Burying beetle Nicrophorus investigator reproduction on Pacific salmon carcasses

Abstract.  1. In many undisturbed watersheds along the Pacific Rim, anadromous salmon ( Oncorhynchus spp.) provide a predictable source of carrion to the riparian zone, largely due to horizontal transfer of salmon carcasses by bears ( Ursus spp.) and other vertebrates.
2. Burying beetles are important members of the north-temperate carrion fauna, and may utilise salmon carcasses and remnants for breeding. In this study, isotopic and observational data are reported that demonstrate previously unrecognised Nicrophorus investigator (Zetterstedt) reproduction on large salmon carcasses from five watersheds in coastal British Columbia.
3. Stable isotope signatures (δ¹⁵N and δ¹³C) of adult beetles collected in autumn indicate a diet of salmon origin in all but one individual from all watersheds, suggesting that this beetle–salmon association is widespread. Comparison of autumn isotope signatures to individuals collected randomly in summer suggests that isotope signatures represent the larval carrion source from the previous autumn rather than immediate adult diet.
4. In a survey of N. investigator use of salmon carcasses from two watersheds, 35 broods were observed on chum and pink salmon carcasses, including 16 natural brood complexes containing over 100 larvae, and five ranging from 250 to 750 larvae.
5. Overall, north-coastal populations of N. investigator breed on the rich and reliable salmon resource and may exhibit a system of communal breeding on these carcasses. This is most relevant when the dramatic reduction in salmon spawning biomass over the last century is considered.     

Stable isotope variability of meso-zooplankton along a gradient of dissolved organic carbon

1.  The δ¹³C and δ¹⁵N signatures of zooplankton vary with dissolved organic carbon (DOC), but inconsistent and limited taxonomic resolution of previous studies have masked differences that may exist among orders, genera or species and are attributable to dietary and/or habitat differences. Here we investigate differences among the isotopic signatures of five zooplankton taxa ( Daphnia , Holopedium , large Calanoida, small Calanoida and Cyclopoida) in Precambrian shield lakes with a sixfold range of DOC concentration.
2.  δ¹³C signatures of Daphnia , small calanoids and large calanoids became more depleted with increasing lake DOC, whereas Holopedium and cyclopoid δ¹³C became enriched with increasing DOC concentration.
3.  The variability of δ¹³C and δ¹⁵N isotopic signatures among zooplankton groups was reduced in high-DOC, compared to low-DOC lakes, especially for δ¹³C. Differences in δ¹³C and POM-corrected δ¹⁵N accounted for up to 33.7% and 19.5% of the variance, respectively, among lakes of varying DOC concentration.
4.  The narrow range of signatures found in higher DOC lakes suggests that different taxa have similar food sources and/or habitats. In contrast, the wide range of signatures in low-DOC lakes suggests that different taxa are exploiting different food sources and/or habitats. Together with the variable trends in zooplankton isotopic signatures along our DOC gradient, these results suggest that food web dynamics within the zooplankton community of temperate lakes will change as climate and lake DOC concentrations change.     

Crop colonisation, feeding, and reproduction by the predatory beetle, Hippodamia convergens, as indicated by stable carbon isotope analysis

Abstract.  1. Habitat management to enhance natural enemy populations in agricultural systems may help regulate levels of crop pests, but little research addresses the behaviour of immigrating beneficial insects.
2. Stable carbon isotopes were used in complementary laboratory and field studies to examine colonisation behaviour of an ephemeral agricultural habitat by the lady beetle, Hippodamia convergens Guérin-Méneville.
3. Under laboratory conditions, H. convergens carbon isotope ratios, δ¹³C, changed after its food supply was shifted from a C₄- to a C₃-based diet of aphids produced on grain sorghum or cotton respectively. Final isotope ratios of adult H. convergens were closer to that of the new C₃-based diet, with most change in δ¹³C occurring within 3 days after the diet shift.
4. The carbon isotope ratios of lady beetle adults collected in cotton fields suggested that grain sorghum was a continuous source for H. convergens until many nearby sorghum fields matured and senesced.
5. When cotton aphid ( Aphis gossypii Glover) prey were absent, carbon isotope ratios of beetle populations did not change over time and virtually no egg production by H. convergens was detected. This indicates that beetles were feeding little on non-aphid resources originating in cotton.
6. With cotton aphids present, beetle isotope ratios decreased towards the carbon isotope ratio of cotton, indicating adult feeding in cotton. As a result, egg masses produced had carbon isotope ratios in the C₃ range of values.
7. The results suggest that some predator species may be retained in habitats without large prey populations, a quality essential in controlling pests in agricultural systems.     

Stable isotope analysis of macroinvertebrates and their food sources in a glacier stream

1. Food sources and trophic structure of the macroinvertebrate community along a longitudinal gradient were examined in a glacier stream of the Swiss Alps (Val Roseg). Analysis of multiple stable isotopes (δ¹³C and δ¹⁵N) and measurement of C : N ratios were used to differentiate between allochthonous and autochthonous organic matter.
2. Although isotopic signatures of algae varied widely among sites and dates, it was possible to discriminate between allochthonous and autochthonous food sources using a site-specific approach.
3. Dominant food sources of herbivorous invertebrates in all main channel sites were epilithic diatoms and the filamentous gold alga Hydrurus foetidus . Allochthonous organic matter was of some importance only in a groundwater-fed stream close to the floodplain margin.
4. Seasonal changes in the δ¹³C signature of the macroinvertebrates corresponded with seasonal changes in δ¹³C of the gold alga H. foetidus . This indicated that the energy base remains autochthonous throughout the year.
5. Because of limited food sources, feeding plasticity of the invertebrate community was high. Both grazers and shredders fed predominantly on algae, whereas gatherer-collectors seemed to be omnivorous.
6. The overall enrichment of δ¹⁵N was 2.25‰ ( r ²=0.99) per trophic level. On a gradient from the glacier site to a downstream forested site trophic enrichment was constant but variation in δ¹⁵N within trophic levels decreased.     

Differential responses of three Agriotes click beetle species to pheromone traps

1 Previous work had suggested that adult male click beetles (Agriotes spp.) show differential responses to species specific pheromone traps. This hypothesis was tested using mark–release–recapture methods to estimate the maximum sampling range and the effective sampling area of traps for three species. 2 Captured beetles of the species Agriotes lineatus, Agriotes obscurus and Agriotes sputator were marked to show the direction of release, the distance of the release point from the trap and the replicate. Analysis of variance showed that there were significant differences in recapture rates between species and release distances. There were no significant differences between release direction and replicates. 3 Calculated linear speeds suggested differences in movement rates in the order: A. lineatus > A. obscurus > A. sputator. There were also substantial differences between the species in the maximum sampling ranges and effective sampling areas of the traps. These placed the species in the same order. 4 The results are used to estimate the minimum cost of mass trapping programmes to prevent males from mating, giving values of €165/ha/year (A. lineatus), €247.5/ha/year (A. obscurus) and €2343/ha/year (A. sputator). 5 Implications for the use of pheromone traps in wireworm pest management are discussed. It is concluded that current U.K. recommendations based on the cumulative total catch of the three species over a sampling season can be improved by considering the spatial relationships between the adult trapping system and larval distribution. The current constraint to this is the general inability to separate wireworms into species.     

Trophic relationships in a tropical stream food web assessed by stable isotope analysis

1. Stable isotope analysis, coupled with dietary data from the literature, was used to investigate trophic patterns of freshwater fauna in a tropical stream food web (Guadeloupe, French West Indies).
2. Primary producers (biofilm, algae and plant detritus of terrestrial origin) showed distinct δ¹³C signatures, which allowed for a powerful discrimination of carbon sources. Both autochthonous (¹³C-enriched signatures) and allochthonous (¹³C-depleted signatures) resources enter the food web. The migrating behaviour of fishes and shrimps between marine and freshwater during their life cycles can be followed by carbon isotopes. Here, shrimp δ¹³C signatures were shown to shift from −16‰ (for juveniles under marine influence) to −24.7‰ (for adults in freshwater habitats). For resident species, δ¹³C values partly reflected the species' habitat preferences along the river continuum : species living in river mouths were ¹³C-enriched in comparison with those collected upstream.
3. Nitrogen isotopic ratios were also discriminating and defined three main trophic guilds among consumers. The δ¹⁵N values of herbivores/detritivores were 5.0–8.4‰, omnivores 8.8–10.2‰ and carnivores 11–12.7‰.
4. Mixing model equations were employed to calculate the possible range of contribution made by respective food sources to the diet of each species. The results revealed the importance of omnivorous species and the dependence of riverine biota on terrestrial subsidies, such as leaf detritus and fruits. Finally, the abundance of shrimps and their feeding habits placed in relief their key role in tropical freshwater food webs. Isotopic analysis provides a useful tool for assessing animal feeding patterns.     

A new method to measure carbon isotope composition of CO2 respired by trees: stem CO2 equilibration

1.  Applying Keeling plot techniques to derive δ¹³C of respiratory input in a closed non-equilibrated chamber can lead to large errors because steady-state diffusion rules are violated in a non-steady-state environment. To avoid these errors, respiratory δ¹³C can be derived using equilibrated closed chambers.
2.  We introduce a new method to obtain stem respired CO₂δ¹³C (δ_{st - r}) with closed equilibrated stem chambers (E-SC). We present a theoretical model describing the equilibration process, test the model against field data and find excellent agreement. The method is further tested by comparing it with closed non-equilibrated stem chambers (NE-SC); we found no difference between these methods.
3.  Our theoretical model to describe CO₂ diffusion from the respiratory pool into the chamber and the equation to derive the δ¹³C of the efflux are general. They could be applied to other ecosystem components (e.g. soils).
4.  Our method is easy to implement, cost effective, minimizes sources of error and allows for rigorous leak detection. One major limitation is its inability to detect rapid change; the equilibration process requires 15 ± 2 h. A second limitation is that it cannot be used for species that produce abundant pitch at sites of stem wounding (e.g. Pseudotsuga menziesii ).
5.  Investigating δ¹³C of CO₂ respired by different ecosystem components is necessary to interpret δ¹³C of ecosystem respiration. This parameter has major implications with respect to global carbon cycle science.     

Evidence for secondary planktonic transport of post‐larvae of seagrass‐associated King George whiting

Stomach contents and stable isotope analyses were used to determine if secondary planktonic dispersal of King George whiting Sillaginodes punctata post‐larvae from shallow inshore habitats in a large embayment in south‐eastern Australia was initiated by wave disturbance. Benthic harpacticoid copepods, which live in and amongst seagrass shoots, were found in the stomachs of S. punctata caught offshore in the plankton only during rough weather. Stable isotope analyses showed that the base of nutritional support, estimated from values of δ¹³C, of S. punctata collected in the plankton changed significantly during rough (waves > 0·25 m) compared to calm (waves < 0·25 m) weather conditions. Values of δ¹³C collected from S. punctata in the plankton during rough weather were more consistent with S. punctata values found in the seagrass. Sillaginodes punctata collected in the plankton and seagrass during rough and calm weather failed to show differences in δ¹⁵N values. Dietary and isotope analyses support a model whereby newly arrived S. punctata larvae can be resuspended from seagrass beds and dispersed offshore by wave action during onshore winds. Secondary planktonic dispersal in S. punctata would provide a mechanism by which seagrass beds further inside Port Phillip Bay are colonized.     

Stable isotopes indicate the extent of freshwater feeding by cormorants Phalacrocorax carbo shot at inland fisheries in England

1. The numbers of cormorants Phalacrocorax carbo feeding at English freshwater fisheries during winter have increased rapidly over the last 20 years, causing concern among fishery managers and anglers.
2. In order to assess the extent of freshwater feeding, stable isotope ratios of carbon and nitrogen (δ¹³C and δ¹⁵N) in feathers of wild cormorants from inland freshwater fisheries were compared with those in the feathers of piscivorous birds with marine diets (captive 'marine-fed' cormorants, free-ranging shags Phalacrocorax aristotelis ) and freshwater diets (juvenile goosanders Mergus merganser ).
3. Isotope signatures of feathers represent the diet at the time of growth. Feathers grown at different times of the year were taken from wild cormorants; each feather type therefore represented the diet over a different temporal scale.
4. Isotopic analyses of feathers indicated that, when shot, nearly all of the cormorants had been feeding entirely on freshwater prey. The mean δ¹³C value of primary feathers growing when birds were shot was –22·2‰, indicative of an entirely freshwater diet.
5. The move to freshwater habitats from coastal breeding grounds occurred over several months, but once established cormorants appear to have fed at freshwater sites throughout the autumn and winter.
6. The suitability of using a two-source isotopic mixing model in order to quantify the extent of freshwater feeding in piscivorous birds is discussed.
7. Although the results indicate long-term residency and feeding in freshwater systems, they do not indicate whether birds were feeding regularly at the sites at which they were shot, or the composition of the diet. It is recommended that further studies using telemetry and multiple isotope analyses be carried out in order to address these issues.     

Trophic structure of a boreal forest arthropod community revealed by stable isotope (δ13C, δ15N) analyses

Terrestrial arthropods are important components of boreal ecosystems but relatively little is known about their trophic structure within communities. We measured δ¹³C and δ¹⁵N values in a broad range of arthropod taxa (Coleoptera, Diptera, Ephemeroptera, Homoptera, Hymenoptera, Lepidoptera, Odonata, Orthoptera, Araneae) from boreal forest in Prince Albert National Park, Saskatchewan, Canada. Isotopic measurements supported previous conventional investigations on foraging niches based on stomach content analysis and direct feeding observations but additional, new information was also obtained using the stable isotope approach. Significant differences were observed in both δ¹⁵N and δ¹³C values between various orders and families or superfamilies within orders. Higher variance in stable isotope values for scavengers (e.g. carrion beetles; Coleoptera, Silphidae) and generalists (e.g. ground beetles; Coleoptera, Carabidae) was found compared to specialists (e.g. grasshoppers; Orthoptera). Consistent isotopic differences between terrestrial and aquatic species were not found. However, aquatic insect δ¹³C values tended to be lower than those of their terrestrial counterparts. We discuss the potential for using stable isotope methods to reconstruct trophic linkages and interaction involving arthropods.     

Support of benthic invertebrates by detrital resources and current autochthonous primary production: results from a whole-lake 13C addition

1. Secondary production of benthic invertebrates in lakes is supported by current autochthonous primary production, and by detritus derived from a combination of terrestrial inputs and old autochthonous production from prior seasons. We quantified the importance of these two resources for the dominant benthic insects in Crampton Lake, a 26 ha, clear-water system.
2. Daily additions of NaH¹³CO₃ to the lake caused an increase in the stable carbon isotope ratios ( δ ¹³C) of the current primary production of phytoplankton and periphyton. We measured the response of four insect groups (taxon-depth combinations) to this manipulation, quantifying their current autochthony (% reliance on current autochthonous primary production) by fitting dynamic mixing models to time series of insect δ ¹³C.
3. The δ ¹³C of all four groups increased in response to the manipulation, although the magnitude of response differed by taxon and by depth, indicating differences in current autochthony. Odonate larvae (Libellulidae and Corduliidae) collected at 1.5 m depth derived 75% of their C from current autochthonous primary production. Chironomid larvae collected at 1.5, 3.5 and 10 m depths derived, respectively, 43%, 39% and 17% of their C from current autochthonous primary production.
4. Both taxon-specific diet preferences and depth-specific differences in resource availability may contribute to differences in current autochthony. Our results demonstrate significant but incomplete support of insect production by current autochthony, and indicate that allochthonous inputs and old autochthonous detritus support a substantial fraction (25–83%) of insect production.     

Changes in feeding niche widths of perch and roach following biomanipulation, revealed by stable isotope analysis

1. We made an empirical test of a recent proposal that feeding niche widths might be determined as variance of stable isotope values. We determined δ ¹³C and δ ¹⁵N values of perch ( Perca fluviatilis ), roach ( Rutilus rutilus ) and their prey from a biomanipulated lake, when the mass removal of fish led to reduced inter- and intra-specific competition and increases in zooplankton abundance and body size.
2. After the first fish removals, both perch and roach mean δ ¹³C values decreased and mean δ ¹⁵N values increased, indicating a greater diet contribution from pelagic sources.
3. Variances of both δ ¹³C and δ ¹⁵N values first increased in both fish populations, indicating a wider food spectrum and expanded feeding niche width following reduced fish abundances. Observed changes were greater for the perch population than for roach.
4. In 2006, the perch population abruptly changed its diet so that most individuals were primarily consuming the abundant young-of-the-year fish, and this was reflected in significantly reduced variances of both δ ¹³C and δ ¹⁵N values.
5. We conclude that isotopic variance can indeed reflect changes in feeding niche width and offers a promising way to study such general ecological concepts.     

Spatial relationships between two Agriotes click-beetle species and wireworms in agricultural fields

1 The production of new insect pheromones for pest monitoring proceeds at a greater rate than their evaluation, with the consequential possibility of premature introduction. Fundamental to their successful deployment is the determination of a consistent relationship between adult male pheromone trap catches and pest damage. In the present study, adult pheromone traps and larval bait traps were used to examine spatial relationships between two species of Agriotes beetle and wireworms at the field scale. 2 The spatial distributions of adult male Agriotes lineatus and Agriotes obscurus in two fields were determined and compared with the distribution of their larvae. Data were assembled as spatially referenced trap counts, and analysed for evidence of aggregation and clustering using Spatial Analysis by Distance IndicEs (SADIE) methodology. Spatial stabilities of adult populations between sampling dates were tested using association tests. Spatial and quantitative linkages between adult and larval trap catches were also tested. Moreover, a new way of adapting SADIE methodologies is presented for situations where two datasets within an area do not share the same sampling points. 3 There was no significant difference in variance : mean relationships for the two species but there were differences in their spatial distributions, and this is a definitive example of the general argument stating that it is important to consider spatial as well as count data in ecological studies. The spatial distribution of A. lineatus varied between sampling occasions at both sites whereas A. obscurus had consistently significant SADIE indices over time at one site, and adult catches could also be linked to larval distributions and counts. It is proposed that observed differences between the two species can be explained by interference between traps and dissimilar movement rates. There was some evidence of an edge effect at the field boundaries. 4 The distance between pheromone traps is related to the time that elapses before adjacent traps interfere with trap captures and this limits the detection of statistically significant spatial patterns. It is shown that the current practice of adding trap counts for different Agriotes species and treating them as numerically equivalent is insufficiently robust to be recommended at this stage. 5 The implications for the use of sex pheromone traps in wireworm pest management are considered. It is concluded that pheromone traps, as currently used, will not reliably indicate where wireworms occur in a field, and that the complexity of interpreting adult male trap counts limits quantitative predictions of population size.     

Contribution of current carbon assimilation in supplying root exudates of Lolium perenne measured using steady-state C labelling

Coupling growth of Lolium perenne L. in sterile solution culture with steady-state ¹³CO₂ labelling allowed quantification of the contribution of C, assimilated either before or after a specific time point, both to plant compartments and root exudates. Plants were grown for 27 days in atmospheres containing CO₂ with δ ¹³C signatures of either −13.5 or −36.1‰. Air supplies to plants were then reciprocally switched to the opposing signature (day 0), plants were destructively harvested and root exudates collected over the next 8 days. Following the switch, C assimilated after day 0 and transported to the roots initially only appeared in root tips, later appearing in both tip and non-tip material. The δ ¹³C signature of the root exudate changed exponentially with time. Assimilation pre- and post-day 0 contributed equally to exudate C at 4.5 days. Beyond day 8, assimilation pre-day 0 still contributed 41.7% of exudate C. Over all 8 days, a linear relationship existed between the δ ¹³C signatures of root tips and exudate, suggesting that all newly assimilated C in the exudate was from root tips. Results imply pulse-labelling approaches to study root exudates are discriminative in the sources of exudates labelled and in the sites from which exudation occurs.     

The effects of preservation on fish tissue stable isotope signatures

The effects of formalin and ethanol preservation on the δ¹³C and δ¹⁵N isotope signatures of Arctic charr Salvelinus alpinus muscle tissue were examined. The lipid content of the tissue samples studied ranged from 3·6 to 6·1% and was not correlated with the magnitude of observed isotopic shifts in preserved samples. Ethanol and formalin significantly depleted and enriched, respectively, the δ¹³C isotope signatures of preserved tissues when compared to control samples. Ethanol did not significantly enrich δ¹⁵N signatures in comparison to controls, whereas formalin did. A meta-analysis of multiple species effects further demonstrated significant preservation effects in fish tissue. Statistical analysis of data obtained by correcting preserved tissue isotope signatures with literature, bootstrapped or meta-analysis derived correction factors demonstrated significant differences between corrected and control sample isotope signatures or failure to produce a unity slope when the data sets were regressed against one another. Species-specific, bootstrapped linear correction models resulted in no such errors. Results suggest that species-specific correction methods should be used for fishes because of the known wide variation in fish tissue lipid content and composition. Accordingly, the use of pilot studies will be required to develop correction factors that properly adjust for preservation effects when interpreting temporal patterns in historic analyses of food webs.     

Nitrogen and carbon isotope ratios in termites: an indicator of trophic habit along the gradient from wood-feeding to soil-feeding

1. Nitrogen and carbon stable-isotope ratios (δ¹⁵N and δ¹³C) of body tissues, mound/nest materials and dietary substrates were determined in termite species with differing trophic habits, sampled from the Mbalmayo Forest Reserve, southern Cameroon.
2. δ¹⁵N of termite tissues was enriched gradually along a spectrum of species representing a trophic gradient from wood- to soil-feeding. Species that could be identified from their general biology and from gut content analysis as feeding on well-rotted wood or as wood/soil interface feeders showed δ¹⁵N intermediate between sound-wood-feeders and soil-feeders. It is proposed that δ¹⁵N is therefore a possible indicator of the functional position of species in the humification process. Differences in δ¹³C were also observed between wood-feeding and soil-feeding forms.
3. High values of δ¹⁵N in soil-feeding termites suggest that nitrogen fixation is of little importance in these species.
4. A wide range of isotope effects (the difference in isotope ratios between termites and their diet) was observed for both nitrogen (Δδ¹⁵N = –1.6 to + 8.8‰) and carbon (Δδ¹³C = –2.2 to + 3.0‰), which suggests a diversity of nutrient acquisition mechanisms within termites and diverse relationships between termites and their intestinal micro-organisms.     

Genetic identification of morphologically cryptic agricultural pests

1 Wireworms are the polyphagous larvae of click beetles and are well known as agricultural pests. Larvae of the genus Agriotes are internationally recognized as economically important pests of potato. Historically associated with crop damage after conversion of grassland, they are an increasing problem even in all-arable rotations.
2 Current studies of Agriotes ecology and behaviour, and consequently control and management, are seriously hampered by the lack of a means of reliably identifying larvae owing to morphological crypsis during this life-stage.
3 Here, sequence data at the mitochondrial 16SrRNA gene are presented for three species of wireworm: Agriotes obscurus, A. lineatus , and A. sputator. A novel terminal restriction fragment length polymorphism (T-RFLP) technique is described that identifies larvae of these species. This technique is shown to be both efficient and reliable. Interestingly, thus far the samples tested have yielded no A. lineatus. Implications for future study of wireworm ecology and control are discussed.     

Feather stable-isotope profiles reveal stopover habitat selection and site fidelity in nine migratory species moving through sub-Saharan Africa

Stable nitrogen (δ¹⁵N), carbon (δ¹³C) and hydrogen (δD) isotope profiles in feathers of nine migratory bird species trapped in Kenya were examined to test the extent to which they were segregated, geographically or by habitat, during an earlier autumn migration stopover in northeast Africa. We examined whether isotopic differences between species varied between years, and whether the isotope profiles of individual species appeared to be consistent. The relationship between mean feather δ¹³C, δ¹⁵N and δD assorted the migrants into several clustered groups. Similar feather isotope values among successive years revealed that each species tended to return to the same or similar stopover areas and selected habitat and diet that generated similar isotopic signatures. Possible explanations are discussed for the existence of these isotopic groups.     

Towards a predictive description of forest canopies from litter properties

1. Specific leaf area (SLA), carbon isotope ratio (δ¹³C) and leaf nitrogen content (N) were measured in litter in 1993 and in the litter layer and canopy in 1996 in a pedunculate Oak ( Quercus robur ) wood at Wytham, Oxford.
2. The frequency distribution of SLA of leaves falling through the late summer and autumn of 1993 changed with a greater contribution of leaves of high SLA earlier in the period. For the same SLA leaves falling earlier in the season had less negative δ¹³C and higher N content than leaves falling later.
3. The frequency distribution of SLA of litter leaves was similar in both years but litter leaves had lower N and less negative δ¹³C in 1996.
4. In the canopy, there was a highly significant negative relationship between height in the canopy and SLA in green and senescent leaves. The relationship between SLA and both δ¹³C ratio and leaf N was different between green, senescent and litter leaves.
5. The frequency distribution of foliage in the canopy estimated from the frequency of SLA in the litter and the vertical pattern of SLA in the canopy was similar to the distribution measured by the 'drop-line' method.     

Absence or masking of metabolic fractionations of 13C in a freshwater benthic food web

1. Although marine research has indicated that metabolic fractionations of ¹³C due to differences in organismal trophic position and proximal composition can complicate the isotopic interpretation of energy flow pathways, such potentially confounding problems have never been examined in freshwater benthic food webs.
2. The δ¹³C values of animals comprising a littoral benthic food web composited from four Canadian Shield lakes showed no relationship with either individual trophic position (δ¹⁵N) or lipid content (C/N ratios).
3. Differences in the relative incorporation of autochthonous and allochthonous energy sources by freshwater benthic organisms will alter their δ¹³C and δ¹⁵N values, thereby masking any possibility of observing ¹³C trophic enrichment.
4. Removal of the possibly confounding influences of lipids through either empirical correction or by analytical extraction may be unnecessary in studies of freshwater benthic food webs. Likewise, a priori adjustments in δ¹³C for freshwater benthic organisms in order to accommodate trophic fractionations which are presumed to occur, based on data from marine offshore food webs, may also be inappropriate.