Stable isotopes are increasingly being used to trace wildlife movements. A fundamental prerequisite of animal isotopic tracking is a good knowledge of spatial isotopic variations in the environment. Few accessible reference maps of the isotopic landscape ("isoscapes") are available for marine predators. Here, we validate for the first time an isotopic gradient for higher trophic levels by using a unique combination of a large number of satellite-tracks and subsequent blood plasma isotopic signatures from a wide-ranging oceanic predator. The plasma δ(13)C and δ(15)N values of wandering albatrosses (n = 45) were highly and positively correlated to the Southern Ocean latitudes at which the satellite-tracked individuals foraged. The well-defined latitudinal baseline carbon isoscapes in the Southern Ocean is thus reflected in the tissue of consumers, but with a positive shift due to the cumulative effect of a slight (13)C-enrichment at each trophic level. The data allowed us to estimate the carbon isotopic position of the main oceanic fronts in the area, and thus to delineate robust isoscapes of the main foraging zones for top predators. The plasma δ(13)C and δ(15)N values were positively and linearly correlated, thus suggesting that latitudinal isoscapes also occur for δ(15)N at the base of the food web in oceanic waters of the Southern Ocean. The combination of device deployments with sampling of relevant tissues for isotopic analysis appears to be a powerful tool for investigating consumers' isoscapes at various spatio-temporal scales.

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The movement and dietary history of individuals can be studied using stable isotope records in archival keratinous tissues. Here, we present a chronology of temporally fine-scale data on the trophic niche of otariid seals by measuring the isotopic signature of serially sampled whiskers. Whiskers of male Antarctic fur seals breeding at the Crozet Islands showed synchronous and regular oscillations in both their δ 13 C and δ 15 N values that are likely to represent their annual migrations over the long term (mean 4.8 years). At the population level, male Antarctic fur seals showed substantial variation in both δ 13 C and δ 15 N values, occupying nearly all the ‘isotopic space’ created by the diversity of potential oceanic habitats (from high Antarctica to the subtropics) and prey (from Antarctic krill to subantarctic and subtropical mesopelagic fishes). At the individual level, whisker isotopic signatures depict a large diversity of foraging strategies. Some seals remained in either subantarctic or Antarctic waters, while the migratory cycle of most animals encompassed a wide latitudinal gradient where they fed on different prey. The isotopic signature of whiskers, therefore, revealed new multi-year foraging strategies of male Antarctic fur seals and is a powerful tool for investigating the ecological niche during cryptic stages of mammals' life.

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We report the trophic structure of a myctophid assemblage by measuring the isotopic niches of 14 species living in Kerguelen waters, southern Indian Ocean. Most of the species show distinct isotopic niches that differ by at least one of the two niche axes (d 13 C habitat and d 15 N trophic position), indicating trophic partitioning within the assemblage. Strong niche segregation occurs within each of the three most common genera of myctophids (Electrona, Gymnoscopelus, and Protomyctophum), illustrating the different mechanisms (habitat and dietary segregation) that allow coexistence of closely related species. Calculated trophic levels (TLs) of myctophids ranged from 3.3 to 4.2, showing that they are secondary and tertiary consumers in the pelagic ecosystem. The positive relationship between TL and standard length of fish points out a structuring effect of size, with larger species (Gymnoscopelus spp.) occupying a higher trophic position than smaller species (Krefftichthys anderssoni and Protomyctophum spp.). Myctophids occupy an intermediate trophic position between macrozooplanktonic crustaceans and seabirds and marine mammals within the pelagic ecosystem. However, the TLs of large myctophids overlap those of crustacean-eating seabirds [e.g., Eudyptes spp. (crested penguins) and Pachyptila belcheri ]. The isotopic niche of myctophids indicates that Aptenodytes patagonicus (king penguin) adults prey upon K. anderssoni when they feed for themselves, thus exemplifying the usefulness of isotopic datasets on potential prey of predators to depict trophic relationships.

BackgroundIndividual variations in the use of the species niche are an important component of diversity in trophic interactions. A challenge in testing consistency of individual foraging strategy is the repeated collection of information on the same individuals.Methodology/Principal FindingsThe foraging strategies of sympatric fur seals (Arctocephalus gazella and A. tropicalis) were examined using the stable isotope signature of serially sampled whiskers. Most whiskers exhibited synchronous δ13C and δ15N oscillations that correspond to the seal annual movements over the long term (up to 8 years). δ13C and δ15N values were spread over large ranges, with differences between species, sexes and individuals. The main segregating mechanism operates at the spatial scale. Most seals favored foraging in subantarctic waters (where the Crozet Islands are located) where they fed on myctophids. However, A. gazella dispersed in the Antarctic Zone and A. tropicalis more in the subtropics. Gender differences in annual time budget shape the seal movements. Males that do not perform any parental care exhibited large isotopic oscillations reflecting broad annual migrations, while isotopic values of females confined to a limited foraging range during lactation exhibited smaller changes. Limited inter-individual isotopic variations occurred in female seals and in male A. tropicalis. In contrast, male A. gazella showed large inter-individual variations, with some males migrating repeatedly to high-Antarctic waters where they fed on krill, thus meaning that individual specialization occurred over years.Conclusions/SignificanceWhisker isotopic signature yields unique long-term information on individual behaviour that integrates the spatial, trophic and temporal dimensions of the ecological niche. The method allows depicting the entire realized niche of the species, including some of its less well-known components such as age-, sex-, individual- and migration-related changes. It highlights intrapopulation heterogeneity in foraging strategies that could have important implications for likely demographic responses to environmental variability.

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The response of the photosynthetic capacity (PmaJ of microphytobenthos to short-term variations of temperature (in the range 5-35' C) was assessed on a seasonal basis. The relationship is described mathematically, and relevant physiologzcal parameters are identi3ed: PMAx, the maximum value of P,n,, achieved at T+l, the optimum temperature. Estimated values of TOPI do not change significant4 throughout the year and remain close to 25" C. It is thus concluded that TOPI is not influenced @ seasonal variations in the daily range of mud surface temperature. Identical conclusions hold for T,,, (ca. 38' C), the thermal threshold beyond which no photosynthesis occurs. Conversely, PMx estimates exhibit substantial variability: PMAx (mean _t root mean square error) is highest in April (1 1.18 i 0.42 p g C. during the beganning of the annual increase in temperature, photoperiod, and maximum imadiance and is lowest in December (3.04 _f 0.16 p g C.[pg Chl a]-'.h-'). From a n ecological point of view, the short-term and seasonal variations of PMAx suggest that the microphytobenthic community takes advantage of the abiotic spring environmental conditions, allowing the onset of the bloom. Nevertheless, no "acclimation strategy" (i.e. shifts in T,, and T,,,, that prevent temperature inhibition in summer or improve photosynthetic rates in winter) is apiarent from our results. Xey index words: intertidal mudJat; microphytobenthos; photosynthetic capacity; seasonal effect; temperature Benthic microalgae inhabiting the surface mud of intertidal flats experience daily large temperature variations during low tides, due to heating of the mud by insolation (Harrison 1985, Harrison and Phizacklea 1985, 1987. Such short-term changes in temperature have a strong impact on microphytobenthic photosynthesis, particularly on the photosynthetic capacity (the so-called Pm,L,, the photosynthetic rate at saturating light levels), because it is temperature dependent. This effect has been described mathematically (Blanchard ' 723 et al. 1996) in order to summarize, with a small set of physiological parameters, the response of microphytobenthic P,,z, to short-term changes in temperature: P,,, increases exponentially up to a maximum value (PMAX) reached at the optimum temperature (Tql), beyond which it declines rapidly to zero when T approaches the maximum temperature (T,,J.Besides the high-frequency variations, microphytobenthos is also exposed to different ranges of temperature on a seasonal scale. However, nothing is known about the acclimation strategy of microphytobenthos to low-frequency temperature variations, despite the fact that the relationship between microphytobenthos photosynthesis and temperature is not constant throughout the year (Colijn and van Buurt 1975, Rasmussen et al. 1983, Grant 1986. From an ecological point of view, this lack of knowledge hinders the predictability of the dynamics of benthic microalgal productivity on intertidal mudflats and creates an obstacle in the quantification of input of matter into the system via...