Variation In Isotope Signatures Research Articles

Stable isotopes in a southern E uropean crossbill population indicate restricted movement between regions with different pine species

The adaptation of irruptive birds to breed where they find abundant food allows them to be highly mobile and ubiquitous and opens a debate on the ecological and evolutionary impact of this behavior. Using δ2H analyses, our aim here was to test whether the density fluctuation in a southern European crossbill population is caused by movements of irruptive northern European crossbills or of crossbills from nearby. Sampling was carried out in the western Pyrenees during four consecutive breeding seasons (2009–2012). During these four years, 2009 was a year with a much higher number of captures per day and this coincided with an invasion of crossbills in northern Europe. Only two out of 160 crossbills showed clear isotopic signatures of a northern European origin (and both were caught in 2009). Moreover, in 2009, we recaptured a bird ringed in The Netherlands. Otherwise, we found no differences among years in δ2H and no significant effect of wing length (indicative of a northern origin) or residency status (long-term recaptures vs. non-recaptures) on δ2H. Wing length also did not vary among these four years. Therefore, our results suggest that annual variation in local crossbill abundances is not only due to the movements of northern irruptive birds, but also to a large part due to the influx and local reproduction of individuals of Iberian origin. The lack of annual variation in isotopic signatures and similarity with values from a nearby mountain area with the same food resource (the pine Pinus sylvestris) suggests that movements are primarily between areas with the same type of resource. Thus, our results support recent morphological and genetic studies that indicate that crossbill populations within Iberia are not structured geographically but by resource use.

Best practices for use of stable isotope mixing models in food-web studies

Stable isotope mixing models are increasingly used to quantify consumer diets, but may be misused and misinterpreted. We address major challenges to their effective application. Mixing models have increased rapidly in sophistication. Current models estimate probability distributions of source contributions, have user-friendly interfaces, and incorporate complexities such as variability in isotope signatures, discrimination factors, hierarchical variance structure, covariates, and concentration dependence. For proper implementation of mixing models, we offer the following suggestions. First, mixing models can only be as good as the study and data. Studies should have clear questions, be informed by knowledge of the system, and have strong sampling designs to effectively characterize isotope variability of consumers and resources on proper spatio-temporal scales. Second, studies should use models appropriate for the question and recognize their assumptions and limitations. Decisions about source grouping or incorporation of concentration dependence can influence results. Third, studies should be careful about interpretation of model outputs. Mixing models generally estimate proportions of assimilated resources with substantial uncertainty distributions. Last, common sense, such as graphing data before analyzing, is essential to maximize usefulness of these tools. We hope these suggestions for effective implementation of stable isotope mixing models will aid continued development and application of this field.

Temporal variability in the isotopic niches of rocky shore grazers and suspension‐feeders

AbstractCarbon and nitrogen stable isotope ratios were measured in rocky inter‐tidal suspension‐feeders (brown mussels and cape reef worms) and grazers (goat's eye limpets and cape sea urchins) to determine the influences of lifestyle and time on the diets of consumers. Niche partitioning between consumer species within the same feeding guild was assessed using isotopic niche area (a proxy for trophic niche). Specimens were collected monthly at a single site in Southeastern South Africa from July 2010 to June 2011. Temporal variations in isotopic signatures were generally greater in the suspension‐feeders compared with the grazers, isotopic niche widths were smaller in the suspension‐feeders and intra‐population variations in isotope signatures were larger in the grazers. No inter‐specific niche overlap (according to standard ellipse areas) was observed within either feeding guild unless standardization calculations were used. Temporal variations in the diets of all the consumers appeared uncoupled from temporal variability in the isotopic signatures of basal resources in the region; as such, shifts in the consumer diets most likely pertained to feeding behaviour and food preferences. Our data provide new insights into how syntopic rocky shore consumers coexist by partitioning their temporally variable food environment.

Assessing sources of human methylmercury exposure using stable mercury isotopes.

Seafood consumption is the primary route of methylmercury (MeHg) exposure for most populations. Inherent uncertainties in dietary survey data point to the need for an empirical tool to confirm exposure sources. We therefore explore the utility of Hg stable isotope ratios in human hair as a new method for discerning MeHg exposure sources. We characterized Hg isotope fractionation between humans and their diets using hair samples from Faroese whalers exposed to MeHg predominantly from pilot whales. We observed an increase of 1.75‰ in δ202Hg values between pilot whale muscle tissue and Faroese whalers’ hair but no mass-independent fractionation. We found a similar offset in δ202Hg between consumed seafood and hair samples from Gulf of Mexico recreational anglers who are exposed to lower levels of MeHg from a variety of seafood sources. An isotope mixing model was used to estimate individual MeHg exposure sources and confirmed that both Δ199Hg and δ202Hg values in human hair can help identify dietary MeHg sources. Variability in isotopic signatures among coastal fish consumers in the Gulf of Mexico likely reflects both differences in environmental sources of MeHg to coastal fish and uncertainty in dietary recall data. Additional data are needed to fully refine this approach for individuals with complex seafood consumption patterns.

Secondary microbial gas formation associated with biodegraded oils from the Liaohe Basin, NE China

A suite of oils and associated gases from the Liaohe Basin NE China, which have similar maturity and source rock origin but with varying degrees of biodegradation, have been analyzed for the molecular compositions of oil and gas as well as for the isotopic values. Isotopic compositions of the gas samples show wide variation (δ13C1 from −55.4‰ to −36.8‰, δ13C2 from −39.4‰ to −28.5‰, δ13C3 from −31.9‰ to −16.4‰ and δ13n-C4 from −26.6‰ to −20.4‰). Most gas samples display clear geochemical evidence of alteration by biodegradation, with relatively high dryness (C1/C1–4>0.97), high ratios of isobutane/n-butane (0.4–1.8) and C2/C3 (0.3–42.3). The carbon isotopic compositions of wet gas components (C3 and C4) in the biodegraded gases are heavier than these in non-biodegraded gases with Δδ13C (C3−C2) of up to 13.4‰, but methane and ethane isotope signatures are variably heavier or lighter than those in non-biodegraded gases. Carbon dioxide contents and isotopic values provide evidence for gas origins. Gases with isotopically heavy carbon dioxide (δ13C>+2‰), and isotopically light methane (δ13C<−50‰) indicate that secondary microbial methane was generated from CO2 reduction under anaerobic conditions. Gases with isotopically heavy methane (>−45‰), together with heavy carbon dioxide (>+8‰), probably indicate a high degree of CO2 conversion to methane under later stages of secondary microbial methane generation. We speculate that the presence of isotopically heavy methane (>−45‰) and isotopically light carbon dioxide (<−8‰) may be indicative of anaerobic methane oxidation but much additional work is needed to evaluate that.It seems likely that most of the variations in isotopic signatures of gases related to biodegraded oil can be described by various sequential biodegradation, carbon dioxide reduction and gas mixing processes. Methanogenesis through carbon dioxide reduction does not necessarily produce isotopically depleted methane. The methane becomes richer in 13C when a large fraction of carbon dioxide is reduced to methane, resulting in newly generated methane that is sometimes heavier than the original thermogenic methane. Anaerobic methane oxidation may exist in the present case study. Mixing of newly generated methane with thermogenic methane already in place in the reservoir, coupled with anaerobic methane oxidation, can result in very complicated isotopic signatures. Care should be taken when gas isotopic values were applied for source and maturity assessment.

Petrogenesis of Miocene Volcanic arc rocks over the Chilean- Pampean flat-slab segment of the Centr

Miocene arc volcanism is manifested widely in the Valle del Cura-El Indio belt region (29°30´–30° South latitude), in the southern Central Andes of Argentina and Chile. The magmas that fed this volcanism are well represented by the Cerro de las Tortolas Formation, which is divided into two volcanic episodes based on petrographic, chemical and age differences: an older basaltic-andesitic event (16–14Ma) and a younger andesitic to dacitic (13–10Ma) one. Representative plagioclase, orthopyroxene, clinopyroxene and amphibole phenocrysts from rock samples already characterized from geochemical and isotopic viewpoints were selected for electron microprobe determination of mineral chemistry. Results indicate an overall homogeneous composition for each of the mineral phases. Equilibrium temperatures were estimated through two-pyroxenes, amphibole-plagioclase and amphibole geothermometers, which show a consistent temperature range between 970 to 850°C. Equilibrium pressure calculated using amphibole composition for volcanic suites produced the most comprehensive results for pressure equilibrium conditions, with results close to 4kb. Changes in the residual mineral assemblages and variations in isotopic signatures indicate that primary magmas were equilibrated at the lower crust with a gradual increase of crustal thickness. These melts evolved towards intermediate magma chambers, where crystallization of phenocrysts occurred at the same temperature and pressure conditions, hence, no increase in depth of intermediate magma chambers is registered although the increase of crustal thickness registered from Lower to Middle Miocene times.

Revisiting the use of δ15N in meso-scale studies of marine food webs by considering spatio-temporal variations in stable isotopic signatures – The case of an open ecosystem: The Bay of Biscay (North-East Atlantic)

Most of the recent framework directives and environmental policies argue for the development and the use of indicators – notably trophodynamic indicators – that should be able to follow ecosystems’ evolution in space and time, particularly under anthropogenic perturbations. In the last decades, the use of stable carbon and nitrogen isotopes ratios has increased exponentially, particularly in studies of marine ecosystems’ trophic structure and functioning. This method is principally based on the assumption that the isotopic composition of a consumer directly reflects that of its food. Nevertheless, few studies have attempted to define the limits of this tool, before using it and drawing ecological conclusions from isotopic analysis. This study aimed to assess the importance of considering spatio-temporal variations in isotopic signatures of consumers when using δ13C and especially δ15N values in open ecosystems with complex food webs, using the Bay of Biscay (North-East Atlantic) as a case study. To this end, more than 140 species from this marine ecosystem were analysed for the isotopic signatures in their muscle tissue. They were sampled from coastal to oceanic and deep-sea areas and at different latitudes, to evaluate spatial variations of isotopic signatures. Selected species were also sampled over several years and in two seasons to account for inter-annual and seasonal variations. In the Bay of Biscay temperate ecosystem, which is subject to both coastal and oceanic influences – two main river inputs and upwelling areas – , δ13C and δ15N values significantly decreased from inshore to offshore species, and to a lesser extent from benthic to pelagic organisms. River discharges appeared to be the first factor influencing δ13C and δ15N values in consumers. From the important spatial variations detected in δ15N values in particular, we suggest that in such contrasted ecosystem, nitrogen isotopic ratios may also be revisited as an indicator of the feeding area. Moreover, we demonstrate that several baselines should be used when calculating trophic levels from δ15N values. From the temporal variations detected, we recommend concentrating on a short time scale for the sampling most organisms.

Stable carbon isotope signatures of methane from a Finnish subarctic wetland

ABSTRACTMethane emissions from Lompolojänkkä, a Finnish aapa mire within the Arctic Circle, were studied by non-intrusive Keeling plot methods, to place better constraints on the seasonal variations in isotopic signature of methane (δ13CCH4) emitted from Arctic wetland. Air samples were collected in Tedlar bags over the wetland at heights of 42 and 280 cm between May and October 2009 and in August 2008. The mixing ratio and δ13C of the methane in the samples were incorporated into Keeling plot analyses to derive bulk δ13CCH4 signatures for the methane inputs to the air above the wetland. The results show an unexpected consistence in δ13CCH4 from early to late summer, clustered around −68.5±0.7‰, but during spring thaw and autumnal freezing, δ13CCH4 is enriched by approximately 2 and 4‰, respectively. The techniques reported in this paper are simple and economical to employ, and give a bulk source signature for the methane inputs to the air above the entire wetland that can be extrapolated to a larger regional area.

Isotopic Ratios Reveal Mixed Seasonal Variation Among Fishes from Two Subtropical Estuarine Systems

Characterizing dietary resources and species interactions in estuaries is challenging, particularly when considering the dynamic nature of these ecosystems, the ranges in body sizes of species, and the potential for trophic roles to vary with ontogeny. We examined the influence of season and location on relationships between body size and δ15N, δ13C, and δ34S values across a range of fishes from two subtropical estuaries. The results suggest that isotopic values of estuarine fishes are independent of body size. However, seasonal variation propagated throughout the assemblage as the majority of fishes integrated different δ15N, δ13C, or δ34S values. The absence of δ15N–, δ13C–, and δ34S–body size relationships suggests that either (1) dietary preference of these fishes do not shift within the range of body sizes sampled, (2) these fishes shift to an alternate diet that is not isotopically distinct, or (3) that spatial and temporal variation in isotopic signatures of prey negate any size-based relationships. Seasonal variability in the isotopic values of these fishes suggests either movement to an alternative habitat or a shift in organic matter source associated with the transition of dry to wet seasons. Moreover, variance distributions of the best-fit models indicate that seasonal dietary preferences of conspecifics do not vary over moderate spatial scales. Seasonal variability among fishes in these estuaries suggests plasticity in feeding strategies that may afford greater adaptive flexibility to these species in response to changes in food availability resulting from variable environmental conditions.

Bone collagen and muscle δ13C in relation to the timing of the migration of Garden Warblers Sylvia borin during return migration from Africa

In migratory birds, seasonal factors interacting at different stages of the annual cycle can influence individual life histories. These have been well documented for Nearctic-Neotropical birds. Yet, seasonal interactions between the wintering, migration, and breeding periods have not been demonstrated for Palearctic-African passerine migrants. We tested whether variation in long- and short-term dietary choice of the Garden Warbler Sylvia borin can influence events during the subsequent spring migration from the African winter quarters to the Palearctic breeding grounds. Using bone collagen and muscle carbon stable isotope (δ13C) analysis, we assessed the relationships between dietary history, body condition, and migration timing in the Garden Warbler during a return migration from Africa. We predicted that Garden Warblers arriving early will have significantly different muscle and collagen δ13C relative to those arriving later. Whereas muscle δ13C (referring to events in the immediate past) was not related to body condition, we found a strong association between body condition and collagen δ13C signatures (representing the integration of long-term events). Collagen and muscle δ13C indicate that birds passing through later originated from moister or cooler geographic areas. The observed variation in isotope signatures might relate to differences in habitat and geographic/population origin, or in dietary intake.

On the Use of Stable Isotopes in Trophic Ecology

Stable isotope analysis (SIA) has proven to be a useful tool in reconstructing diets, characterizing trophic relationships, elucidating patterns of resource allocation, and constructing food webs. Consequently, the number of studies using SIA in trophic ecology has increased exponentially over the past decade. Several subdisciplines have developed, including isotope mixing models, incorporation dynamics models, lipid-extraction and correction methods, isotopic routing models, and compound-specific isotopic analysis. As with all tools, there are limitations to SIA. Chief among these are multiple sources of variation in isotopic signatures, unequal taxonomic and ecosystem coverage, over-reliance on literature values for key parameters, lack of canonical models, untested or unrealistic assumptions, low predictive power, and a paucity of experimental studies. We anticipate progress in SIA resulting from standardization of methods and models, calibration of model parameters through experimentation, and continued development of several recent approaches such as isotopic routing models and compound-specific isotopic analysis.

Carbon isotopic fractionation in eider adipose tissue varies with fatty acid structure: implications for trophic studies

Carbon isotopic fractionation was investigated in fatty acids (FA) of adipose tissue and blood serum of threatened Steller's eiders (Polysticta stelleri) and spectacled eiders (Somateria fischeri) relative to the FA in their diets. Captive eiders were fed a known diet for 180 days with serum sampled at 60, 120 and 180 days immediately after a 12 fast; adipose was collected at 180 days. Essential FA (EFA) in the adipose showed varying degrees of isotope fractionation (0-4‰), depending on FA structure. The δ(13)C values of long-chain FA 20:5n-3 and 22:6n-3 did not differ from those in the diet, while those of 18:2n-6 and 18:3n-3 were ∼2‰ greater than in the diet. The δ(13)C values of free FA (FFA) in serum were not consistent within individuals or sampling dates; fractionation varied randomly, suggesting that FFA were arising from diet, rather than mobilization from adipose tissue. Discrimination factors were used in combination with a mixing model incorporating FA and lipid concentrations to estimate the diet of eiders fed a binary mixture with contrasting isotopic signatures. Diet estimates varied with FA but mean values closely approximated the actual proportions consumed. By tracking EFA, this study avoided the complications in interpretation arising from isotopic routing of carbon in bulk isotope analyses and serves as a basis for the development of compound-specific isotopic methods to trace dietary input in wild eiders. However, our understanding of the processes contributing to the variation in isotopic signatures of FA in nature is currently limited, and we recommend that future research directions focus on elucidating these mechanisms.

Seasonal Trophic Shift of Littoral Consumers in Eutrophic Lake Taihu (China) Revealed by a Two-Source Mixing Model

We evaluated the seasonal variation in the contributions of planktonic and benthic resources to 11 littoral predators in eutrophic Lake Taihu (China) from 2004 to 2005. Seasonal fluctuations in consumer σ13C and σ15N were attributed to the combined impacts of temporal variation in isotopic signatures of basal resources and the diet shift of fishes. Based on a two-end-member mixing model, all target consumers relied on energy sources from coupled benthic and planktonic pathways, but the predominant energy source for most species was highly variable across seasons, showing seasonal trophic shift of littoral consumers. Seasonality in energy mobilization of consumers focused on two aspects: (1) the species number of consumers that relied mainly on planktonic carbon showed the lowest values in the fall and the highest during spring/summer, and (2) most consumer species showed seasonal variation in the percentages of planktonic reliance. We concluded that seasonal trophic shifts of fishes and invertebrates were driven by phytoplankton production, but benthic resources were also important seasonally in supporting littoral consumers in Meiliang Bay. Energy mobilization of carnivorous fishes was more subject to the impact of resource availability than omnivorous species.

Effects of food quality on tissue-specific isotope ratios in the mussel Perna perna

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.

Are spatial variations in the diets of hydrothermal fauna linked to local environmental conditions?

Abstract Trophic relationships in Bathymodiolus azoricus mussel bed communities on the Tour Eiffel hydrothermal edifice (Lucky Strike) were assessed using δ13C and δ15N signatures from 14 hydrothermal species. The nutritional basis of B. azoricus was also investigated with δ34S. Faunal samples and environmental data (temperature, pH, total dissolved sulfide, iron and copper concentrations) were collected from 12 different locations on the edifice. Chemical conditions varied between microhabitats, and were all correlated to temperature. Carbon and nitrogen isotopic results revealed the presence of two, apparently independent, trophic groups. The first was composed of symbiont-bearing fauna (B. azoricus and their associated polychaetes Branchipolynoe seepensis), while the second enclosed heterotrophic fauna (bacterivores, detritivores, scavengers, predators). A majority of mussels displayed δ13C values ranging from −27‰ to −34‰, supporting thiotrophy as the dominant nutritional pathway at Tour Eiffel, with methanotrophy and filter feeding emerging as secondary strategies. This result was corroborated by δ34S signatures. However, higher δ13C values in larger mussels suggested that, as they grow, B. azoricus mussels rely more heavily on their methanotrophic endosymbionts. Significant spatial variability in isotopic signatures for single faunal species was observed at the scale of the edifice for three species (B. azoricus, B. seepensis, Amathys lutzi), and environmental conditions explained variation in isotopic signatures for one-third of the species. This confirms the hypothesis raised by several authors on the role of hydrothermal fluids on the trophic network at small spatial scales. We suggest that vent fluid characteristics, by influencing microbial production, are key factors in the variation of local carbon sources at vents.

Isotopic variability in lake littoral organisms presents a challenge for food web studies

Most studies of lake ecosystems, including those that use stable isotope analysis (SIA), have traditionally concentrated on the pelagic system (Beaudoin et al. 2001, Vander Zanden & Vadeboncoeur 2002). Hence, temporal, spatial, and interspecies variability in isotopic signatures of pelagic food web components (especially zooplankton) have been the focus of recent studies (e.g., Matthews & Mazumder 2003, 2007). Recent SIA studies have also clearly indicated, however, that benthic production and the littoral areas of lakes may contribute more than previously thought to the whole lake ecosystem energy budget (Hecky & Hesslein 1995, Vander Zanden & Vadeboncoeur 2002, Vander Zanden et al. 2006). Studies of lake food webs based on SIA often assume single isotope values to represent baseline values for pelagic and littoral food webs. These values may then be used as “end members” in isotope mixing models, such as to assess trophic positions or relative contributions from different energy sources. But such single values ignore temporal, spatial or especially inter-specific variation in stable isotope values for these primary consumers, as shown for pelagic consumers by Matthews & Mazumder (2003). Such variability may have considerable impacts on the outputs from mixing models and hence on the inferences drawn about food web structure. Lake littoral habitats have received much less attention in this respect, despite being important feeding areas for many fish species and therefore contributing significantly to the observed stable isotope signatures at higher trophic levels. Littoral habitats are also inherently more heterogeneous than pelagic habitats, and primary production from phytoplankton, periphyton, and macrophytes, combined with allochthonous sources of energy, potentially result in complex and variable isotopic signatures in littoral primary consumers (Syvaranta et al. 2006). We present stable isotope data for littoral invertebrate consumer populations (benthic invertebrates and zooplankton) from Lake Jyvasjarvi in central Finland and show how variation in isotope signatures between different littoral invertebrate consumers can be reflected at higher trophic levels.

Variability of Stable Isotope Ratios of Glassfish (Ambassis jacksoniensis) from Mangrove/Saltmarsh Environments in Southeast Australia and Implications for Choosing Sample Size

Stable isotopes offer an accurate and cost-effective way to understand critical pathways of energy, and their use has application where aquatic habitats have been degraded. Isotope ratios can offer insights into appropriate conservation and rehabilitation techniques to manage these valuable resources. To date, the literature shows little attention to spatial and temporal variation in isotope signatures of samples taken from estuarine environments. This study reports on an investigation of sample size by which to determine isotopic signatures within a single species of fish, Ambassis jacksoniensis. Provided are two ways by which to determine how many samples are needed, and the results imply that care may be needed in interpreting previous studies based on small sample size. A minimum of five individuals would appear necessary in some cases.

Temporal and spatial variability in stable isotope ratios of SPM link to local hydrography and longer term SPM averages suggest heavy dependence of mussels on nearshore production

Temporal changes in hydrography affect suspended particulate matter (SPM) composition and distribution in coastal systems, potentially influencing the diets of suspension feeders. Temporal variation in SPM and in the diet of the mussel Perna perna, were investigated using stable isotope analysis. The δ13C and δ15 N ratios of SPM, mussels and macroalgae were determined monthly, with SPM samples collected along a 10 km onshore–offshore transect, over 14 months at Kenton-on-Sea, on the south coast of South Africa. Clear nearshore (0 km) to offshore (10 km) carbon depletion gradients were seen in SPM during all months and extended for 50 km offshore on one occasion. Carbon enrichment of coastal SPM in winter (June–August 2004 and May 2005) indicated temporal changes in the nearshore detrital pool, presumably reflecting changes in macroalgal detritus, linked to local changes in coastal hydrography and algal seasonality. Nitrogen patterns were less clear, with SPM enrichment seen between July and October 2004 from 0 to 10 km. Nearshore SPM demonstrated cyclical patterns in carbon over 24-h periods that correlated closely with tidal cycles and mussel carbon signatures, sampled monthly, demonstrated fluctuations that could not be correlated to seasonal or monthly changes in SPM. Macroalgae showed extreme variability in isotopic signatures, with no discernable patterns. IsoSource mixing models indicated over 50% reliance of mussel tissue on nearshore carbon, highlighting the importance of nearshore SPM in mussel diet. Overall, carbon variation in SPM at both large and small temporal scales can be related to hydrographic processes, but is masked in mussels by long-term isotope integration.

The importance of quantifying inherent variability when interpreting stable isotope field data

Stable isotope data are often used to assess diet, trophic level, trophic niche width and the extent of omnivory. Notwithstanding ongoing discussions about the value of these approaches, variations in isotopic signatures among individuals depend on inherent variability as well as differences in feeding habitats. Remarkably, the relative contributions of diet variation and inherent variability to differences in delta(15)N and delta(13)C among individuals have not been quantified for the same species at the same life history stages, and inherent variability has been ignored or assumed. We quantified inherent variability in delta(13)C and delta(15)N among individuals of a marine fish (the European sea bass, Dicentrarchus labrax) reared in a controlled environment on a diet of constant isotopic composition and compared it with variability in delta(13)C and delta(15)N among individuals from wild bass populations. The analysis showed that inherent variability among reared individuals on a controlled diet was equivalent to a large proportion of the observed variability among wild individuals and, therefore, that inherent variability should be measured to establish baseline variability in wild populations before any assumptions are made about the influence of diet. Given that inherent variability is known to be dependent on species, life history stage and the environment, our results show that it should be quantified on a case-by-case basis if diet studies are intended to provide absolute assessments of dietary habits.

Predation by jellyfish on large and emergent zooplankton: Implications for benthic–pelagic coupling

Stable carbon isotopes were used to determine the contribution of emergent demersal zooplankton to the diet of the scyphozoan jellyfish Catostylus mosaicus at Smiths Lake, New South Wales, Australia. A preliminary study in 2004 indicated that there was no difference in the δ 13C of ectodermal tissue and mesoglea of the medusae. In 2005, medusae and zooplankton present during the day and night were sampled and isotopic signatures were modelled using IsoSource. Modelling indicated that: (1) mollusc veligers and copepods sampled during the day contributed <13% of the carbon to the jellyfish; (2) copepods sampled at night contributed up to 25%; and (3) the large, emergent decapod Lucifer sp. contributed 88–94%. We hypothesised, therefore, that medusae derive most of their carbon from emergent species of zooplankton. In 2006, sampling done in 2005 was repeated three times over a period of 4 weeks to measure short-term temporal variation in isotopic signatures of medusae and zooplankton, and emergent demersal zooplankton was specifically sampled using emergence traps. Short-term temporal variation in isotopic signatures was observed for some taxa, however, actual variations were small (<1.5‰) and the values of medusae and zooplankton remained consistent relative to each other. IsoSource modelling revealed that mysid shrimp and emergent copepods together contributed 79–100% of the carbon to the jellyfish, and that the maximum possible contribution of daytime copepods and molluscs was only 22%. Jellyfish apparently derive most of their carbon from emergent zooplankton and by capturing small numbers of relatively large taxa, such as Lucifer sp. or mysid shrimp. Small but abundantly captured zooplankton (such as mollusc veligers) contribute only minor amounts of carbon. Jellyfish have a major role in the transfer of carbon between benthic and pelagic food webs in coastal systems.