Atmospheric nitrogen deposition is associated with elevated phosphorus limitation of lake zooplankton

Here, we present data that for the first time suggests that the effects of atmospheric nitrogen (N) deposition on nutrient limitation extend into the food web. We used a novel and sensitive assay for an enzyme that is over‐expressed in animals growing under dietary phosphorus (P) deficiency (alkaline phosphatase activity, APA) to assess the nutritional status of major crustacean zooplankton taxa in lakes across a gradient of atmospheric N deposition in Norway. Lakes receiving high N deposition had suspended organic matter (seston) with significantly elevated carbon:P and N:P ratios, indicative of amplified phytoplankton P limitation. This P limitation appeared to be transferred up the food chain, as the cosmopolitan seston‐feeding zooplankton taxa Daphnia and Holopedium had significantly increased APA. These results indicate that N deposition can impair the efficiency of trophic interactions by accentuating stoichiometric food quality constraints in lake food webs.     

Major contribution of sulfide-derived sulfur to the benthic food web in a large freshwater lake

In freshwater systems, contributions of chemosynthetic products by sulfur-oxidizing bacteria in sediments as nutritional resources in benthic food webs remain unclear, even though chemosynthetic products might be an important nutritional resource for benthic food webs in deep-sea hydrothermal vents and shallow marine systems. To study geochemical aspects of this trophic pathway, we sampled sediment cores and benthic animals at two sites (90 and 50 m water depths) in the largest freshwater (mesotrophic) lake in Japan: Lake Biwa. Stable carbon, nitrogen, and sulfur isotopes of the sediments and animals were measured to elucidate the sulfur nutritional resources for the benthic food web precisely by calculating the contributions of the incorporation of sulfide-derived sulfur to the biomass and of the biogeochemical sulfur cycle supporting the sulfur nutritional resource. The recovered sediment cores showed increases in ³⁴S-depleted sulfide at 5 cm sediment depth and showed low sulfide concentration with high δ³⁴S in deeper layers, suggesting an association of microbial activities with sulfate reduction and sulfide oxidation in the sediments. The sulfur-oxidizing bacteria may contribute to benthic animal biomass. Calculations based on the biomass, sulfur content, and contribution to sulfide-derived sulfur of each animal comprising the benthic food web revealed that 58%–67% of the total biomass sulfur in the benthic food web of Lake Biwa is occupied by sulfide-derived sulfur. Such a large contribution implies that the chemosynthetic products of sulfur-oxidizing bacteria are important nutritional resources supporting benthic food webs in the lake ecosystems, at least in terms of sulfur. The results present a new trophic pathway for sulfur that has been overlooked in lake ecosystems with low-sulfate concentrations.     

Global change‐driven effects on dissolved organic matter composition: Implications for food webs of northern lakes

Northern ecosystems are experiencing some of the most dramatic impacts of global change on Earth. Rising temperatures, hydrological intensification, changes in atmospheric acid deposition and associated acidification recovery, and changes in vegetative cover are resulting in fundamental changes in terrestrial–aquatic biogeochemical linkages. The effects of global change are readily observed in alterations in the supply of dissolved organic matter (DOM)—the messenger between terrestrial and lake ecosystems—with potentially profound effects on the structure and function of lakes. Northern terrestrial ecosystems contain substantial stores of organic matter and filter or funnel DOM, affecting the timing and magnitude of DOM delivery to surface waters. This terrestrial DOM is processed in streams, rivers, and lakes, ultimately shifting its composition, stoichiometry, and bioavailability. Here, we explore the potential consequences of these global change‐driven effects for lake food webs at northern latitudes. Notably, we provide evidence that increased allochthonous DOM supply to lakes is overwhelming increased autochthonous DOM supply that potentially results from earlier ice‐out and a longer growing season. Furthermore, we assess the potential implications of this shift for the nutritional quality of autotrophs in terms of their stoichiometry, fatty acid composition, toxin production, and methylmercury concentration, and therefore, contaminant transfer through the food web. We conclude that global change in northern regions leads not only to reduced primary productivity but also to nutritionally poorer lake food webs, with discernible consequences for the trophic web to fish and humans.     

Trophic cascades in the bryosphere: the impact of global change factors on top‐down control of cyanobacterial N2‐fixation

Trophic cascades in which predators regulate densities of organisms at lower trophic levels are important drivers of population dynamics, but effects of trophic cascades on ecosystem‐level fluxes and processes, and the conditions under which top‐down control is important, remain unresolved. We manipulated the structure of a food web in boreal feather mosses and found that moss‐inhabiting microfauna exerted top‐down control of N₂‐fixation by moss‐associated cyanobacteria. However, the presence of higher trophic levels alleviated this top‐down control, likely through feeding on bacterivorous microfauna. These effects of food‐web structure on cyanobacterial N₂‐fixation were dependent on global change factors and strongly suppressed under N fertilisation. Our findings illustrate how food web interactions and trophic cascades can regulate N cycling in boreal ecosystems, where carbon uptake is generally strongly N‐limited, and shifting trophic control of N cycling under global change is therefore likely to impact ecosystem functioning.     

From clear lakes to murky waters – tracing the functional response of high‐latitude lake communities to concurrent ‘greening’ and ‘browning’

Climate change and the intensification of land use practices are causing widespread eutrophication of subarctic lakes. The implications of this rapid change for lake ecosystem function remain poorly understood. To assess how freshwater communities respond to such profound changes in their habitat and resource availability, we conducted a space‐for‐time analysis of food‐web structure in 30 lakes situated across a temperature‐productivity gradient equivalent to the predicted future climate of subarctic Europe (temperature +3°C, precipitation +30% and nutrient +45 μg L^?1 total phosphorus). Along this gradient, we observed an increase in the assimilation of pelagic‐derived carbon from 25 to 75% throughout primary, secondary and tertiary consumers. This shift was overwhelmingly driven by the consumption of pelagic detritus by benthic primary consumers and was not accompanied by increased pelagic foraging by higher trophic level consumers. Our data also revealed a convergence of the carbon isotope ratios of pelagic and benthic food web endmembers in the warmest, most productive lakes indicating that the incorporation of terrestrial derived carbon into aquatic food webs increases as land use intensifies. These results, reflecting changes along a gradient characteristic of the predicted future environment throughout the subarctic, indicate that climate and land use driven eutrophication and browning are radically altering the function and fuelling of aquatic food webs in this biome.     

Piscivore enhancement effects on food webs depend on planktivore body size and species composition in replicated whole lake experiments

Recent syntheses of trophic cascade and biomanipulation research have suggested that the effects of piscivores on planktivorous fish populations are reduced, when planktivores are capable of outgrowing predator gape limitation and in systems with complex food web interactions. These hypotheses, however, have not been tested in long-term, whole-lake, experiments where processes such as fish recruitment and compensatory food web responses may be important. We conducted a replicated whole-lake experiment to test for the effects of supplemental piscivore introductions on food webs of eutrophic lakes dominated by deep-bodied planktivores. Responses to piscivore enhancement were compared between lakes differing in food web structure due to the presence of omnivorous gizzard shad (Dorosoma cepedianum). A significant decrease in the relative abundance of juvenile planktivorous fish, and an increase in total benthic macroinvertebrate density was observed in lakes containing mainly bluegills (Lepomus machrochirus). In contrast, lakes containing gizzard shad exhibited no significant responses to piscivore manipulation. Our results support the hypothesis that food webs in lakes dominated by deep-bodied planktivorous fish species respond weakly to piscivore enhancement. In addition, our findings support the hypothesis that cascading trophic interactions are weaker in lake ecosystems with more complex food web interactions such as those containing gizzard shad.     

Long‐term effects of climate change on carbon flows through benthic secondary production in small lakes

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Highly unsaturated fatty acids in nature: what we know and what we need to learn

The supply and demand of omega‐3 highly unsaturated fatty acids (ω‐3 HUFA) in natural ecosystems may lead to resource limitation in a diverse array of animal taxa. Here, we review why food quality in terms of ω‐3 HUFAs is important, particularly for neural tissue, across a diversity of animal taxa ranging from invertebrate zooplankton to vertebrates (including humans). Our review is focused on ω‐3 HUFAs rather than other unsaturated fatty acids because these compounds are especially important biochemically, but scarce in nature. We discuss the dichotomy between ω‐3 HUFA availability between aquatic primary producers, which are often rich in these compounds, and terrestrial primary producers, which are contain little to none of them. We describe the use of fatty acids as qualitative and quantitative tracers for reconstructing animal diets in natural ecosystems. Next, we discuss both direct and indirect ecological implications of ω‐3 HUFA limitation at the individual, population, food web, and ecosystem scales, which include: changes in behavior, species composition, secondary production rates, trophic transfer efficiency and cross‐ecosystem subsidies. We finish by highlighting future research priorities including a need for more research on ω‐3 HUFAs in terrestrial systems, more research their importance for higher order consumers, and more research on the food web and ecosystem‐scale effects of ω‐3 HUFA limitation. Synthesis Mismatches between the supply of and demand for omega‐3 highly unsaturated fatty acids (ω‐3 HUFA) in natural ecosystems have the potential to result in resource limitation across a diverse array of ecosystems. We combined perspectives from ecology and nutritional science to develop a unified synthesis of ω‐3 HUFA ecology. We reviewed the importance of ω‐3 HUFAs for animals, the striking differences in ω‐3 HUFA availability at the base of terrestrial versus aquatic food webs, and the implications of ω‐3 HUFA limitation for food webs. We finished by highlighting research priorities in the field including more research on ω‐3 HUFAs in terrestrial systems, on higher order consumers, and at the food web and ecosystem‐scales.     

Lake size and fish diversity determine resource use and trophic position of a top predator in high‐latitude lakes

Prey preference of top predators and energy flow across habitat boundaries are of fundamental importance for structure and function of aquatic and terrestrial ecosystems, as they may have strong effects on production, species diversity, and food‐web stability. In lakes, littoral and pelagic food‐web compartments are typically coupled and controlled by generalist fish top predators. However, the extent and determinants of such coupling remains a topical area of ecological research and is largely unknown in oligotrophic high‐latitude lakes. We analyzed food‐web structure and resource use by a generalist top predator, the Arctic charr Salvelinus alpinus (L.), in 17 oligotrophic subarctic lakes covering a marked gradient in size (0.5–1084 km²) and fish species richness (2–13 species). We expected top predators to shift from littoral to pelagic energy sources with increasing lake size, as the availability of pelagic prey resources and the competition for littoral prey are both likely to be higher in large lakes with multispecies fish communities. We also expected top predators to occupy a higher trophic position in lakes with greater fish species richness due to potential substitution of intermediate consumers (prey fish) and increased piscivory by top predators. Based on stable carbon and nitrogen isotope analyses, the mean reliance of Arctic charr on littoral energy sources showed a significant negative relationship with lake surface area, whereas the mean trophic position of Arctic charr, reflecting the lake food‐chain length, increased with fish species richness. These results were supported by stomach contents data demonstrating a shift of Arctic charr from an invertebrate‐dominated diet to piscivory on pelagic fish. Our study highlights that, because they determine the main energy source (littoral vs. pelagic) and the trophic position of generalist top predators, ecosystem size and fish diversity are particularly important factors influencing function and structure of food webs in high‐latitude lakes.     

Invasive plants have different effects on trophic structure of green and brown food webs in terrestrial ecosystems: a meta‐analysis

Although invasive plants are a major source of terrestrial ecosystem degradation worldwide, it remains unclear which trophic levels above the base of the food web are most vulnerable to plant invasions. We performed a meta‐analysis of 38 independent studies from 32 papers to examine how invasive plants alter major groupings of primary and secondary consumers in three globally distributed ecosystems: wetlands, woodlands and grasslands. Within each ecosystem we examined if green (grazing) food webs are more sensitive to plant invasions compared to brown (detrital) food webs. Invasive plants have strong negative effects on primary consumers (detritivores, bacterivores, fungivores, and/or herbivores) in woodlands and wetlands, which become less abundant in both green and brown food webs in woodlands and green webs in wetlands. Plant invasions increased abundances of secondary consumers (predators and/or parasitoids) only in woodland brown food webs and green webs in wetlands. Effects of invasive plants on grazing and detrital food webs clearly differed between ecosystems. Overall, invasive plants had the most pronounced effects on the trophic structure of wetlands and woodlands, but caused no detectable changes to grassland trophic structure.     

Bottom‐up and top‐down effects of browning and warming on shallow lake food webs

Productivity and trophic structure of aquatic ecosystems result from a complex interplay of bottom‐up and top‐down forces that operate across benthic and pelagic food web compartments. Projected global changes urge the question how this interplay will be affected by browning (increasing input of terrestrial dissolved organic matter), nutrient enrichment and warming. We explored this with a process‐based model of a shallow lake food web consisting of benthic and pelagic components (abiotic resources, primary producers, grazers, carnivores), and compared model expectations with the results of a browning and warming experiment in nutrient‐poor ponds harboring a boreal lake community. Under low nutrient conditions, the model makes three major predictions. (a) Browning reduces light and increases nutrient supply; this decreases benthic and increases pelagic production, gradually shifting productivity from the benthic to the pelagic habitat. (b) Because of active habitat choice, fish exert top‐down control on grazers and benefit primary producers primarily in the more productive of the two habitats. (c) Warming relaxes top‐down control of grazers by fish and decreases primary producer biomass, but effects of warming are generally small compared to effects of browning and nutrient supply. Experimental results were consistent with most model predictions for browning: light penetration, benthic algal production, and zoobenthos biomass decreased, and pelagic nutrients and pelagic algal production increased with browning. Also consistent with expectations, warming had negative effects on benthic and pelagic algal biomass and weak effects on algal production and zoobenthos and zooplankton biomass. Inconsistent with expectations, browning had no effect on zooplankton and warming effects on fish depended on browning. The model is applicable also to nutrient‐rich systems, and we propose that it is a useful tool for the exploration of the consequences of different climate change scenarios for productivity and food web dynamics in shallow lakes, the worldwide most common lake type.     

Effects of Multi-chain Omnivory on the Strength of Trophic Control in Lakes

Omnivory has been implicated in both diffusing and intensifying the effects of consumer control in food chains. Some have postulated that the strong, community level, top-down control apparent in lakes is not expressed in terrestrial systems because terrestrial food webs are reticulate, with high degrees of omnivory and diverse plant communities. In contrast, lake food webs are depicted as simple linear chains based on phytoplankton-derived energy. Here, we explore the dynamic implications of recent evidence showing that attached algal (periphyton) carbon contributes substantially to lake primary and secondary productivity, including fish production. Periphyton production represents a cryptic energy source in oligotrophic and mesotrophic lakes that is overlooked by previous theoretical treatment of trophic control in lakes. Literature data demonstrate that many fish are multi-chain omnivores, exploiting food chains based on both littoral and pelagic primary producers. Using consumer-resource models, we examine how multiple food chains affect fourth-level trophic control across nutrient gradients in lakes. The models predict that the stabilizing effects of linked food chains are strongest in lakes where both phytoplankton and periphyton contribute substantially to production of higher trophic levels. This stabilization enables a strong and persistent top down control on the pelagic food chain in mesotrophic lakes. The extension of classical trophic cascade theory to incorporate more complex food web structures driven by multi-chain predators provides a conceptual framework for analysis of reticulate food webs in ecosystems.     

Uncovering trophic positions and food resources of soil animals using bulk natural stable isotope composition

Despite the major importance of soil biota in nutrient and energy fluxes, interactions in soil food webs are poorly understood. Here we provide an overview of recent advances in uncovering the trophic structure of soil food webs using natural variations in stable isotope ratios. We discuss approaches of application, normalization and interpretation of stable isotope ratios along with methodological pitfalls. Analysis of published data from temperate forest ecosystems is used to outline emerging concepts and perspectives in soil food web research. In contrast to aboveground and aquatic food webs, trophic fractionation at the basal level of detrital food webs is large for carbon and small for nitrogen stable isotopes. Virtually all soil animals are enriched in ¹³C as compared to plant litter. This ‘detrital shift’ likely reflects preferential uptake of ¹³C‐enriched microbial biomass and underlines the importance of microorganisms, in contrast to dead plant material, as a major food resource for the soil animal community. Soil organic matter is enriched in ¹⁵N and ¹³C relative to leaf litter. Decomposers inhabiting mineral soil layers therefore might be enriched in ¹⁵N resulting in overlap in isotope ratios between soil‐dwelling detritivores and litter‐dwelling predators. By contrast, ¹³C content varies little between detritivores in upper litter and in mineral soil, suggesting that they rely on similar basal resources, i.e. little decomposed organic matter. Comparing vertical isotope gradients in animals and in basal resources can be a valuable tool to assess trophic interactions and dynamics of organic matter in soil. As indicated by stable isotope composition, direct feeding on living plant material as well as on mycorrhizal fungi is likely rare among soil invertebrates. Plant carbon is taken up predominantly by saprotrophic microorganisms and channelled to higher trophic levels of the soil food web. However, feeding on photoautotrophic microorganisms and non‐vascular plants may play an important role in fuelling soil food webs. The trophic niche of most high‐rank animal taxa spans at least two trophic levels, implying the use of a wide range of resources. Therefore, to identify trophic species and links in food webs, low‐rank taxonomic identification is required. Despite overlap in feeding strategies, stable isotope composition of the high‐rank taxonomic groups reflects differences in trophic level and in the use of basal resources. Different taxonomic groups of predators and decomposers are likely linked to different pools of organic matter in soil, suggesting different functional roles and indicating that trophic niches in soil animal communities are phylogenetically structured. During last two decades studies using stable isotope analysis have elucidated the trophic structure of soil communities, clarified basal food resources of the soil food web and revealed links between above‐ and belowground ecosystem compartments. Extending the use of stable isotope analysis to a wider range of soil‐dwelling organisms, including microfauna, and a larger array of ecosystems provides the perspective of a comprehensive understanding of the structure and functioning of soil food webs.     

Highly resolved early Eocene food webs show development of modern trophic structure after the end-Cretaceous extinction

Generalities of food web structure have been identified for extant ecosystems. However, the trophic organization of ancient ecosystems is unresolved, as prior studies of fossil webs have been limited by low-resolution, high-uncertainty data. We compiled highly resolved, well-documented feeding interaction data for 700 taxa from the 48 million-year-old latest early Eocene Messel Shale, which contains a species assemblage that developed after an interval of protracted environmental and biotal change during and following the end-Cretaceous extinction. We compared the network structure of Messel lake and forest food webs to extant webs using analyses that account for scale dependence of structure with diversity and complexity. The Messel lake web, with 94 taxa, displays unambiguous similarities in structure to extant webs. While the Messel forest web, with 630 taxa, displays differences compared to extant webs, they appear to result from high diversity and resolution of insect–plant interactions, rather than substantive differences in structure. The evidence presented here suggests that modern trophic organization developed along with the modern Messel biota during an 18 Myr interval of dramatic post-extinction change. Our study also has methodological implications, as the Messel forest web analysis highlights limitations of current food web data and models.     

Body sizes,cumulative and allometric degree distributions across natural food webs

The distributions of body masses and degrees (i.e. the number of trophic links) across species are key determinants of food‐web structure and dynamics. In particular, allometric degree distributions combining both aspects in the relationship between degrees and body masses are of critical importance for the stability of these complex ecological networks. They describe decreases in vulnerability (i.e. the number of predators) and increases in generality (i.e. the number of prey) with increasing species’ body masses. We used an entirely new global body‐mass database containing 94 food webs from four different ecosystem types (17 terrestrial, 7 marine, 54 lake, 16 stream ecosystems) to analyze (1) body mass distributions, (2) cumulative degree distributions (vulnerability, generality, linkedness), and (3) allometric degree distributions (e.g. generality – body mass relationships) for significant differences among ecosystem types. Our results demonstrate some general patterns across ecosystems: (1) the body masses are often roughly log‐normally (terrestrial and stream ecosystems) or multi‐modally (lake and marine ecosystems) distributed, and (2) most networks exhibit exponential cumulative degree distributions except stream networks that most often possess uniform degree distributions. Additionally, with increasing species body masses we found significant decreases in vulnerability in 70% of the food webs and significant increases in generality in 80% of the food webs. Surprisingly, the slopes of these allometric degree distributions were roughly three times steeper in streams than in the other ecosystem types, which implies that streams exhibit a more pronounced body mass structure. Overall, our analyses documented some striking generalities in the body‐mass (allometric degree distributions of generality and vulnerability) and degree structure (exponential degree distributions) across ecosystem types as well as surprising exceptions (uniform degree distributions in stream ecosystems). This suggests general constraints of body masses on the link structure of natural food webs irrespective of ecosystem characteristics.     

Predator and detritivore niche width helps to explain biocomplexity of experimental detritus-based food webs in four aquatic and terrestrial ecosystems

In the study of food webs, the existence and explanation of recurring patterns, such as the scale invariance of linkage density, predator–prey ratios and mean chain length, constitute long-standing issues. Our study focused on litter-associated food webs and explored the influence of detritivore and predator niche width (as δ¹³C range) on web topological structure. To compare patterns within and between aquatic and terrestrial ecosystems and take account of intra-habitat variability, we constructed 42 macroinvertebrate patch-scale webs in four different habitats (lake, lagoon, beech forest and cornfield), using an experimental approach with litterbags. The results suggest that although web differences exist between ecosystems, patterns are more similar within than between aquatic and terrestrial web types. In accordance with optimal foraging theory, we found that the niche width of predators and prey increased with the number of predators and prey taxa as a proportion of total taxa in the community. The tendency was more marked in terrestrial ecosystems and can be explained by a lower per capita food level than in aquatic ecosystems, particularly evident for predators. In accordance with these results, the number of links increased with the number of species but with a significantly sharper regression slope for terrestrial ecosystems. As a consequence, linkage density, which was found to be directly correlated to niche width, increased with the total number of species in terrestrial webs, whereas it did not change significantly in aquatic ones, where connectance scaled negatively with the total number of species. In both types of ecosystem, web robustness to rare species removal increased with connectance and the niche width of predators. In conclusion, although limited to litter-associated macroinvertebrate assemblages, this study highlights structural differences and similarities between aquatic and terrestrial detrital webs, providing field evidence of the central role of niche width in determining the structure of detritus-based food webs and posing foraging optimisation constraints as a general mechanistic explanation of food web complexity differences within and between ecosystem types.     

Unravelling the complex structure of forest soil food webs: higher omnivory and more trophic levels

Food web topologies depict the community structure as distributions of feeding interactions across populations. Although the soil ecosystem provides important functions for aboveground ecosystems, data on complex soil food webs is notoriously scarce, most likely due to the difficulty of sampling and characterizing the system. To fill this gap we assembled the complex food webs of 48 forest soil communities. The food webs comprise 89 to 168 taxa and 729 to 3344 feeding interactions. The feeding links were established by combining several molecular methods (stable isotope, fatty acid and molecular gut content analyses) with feeding trials and literature data. First, we addressed whether soil food webs (n = 48) differ significantly from those of other ecosystem types (aquatic and terrestrial aboveground, n = 77) by comparing 22 food web parameters. We found that our soil food webs are characterized by many omnivorous and cannibalistic species, more trophic chains and intraguild‐predation motifs than other food webs and high average and maximum trophic levels. Despite this, we also found that soil food webs have a similar connectance as other ecosystems, but interestingly a higher link density and clustering coefficient. These differences in network structure to other ecosystem types may be a result of ecosystem specific constraints on hunting and feeding characteristics of the species that emerge as network parameters at the food‐web level. In a second analysis of land‐use effects, we found significant but only small differences of soil food web structure between different beech and coniferous forest types, which may be explained by generally strong selection effects of the soil that are independent of human land use. Overall, our study has unravelled some systematic structures of soil food‐webs, which extends our mechanistic understanding how environmental characteristics of the soil ecosystem determine patterns at the community level.     

Spatial and temporal variabilities of δ<Superscript>13</Superscript>C and δ<Superscript>15</Superscript>N within lower trophic levels of a large lake: implications for estimating trophic relationships of consumers

Stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) often have unique values among lake habitats (e.g. benthic, littoral, pelagic), providing a widely used tool for measuring the structure and energy flow in aquatic food webs. However, there has been little recognition of the spatial and temporal variabilities of these isotopes within habitats of aquatic ecosystems. To address this, δ¹³C and δ¹⁵N were measured in seston, zebra mussels (Dreissena polymorpha) and young-of-year (YOY) yellow (Perca flavescens), and white perch (Morone americana) collected from four sites across the offshore habitat of the western basin of Lake Erie during June–September 2009. Values of δ¹³C and δ¹⁵N showed significant spatial and temporal variations, with month accounting for >50% of the variation, for both stable isotopes and all the species except seston. Such variation in isotope values has the potential to significantly influence or confound interpretation of stable isotopes in measures, such as trophic position (TP) which use lower trophic level organisms as their baseline. For example, TP was found to vary up to 0.7 for yellow and white perch (TP = δ¹⁵N_fish − δ¹⁵N_{zebra mussel}/diet-tissue fractionation factor) depending on the zebra mussel data used (e.g., from a different location or a different collection month). As the use of stable isotopes continues to move from qualitative to more quantitative measures of trophic structure, food web research must recognize the importance of stable isotopes' variability in lower trophic level organisms, especially in large lake systems.     

Variable littoral‐pelagic coupling as a food‐web response to seasonal changes in pelagic primary production

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