Benthic-pelagic trophic interactions within the fish assemblage in the western Bering Sea shelf area according to stomach content analysis and ratios of C and N stable isotopes

The composition, abundance, diet and trophic status of zooplankton, bottom invertebrates, fish and nekton were analyzed based on the data collected by the staff of the TINRO-Center during complex bottom trawl catches on the Bering Sea shelf in the fall of 2004. The stomach contents of mass fish species were analyzed and the nitrogen and carbon isotopic composition of 36 mass species of plankton, benthos, nekton and nektobenthos, which together make up the basis of pelagic and bottom communities, was determined. It was found that zooplankton noticeably differ from benthic invertebrates in carbon isotopic composition: δ¹³C values in zooplankton varied from −20.3‰ to −17.9‰; in benthos—from −17.5‰to −13.0‰; and in fish—from −19.2‰ (juvenile walleye pollock) to −15.3‰ (saffron cod). The levels of ¹³C isotope in the tissues of fish depended mostly on the share of pelagic or benthic animals in their diet. δ¹⁵N values in the studied species ranged from 8.6‰ (in sea urchins) to 17.2‰ (in large Pacific cods), which corresponds to a trophic level of 2.8. Obviously the δ¹⁵N values reflect the degree of predation and generally show the ratio of primary, secondary and tertiary consumers in a fish’s diet. Trophic interactions manifest a high degree of interdependence between benthic and pelagic communities (even without taking into account such lower components of the food web as phytoplankton, bacteria, and protozoa) occurring in most nektonic species that depend on both bottom and pelagic food.     

Stable isotopes and trophic positions of littoral fishes from a Mediterranean marine protected area

Stable isotope analyses were employed to explore feeding and foraging habitats and trophic levels of littoral fishes in a western Mediterranean Marine Protected Area (Egadi Islands, Sicily, Italy). Carbon and nitrogen stable isotope ratios were measured in primary producers, invertebrates and fishes collected in December 2001 and January 2002. Fishes of the littoral region of the Egadi Islands had isotopic signatures that fell into a wider range for δ ¹³C (about 6‰) than for δ ¹⁵N (about 3‰). Carbon isotope ratios were consistent with a food web based on mixed sources and two trophic pathways leading to different fish species. Differences in the isotopic composition between islands were higher for benthivorous than for planktivorous fishes. The overall picture gained from this study is of a isotopic distinction between planktivorous and benthivorous fishes, resource partitioning facilitating the coexistence of similar species within the same ecosystem, and spatial variability in the isotopic signatures and trophic level of fishes. Asymmetrical analysis of variance showed that estimated trophic levels were lower in the area with the highest level of protection (Zone A) for only two out of the nine fishes analysed. As a consequence, overall spatial differences do not seem to be a consequence of protection, since in most cases trophic levels did not change significantly between zone A and zones C where professional fishing (trawling apart) is permitted, but of natural sources of variation (e.g. variability in food availability and site-specific food preferences of fishes). However, the results of this study suggest a different response at the species compared to the community level.     

Isotopic signatures and trophic status of Ramaria

The genus Ramaria is composed of several subgenera that often correspond to specific trophic strategies. Because carbon and nitrogen isotopes can be used to assess fungal trophic status and nitrogen sources, we accordingly carried out an extensive survey of isotopic patterns in archived specimens of Ramaria from Germany and other locations. Isotopic patterns in species generally corresponded to subgeneric affiliations and to the range of different potential substrates, with fungi fruiting on wood and litter (subgenera Asteroramaria and Lentoramaria) much lower in δ¹⁵N (≈−3‰) than ectomycorrhizal taxa (≈12‰) (subgenus Ramaria) or taxa fruiting on soil (≈13‰) (subgenus Echinoramaria). Conversely, fungi fruiting on wood and litter were higher in δ¹³C (−23‰) than those fruiting on soil (≈−27‰), with ectomycorrhizal fungi intermediate (≈−24.5‰). Fungi colonizing mineral soil horizons were about 3‰ enriched in ¹⁵N relative to those colonizing both mineral and organic horizons. The high δ¹⁵N and low δ¹³C signatures of taxa fruiting on soil remains unexplained. The high degree of fidelity of isotopic signatures with subgeneric classifications and life history traits suggests that sporocarps are good integrators of patterns of carbon and nitrogen cycling for specific taxa. Archived specimens represent a useful trove of life history information that could be mined without requiring extensive supporting isotopic data from other ecosystem pools.     

Piscivorous birds as top predators and fishery competitors in the lagoon ecosystem

Trophic patterns of omnivorous freshwater shrimps, Exopalaemon modestus and Macrobrachium nipponensis, were investigated in two shallow eutrophic lakes by using stable isotope analysis. δ¹⁵N and δ¹³C of M. nipponensis and E. modestus increased with increasing body weight, which might be attributed to larger individuals ingesting organisms that feed higher up the food chain and/or increased assimilation of benthic food items with enriched isotopic signatures. Of the freshwater shrimps occurring in the studied lakes, those from Lake Taihu had significantly elevated δ¹⁵N and δ¹³C values (4.3‰ and 1.8‰, respectively) compared with those from the less eutrophic Lake Chaohu, indicating that the isotopic signature might partially reflect the trophic states of their habitats. Mixing model results suggested that the benthic food web provides the primary carbon source for both shrimp species, and that E. modestus assimilated relatively more pelagic food sources than M. nipponensis in these lakes. Handling editor: S. Wellekens     

Challenges using stable isotopes for estimating trophic levels in marine amphipods

In marine food web studies, stable isotopes of nitrogen (δ¹⁵N) and carbon (δ¹³C) are widely used to estimate organisms’ trophic levels (TL) and carbon sources, respectively. For smaller organisms, whole specimens are commonly analyzed. However, this “bulk method” involves several pitfalls since different tissues may fractionate stable isotopes differently. We compared the δ¹⁵N and δ¹³C values of exoskeleton versus soft tissue, in relation to whole specimens, of three common Arctic amphipods in Svalbard waters: the benthic Anonyx nugax, the sympagic (ice-associated) Gammarus wilkitzkii and the pelagic Themisto libellula. The δ¹⁵N values of the exoskeletons were significantly lower than those of the soft tissues for A. nugax (10.5 ± 0.7‰ vs. 15.7 ± 0.7‰), G. wilkitzkii (3.3 ± 0.3‰ vs. 8.3 ± 0.4‰) and T. libellula (8.6 ± 0.3‰ vs.10.8 ± 0.3‰). The differences in δ¹³C values between exoskeletons and soft tissues were insignificant, except for A. nugax (−21.2 ± 0.2‰ vs. −20.3 ± 0.2‰, respectively). The δ¹⁵N values of whole organisms were between those of the exoskeletons and the soft tissues, being similarly enriched in ¹⁵N as the exoskeletons (except G. wilkitzkii) and depleted in ¹⁵N by 1.2–3.7‰ compared to the soft tissues. The δ¹⁵N-derived TLs of the soft tissues agreed best with the known feeding preferences of the three amphipods, which suggest a potential underestimation of 0.5–1.0 TL when stable isotope analyses are performed on whole crustaceans with thick exoskeletons. The insignificant or small differences in δ¹³C values among exoskeletons, soft tissues and whole specimens, however, suggest low probability for misinterpretations of crustaceans’ primary carbon source in marine ecosystems with distinctly different δ¹³C-carbon sources.     

Isotopic enrichment in herbivorous insects: a comparative field-based study of variation

Researchers will be able to use stable isotope analysis to study community structure in an efficient way, without a need for extensive calibrations, if isotopic enrichment values are consistent, or if variation in enrichment values can be predicted. In this study, we generated an experimental data set of δ¹⁵N and δ¹³C enrichment means for 22 terrestrial herbivorous arthropods feeding on 18 different host plants. Mean enrichments observed across a single trophic transfer (plants to herbivores) were −0.53±0.26‰ for δ¹³C (range: −3.47‰ to 1.89‰) and 1.88±0.37‰ for δ¹⁵N (range: −0.20‰ to 6.59‰). The mean δ¹³C enrichment was significantly lower than that reported in recent literature surveys, whereas the mean δ¹⁵N enrichment was not significantly different. The experimental data set provided no support for recent hypotheses advanced to explain variation in enrichment values, including the proposed roles for consumer feeding mode, development type, and diet C:N ratio. A larger data set, formed by combining our experimental data with data from the literature, did suggest possible roles for feeding mode, nitrogen recycling, herbivore life stage, and host plant type. Our results indicate that species enrichment values are variable even in this relatively narrow defined group of organisms and that our ability to predict enrichment values of terrestrial herbivorous arthropods based on physiological, ecological, or taxonomic traits is low. The primary implications are that (1) mean enrichment may have to be measured empirically for each trophic link of interest, rather than relying on estimates from a broad survey of animal taxa and (2) the advantage of using stable isotope analysis to probe animal communities that are recalcitrant to other modes of study will be somewhat diminished as a consequence.Electronic Supplementary Material Supplementary material is available for this article at     

Research article: small-scale spatial variation of δ13C and δ15N isotopes in Antarctic carbon sources and consumers

Regional food web studies that fail to account for small-scale isotopic variability can lead to a mismatch between an organism’s inferred and true trophic position. Misinterpretation of trophic status may result, substantially limiting spatial and temporal comparability of food web studies. We sampled several carbon sources and consumers in a nested design to assess the variability of food web members across small spatial scales (100 s of m to several km) in regions around the Windmill Islands and Vestfold Hills in East Antarctica. For carbon sources, δ¹³C in sea ice POM was particularly variable between locations (km apart) and between sites (100 s of m apart) with replicate samples varying by up to 16‰. Macroalgae δ¹³C was less variable (replicate samples ranging up to 6.9‰ for the red alga Iridaea cordata), yet still differed between locations. Sediment POM and pelagic POM were the least variable, displaying minimal differences between locations or sites for δ¹³C and δ¹⁵N. Three out of eight consumers were significantly different between locations for δ¹³C, and five out of eight for δ¹⁵N, with the fish Trematomus bernacchii the most variable for both δ¹³C and δ¹⁵N. At smaller scales, the amphipod Paramorea walkeri showed significant variation between sites in δ¹³C but not in δ¹⁵N. We attribute small-scale variability to the dynamic physical environment for carbon sources in coastal systems and a close coupling of diet to habitat for consumers. We highlight the need to account for small-scale spatial variation in sampling designs for regional food web studies.     

Interspecific and nutrient-dependent variations in stable isotope fractionation: experimental studies simulating pelagic multitrophic systems

Stable isotope signatures of primary producers display high inter- and intraspecific variation. This is assigned to species-specific differences in isotope fractionation and variable abiotic conditions, e.g., temperature, and nutrient and light availability. As consumers reflect the isotopic signature of their food source, such variations have direct impacts on the ecological interpretation of stable isotope data. To elucidate the variability of isotope fractionation at the primary producer level and the transfer of the signal through food webs, we used a standardised marine tri-trophic system in which the primary producers were manipulated while the two consumer levels were kept constant. These manipulations were (1) different algal species grown under identical conditions to address interspecific variability and (2) a single algal species cultivated under different nutrient regimes to address nutrient-dependent variability. Our experiments resulted in strong interspecific variation between different algal species (Thalassiosira weissflogii, Dunaliella salina, and Rhodomonas salina) and nutrient-dependent shifts in stable isotope signatures in response to nutrient limitation of R. salina. The trophic enrichment in ¹⁵N and ¹³C of primary and secondary consumers (nauplii of Acartia tonsa and larval herring) showed strong deviations from the postulated degree of 1.0‰ enrichment in δ¹³C and 3.4‰ enrichment in δ¹⁵N. Surprisingly, nauplii of A. tonsa tended to keep “isotopic homeostasis” in terms of δ¹⁵N, a pattern not described in the literature so far. Our results suggest that the diets’ nutritional composition and food quality as well as the stoichiometric needs of consumers significantly affect the degree of trophic enrichment and that these mechanisms must be considered in ecological studies, especially when lower trophic levels, where variability is highest, are concerned.     

Land–Ocean Coupling of Carbon and Nitrogen Fluxes on Sandy Beaches

Rivers link oceans with the land, creating global hot spots of carbon processing in coastal seas. Coastlines around the world are dominated by sandy beaches, but beaches are unusual in that they are thought to rely almost exclusively on marine imports for food. No significant connections to terrestrial production having been demonstrated. By contrast, we isotopically traced carbon and nitrogen pathways leading to clams (Donax deltoides) on beaches. Clams from areas influenced by river plumes had significantly different isotope signatures (δ¹³C: −18.5 to −20.2‰; δ¹⁵N: 8.3–10.0‰) compared with clams remote from plumes (δ¹³C: −17.5 to −19.5‰; δ¹⁵N: 7.6–8.7‰), showing that terrestrial carbon and sewage, both delivered in river plumes, penetrate beach food webs. This is a novel mechanism of trophic subsidy in marine intertidal systems, linking the world’s largest shore ecosystem to continental watersheds. The same clams also carry pollution signatures of sewage discharged into rivers, demonstrating that coastal rivers connect ecosystems in unexpected ways and transfer contaminants across the land–ocean boundary. The links we demonstrate between terrigenous matter and the largest of all marine intertidal ecosystems are significant given the immense social, cultural, and economic values of beaches to humans and the predicted consequences of altered river discharge to coastal seas caused by global climate change.     

Contributions of stable-isotope data to elucidating food webs of Mediterranean rocky littoral fishes

The food webs of rocky infra-littoral ecosystems in the Mediterranean have been little studied. In this investigation stable isotopes and dietary data were compared in an attempt to describe features of the food webs concerned. δ¹³C and δ¹⁵N were determined for plants, invertebrates and fishes from the Bay of Calvi, Corsica. Dietary data were derived from the literature. δ¹³C of plants ranged from –8.59‰ to –33.74‰, of benthic invertebrates from –17.0‰ to –20.52‰, of planktonic invertebrates from –20.08‰ to –22.34‰ and of fishes from –16.27‰ to –19.59‰. δ¹⁵N was generally greater at higher trophic levels. δ¹⁵N of plants was 0.95–2.92‰, of benthic invertebrates 1.69–6.54‰, of planktonic invertebrates 3.51–6.82‰ and of fishes 4.63–9.77‰. ¹³C enrichment tended to be associated with benthic food chains and ¹³C depletion with planktonic chains. Stable-isotope data suggested more varied diets for many species than implied by gut-contents data. Omnivory and trophic plasticity were widespread, and many consumers fed lower down the food chain than previous studies had suggested. Both stable-isotope and gut-contents analysis resolved differences between fishes feeding on planktonic and benthic prey and indicated that the herbivorous fish Sarpa salpa fed on a diet substantially different from that of other fishes. Zooplankton were important in the diets of several consumers (both primary and secondary), as was plankton derived detritus. One species of fish previously identified as planktivorous was shown to feed largely on benthic organisms, whilst several species of benthic invertebrates may feed on plankton-derived detritus. Although herbivores seemed to obtain most of their C from macroalgae, δ¹⁵N data suggested that many of these animals supplemented their intake of N, although gut-contents analysis did not provide evidence for such uptake. The isotopic data have elucidated several features of the food web which we would not otherwise have detected. Received: 26 April 1999 / Accepted: 24 September 1999     

Spatial variability in Florida Bay particulate organic matter composition: combining flow cytometry with stable isotope analyses

Long-term management plans for restoration of natural flow conditions through the Everglades increase the importance of understanding potential nutrient impacts of increased freshwater delivery on Florida Bay biogeochemistry. Planktonic communities respond quickly to changes in water quality, thus spatial variability in community composition and relationships to nutrient parameters must be understood in order to evaluate future downstream impacts of modifications to Everglades hydrology. Here we present initial results combining flow cytometry analyses of phytoplankton and bacterial populations (0.1–50 μm size fraction) with measurements of δ¹³C and δ¹⁵N composition and dissolved inorganic nutrient concentrations to explore proxies for planktonic species assemblage compositions and nutrient cycling. Particulate organic material in the 0.1–50 μm size fraction was collected from five stations in Northeastern and Western Florida Bay to characterize spatial variability in species assemblage and stable isotopic composition. A dense bloom of the picocyanobacterium, Synechococcus elongatus, was observed at Western Florida Bay sites. Smaller Synechococcus sp. were present at Northeast sites in much lower abundance. Bacteria and detrital particles were also more abundant at Western Florida Bay stations than in the northeast region. The highest abundance of detritus occurred at Trout Creek, which receives freshwater discharge from the Everglades through Taylor Slough. In terms of nutrient availability and stable isotopic values, the S. elongatus population in the Western bay corresponded to low DIN (0.5 μM NH ₄ ⁺ ; 0.2 μM NO ₃ ⁻ ) concentrations and depleted δ¹⁵N signatures ranging from +0.3 to +0.8‰, suggesting that the bloom supported high productivity levels through N₂-fixation. δ¹⁵N values from the Northeast bay were more enriched (+2.0 to +3.0‰), characteristic of N-recycling. δ¹³C values were similar for all marine Florida Bay stations, ranging from −17.6 to −14.4‰, however were more depleted at the mangrove ecotone station (−25.5 to −22.3‰). The difference in the isotopic values reflects differences in carbon sources. These findings imply that variations in resource availability and nutrient sources exert significant control over planktonic community composition, which is reflected by stable isotopic signatures.     

Influence of functional feeding groups and spatiotemporal variables on the δ<Superscript>15</Superscript>N signature of littoral macroinvertebrates

The δ¹⁵N trophic enrichment in littoral food webs is not well known despite the importance of macroinvertebrates in lacustrine energy fluxes. We wanted to assess the influence of functional feeding group (grazer, collector, shredder, predator, predator–hematophagous, predator–sucker) and spatiotemporal variables (year, month, station of sampling) on littoral macroinvertebrate δ¹⁵N signatures. For 2 years, during the plant growth period phytophilous littoral macroinvertebrates were sampled in Lake St. Pierre, a large fluvial lake of the St. Lawrence River, Québec, Canada. The δ¹⁵N analyses showed that station was the most important factor for explaining δ¹⁵N variation, followed by sampling month and functional feeding group. The organisms sampled in the stations of the south shore, which experienced greater macrophyte abundance, slower currents, and stronger NO₃ depletion exhibited higher δ¹⁵N values than those sampled on the north shore. Grazer-to-predator δ¹⁵N enrichment valued 1.6‰, which is inferior to the 3.4‰ generally admitted in food-web research. Shredders exhibited the lowest δ¹⁵N values and predators–hematophagous the highest. δ¹⁵N signature of invertebrates increased 3‰ through the summer between May and September. Only samples collected within a short period should be pooled to avoid an error value equivalent to one trophic level (1.6) enrichment. Furthermore, it is recommended not to pool macroinvertebrate samples collected at stations with differing watershed land uses.     

δ13C and δ15N indicators of fish and shrimp community diet and trophic structure in a mangrove ecosystem in Thailand

Stable carbon and nitrogen isotope ratios were used to elucidate primary carbon sources and trophic relationships of the fish and shrimp community in the Klong Ngao mangrove ecosystem, southern Thailand. There were no significant differences in isotopic compositions of biota between mangrove and offshore sites (Welch–Aspin test). The δ¹⁵N values of eight fish species and two shrimp species at both sites were also not significantly different by the test, meaning that at both sites they feed on the same diets due to the discharge of large quantities of mangrove sediments. The δ¹⁵N isotopic enrichment of consumers suggested that there are four trophic levels in the Klong Ngao food web, with at least two fish species capable of switching feeding strategies and thus altering their apparent trophic positions. Phytoplankton culture experiments indicated that mangrove-derived sediments could play an important role in stimulating phytoplankton growth for low turbidity offshore areas, thus providing an alternate food source. The isotopic associations among sources and consumers indicated that mangroves were the major carbon source supporting aquatic food webs in the Klong Ngao ecosystem.     

Effects of food quality on tissue-specific isotope ratios in the mussel <Emphasis Type="Italic">Perna perna</Emphasis>

Investigations into trophic ecology and aquatic food web resolution are increasingly accomplished through stable isotope analysis. The incorporation of dietary and metabolic changes over time results in variations in isotope signatures and turnover rates of producers and consumers at tissue, individual, population and species levels. Consequently, the elucidation of trophic relationships in aquatic systems depends on establishing standard isotope values and tissue turnover rates for the level in question. This study investigated the effect of diet and food quality on isotopic signatures of four mussel tissues: adductor muscle, gonad, gill and mantle tissue from the brown mussel Perna perna. In the laboratory, mussels were fed one of the two isotopically distinct diets for 3 months. Although not all results were significant, overall δ¹³C ratios in adductor, mantle and gill tissues gradually approached food source signatures in both diets. PERMANOVA analyses revealed significant changes over time in tissue δ¹³C (mantle and gill) with both diets and in δ¹⁵N (all tissues) and C:N ratios (mantle and gill) for one diet only. The percentage of replaced carbon isotopes were calculated for the 3 month period and differed among tissues and between diets. The tissue with the highest and lowest amount of replaced isotopes over 81 days were mantle tissue on the kelp diet (33.89%) and adductor tissue on the fish food diet (4.14%), respectively. Percentages could not be calculated for any tissue in either diet for δ¹⁵N due to the lack of significant change in tissue nitrogen. Fractionation patterns in tissues for both diets can be linked to nutritional stress, suggesting that consumer isotopic signatures are strongly dependent on food quality, which can significantly affect the degree of isotopic enrichment within a trophic level.     

Stable carbon and nitrogen incorporation in blood and fin tissue of the catfish <Emphasis Type="Italic">Pterygoplichthys disjunctivus</Emphasis> (Siluriformes,Loricariidae)

A feeding trial was performed in the laboratory with the catfish species Pterygoplichthys disjunctivus to determine stable carbon (¹³C) and nitrogen (¹⁵ N) turnover rates and discrimination factors in non-lethally sampled tissues (red blood cells, plasma solutes, and fin). A second feeding trial was conducted to determine what P. disjunctivus could assimilate from low-quality wood-detritus—refractory polysaccharides (e.g., cellulose), or soluble wood-degradation products inherent in wood-detritus. This was performed by feeding the fish an artificial wood-detritus diet with fibrous (δ¹³C = −26.36‰; δ¹⁵  N = 2.13‰) and soluble portions (δ¹³C = −11.82‰; δ¹⁵  N = 3.39‰) that had different isotopic signatures and monitoring the dynamics of isotopic incorporation in the different tissues over time. Plasma solutes turned over more quickly than red blood cells for ¹³C and ¹⁵ N. However, in contrast to previous studies of juvenile fishes, C and N incorporation was primarily driven by catabolic tissue turnover as opposed to growth rate. Tissue-diet discrimination factors for ¹⁵ N varied from 4.08 to 5.17‰, whereas they were <2‰ for ¹³C (and less than 0.3‰ for plasma and red blood cells). The results of trial two suggested that P. disjunctivus could not assimilate refractory polysaccharides. Moreover, the δ¹³C and δ¹⁵ N signatures of wild-caught P. disjunctivus from Florida confirmed their detrital trophic standing in Floridian aquatic ecosystems.     

Effects of nutrient enrichment on C and N stable isotope ratios of invertebrates,fish and their food resources in boreal streams

Carbon and nitrogen stable isotopes are frequently used to study energy sources and food web structure in ecosystems, and more recently, to study the effects of anthropogenic stress on aquatic ecosystems. We investigated the effect of nutrient enrichment on δ¹³C and δ¹⁵N in fine (FPOM), coarse (CPOM) particulate organic matter, periphyton, invertebrates and fish in nine boreal streams in south-central Sweden. In addition, we analysed the diet of benthic consumers using stable isotope data. Increases in δ¹⁵N of periphyton (R ² = 0.88), CPOM (0.78), invertebrates (0.92) and fish (0.89) were related to nutrient enrichment. In contrast, δ¹³C signatures did not change along the nutrient gradient. Our results show that δ¹⁵N has potential as a sensitive indicator of nutrient enrichment in boreal streams. Carbon and nitrogen isotopes failed to elucidate putative diets of selected aquatic consumers. Indeed, comparison of low- and high-impact sites showed that δ¹³C of many consumers were found outside the ranges of basal resource δ¹³C. Moreover, ranges of basal resource δ¹³C and δ¹⁵N overlapped at both low and high sites, making discrimination between the importance of allochthonous and autochthonous production difficult. Our findings show that a fractionation rate of 3.4‰ is not always be appropriate to assess trophic interactions, suggesting that more studies are needed on fractionation rates along gradients of impairment. Handling editor: M. Power     

δ<Superscript>15</Superscript>N dynamics of ammonium and particulate nitrogen in a temperate eutrophic estuary

We monitored the stable nitrogen isotopic composition (δ¹⁵N) of suspended matter and ammonium in the freshwater stretch of the Scheldt estuary (Belgium) over a full year to investigate for seasonal evolution and possible co-variation between isotopic signatures. The δ¹⁵N value of ammonium remained rather constant during winter (average = +11.4‰) but increased significantly with the spring and summer bloom, reaching values as high as +70‰. This enrichment of the ammonium pool in ¹⁵N coincided with significant ammonium depletion during summer period, suggesting a close causal relationship. Based on a semi-closed system approach we deduced an apparent fractionation factor associated with NH₄⁺ utilization (i.e. combining effects of uptake and nitrification) of 18.4‰ (SE = 2.0‰), which is similar to values reported in literature. Observed variations of ammonium δ¹⁵N could account for about 69% of δ¹⁵N variation in suspended matter.     

Feeding habits of Hymenoptera and Isoptera in a tropical rain forest as revealed by nitrogen and carbon isotope ratios

Despite the recognition of the functional role of Hymenoptera (ants, bees and wasps) and Isoptera (termites) in tropical ecosystems, their detailed feeding habits are not well known. To examine the feeding habits of these groups, we measured nitrogen (N) and carbon (C) stable isotope ratios (δ¹⁵N and δ¹³C) of hymenopterans (12 families, ≥16 genera and ≥32 species) and isopterans (one family and 10 species) collected in a tropical rain forest, Sarawak, Malaysia. We compared the isotopic signatures of these insects to those previously reported for other consumers collected in the same forest. The δ¹⁵N and δ¹³C values of these insects overlapped with those of the other consumers, indicating that they have access to diverse C and N sources in the forest. The δ¹⁵N values of ants and termites indicated that their feeding habits range along a continuum from herbivory (i.e. dependent on honeydew and nectar) to predation and from wood-feeders to soil-feeders, respectively. In addition, the δ¹⁵N values of wasps varied greatly from −0.1‰ (Braconidae sp.) to 8.6‰ (Bembix sp.), suggesting that their feeding habits also range from omnivory to predation. The ant species Camponotus gigas had δ¹³C values similar to those of invertebrate detritivores and omnivores rather than to those of invertebrate herbivores, although the diet of this species consists mostly of honeydew. This discrepancy suggests that the ant uses carbohydrates as an energy source, the isotopic signatures of which are not well retained in the body tissues. Values of both δ¹⁵N and δ¹³C of the predatory army ant Leptogenys diminuta and the soil-feeding termite Dicuspiditermes nemorosus did not differ significantly, indicating that both trophic level and the humification of feeding substrates can increase the isotopic signatures of terrestrial consumers.     

Trophic ecology of the invasive argentine ant: spatio-temporal variation in resource assimilation and isotopic enrichment

Studies of food webs often employ stable isotopic approaches to infer trophic position and interaction strength without consideration of spatio-temporal variation in resource assimilation by constituent species. Using results from laboratory diet manipulations and monthly sampling of field populations, we illustrate how nitrogen isotopes may be used to quantify spatio-temporal variation in resource assimilation in ants. First, we determined nitrogen enrichment using a controlled laboratory experiment with the invasive Argentine ant (Linepithema humile). After 12 weeks, worker δ¹⁵N values from colonies fed an animal-based diet had δ¹⁵N values that were 5.51% greater compared to colonies fed a plant-based diet. The shift in δ¹⁵N values in response to the experimental diet occurred within 10 weeks. We next reared Argentine ant colonies with or without access to honeydew-producing aphids and found that after 8 weeks workers from colonies without access to aphids had δ¹⁵N values that were 6.31% larger compared to colonies with access to honeydew. Second, we sampled field populations over a 1-year period to quantify spatio-temporal variability in isotopic ratios of L. humile and those of a common native ant (Solenopsis xyloni). Samples from free-living colonies revealed that fluctuations in δ¹⁵N were 1.6–2.4‰ for L. humile and 1.8–2.9‰ for S. xyloni. Variation was also detected among L. humile castes: time averaged means of δ¹⁵N varied from 1.2 to 2.5‰ depending on the site, with δ¹⁵N values for queens ≥ workers > brood. The estimated trophic positions of L. humile and S. xyloni were similar within a site; however, trophic position for each species differed significantly at larger spatial scales. While stable isotopes are clearly useful for examining the trophic ecology of arthropod communities, our results suggest that caution is warranted when making ecological interpretations when stable isotope collections come from single time periods or life stages.     

Ontogenetic dietary shifts in European common frog (Rana temporaria) revealed by stable isotopes

During migrations and ontogeny amphibians change their habitat and feeding, and thus are important in linking terrestrial and aquatic ecosystems. We measured δ ¹³C and δ ¹⁵N values of early stages (egg, embryo, tadpole) and toes of adult frogs Rana temporaria, collected from a small wetland in Lithuania. We compared the isotopic composition of these tissues with potential food sources, excrements of tadpoles, and filled intestinal tracts. We found that δ ¹³C values in R. temporaria tadpoles were markedly depleted in comparison to adults, eggs or embryos, demonstrating a terrestrial to aquatic shift in energy sources. After the onset of feeding, tadpoles approached isotopic equilibrium with available food (algae and litter). Tadpoles had higher δ ¹⁵N than both algae and litter, differing by 3.6 and 2.4‰, respectively, and similar δ ¹³C to these sources. However, tadpole excrements and body tissue diverged, with mean δ ¹³C values of excrements (−30.3 ± 1.6‰ SD) more similar to litter (−31.7 ± 1.2‰ SD) and body tissue δ ¹³C (−34.8 ± 0.7‰ SD) more similar to algae (−34.2 ± 4.1‰ SD). This suggests that algal resources are critical in early life stages of this anuran, particularly at stages characterized by high growth and low development (stages: 25–35).