Susan Waldron

Factors That Influence Assimilation Rates and Fractionation of Nitrogen and Carbon Stable Isotopes in Avian Blood and Feathers

By switching great skuas Catharacta skua from one isotopically distinct diet to another, we measured diet‐tissue discrimination factors and tested the assumption that dietary nitrogen and carbon isotope signatures are incorporated into blood and feathers at similar rates. We also examined the effects of metabolic rate and looked for evidence of isotopic routing. We found that blood δ15N and δ13C signatures altered after the diet switch at similar rates (14.4 d and 15.7 d, respectively). Qualitative analyses imply that the same was true with feathers. Mass balance calculations suggest that only a small amount of lipid is likely to be involved in the synthesis of blood and feathers. Differences in diet‐tissue discrimination factors before and after the diet switch may mean that toward the end of the experiment, some of the nutrients for blood synthesis had been coming from stores. Repeated measures mixed models provided evidence that increases in metabolic rate might accelerate fractional turnover rates in blood. There is a need for more laboratory‐based experimental isotope studies in order to address further questions that this study has raised.

Stable isotope ratios indicate that body condition in migrating passerines is influenced by winter habitat

Although predicted some time ago, there has been little success in demonstrating that the overall fitness of migratory birds depends on the combined influences of their experiences over all seasons. We used stable carbon isotope signatures (δ13C) in the claws of migrating black–throated blue warblers Dendroica caerulescens to infer their wintering habitats and investigated whether winter habitat selection can be linked to condition during migration. Resident bird species with low δ13C corresponded to selection of more mesic habitats, and migrating birds with low δ13C were in better condition than conspecifics with higher δ13C signatures. These findings concur with empirical observations on the wintering grounds, where dominants (mostly males) tend to exclude subordinates from mesic areas (considered to be high–quality habitats). We believe that variation in condition during migration may be one of the key factors determining differences in arrival times and condition at the breeding areas, which in turn have a major influence on reproductive success.

Ebullition of methane‐containing gas bubbles from near‐surface Sphagnum peat

[1] To date, very little information has been available on the build-up and release of biogenic gas bubbles in poorly-decomposed bog peats near the peatland surface (upper 1 m). We investigated the importance of ebullition of biogenic gas bubbles as a mechanism for the transport of CH 4 to the atmosphere in eight cores (24 cm diameter, 22 cm depth) of poorly-decomposed, near-surface bog peat. Ebullition was recorded in all but one sample but varied greatly between samples. Maximum rates of CH4 efflux via ebullition were also highly variable, ranging from 2.2 to 83.0 mg CH 4 m -2 day -1 . These rates are similar to rates of diffusive CH 4 efflux. Our results also show that wetland methane models are likely to need revision because they assume that unrealistically high CH 4 pore-water concentrations are required before bubbles can be produced and because, in part, they do not account for gas bubble build-up prior to ebullition.

Identifying migratory Salmo trutta using carbon and nitrogen stable isotope ratios

Many Salmo trutta populations consist of non-anadromous (freshwater-resident) brown trout and anadromous (sea-run migratory) sea trout. Although adult brown trout and sea trout can usually be identified using differences in size and body colouration, it is not possible to easily identify eggs/alevins as the progeny of brown trout or sea trout. In this study we show that delta(13)C and delta(15)N, measured using a continuous flow isotope ratio mass spectrometer (CF-IRMS), can accurately identify fish eggs as the progeny of freshwater-resident (delta(13)C(egg) = -25.7 +/- 1.9 per thousand,delta(15)N(egg) = 9.2 +/- 1.8 per thousand) or migratory (delta(13)C(egg) = -19.9 +/- 1.1 per thousand, delta(15)N(egg) = 14. 3 +/- 1.5 per thousand) adult female Salmo trutta. Case studies show that stable isotope analysis is a more reliable technique for distinguishing anadromous adult fish than differentiation using morphological characteristics. For example, stable isotope analysis of brown trout from Loch Eck, Scotland, revealed that some individuals possessed delta(13)C and delta(15)N signatures indicative of marine feeding despite visual identification as freshwater-resident fish. It is most likely that these fish are misidentified sea trout although it possible that these fish may be brown trout that have adopted an estuarine feeding strategy to avoid interspecific competition for food within Loch Eck with salmon, powan and Arctic charr. Most stable isotope studies of fish ecology use terminal tissue sampling to provide sufficient biological material for isotopic analysis; however, our study suggests that adipose fin tissue could provide a comparable measure of delta(13)C and delta(15)N. Such a strategy would be invaluable when studying the trophic ecology or migration patterns of fish of high conservation value.

Bioamplification of mercury in great skua Catharacta skua chicks: The influence of trophic status as determined by stable isotope signatures of blood and feathers

Abstract Biomagnification of mercury was investigated via combined mercury and stable isotope analysis of the blood and feathers of great skua chicks from two colonies in the north-east Atlantic. There were significant positive correlations between δ15N signatures and mercury concentrations in the blood from chicks at both colonies suggesting that dietary specialization influences intra-specific variability in mercury burdens. The relationships were slightly weaker in feathers and therefore blood is probably a better monitoring unit. Adult blood was also assessed and in terms of biomonitoring may provide an index of mercury intake over the whole breeding season.

The global influence of the hydrogen isotope composition of water on that of bacteriogenic methane from shallow freshwater environments

We propose that the hydrogen isotope composition of recently produced microbial methane, δD(CH4), in sulfate-poor, shallow freshwater environments, is directly related to the hydrogen isotopic composition of the system water δD(H2O). As δD(H2O) varies globally, systematic differences in δD(CH4) as a function of δD(H2O) should be observed. From available mean paired measurements from 46 sites, the relationship for δD(CH4) and δD(H2O) in the natural environment can be defined as δD(CH4) = 0.675δD(H2O) −284‰ (p < 0.0001). This relationship is statistically distinct from that generated by considering three separate laboratory-based anaerobic inoculations that contain similar methanogenic communities to the natural freshwater samples and therefore, are likely to produce methane by similar metabolic pathways: δD(CH4) = 0.444δD(H2O) −321‰ (p < 0.0001). We suggest that the relationship arising from the laboratory incubations defines the δD(CH4) of methane produced at source in shallow freshwater environments. We can approximate that 50% of the variation in natural δD(CH4) samples can be explained by δD(H2O), with isotopic fractionation postproduction, or mixing with gas already fractionated likely responsible for most of the noise in the natural system and difference of the natural sample relationship to the laboratory relationship. Methanogenic pathway may also influence δD(CH4), but the foundations for this hypothesis need to be reconsidered, and field and laboratory data exist that do not support it. The relationships presented here describe δD of methane from only shallow (subsurface) freshwater environments; paired δD(CH4)–δD(H2O) values from other environments (e.g., marine, glacial drift) suggest that a different relationship is needed to describe the influence of δD(H2O) on δD(CH4).

Enigmatic stable isotope dynamics of deep peat methane

The carbon and hydrogen stable isotope ratios of methane and carbon dioxide from Ellergower Moss, a raised peat bog in SW Scotland, were measured to characterize the isotopic composition of peatland gas flux that is not subject to near-surface bacterial oxidation. Two profiles of gas samples, collected in situ under pools at 1 m intervals from the surface to a depth of 5 m, gave mean isotopic signatures of δ13C(CH4) = −76±3‰ (n=22), δ13C(CO2) =3.8±5.3‰ (n=22), and δD(CH4) = −294±39‰ (n=21). The profiles revealed two isotopically distinct methane reservoirs within Ellergower Moss, with a boundary layer at 2–2.5 m. The zonation appears to be independent of methanogenic pathway and of changes in δ13C or δD of bulk substrate, but there is a positive correlation between an increase in δD(CH4) and reported increasing partial pressure of methane with depth (r= 0.943 and n = 18, >99.9% significant). To reflect C and H cycling within the peat and gas storage mechanisms, we propose the zones can be termed dynamic (0–2 m) and static (from 3–5 m). The δD(CH4) profiles exhibit the zonation and boundary most clearly, with a 52±18‰ difference between methane in the dynamic zone (−332±17‰) and that in the static zone (−260±9‰). Only if the static zone is breached, will its methane be released to the atmosphere; thus unoxidized peatland flux to the atmosphere will be similar in composition to that produced in the dynamic reservoir, where δ13C(CH4) = −76±4‰ and δD(CH4) = −332±17‰.

Annual Variation in Great Skua Diets: The Importance of Commercial Fisheries and Predation on Seabirds Revealed by Combining Dietary Analyses

Abstract This study combines conventional dietary assessment with stable isotope techniques to describe Great Skua (Stercorarius skua) diet. Stable carbon and nitrogen isotope ratios in feathers of Great Skua chicks collected over three years were analyzed in conjunction with pellet and regurgitate collections. A significant drop in trophic status was detected in 1997, which likely resulted from an increase in herring and mackerel in the diet. These items were almost certainly obtained from a commercial trawler, as evidenced by a significant drop in territorial attendance during the ships absence. Feathers yielded significantly different δ13C values among years, and part of this may have been related to a period of enhanced phytoplankton growth during 1996. A combined approach, such as the one described here, is likely to become increasingly useful in elucidating the diets of polyphagous birds. Variación Anual en la Dieta de Stercorarius skua: La Importancia de Pesqueros Comerciales y Predación de Aves Marinas Revelada por la Combinación de Análisis de Dieta Resumen. Este estudio combina evaluaciones de dieta convencionales con técnicas de isótopos estables para describir la dieta de Stercorarius skua. Analizamos la proporción de isótopos estables de carbono y nitrógeno en las plumas de pichones de S. skua colectadas durante más de tres años, en conjunto con colecciones de pellets y regurjitados. Un descenso significativo en la posición trófica fue detectado en 1997 probablemente como consecuencia del incremento de arenque y caballa en la dieta. Estos ítems fueron casi con seguridad obtenidos de un pesquero de arrastre comercial, como lo sugiere la caída significativa de la presencia en el territorio durante la ausencia del barco. Las plumas generaron valores significativamente diferentes de δ13C entre años, parte de lo cual puede haber estado relacionado con un período de gran crecimiento del fitoplancton durante 1996. Un enfoque combinado como el descripto aquí será probablemente cada vez más útil para elucidar la dieta de aves polífagas.

Does breeding site fidelity drive phenotypic and genetic sub-structuring of a population of Arctic charr?

There is now increasing acceptance that divergence of phenotypic traits, and the genetic structuring that underlie such divergence, can occur in sympatry. Here we report the serendipitous discovery of a sympatric polymorphism in the upper Forth catchment, Scotland, in a species for which high levels of phenotypic variation have been reported previously, the Arctic charr, Salvelinus alpinus. Attempting to determine the proximate mechanisms through which this pattern of phenotypic variation is maintained, we examine the use of the available feeding resource and the genotypic and phenotypic structure of charr in this system. We show clear differences in head morphology between charr from three very closely connected lakes with no barrier to movement (Lochs Doine, Voil and Lubnaig) and also differences in muscle stable isotope signatures and in stomach contents. There were significant differences at 6 microsatelite loci (between Lubnaig and the other two lochs) and very low estimates of effective migration between populations. We conclude that, despite living in effective sympatry, strong genetic and phenotypic sub-structuring is likely maintained by very high levels of site fidelity, especially during spawning, resulting in functional allopatric divergence of phenotype.

The utility of carbon and nitrogen isotope analyses to trace contributions from fish farms to the receiving communities of freshwater lakes: a pilot study in Esthwaite Water, UK

A pilot study was conducted to assess the potential for stable isotope analyses to reveal the fate of waste pelleted food material from fish farms in freshwater food webs. Esthwaite Water (Cumbria, UK) was selected as the study site, as it hosts an established salmonid farm, and a wealth of complementary limnological data exists. Salmonid pellet feed consists of primarily marine-derived material and thus exhibits carbon and nitrogen stable isotopic compositions distinct to most freshwater organic material. Comparison of the isotopic ratios of organisms at the cage site with an unaffected control site, supports incorporation of pellet-derived material to the diet of planktonic and benthic communities. Moreover, after allowing for a number of trophic transfers, stable isotope analyses revealed the predatory cladoceran Leptodora kindti also utilised pellet material, while roach were probably short-circuiting the food chain by directly consuming particulate pellet material, as well as via ingestion of their zooplankton prey. Isotope data substituted into a simple two-source mixing model suggested that approximately 65% of Daphnia, and >80% of roach body carbon may be derived from pellet material in the plankton, and that chironomid larvae may incorporate >50% in the sediment environs. However, contributions calculated from both δ13C and δ15N values were inconsistent, which may simply be due to the constraints of the model and parameters used, but may also reflect different routing of isotopes from the original pellet source, via soluble or particulate routes.