Dietary and isotopic specialization: the isotopic niche of three <Emphasis Type="Italic">Cinclodes</Emphasis> ovenbirds

By comparing the isotopic composition of tissues deposited at different times, we can identify individuals that shift diets over time and individuals with constant diets. We define an individual as an isotopic specialist if tissues deposited at different times have similar isotopic composition. If tissues deposited at different times differ in isotopic composition we define an individual as an isotopic generalist. Individuals can be dietary generalists but isotopic specialists if they feed on the same resource mixture at all times. We assessed the degree of isotopic and dietary specialization in three related Chilean bird species that occupy coastal and/or freshwater environments: Cinclodes oustaleti, Cinclodes patagonicus, and Cinclodes nigrofumosus. C. oustaleti individuals were both isotopic and dietary generalists. Tissues deposited in winter (liver and muscle) had distinct stable C (δ¹³C) and stable N isotope ratio (δ¹⁵N) values from tissues deposited in the summer (wing feathers) suggesting that birds changed the resources that they used seasonally from freshwater habitats in the summer to coastal habitats in the winter. Although the magnitude of seasonal isotopic change was high, the direction of isotopic change varied little among individuals. C. patagonicus included both isotopic specialists and generalists, as well as dietary specialists and generalists. The isotopic composition of the feathers and liver of some C. patagonicus individuals was similar, whereas that of others differed. In C. patagonicus, there were large inter-individual differences in the magnitude and the direction of seasonal isotopic change. All individuals of C. nigrofumosus were both isotopic and dietary specialists. The distribution of δ¹³C and δ¹⁵N values overlapped broadly among tissues and clustered in a small, and distinctly intertidal, region of δ space. Assessing individual specialization and unraveling the factors that influence it, have been key questions in animal ecology for decades. Stable isotope analyses of several tissues in appropriate study systems provide an unparalleled opportunity to answer them.     

Natural selection influences AFLP intraspecific genetic variability and introgression patterns in Atlantic eels

Investigating patterns of genetic variation in hybridizing species provides an opportunity to understand the impact of natural selection on intraspecific genetic variability and interspecific gene exchange. The Atlantic eels Anguilla rostrata and A. anguilla each occupy a large heterogeneous habitat upon which natural selection could differentially shape genetic variation. They also produce viable hybrids only found in Iceland. However, the possible footprint of natural selection on patterns of genetic variation within species and introgressive hybridization in Icelandic eels has never been assessed. We revisited amplified fragment length polymorphism data collected previously using population genomics and admixture analyses to test if (i) genetic variation could be influenced by non-neutral mechanisms at both the intra- and interspecific levels, and if (ii) selection could shape the spatio-temporal distribution of Icelandic hybrids. We first found candidate loci for directional selection within both species. Spatial distributions of allelic frequencies displayed by some of these loci were possibly related with the geographical patterns of life-history traits in A. rostrata , and could have been shaped by natural selection associated with an environmental gradient along European coasts in A. anguilla . Second, we identified outlier loci at the interspecific level. Non-neutral introgression was strongly suggested for some of these loci. We detected a locus at which typical A. rostrata allele hardly crossed the species genetic barrier, whereas three other loci showed accelerated patterns of introgression into A. anguilla in Iceland. Moreover, the level of introgression at these three loci increased from the glass eel to the yellow eel stage, supporting the hypothesis that differential survival of admixed genotypes partly explains the spatio-temporal pattern of hybrid abundance previously documented in Iceland.     

Trophic structure of common marine species in the Bohai Strait,North China Sea,based on carbon and nitrogen stable isotope ratios

Trophic relationships among common coastal species in the Bohai Strait, North China Sea, were investigated in this study using stable isotope ratios of carbon (¹³C/¹²C) and nitrogen (¹⁵N/¹⁴N) as tracers. During the research cruises of this study, 16 species of invertebrates and 18 species of fishes were collected, as were various food sources (phytoplankton, macroalgae, and sediment). The carbon and nitrogen isotope ratios of all collected samples were measured and used for trophic level analyses. The results showed that the grazers Oregonia gracilis, Notoacmea schrenckii, Chlorostoma rustica, and Chelon affinis, preferentially consumed Zostera marina (p < 0.01), whereas Anthocidaris crassispina preferred Ulva conglobata (p < 0.01). Two trophic models based on nitrogen isotope ratios were built to identify the trophic level of each species. The model that combined food sources was more appropriate than the model that used a single primary producer to identify the relative trophic positions of primary consumers. Scombermorus niphonius, an important fishery resource, was at the top trophic level. The seastar Asterias amurensis was at the highest level among the invertebrates and directly threatened production of shellfish. Based on the trophic level and food source relationships identified in this study, we gave some advice for ecological restoration, such as optimizing the structure of food source distribution, limiting seastar numbers, and improving the applicability of habitat for fishery species.     

Influence of timing of fishing on trophic levels and diets of typical fish and invertebrate species in the Bohai Strait over a single year based on carbon and nitrogen isotope analysis

Fishing is the most widespread human exploitation of marine resources, which has an annual cyclical influence on aquatic species in Chinese offshore waters. This study used carbon and nitrogen isotopic ratios as tracers to reveal the changes in trophic level and dietary composition of offshore organisms during four cruises in March, June, August and November 2014. The results indicated that the trophic levels of fishes declined during two fishing periods, from March (average trophic level = 3.36) to June (3.01), and from August (2.99) to November (2.57), while most invertebrates did not show this trend. The self-restoring ability of this ecosystem was reflected in the trophic level changes after the closed fishing season (from June 1 to September 1). The trophic levels of fishes remained stable, and some species even recovered such as Enchelyopus elongates (trophic level increased from 2.84 in June to 2.86 in August), Cryptocentrus filifer (from 3.10 to 3.12), and Ernogrammus hexagrammus (from 2.91 to 2.96). According to the trophic results, we selected the invertebrates Octopus minor and Asterias amurensis from the top trophic levels for dietary composition analysis. The composition of their diets changed significantly after fishing periods, and the proportions of some smaller and “non-commercial” species increased, such as Notoacmea schrenckii and Chlorostoma rustica. After the closed fishing season, the larger and “commercial” species contributed a greater proportion to their diet composition. These results indicated that the closed fishing season should be prolonged to give the ecosystem enough time to restore itself and further halt the trend of this fishery towards environment deterioration.     

The diet composition of immature loggerheads: Insights on trophic niche, growth rates, and fisheries interactions

For immature animals, diet quality and composition influence expression of life history traits such as growth rates and ultimately life stage duration and age to maturity. Circumglobally distributed loggerhead turtles (Caretta caretta) exhibit a multi-decade immature stage that generally occupies neritic habitats and is characterized by slow growth and an omnivorous diet. Although adult nesting populations are geographically distinct, foraging areas for immature loggerheads show a high degree of mixing of individuals that originate from multiple nesting stocks. Furthermore, despite their generalist foraging ecology, immature loggerheads have been observed to supplement their natural diets with fish from fishery discards and/or caught in fishing gear. However, whether trophic opportunism results in variation in loggerhead growth rates within or among feeding areas has not been investigated. In Core Sound, North Carolina (NC), USA, immature loggerheads demonstrate highly variable size-specific growth rates, in contrast to other studies that report discernible somatic growth functions in immature marine turtles. To determine whether inter-individual variation in growth rates at this site was due to variation in diet composition, and specifically variation in consumption of fish, we analyzed carbon and nitrogen stable isotope ratios of loggerhead blood plasma and of tissue samples of putative loggerhead prey, as well as commercially important fish species. Our results indicated that growth rates were not related to trophic levels at which individual turtles fed, but rather probably reflected inter-individual variation in overwintering or foraging behavior (i.e. nearshore vs. offshore). Furthermore, loggerhead diets were highly diverse, and comprised mainly blue crabs and/or whelks, as well as small proportions of cannonball jellies. Fish were unimportant dietary components for loggerheads. Although loggerheads in NC do not appear to feed on fish catch or discards, immature turtles showed dietary preferences for prey items that are also valuable to or are commonly taken as bycatch in commercial fisheries (e.g. blue crabs and whelks, respectively) in the region. Thus, the status of these prey items/fishery stocks as well as trends in loggerhead populations should be monitored to mitigate potential competitive interactions between fisheries activities and loggerhead turtles.     

Differential effect of inter- and intraspecific competition on the performance of invasive and native Taraxacum species

Inter- and intraspecific competitive abilities are significant determinants of invasive success and the ecological impact of non-native plants. We tested two major hypotheses on the competitive ability of invasive species using invasive (Taraxacum officinale) and native (T. platycarpum) dandelions: differential interspecific competitive ability between invasive and native species and the kin recognition of invasive species. We collected seeds from two field sites where the two dandelion species occurred nearby. Plants were grown alone, with kin (plants from the same maternal genotype) or strangers (plants from different populations) of the same species, or with different species in a growth chamber, and the performance at the early developmental stage between species and treatments was compared. The invasive dandelions outcompeted the native dandelions when competing against each other, although no difference between species was detected without competition or with intraspecific competition. Populations of native species responded to interspecific competition differently. The effect of kinship on plant performance differed between the tested populations in both species. A population produced more biomass than the other populations when grown with a stranger, and this trend was manifested more in native species. Our results support the hypothesis that invasive plants have better competitive ability than native plants, which potentially contributes to the establishment and the range expansion of T. officinale in the introduced range. Although kin recognition is expected to evolve in invasive species, the competitive ability of populations rather than kinship seems to affect plant growth of invasive T. officinale under intraspecific competition.     

The influence of productivity and width of littoral zone on the trophic position of a large-bodied omnivore

Omnivory is common in many food webs. Omnivores in different habitats can potentially change their feeding behaviour and alter their trophic position and role according to habitat conditions. Here we examine the trophic level and diet of the omnivorous signal crayfish (Pacifastacus leniusculus) in gradients of trophic status and lake size, both of which have been previously suggested to affect trophic position of predators separately or combined as productive space. We found the trophic position of omnivorous crayfish to be positively correlated with lake trophic status, but found no evidence for any influence of lake size or productive space on crayfish trophic position. The higher trophic position of crayfish in eutrophic lakes was largely caused by a shift in crayfish diet and not by an increase in trophic links in basal parts of the food web. Hence, our results support the “productivity hypothesis,” suggesting that food chains can be longer in more productive systems. Furthermore, stable isotope data indicated that larger crayfish are more predatory than smaller crayfish in lakes with wider littoral zones. Wider littoral zones promoted the development of intrapopulation differences in trophic position whereas narrow littoral zones did not. Hence, differences in habitat quality between and within lakes seem to influence the trophic positions of omnivorous crayfish. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Temporal and spatial variability of the oxygen isotopic composition of massive corals from the South China Sea: Influence of the Asian monsoon

Coral colonies of Porites lutea and P. lobata were collected from areas to the east and south offshore of Hainan Island, South China Sea. Oxygen isotope ratios, ¹⁸O/¹⁶O, were analyzed along the major axis of growth at a resolution of 25–28 samples within an annual band from three colonies, giving data series lengths of 15, 20 and 30 years. Coral δ¹⁸O correlates well with sea surface temperature (SST) throughout most years, particularly when the Asian winter monsoon prevails over the region. Due to the same seasonal phases of SST and sea surface salinity (SSS), it is estimated that most of the seasonal features of coral δ¹⁸O are controlled by changes in SST with a significant contribution of SSS. This indicates the importance of temperature, as determined by the Asian winter monsoon, as a control on seasonal coral δ¹⁸O variations in the South China Sea. Given the fact that annual SSS maxima show minor inter-annual changes, the inter-annual change of annual coral δ¹⁸O maximum mainly reflects the SST induced by the winter monsoon. While the coral δ¹⁸O is mainly controlled by SST, deviations of the δ¹⁸O from SST correlate statistically with sea surface salinity, particularly for summer extreme events when SST reaches its annual maximum with minor inter-annual change. Therefore, we suggest that the inter-annual variation of the annual coral δ¹⁸O minima is mainly controlled by seawater composition. Seawater composition analysis indicates that its δ¹⁸O and salinity are simultaneously affected by freshwater inputs primarily from precipitation, which is mainly supplied by the Asian summer monsoon. Thus seasonal alternations of the winter and summer monsoon mainly control the coral δ¹⁸O, thereby allowing reconstruction of these monsoonal variables. The three coral records consistently reveal clear inter-decadal trends in δ¹⁸O — a gradual increase from 1968 to 1987, and a subsequent decrease from 1987 to 2003. These inter-decadal trends are roughly consistent with salinity changes — but not with temperature and precipitation, suggesting that seawater composition controlled by freshwater inputs from distant source areas, rather than from local precipitation, is responsible for the trends in seawater composition and coral δ¹⁸O over the last 30 years. Comparison of the three coral records indicates that spatial variations in coral δ¹⁸O coincide with variations in seawater salinity and local precipitation, but not with temperature. This confirms the dominant role of seawater composition, mainly controlled by monsoonal precipitation, on the spatial variability of coral δ¹⁸O in the South China Sea.     

Cyclicity and variability in prey dynamics strengthens predator numerical response: the effects of vole fluctuations on white stork productivity

Theory predicts that optimality of life-long investment in reproduction is, among other factors, driven by the variability and predictability of the resources. Similarly, during the breeding season, single resource pulses characterized by short periods and high amplitudes enable strong numerical responses in their consumers. However, it is less well established how spatio-temporal dynamics in resource supplies influence the spatio-temporal variation of consumer reproduction. We used the common vole (Microtus arvalis)—white stork (Ciconia ciconia) resource—consumer model system to test the effect of increased temporal variation and periodicity of vole population dynamics on the strength of the local numerical response of storks. We estimated variability, cycle amplitude, and periodicity (by means of direct and delayed density dependence) in 13 Czech and Polish vole populations. Cross-correlation between annual stork productivity and vole abundance, characterizing the strength of the local numerical response of storks, increased when the vole population fluctuated more and population cycles were shorter. We further show that the onset of incubation of storks was delayed during the years of higher vole abundance. We demonstrate that high reproductive flexibility of a generalist consumer in tracking the temporal dynamics of its resource is driven by the properties of the local resource dynamics and we discuss possible mechanisms behind these patterns.     

Temporal and spatial partitioning of water resources among eight woody species in a Hawaiian dry forest

Lowland dry forests are unique in Hawaii for their high diversity of tree species compared with wet forests. We characterized spatial and temporal partitioning of soil water resources among seven indigenous and one invasive dry forest species to determine whether the degree of partitioning was consistent with the relatively high species richness in these forests. Patterns of water utilization were inferred from stable hydrogen isotope ratios (δD) of soil and xylem water, zones of soil water depletion, plant water status, leaf phenology, and spatial patterns of species distribution. Soil water δD values ranged from –20‰ near the surface to –48‰ at 130 cm depth. Metrosideros polymorpha, an evergreen species, and Reynoldsia sandwicensis, a drought-deciduous species, had xylem sap δD values of about –52‰, and appeared to obtain their water largely from deeper soil layers. The remaining six species had xylem δD values ranging from –33 to –42‰, and apparently obtained water from shallower soil layers. Xylem water δD values were negatively correlated with minimum annual leaf water potential and positively correlated with leaf solute content, an integrated measure of leaf water deficit. Seasonal patterns of leaf production ranged from dry season deciduous at one extreme to evergreen with near constant leaf expansion rates at the other. Species tapping water more actively from deeper soil layers tended to exhibit larger seasonality of leaf production than species relying on shallower soil water sources. Individuals of Myoporum sandwicense were more spatially isolated than would be expected by chance. Even though this species apparently extracted water primarily from shallow soil layers, as indicated by its xylem δD values, its nearly constant growth rates across all seasons may have been the result of a larger volume of soil water available per individual. The two dominant species, Diospyros sandwicensis and Nestegis sandwicensis, exhibited low leaf water potentials during the dry season and apparently drew water mostly from the upper portion of the soil profile, which may have allowed them to exploit light precipitation events more effectively than the more deeply rooted species. Character displacement in spatial and temporal patterns of soil water uptake was consistent with the relatively high diversity of woody species in Hawaiian dry forests. Received: 20 May 1999 / Accepted: 2 March 2000     

Stable isotopic variations in modern herbivore tooth enamel, plants and water on the Tibetan Plateau: Implications for paleoclimate and paleoelevation reconstructions

Stable isotope analyses of fossil teeth and other authigenic minerals have been used to reconstruct the paleoenvironment and paleoelevation of the Tibetan Plateau. The accuracy of such reconstructions is limited by the lack of a comprehensive modern comparative database from the region. We analyzed the carbon and oxygen isotopic compositions (δ¹³C and δ¹⁸O values) of tooth enamel from modern herbivores, the δ¹³C values of grasses and the δ¹⁸O values of water samples collected from various elevations within the Tibetan Plateau to examine their relationships with modern environment/elevation. The δ¹³C values of enamel samples from horses, yaks and goats display a narrow range of variation, with a mean of − 10.7 ± 1.4‰ (n = 301), indicating that these modern herbivores were feeding predominantly on C3 plants, consistent with the current dominance of C3 vegetation in the region. Some of the samples have δ¹³C values between − 7.3 and − 10‰. Although these higher δ¹³C values could suggest consumption of some C4 plants by the animals, the lack of significant seasonal δ¹³C variations within individual teeth indicates that these higher enamel δ¹³C values are due to consumption of C3 plants experiencing water stress and/or some CAM plants rather than C4 plants. Our data show that the conservative “cut-off” δ¹³C value for a pure C3 diet within the Tibetan Plateau should be − 8‰ for modern herbivores and − 7‰ (or even − 6.5‰) for fossils if the region was as arid in the past as today. In contrast to the small intra-tooth δ¹³C variations within individual teeth, serial enamel samples display large intra-tooth δ¹⁸O variations, reflecting seasonal variations in the δ¹⁸O of meteoric water. The mean δ¹⁸O values of tooth enamel from yaks and horses show a strong correlation with water δ¹⁸O values, confirming that the δ¹⁸O of tooth enamel from obligate drinker generally tracks the δ¹⁸O of meteoric water. Unfortunately, elevation alone cannot explain most of the variance in the δ¹⁸O of precipitation and tooth enamel, suggesting that quantitative reconstruction of the paleoelevation of the Tibetan Plateau using re-constructed δ¹⁸O values of paleo-meteoric water from fossil enamel or other oxygen-bearing minerals is not warranted. For a given environment, horses have the lowest enamel–δ¹⁸O values while goats display the highest enamel–δ¹⁸O values among the species studied. The large inter-species δ¹⁸O variations are likely due to differences in physiology and diet/drinking behavior of the animals. This underscores the importance of species-specific studies when interpreting δ¹⁸O data of fossil mammalian teeth in a stratigraphic sequence as a record of paleoclimate changes.     

Rethinking the nature of intraspecific variability and its consequences on species coexistence

Intraspecific variability (IV) has been proposed to explain species coexistence in diverse communities. Assuming, sometimes implicitly, that conspecific individuals can perform differently in the same environment and that IV increases niche overlap, previous studies have found contrasting results regarding the effect of IV on species coexistence. We aim at showing that the large IV observed in data does not mean that conspecific individuals are necessarily different in their response to the environment and that the role of high‐dimensional environmental variation in determining IV has largely remained unexplored in forest plant communities. We first used a simulation experiment where an individual attribute is derived from a high‐dimensional model, representing “perfect knowledge” of individual response to the environment, to illustrate how large observed IV can result from “imperfect knowledge” of the environment. Second, using growth data from clonal Eucalyptus plantations in Brazil, we estimated a major contribution of the environment in determining individual growth. Third, using tree growth data from long‐term tropical forest inventories in French Guiana, Panama and India, we showed that tree growth in tropical forests is structured spatially and that despite a large observed IV at the population level, conspecific individuals perform more similarly locally than compared with heterospecific individuals. As the number of environmental dimensions that are well quantified at fine scale is generally lower than the actual number of dimensions influencing individual attributes, a great part of observed IV might be represented as random variation across individuals when in fact it is environmentally driven. This mis‐representation has important consequences for inference about community dynamics. We emphasize that observed IV does not necessarily impact species coexistence per se but can reveal species response to high‐dimensional environment, which is consistent with niche theory and the observation of the many differences between species in nature.     

The effect of FISH and CARD-FISH on the isotopic composition of C- and N-labeled Pseudomonas putida cells measured by nanoSIMS

The use of nanoSIMS for the exploration of microbial activities in natural habitats often implies that stable isotope tracer experiments are combined with in situ hybridization techniques (i.e. fluorescence in situ hybridization (FISH) or catalyzed reporter deposition (CARD)-FISH). In this study, Pseudomonas putida grown on ¹³C- and ¹⁵N-labeled carbon and nitrogen, collected in exponential growth and stationary phases, was hybridized and analyzed by nanoSIMS. It was shown that ¹³C and ¹⁵N fractions decreased after FISH and CARD-FISH in comparison to chemically untreated cells. However, the fractions were influenced differently by various treatments. After paraformaldehyde fixation of exponentially growing cells, a reduction of the ¹³C and ¹⁵N fractions was measured from 94 ± 1.2% and 89.5 ± 3.8% to 90.2 ± 0.8% and 64 ± 4.6%, respectively, indicating that nitrogen isotopic composition was most influenced. A further decrease of the ¹³C and ¹⁵N fractions to 80.7 ± 6.5 and 59.5 ± 4.1%, respectively, was measured after FISH, while CARD-FISH decreased the fractions to 57.4 ± 3.0% and 47.1 ± 4.1%, respectively. The analysis of cells collected in different growth phases revealed that the effect of various treatments seemed to be dependent on the cell's physiological state. In addition, a mathematical model that can be used in further studies was developed in order to calculate the amount of carbon introduced into the cells by chemical treatments. These results can be valuable for environmental FISH-nanoSIMS studies where the isotopic composition of single cells will be used to quantitatively assess the importance of specific populations to certain biochemical processes and determine budget estimations.     

Evidence of stenotopy and anthropogenic influence on carbon source for two major riverine fishes of the Lake Victoria watershed

Stable isotope analysis was used to investigate the migratory status and to determine the relative contribution of allochthonous and autochthonous sources of carbon for the major riverine fish species ( Barbus altianalis and Labeo victorianus ) in rivers draining the Kenyan side of Lake Victoria. The two fish species derived carbon from both C4 and C3 plant sources, although L. victorianus exhibited less enriched isotopic carbon values. Fish samples from stations under direct influence of effluents from sugar factories exhibited enriched δ¹³C signals. Assuming that this reflects carbon sourcing from riparian C4 plants, it suggests that carbon from terrestrial sources can be a major energy source in some rivers. This heavy carbon enrichment associated with sugar factories was spatially restricted and occurred in all seasons, implying that sub-populations of the two fish species are non-migratory. The large migratory populations of these two species, for which Lake Victoria was once famous, may be no more.     

Niche differentiation among invasive crayfish and their impacts on ecosystem structure and functioning

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Diet overlap among two Asian carp and three native fishes in backwater lakes on the Illinois and Mississippi rivers

Bighead and silver carp are well established in the Mississippi River basin following their accidental introduction in the 1980s. Referred to collectively as Asian carp, these species are filter feeders consuming phytoplankton and zooplankton. We examined diet overlap and electivity of Asian carp and three native filter feeding fishes, bigmouth buffalo, gizzard shad, and paddlefish, in backwater lakes of the Illinois and Mississippi rivers. Rotifers, Keratella spp., Brachionus spp., and Trichocerca spp., were the most common prey items consumed by Asian carp and gizzard shad, whereas crustacean zooplankton were the preferred prey of paddlefish. Bigmouth buffalo diet was broad, including both rotifers and crustacean zooplankton. Dietary overlap with Asian carp was greatest for gizzard shad followed by bigmouth buffalo, but we found little diet overlap for paddlefish. Diet similarity based on taxonomy correlated strongly with diet similarity based on size suggesting filtration efficiency influenced the overlap patterns we observed. Although rotifers were the most common prey item consumed by both bighead and silver carp, we found a negative relation between silver carp CPUE and cladoceran density. The competitive effect of Asian carp on native fishes may be forestalled because of the high productivity of Illinois and Mississippi river habitats, yet the potential for negative consequences of Asian carp in less productive ecosystems, including Lake Michigan, should not be underestimated.     

Seasonal and environmental influences on recruitment patterns and habitat usage among resident and transient fishes in a World Heritage Site subtropical estuary

This study investigated whether the fish communities inhabiting shallow non‐vegetated habitats in two divergent bays in a subtropical World Heritage Site estuarine system differed according to wet (spring–summer) and dry (autumn–winter) seasons or polyhaline and mesohaline zones, within the broader objective of facilitating spatio‐temporal management. Species richness (total of 74 taxa; total length, L_T = 11–552 mm) and abundance (51 109 individuals) were mostly greater in the wet than dry season and in polyhaline than mesohaline areas. There was a major effect of rainfall on recruitment, particularly among transient fishes, which could be the result of enhanced survival of young via greater productivity (food resources) and protection from predators (via turbidity reducing visual cues). Salinity had strong interactive effects with rainfall and temperature in one bay, with greater species richness and overall abundances as well as large abundances of four key species [Anchoa januaria and Atherinella brasiliensis (pelagic residents), Cetengraulis edentulus (pelagic transient) and Diapterus rhombeus (demersal transient)] during the wet season in polyhaline areas; possibly reflecting a biodiversity hotspot that might be affected by distance to the estuary mouth and convergence hydrology. Regionally, the results support enforcing spatio‐temporal restrictions to minimize anthropogenic activities within statutory (but not always enforced) protected areas. Globally, the data reiterate the need to identify and understand biotic and abiotic effects on estuarine ichthyofaunal distributions and abundances as a precursor to their management.     

Biological and physical influences on the carbon isotope content of CO<Subscript>2</Subscript> in a subalpine forest snowpack,Niwot Ridge,Colorado

Considerable research has recently been devoted to understanding biogeochemical processes under winter snow cover, leading to enhanced appreciation of the importance of many winter ecological processes. In this study, a comprehensive investigation of the stable carbon isotope composition (δ¹³C) of CO₂ within a high-elevation subalpine forest snowpack was conducted. Our goals were to study the δ¹³C of biological soil respiration under snow in winter, and to assess the relative importance of diffusion and advection (ventilation by wind) for gas transport within snow. In agreement with other studies, we found evidence of an active microbial community under a roughly 1-m deep snowpack during winter and into spring as it melted. Under-snow CO₂ mole fractions were observed up to 3,500 μmol mol⁻¹, and δ¹³C of CO₂ varied from ~−22 to ~−8‰. The δ¹³C of soil respiration calculated from mixing relationships was −26 to −24‰, and although it varied in time, it was generally close to that of the bulk organic horizon (−26.0‰). Subnivean CO₂ and δ¹³C were quite dynamic in response to changes in soil temperature, liquid water availability, and wind events. No clear biologically-induced isotopic changes were observed during periods when microbial activity and root/rhizosphere activity were expected to vary, although such changes cannot be eliminated. There was clear evidence of isotopic enrichment associated with diffusive transport as predicted by theory, but simple diffusive enrichment (4.4‰) was not observed. Instead, ventilation of the snowpack by sustained wind events in the forest canopy led to changes in the diffusively-enriched gas profile. The isotopic influence of diffusion on gases in the snowpack and litter was greatest at greater depths, due to the decreased relative contribution of advection at depth. There were highly significant correlations between the apparent isotopic content of respiration from the soil with wind speed and pressure. In summary, physical factors influencing gas transport substantially modified and potentially obscured biological factors in their effects on δ¹³C of CO₂ within this subalpine forest snowpack.     

Biosynthetic and environmental effects on the stable carbon isotopic compositions of anteiso- (3-methyl) and iso- (2-methyl) alkanes in tobacco leaves

Nicotiana tabacum is the only plant known to synthesise large quantities of anteiso- (3-methyl) alkanes and iso- (2-methyl) alkanes. We investigated the carbon isotope ratios of individual long-chain n-alkanes, anteiso- and iso-alkanes (in the C₂₉-C₃₃ carbon number range) extracted from tobacco grown in chambers under controlled conditions to confirm the pathway used by the tobacco plant to synthesise these particular lipids and to examine whether environmental data are recorded in these compounds. Tobacco was grown under differing temperatures, water availabilities and light intensities in order to control its stable carbon isotope ratios and evaluate isotopic fractionations associated with the synthesis of these particular lipids. The anteiso-alkanes were found to have a predominant even-carbon number distribution (maximising at C₃₂), whereas the iso-alkanes exhibit an odd-carbon number distribution (maximising at C₃₁). Iso-alkanes were relatively more abundant than the anteiso-alkanes and only two anteiso-alkanes (C₃₀ and C₃₂) were observed.The anteiso-alkanes and iso-alkanes were found to be enriched in ¹³C by 2.8-4.3‰ and 0-1.8‰ compared to the n-alkanes, respectively, consistent with different biosynthetic precursors. The assumed precursor for the odd-carbon-numbered iso-alkanes is iso-butyryl-CoA (a C₄ unit derived from valine) followed by subsequent elongation of C₂ units and then decarboxylation. The assumed precursor for even-carbon-numbered anteiso-alkanes is α-methylbutyryl-CoA (a C₅ unit derived from isoleucine) and subsequent elongation by C₂ units followed by decarboxylation. The ratio of carbon atoms derived from α-methylbutyryl-CoA and subsequent C₂ units (from malonyl-CoA) is 1:5 for the biosynthesis of a C₃₀anteiso-alkane. The ratio of carbon atoms derived from iso-butyryl-CoA and subsequent C₂ units (from malonyl-CoA) is 4:25 for the synthesis of a C₂₉iso-alkane. An order of ¹³C depletion n-alkanes > iso-alkanes > anteiso-alkanes is evident from compound specific isotope data. This trend can probably be attributed to the ratio of the two different sources of carbon atoms in the final wax components.Higher water availability generally results in more depleted stable carbon isotope ratios due to maximised discrimination during carboxylation, associated with less diffusional limitation. This was confirmed in the present study by compound specific isotope analyses of iso-alkanes, anteiso-alkanes and n-alkane lipids extracted from the tobacco leaves. Likewise, light intensity has been shown to influence plant bulk δ¹³C in previous studies. The carbon isotope ratios of n-alkanes in tobacco grown under low-light conditions were about 2‰ more depleted in ¹³C than those of lipids extracted from tobacco grown under elevated light conditions. A similar order of difference is observed for the iso-alkanes and anteiso-alkanes (1.8‰ and 1.9‰, respectively). A negligible depletion in carbon isotope ratios was observed for the iso-alkanes and anteiso-alkanes extracted from tobacco grown under elevated temperatures. These results are consistent with the work of Farquhar [Farquhar, G.D., 1980. Carbon isotope discrimination by plants: effects of carbon dioxide concentration and temperature via the ratio of intercellular and atmospheric CO₂ concentrations. In: Pearman, G.I. (Ed.), Carbon Dioxide and Climate: Australian Research. Springer, Berlin, pp. 105-110] where temperature appears to have only a minor effect on plant bulk δ¹³C.