials demonstrated that two of five common snails (Tegula fasciata–smooth tegula and Smaragdia viridis–emerald nerite) grazed unepiphytized turtlegrass blades and that smooth tegula abundance had a positive relationship with scarring intensity. Subsequent surveys at three Caribbean sites (separated by >150 km) also showed a high occurrence of snail-induced scars on turtlegrass blades. Finally, simulated herbivory experiments and field observations of a turtlegrass bed in Florida, United States suggests that herbivore damage could facilitate fungal growth in live seagrass tissue through mechanical opening of tissue. Combined, these findings reveal that snail grazing on live turtlegrass blades in the Caribbean can be common. Based on these results, we hypothesize that small grazers could be exerting top-down control over turtlegrass growth directly via grazing and/or indirectly by facilitating microbial infection in live seagrass tissue. Further studies are needed to determine the generality and relative importance of direct and indirect effects of gastropod grazing on turtlegrass health.

Invertebrate Grazing on Live Turtlegrass (Thalassia testudinum): A Common Interaction That May Facilitate Fungal Growth

Invertebrate Grazing on Live Turtlegrass (Thalassia testudinum): A Common Interaction That May Facilitate Fungal Growth

A sprinkling of gold dust: Pine pollen as a carbon source in Baltic Sea coastal food webs

AbstractAllochthonous subsidies to marine ecosystems have mainly focused on biogeochemical cycles, but there has also been recent interest in how terrestrial carbon (C) influences marine food webs. In the Baltic Sea, pine (Pinus sylvestris) pollen is found in large amounts in shallow bays in early summer. Pollen is a significant C‐source in freshwater ecosystems and may also be important in coastal food webs. We examined the consumption of pollen and autochthonous resources by benthic invertebrates in shallow bays of the Baltic Sea. We used stable isotopes to estimate diets and reconstructed consumer‐resource networks (food webs) for grazers and particulate organic matter (POM)‐feeders to compare how these different guilds used pollen. We found thatP. sylvestrispollen was consumed in small amounts by a variety of animals and in some cases made up a sizeable proportion of invertebrates' diets. However, invertebrates generally depended less on pollen than other resources. The degree of pollen consumption was related to feeding traits, with generalist invertebrate grazers consuming more pollen (> 10% of diet) than the more specialist POM‐feeders (< 5% of diet contributed by pollen). POM‐feeders may consume additional microbially‐degraded pollen which was not identifiable in our model. We suggest that pollen is a small but substantial allochthonous C‐source in shallow bay food webs of the Baltic Sea, with the potential to affect the dynamics of these ecosystems.

Near-daily reconstruction of tropical intertidal limpet life-history using secondary-ion mass spectrometry

Measurements of life-history traits can reflect an organism’s response to environment. In wave-dominated rocky intertidal ecosystems, obtaining in-situ measurements of key grazing invertebrates are constrained by extreme conditions. Recent research demonstrates mollusc shells to be high-resolution sea-surface temperature proxies, as well as archival growth records. However, no prior molluscan climate proxy or life-history reconstruction has been demonstrated for the tropical rocky intertidal environment—a zone influenced by warmer waters, mixed tides, trade-wind patterns, and wave-action. Here, we show near-daily, spatiotemporal oxygen isotope signatures from the tropical rocky intertidal environment by coupling secondary ion mass spectrometry analysis of oxygen isotopes with the sclerochronology of an endemic Hawaiian intertidal limpet Cellana sandwicensis, that is a significant biocultural resource harvested for consumption. We also develop a method for reliable interpretation of seasonal growth patterns and longevity in limpets. This study provides a robust approach to explore tropical intertidal climatology and molluscan life-history.

Longitudinal variation in the nutritional quality of basal food sources and its effect on invertebrates and fish in subalpine rivers.

There is growing recognition of the importance of food quality over quantity for aquatic consumers. In streams and rivers, most previous studies considered this primarily in terms of the quality of terrestrial leaf litter and importance of microbial conditioning. However, many recent studies suggest that algae are a more nutritional food source for riverine consumers than leaf litter. To date, few studies have quantified longitudinal shifts in the nutritional quality of basal food resources in river ecosystems and how these may affect consumers. We conducted a field investigation in a subalpine river ecosystem in Austria to investigate longitudinal variations in diet quality of basal food sources (submerged leaves and periphyton) and diet source dependence of stream consumers (invertebrate grazers, shredders, filterers and predators, and fish). Fatty acid (FA) profiles of basal food sources and their consumers were measured. Our results indicate systematic differences between the FA profiles of terrestrial leaves and aquatic biota, that is periphyton, invertebrates and fish. Submerged leaves contained very low proportions of long-chain polyunsaturated fatty acids (LC-PUFAs), which were conversely rich in aquatic biota. While the FA composition of submerged leaves remained similar among sites, the LC-PUFAs of periphyton increased longitudinally, which was associated with increasing nutrients from upstream to downstream. Longitudinal variations in periphyton LC-PUFAs were reflected in the LC-PUFAs of invertebrate grazers and shredders, and further tracked by invertebrate predators and fish. However, brown trout Salmo trutta contained a large proportion of docosahexaenoic acid (DHA, 22:6ω3), a LC-PUFA almost entirely missing in basal sources and invertebrates. The fish accumulated eicosapentaenoic acid (EPA, 20:5ω3) from invertebrate prey and may use this FA to synthesize DHA. Our results provide a nutritional perspective for river food web studies, emphasizing the importance of algal resources to consumer somatic growth and the need to account for the longitudinal shifts in the quality of these basal resources.

Combining Hydrologic Simulations and Stream‐network Models to Reveal Flow‐ecology Relationships in a Large Alpine Catchment

AbstractFlow regimes profoundly influence river organisms and ecosystem functions, but regulatory approaches often lack the scientific basis to support sustainable water allocation. In part, this reflects the challenge of understanding the ecological effects of flow variability over different temporal and spatial domains. Here, we use a process‐based distributed hydrological model to simulate 23 years of natural flow regime in 100 bioassessment sites across the Adige River network (NE Italy), and to identify typical nivo‐glacial, nivo‐pluvial, and pluvial reaches. We then applied spatial stream‐network models (SSN) to investigate the relationships between hydrologic and macroinvertebrate metrics while accounting for network spatial autocorrelation and local habitat conditions. Macroinvertebrate metrics correlated most strongly with summer, winter, and temporal variation in streamflow, but effects varied across flow regime types. For example (i) taxon richness appeared limited by high summer flows and high winter flows in nivo‐glacial and pluvial streams, respectively; (ii) invertebrate grazers increased proportionally with the annual coefficient of flow variation in nivo‐glacial streams but tended to decline with flow variation in pluvial streams. Although local land‐use and water quality also affected benthic communities, most variation in macroinvertebrate metrics was associated with spatial autocorrelation. These findings highlight the importance of developing environmental flow management policies in ways that reflect specific hydro‐ecological and land use contexts. Our analyses also illustrate the importance of spatially‐explicit approaches that account for auto‐correlation when quantifying flow‐ecology relationships.