Basal Resources Research Articles

Climate-Induced Expansion of Consumers in Seagrass Ecosystems: Lessons From Invasion Ecology

A warming climate is driving the poleward expansion of tropical, subtropical, and temperate plant and animal distributions. These changes have and continue to lead to the colonization of novel organisms into areas beyond their historical ranges. While the full scope of ecological impacts remains unclear, these expansions could alter density-dependent interactions, habitat occupancy patterns, and food web dynamics– similar to exotic species impacts in invaded ecosystems. Seagrasses are habitats of particular interest, given their widespread distribution and ecosystem services. While multiple recent studies report on the effects of the return of larger tropical herbivores in seagrass beds in warming subtropical waters, less is known about the addition of mid-trophic level consumers. These consumers are often key determinants of energy and nutrient transfers from basal resources to higher order predators. Here, we discuss the potential impacts of these distribution changes on temperate and subtropical seagrass communities using information derived from invasive species studies. Notably, we outline several scenarios and generate predictions about how their establishment might occur and speculate on impacts of warmer water consumers as they move poleward. We also discuss potential confounding factors of detecting changes in these consumer distributions. Following the invasive species literature, we offer a framework for generating hypotheses and predicting effects from these range-expanding organisms. Given that climates are predicted to continue to warm into the future, thus facilitating additional species expansions, our goal is to guide future research efforts and provide information for rapid dissemination and utility for this growing subdiscipline of marine ecology.

Community dynamics of estuarine forage fishes are associated with a latitudinal basal resource regime

AbstractForage fishes are an important component of marine, estuarine, and aquatic food webs that facilitate the transfer of energy and nutrients from primary producers to upper trophic levels. Previous studies of forage fishes have focused primarily on pelagic planktivorous species in pelagic environments. However, benthically associated taxa can be just as important as planktivorous species, particularly in highly productive estuarine environments that provide critical habitat for many predators. In this study, we analyzed a 20‐year forage fish community composition and abundance dataset across four eastern Gulf of Mexico estuaries spanning a broad latitudinal gradient to investigate spatiotemporal variability in community structure and quantify associations with habitat. Our analyses revealed significant regional structuring of forage fish communities, coupled with a strong association with habitat characteristics related to latitudinal effects and basal resource regime. Communities in the two northern estuaries and two southern estuaries were associated primarily with planktonically reliant and benthically reliant taxa, respectively. Despite regional differences, we uncovered a coherent annual cycle in forage fish communities across all estuaries related to seasonal shifts in abundances of several abundant and ubiquitous species. We additionally revealed significant subdecadal periodicity potentially associated with bottom‐up effects of global climatic cycles. The significant association of forage fish communities with habitat regime shown in this study underlies the importance of continued monitoring of these communities. This study represents a novel approach to assess this critical ecosystem component in diverse estuarine systems globally.

Stable carbon and nitrogen isotopes explain methylmercury concentrations in stream food webs of Lake George, New York (USA).

Mercury has been studied extensively in lakes due to health risks associated with the consumption of contaminated fish, while stream ecosystems have received less attention. To better understand mercury bioavailability in the lower food web of streams, we collected macroinvertebrates (predators and detritivore) along with autochthonous (epilithic algae) and allochthonous (leaf litter) basal resources in eight streams entering Lake George. Samples were analyzed for methylmercury (MeHg), total mercury, and carbon and nitrogen isotopes (δ13C & δ15N) to determine how mercury concentrations in basal resources, biomagnification rates, and environmental factors (watershed characteristics and water chemistry) effected MeHg concentrations in predatory macroinvertebrates. While biomagnification rates, calculated as trophic magnification slope, explained between 68% and 98% of MeHg variability within a stream food web, the range was small (0.310-0.387) resulting in the biotic components following a consistent pattern of increasing MeHg among streams. The stream order was negatively related to basin slope for all biotic components and explained 70% of MeHg variability in predatory macroinvertebrates. Methylmercury concentrations were significantly and negatively related to δ13C in predators, epilithic algae, and leaf litter. We believe the biofilms on leaf litter utilized bacterial-respired carbon dioxide decreasing δ13C (<-28‰) and increasing MeHg while epilithic algal δ13C increased due to enhanced primary production resulting in biodilution of MeHg. Methylmercury in basal resources responded to δ13C similarly but through different processes. Our findings show shallow slopes elevate MeHg in basal resources and explain most of the predator MeHg variation among streams with little influence of biomagnification rates.

Autochthony and isotopic niches of benthic fauna at shallow-water hydrothermal vents

The food webs of shallow-water hydrothermal vents are supported by chemosynthetic and photosynthetic autotrophs. However, the relative importance of these two basal resources for benthic consumers and its changes along the physicochemical gradient caused by vent plumes are unknown. We used stable carbon and nitrogen isotopes (i.e., δ13C and δ15N) and Bayesian mixing models to quantify the dietary contribution of basal resources to the benthic fauna at the shallow-water vents around Kueishan Island, Taiwan. Our results indicated that the food chains and consumer production at the shallow-water vents were mainly driven by photoautotrophs (total algal contribution: 26–54%) and zooplankton (19–34%) rather than by chemosynthetic production (total contribution: 14–26%). Intraspecific differences in the trophic support and isotopic niche of the benthic consumers along the physicochemical gradient were also evident. For instance, sea anemone Anthopleura sp. exhibited the greatest reliance on chemosynthetic bacteria (26%) and photoautotrophs (66%) near the vent openings, but zooplankton was its main diet in regions 150–300 m (32–49%) and 300–700 m (32–78%) away from the vent mouths. The vent-induced physicochemical gradient structures not only the community but also the trophic support and isotopic niche of vent consumers.

Substrate composition influences amino acid carbon isotope profiles of fungi: implications for tracing fungal contributions to food webs

Fungi link detrital resources and metazoan consumers through their role as decomposers. However, fungal contributions to metazoans may be misestimated in amino acid isotope studies because fungi are capable of both synthesizing amino acids (AAs) de novo and absorbing AAs from their environment. While fungi cultured in AA-free media have been used to represent fungi in studies of natural environments, fungi likely gain energetic benefits by taking up substrate AAs directly in situ. Consequently, fungi cultured on AA-free media may not be representative of the true variability of natural fungal δ13 CAA profiles. Therefore, the objective of this experiment was to determine the effect of substrate AA availability on yeast δ13 CAA profiles. We found that yeasts cultured in media of relatively higher AA content had different δ13 CAA profiles than yeasts grown in AA-free media, in part because yeasts utilized two essential AAs (Leu and Val) directly from media substrates when available in sufficient amounts. Furthermore, these differences among yeast δ13 CAA profiles remained after normalization of δ13 CAA values. We recommend further characterization of the variation in fungal δ13 CAA profiles and the incorporation of this potential variability into interpretations of basal resource use by metazoans.

Dietary fatty acid profiles shape crayfish biosynthesis and performance: Implications for riverine food webs

Abstract Alteration of riverine flows can modify the structure and function of ecosystems, changing energy pathways and patterns of micronutrient transfer between trophic levels. Fatty acids (FAs) commonly are used to evaluate food quality, since some FAs required for somatic growth and physiological functions in animals must be obtained from their diet. FAs also are used in food‐web studies as biotracers as a consequence of their constrained metabolic biosynthesis by animals. However, their utility may be confounded by selective retention or modification of dietary FAs by consumers. We conducted a 70‐day feeding trial to compare growth and survival of an abundant and widespread mesoconsumer (Cherax destructor, the common yabby or crayfish) fed three contrasting diets: a poor‐quality detritus‐based diet; a high protein invertebrate diet; and a high‐quality commercial aquaculture pellet. Fatty acid profiles were obtained for each dietary treatment and contrasted with crayfish FA profiles at the end of the experiment to examine patterns of FA retention and integration. We also collected wild crayfish from floodplain wetland and river habitats, and obtained FA profiles from their stomach contents and body tissue to compare with experimental crayfish. Experimental crayfish fed high‐quality commercial pellets doubled in mass during the 70‐day assay, invertebrate fed crayfish growth was intermediate, and growth of crayfish fed detritus was negligible. Fatty acid profiles of crayfish fed our three contrasting diets differed significantly at the end of the experiment. Proportions of the polyunsaturated omega‐6 FA linoleic acid (LIN, 18:2ω6) in crayfish followed the same inequality observed in growth and diets: pellets &gt; invertebrates &gt; detritus. Pellet‐fed crayfish preferentially assimilated greater proportions of FAs 20:4ω6 (ARA), 20:5ω3 (EPA) 18:1ω9 (OA) and 16:1ω7 (POA) into their tissue. Fatty acid profiles of floodplain crayfish differed to profiles of riverine crayfish, and floodplain crayfish had higher proportions of essential FAs ARA and LIN in their tissues. Fatty acid biosynthesis by crayfish was best described by a hypothesis of FA allostasis rather than homeostasis; in this, FA profiles of crayfish were shaped by their diet, and selective integration and modification of high‐quality FAs from basal resources rich in these micronutrients led to higher proportions in crayfish tissues. Here we present evidence for the conversion of shorter‐chain essential FAs by freshwater crayfish to compensate for a lack of long‐chain FAs in their diet. We provide a necessary step for improving our understanding of micronutrient dynamics and the transfer of essential molecules between trophic levels in lowland river food webs. Floodplain habitats are known to provide higher‐quality basal food resources for mesoconsumers than riverine habitats, and here we identify one mechanism by which that may be extended to subsequent trophic levels.

Autotrophs are important contributors to benthic macroinvertebrate food webs in two tropical first‐order forest streams

Abstract In tropical first‐order streams, the availability of basal resources changes according to tropical seasonality, i.e., dry and rainy seasons, with consequences for the food web structure. However, the seasonal variation in autotrophic and detrital pathways remains unclear. In this study, we aimed to establish whether the dominant pathway of the benthic macroinvertebrate food web in two tropical first‐order streams in the Lacandona rainforest, Mexico, relies on autotrophic or detrital food resources. We evaluated the assimilation of autotrophic and detrital food resources through carbon and nitrogen stable isotope (δ13C and δ15N) analysis. We included the biomass of the macroinvertebrates sustained by the autotrophic and detrital pathways and the food web structure—trophic position, size structure, and degree of omnivory. The biomass sustained by the detrital pathway remained similar throughout the year in both streams. In contrast, the biomass sustained by the autotrophic pathway was higher in the dry season than in the rainy season. The benthic macroinvertebrate food web had four levels, a poorly developed size structure, and an important degree of omnivory in both streams and in both seasons. It is likely that omnivory was a response to high resource variability and a strategy to reduce intraguild competition. Autotrophs had a relatively higher importance than detritus since a much lower algae biomass sustained an equal or higher macroinvertebrate biomass, particularly in the dry season when the biomass of autotrophs increased. Seasonality influenced the macroinvertebrate food web by modifying the food resource availability and transference to higher trophic levels. However, the high frequency of omnivory reduced the seasonal changes in the food web structure. Our results suggest that the autotrophic pathway plays an important role in maintaining the benthic macroinvertebrate food web in tropical streams. The relative importance of autotrophs in the macroinvertebrate food web is associated with seasonality, the rainfall pattern, and its effect on the stream hydrology.

Enrichment in biodiversity and maturation of the soil food web under conservation agriculture is associated with suppression of rice-parasitic nematodes

Meloidogyne spp. and Hirschmanniella spp. are among the most damaging plant-parasitic nematodes (PPNs). They threaten rice production, the main staple food in Asia. Cropping systems that promote natural biocontrol and plant tolerance to diseases are put forward as sustainable solutions to protect rice from these pests. In particular, cropping systems managed under conservation agriculture (CA) are promising because they improve soil health and functioning. We investigated the effects of two cropping system components in a Cambodian field, (i) CA practices, i.e., no-tillage with a cover crop Stylosanthes guianensis (cv. Nina), versus conventional plow-based tillage with no cover crop, and (ii) using IR504, IR64, Azucena and Zhonghua 11 rice varieties, on PPNs in roots and on communities (bacteria, fungi and nematodes) in the rhizosphere. We used a sequencing approach via amplicon barcoding to target microbial marker genes (16 S and ITS rRNA gene) and a microscopic approach to identify and quantify nematodes in the rhizosphere compartment. The variety had less effect than agricultural practices on the infection by PPNs and on the assembly of the three rhizosphere communities. Under CA, the abundance of PPNs extracted from the roots was reduced by 88%. Soil quality was substantially improved (+83% of total Kjeldahl nitrogen, +34% of available phosphorus, +10% of exchangeable potassium, +110% of soil organic carbon, +30% for the cation exchange capacity), thus providing more basal resources for microbial decomposers, especially fungi (+164% putative saprotrophs). Characterization of the three rhizosphere communities revealed a shift in the structure associated with soil enrichment. Both microbial richness (+3% for bacteria and +38% for fungi) and diversity (Shannon index, +11% for fungi and +5% for nematodes) increased. The relative abundance of taxa was modified by CA with notably more mycorrhizal fungi (+329% Glomeromycota spp.) and fewer Pratylenchidae nematodes (−92% Hirschmanniella spp.) in the rhizosphere. The reassembly of the communities using CA was associated with regulation of PPN populations. The reduction in Meloidogyne spp. abundance in roots (−64%) was correlated with the maturity of the food web (maturity index, +10% under CA) and with the increase in the relative abundance of omnivorous nematodes in the rhizosphere (+68% under CA). Seven years of CA in this field enabled the whole soil food web to mature thus creating a favorable niche for potentially predatory nematodes and microbes antagonistic against PPNs. This study confirms that CA is an alternative to nematicides to limit infection by PPNs in rice cropping systems.

Understanding the trophic relationships amongst arthropods in olive grove by δN15and δC13stable isotope analysis

AbstractSo far the limited knowledge about the diet of omnivores and generalist predators in olive groves has not allowed the clarification of their role as natural enemies. Stable isotope (δN15and δC13) analysis is a useful tool for predicting the trophic position of a high number of species, the variety of basal resources and the flux of energy in the food web. We have explored the possible consumers of two important pests in the olive grove: adults ofPrays oleaeBern (Lepidoptera: Praydidae) during its anthophagous generation and the pupae ofBactrocera oleae(Rossi) (Diptera: Tephritidae). In addition, we have included in the study the adult instar of the secondary pestEuphyllura olivina(Costa) (Hemiptera: Psyllidae). These arthropods were clustered in isotopic groups according to their similarities in δN15and δC13content assuming that, in this case, they shared similar feeding habits. Using this criterion, we obtained eight isotopically different groups of predators and eleven groups of phytophagous insects. We have used Bayesian mixing models in order to identify the potential predators of the different pests included in our study and the proportion that each pest occupied in the diet of the identified predators. Different taxa of spiders andAnthocoris nemoralis(Fabricius) (Hemiptera: Anthocoridae) have emerged as potential predators ofB. oleae. Moreover, Gnaphosidae (Araneae) showed suitable isotopic characteristics to be identified as potential predator ofP. oleaeandA. nemoralishas been confirmed as a predator ofE. olivina. The presence ofE. olivinaas an abundant resource encourages the settlement of natural enemies in olive orchards which feed on this secondary pest, but are also ready to feed on any insect outbreak, for exampleP. oleae.

Low-pH seawater alters indirect interactions in rocky-shore tidepools.

Ocean acidification is expected to degrade marine ecosystems, yet most studies focus on organismal‐level impacts rather than ecological perturbations. Field studies are especially sparse, particularly ones examining shifts in direct and indirect consumer interactions. Here we address such connections within tidepool communities of rocky shores, focusing on a three‐level food web involving the keystone sea star predator, Pisaster ochraceus, a common herbivorous snail, Tegula funebralis, and a macroalgal basal resource, Macrocystis pyrifera. We demonstrate that during nighttime low tides, experimentally manipulated declines in seawater pH suppress the anti‐predator behavior of snails, bolstering their grazing, and diminishing the top‐down influence of predators on basal resources. This attenuation of top‐down control is absent in pools maintained experimentally at higher pH. These findings suggest that as ocean acidification proceeds, shifts of behaviorally mediated links in food webs could change how cascading effects of predators manifest within marine communities.

Response of nutrient limitation to invasive fish suppression: How carcasses and analog pellets alter periphyton

The native Yellowstone Cutthroat Trout (Oncorhynchus clarkii bouvieri Jordan and Gilbert, 1883) population in Yellowstone Lake, Yellowstone National Park, Wyoming, USA, is in decline because of competition from the introduced, invasive Lake Trout (Salvelinus namaycush Walbaum in Artedi, 1792). Gillnetting is used to suppress adult Lake Trout; however, methods are being developed to suppress embryos, including adding Lake Trout carcasses and carcass-analog pellets to spawning sites. Decomposing carcasses and analog pellets cause decreased dissolved oxygen concentrations thereby leading to Lake Trout embryo mortality, but the effects of these methods on primary producers are unknown. We deployed in-situ nutrient diffusing substrates (NDS) at 3 spawning sites. The 1st site was treated with carcasses, the 2nd site was treated with analog pellets, and a 3rd lacked treatment (control). To estimate how suppression measures may alter nutrient limitation, we measured algal biomass in 6 NDS amendments at each site: nothing (control), N, P, N + P, ground carcasses, or pulverized analog pellets. We deployed 5 replicates of each amendment at each site before and after treating whole sites. N and P co-limited periphyton before carcasses or analog pellets were added to spawning sites (p < 0.01); however, nutrients were not limiting after the treatments were added to spawning sites (p = 0.31–1). Algal biomass was 4× higher after whole-site carcass treatments. In contrast, analog pellets appeared to suppress algal biomass in the amendments (20% of NDS at the control site post-treatment) and in the treatment plot (33% of pre-treatment biomass at analog pellet site). We also measured how individual ingredients in analog pellets altered periphyton biomass, which suggested that vitamin E, estrogen, and soybean oil ingredients reduced the growth of primary producers. Suppression methods may stimulate or reduce algal biomass, depending on the methods used, which could have cascading effects on food webs and potentially reduce the success of the control measures. Estimating how different Lake Trout suppression methods may alter basal resources in the littoral zone of Yellowstone Lake will help natural resource agencies develop effective plans to control invasive predators at early life stages while minimally altering ecosystems.

Trophic niche but not abundance of Collembola and Oribatida changes with drought and farming system.

Higher frequencies of summer droughts are predicted to change soil conditions in the future affecting soil fauna communities and their biotic interactions. In agroecosystems drought effects on soil biota may be modulated by different management practices that alter the availability of different food resources. Recent studies on the effect of drought on soil microarthropods focused on measures of abundance and diversity. We here additionally investigated shifts in trophic niches of Collembola and Oribatida as indicated by stable isotope analysis (13C and 15N). We simulated short-term summer drought by excluding 65% of the ambient precipitation in conventionally and organically managed winter wheat fields on the DOK trial in Switzerland. Stable isotope values suggest that plant litter and root exudates were the most important resources for Collembola (Isotoma caerulea, Isotomurus maculatus and Orchesella villosa) and older plant material and microorganisms for Oribatida (Scheloribates laevigatus and Tectocepheus sarekensis). Drought treatment and farming systems did not affect abundances of the studied species. However, isotope values of some species increased in organically managed fields indicating a higher proportion of microorganisms in their diet. Trophic niche size, a measure of both isotope values combined, decreased with drought and under organic farming in some species presumably due to favored use of plants as basal resource instead of algae and microorganisms. Overall, our results suggest that the flexible usage of resources may buffer effects of drought and management practices on the abundance of microarthropods in agricultural systems.

Predator Group Composition Indirectly Influences Food Web Dynamics through Predator Growth Rates.

AbstractConsiderable theoretical work predicts that intraspecific trait variation can have profound ecological consequences by altering species interactions. Because of their high flexibility, behavioral traits may be especially relevant in mediating how species respond to one another, thus affecting food web dynamics and ecosystem functioning. However, empirical evidence supporting this idea is limited. Here, we generated predator groups where we manipulated the composition of behavioral types within the groups to assess effects on predator growth rates, prey communities, basal resources, and ecosystem functioning in replicated outdoor ponds. Using European perch (Perca fluviatilis), we created three types of predator populations: two where all individuals expressed either bold or shy phenotypes and one that contained a mix of individuals of the two behavioral types. Bold perch grew faster in mixed populations, indicating that predator growth depends on each individual's behavioral type and that of its group members. However, there was no evidence that the behavioral composition of the perch population directly altered the dynamics of lower trophic levels. Instead, final perch biomass, not behavioral composition, had the strongest influence on lower trophic levels. Thus, the central question may not be whether predator behavior matters at all for trophic dynamics but rather when behavioral effects will predominate over effects of other influences, such as predator biomass variation.

Phytoplankton Survival in Hindgut of Invasive Silver Carp (Hypophthalmichthys molitrix)

The ability of organisms to survive ingestion and digestion by their predators, or endozoochory, is a fascinating ecological phenomenon that can facilitate predator-mediated dispersal of prey and alter interaction strengths within ecological networks. However, the role of endozoochory in the context of invasive species is considered less often. Throughout the United States, Silver Carp (Hypophthalmichthys molitrix) are prolific invaders that often alter food web structure of recipient ecosystems through the consumption of basal resources. Despite the biogeochemical and food web effects of Silver Carp, there is limited understanding of plankton prey survival after Silver Carp consumption and digestion, and even less known about the ecological effects of selective diets and potential survival. In this study, we quantify hindgut contents of Silver Carp collected from Kentucky Lake, Kentucky, Tennessee River Valley, United States. We found the majority (83%) of phytoplankters within hindguts of Silver Carp showed little digestion prior to egestion. Our study suggests digestion limitations of Silver Carp may have important ecological implications for invaded environments. These results may be applicable in understanding how this rapidly spreading invasive fish can influence food web dynamics and biogeochemical cycles pertinent to toxic algal blooms within recently invaded ecosystems, and forecasting invasion in the near future.

Influence of biomimicry structures on ecosystem function in a Rocky Mountain incised stream

AbstractRising levels of stream degradation have motivated a boom in restoration projects across the globe. However, postrestoration monitoring is still frequently lacking and does not always incorporate biotic responses to changes in the physical template. Beaver mimicry structures (BMSs) are becoming a popular tool to restore degraded streams throughout the American West, but relatively little is known about how these installations influence both biotic and abiotic factors, with consequences for ecosystem functioning. We monitored basal resources, organic and inorganic material standing stocks, and macroinvertebrate density, biomass, and production to quantify functional responses to BMS installation. We compared conditions at BMS sites to naturally occurring beaver dam and reference riffle sites in a low‐gradient stream in southwest Montana. Thermal ranges were contracted, and daily maximum temperatures were higher, in the BMS treatment compared to the reference riffle treatment. Fine sediment standing stock and basal resources were similar in Beaver and BMS treatments, and both treatments were higher than reference riffles. All treatments differed in macroinvertebrate density, which was highest in the Beaver treatment, followed by Mimic and then Reference treatment. Biomass and secondary production were higher in Beaver and BMS treatments compared to the Reference treatment, but only Beaver and Reference treatments differed significantly, likely due to differences in physical habitat and basal resource availability. Consequently, production of collector–gatherers in the BMS treatment and shredders in the Beaver treatment was higher than in reference riffles. Changes to local hydrology and sediment dynamics resulting from BMS influence biotic functional responses like organic material standing stock and secondary production, creating habitat and ecosystem function distinct from riffles and similar to target conditions of natural beaver dams. To continue to improve BMS as a standard restoration practice, future research could consider the extent of degradation, increasing temporal scale of monitoring. Alterations to aquatic–terrestrial subsidies and impacts to fishes.

Trophic position of dolphins tracks recent changes in the pelagic ecosystem of the Macaronesian region (NE Atlantic)

Dolphins play a key role in marine food webs as predators of mid-trophic-level consumers. Because of their mobility and relatively long life span, they can be used as indicators of large-scale changes in the ecosystem. In this study, we calculated the trophic position (TP) of 5 dolphin species from the Canary, Madeira and Azores Islands using bulk and compound-specific stable isotope ratios from muscle tissue to assess trophic adaptations to recent changes in the availability of feeding resources. Dolphin TP values were then compared with those of 7 other species of cetaceans from this region. Analysis of stable nitrogen isotopes in amino acids of the common dolphin indicated non-significant effects of changes in the basal resources of the food web and thus supported the use of bulk samples for TP estimations. Dolphins occupied an intermediate TP (mean: 3.91 to 4.20) between fin (3.25) and sperm whales (4.95). Species-specific TP were equivalent among islands. However, TP increased for the common dolphin and decreased for the bottlenose dolphin (the latter also becoming more oceanic) between 2000 and 2018 in the Canary Islands. These results suggest different impacts of recent changes in the oceanography and in the pelagic food web of the Macaronesian region on the trophic ecology of dolphin species.

Ecosystem Response Through a Resilience Lens: Do Differences in the Illinois River Over 150 Y Indicate Regime Shifts?

AbstractMost of the world's large river ecosystems are subject to multiple anthropogenic stressors including climate change, disconnection between the main river channel and floodplain, flow regulation, and land‐use modification. Viewed through a resilience lens, these ecosystems are potentially in a new regime, exhibiting a different structure, function, and set of feedbacks. We investigated the trophic status and food web character response to anthropogenic stressors over 150 years in the Illinois River. Differences in stable isotope ratios, community niche space, basal resource contribution to higher levels in the food web, mean trophic position of fish functional feeding guilds, and food chain length among four stressor phases suggest changes in ecosystem function in the Illinois River. Moreover, the river did not return to pre‐stressor conditions following watershed‐wide restoration efforts. Differences in trophic status and food web character also varied between the upper‐ and lower‐river zones of the Illinois River over time. This spatial divergence further highlights the complexity of responses to multiple stressors in this ecosystem. Overall, differences in trophic status and food web character suggest a shift to a new regime–an Anthropocene regime–and one that has a reduced capacity to absorb additional disturbances. Given human pressures are a global issue, our results based on a 150 years data set, could assist in the management of riverine landscapes that face regime shifts due to human stressors.

Invasive carp alter trophic niches of consumers and basal resources in African reservoirs

Environmental pollution and biological invasions are key drivers of biodiversity change. However, the effects of invasion and pollution on food webs remain largely unexplored. Here, we used stable isotopes to examine the effects of common carp Cyprinus carpio and pollution on trophic dynamics in six small reservoirs. Our results revealed that the trophic niche widths of invertebrates, vertebrates, and invasive carp did not significantly differ among reservoirs with different pollution statuses. However, we found low niche conservatism among reservoirs, suggesting that while niche width may remain consistent, there is a shift in the position of the niches in isotopic space under both pollution and invasion scenarios. Niche conservatism among reservoirs was generally higher in invertebrates, but this was also regardless of reservoir condition (i.e. presence or absence of pollution and invasion). These results suggest that invasion by species coupled with organic pollution may cause subtle yet differing effects on components of a food web (basal end-members, invertebrates and vertebrates). Our findings provide a baseline measure of the potential in the development of detection and response strategies for carp invasions and organic pollution.

Basal resources of river food webs largely affect the fatty acid composition of freshwater fish

1.Overfishing, altered flow regimes, loss of connectivity, pollution and other direct human disturbances have had a significant impact on freshwater fish biodiversity. While direct effects of these disturbances are well documented, some can also lead to changes in the nutritional composition at the base of freshwater food webs and may affect fish growth and reproduction. Thus far, little is known about how changes in the nutritional composition at the base of freshwater food webs are reflected in fish production and fatty acid composition.2.We conducted a field study in subalpine rivers in Austria to examine how variations in the food quality of basal food sources affect the biochemical composition of freshwater fish. Basal food sources (terrestrial leaves and periphyton), the abundant fish taxa brown trout (Salmo trutta) and European bullhead (Cottus gobio) and their macroinvertebrate prey were collected and their stable carbon isotopes (δ13C) and fatty acid (FA) compositions were measured.3.Our results show that periphyton was the major source of dietary carbon for fish and most invertebrates, except shredders, and a high-quality food for all consumers that provides long-chain polyunsaturated FA (LC-PUFA). In contrast, submerged leaves contained very low content of LC-PUFA and were low-quality food for consumers.4.Changes in periphyton FA across these river ecosystems were tracked in macroinvertebrate FA profiles and in the FA composition of muscle tissues in both fish species. In contrast, the FA composition of fish brain remained unaltered across the study streams.5.The docosahexaenoic acid (DHA, 22:6ω3) content in muscle tissues of S. trutta remained stable and did not correspond to changes of periphyton FA, indicating strong regulation of DHA in this species which is independent of aquatic food sources. In contrast, muscle DHA and eicosapentaenoic acid (EPA, 20:5ω3) content of C. gobio significantly declined when periphyton quality decreased, reflecting the strong impact of the nutritional quality of periphyton on the FA composition of this bottom-dwelling fish.6.Reduced nutritional quality of periphyton led to reduced LC-PUFA in muscle tissues of bottom-dwelling bullhead and even piscivorous brown trout, suggesting that changes of FA composition at the base of river food webs cannot be compensated by intermediate consumers, such as benthic invertebrates, that also convey dietary FA to fishes. This study highlights how subtle human disturbance to streams can affect freshwater fish species through changes to the nutritional quality of their basal food sources.

Water diversion and pollution interactively shape freshwater food webs through bottom-up mechanisms.

Water diversion and pollution are two pervasive stressors in river ecosystems that often co-occur. Individual effects of both stressors on basal resources available to stream communities have been described, with diversion reducing detritus standing stocks and pollution increasing biomass of primary producers. However, interactive effects of both stressors on the structure and trophic basis of food webs remain unknown. We hypothesized that the interaction between both stressors increases the contribution of the green pathway in stream food webs. Given the key role of the high-quality, but less abundant, primary producers, we also hypothesized an increase in food web complexity with larger trophic diversity in the presence of water diversion and pollution. To test these hypotheses, we selected four rivers in a range of pollution subject to similar water diversion schemes, and we compared food webs upstream and downstream of the diversion. We characterized food webs by means of stable isotope analysis. Both stressors directly changed the availability of basal resources, with water diversion affecting the brown food web by decreasing detritus stocks, and pollution enhancing the green food web by promoting biofilm production. The propagation of the effects at the base of the food web to higher trophic levels differed between stressors. Water diversion had little effect on the structure of food webs, but pollution increased food chain length and trophic diversity, and reduced trophic redundancy. The effects at higher trophic levels were exacerbated when combining both stressors, as the relative contribution of biofilm to the stock of basal resources increased even further. Overall, we conclude that moderate pollution increases food web complexity and that the interaction with water abstraction seems to amplify this effect. Our study shows the importance of assessing the interaction between stressors to create predictive tools for a proper management of ecosystems.