The role of stable isotopes and mercury concentrations to describe seabird foraging ecology in tropical environments

Nitrogen (δ¹⁵N) and carbon (δ¹³C) stable isotopes and contaminants, such as mercury, have been widely used to characterise foraging ecology of temperate and polar seabirds. In this study, for the first time, we used isotopic signatures and mercury levels of feathers and blood of eight tropical seabird species, that forage in a range-gradient between inshore and offshore areas, to describe the foraging habits of a large tropical seabird community (from two neighboring islands of the Seychelles archipelago, western Indian Ocean) during both the breeding and inter-breeding periods. Overall, we found a high overlap in both δ¹⁵N and δ¹³C signatures among species. The high inter-specific overlap in δ¹⁵N values was expected, given the similarities in the diet of the species from this community. However, several unexpected results, such as (1) the consistently higher δ¹⁵N signatures of white terns (Gygis alba), (2) the large variation in inter-specific differences in δ¹⁵N signatures among the sampling groups (season, age, island and tissue) and (3) the consistent low δ¹⁵N values of breeding birds during the northwest monsoon (austral summer), suggest that δ¹⁵N signatures cannot be used as indicators of seabird trophic levels in this community. The high inter-specific overlap in δ¹³C signatures and the absence, during the breeding season, of a δ¹³C gradient that follows the inshore-offshore foraging gradient within the community can be explained by the habitat homogeneity of the Seychelles continental shelf and suggest that birds forage mostly within the limits of this “plateau”. On the other hand, the similarities in δ¹³C values between the breeding and inter-breeding periods in species that are known to show post-breeding dispersal, strongly support the hypothesis of a lack of latitudinal variation in δ¹³C signatures of POM in the central Indian Ocean, and the consequent inaccuracy of δ¹³C values to track seabird movements within this geographic area. Inter-specific differences in mercury levels seem to be related to prey size, while consistent higher mercury concentrations in one of the studied islands suggest different island mercury-backgrounds and possible segregation in foraging areas between the seabirds of the two islands. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Stable isotopes identify age- and sex-specific dietary partitioning and foraging habitat segregation in southern giant petrels breeding in Antarctica and southern Patagonia

We examined the isotopic signatures (δ¹³C, δ¹⁵N) of adult body feathers from southern giant petrels Macronectes giganteus collected at two breeding colonies in Antarctica (Potter Peninsula and Cape Geddes) and one in southern Patagonia (Observatorio Island), as well as in whole blood collected from adults of both sexes at each Antarctic colonies and from chicks at Potter Peninsula. As body feather moult is a continuous process in giant petrels, feathers provide an integrated annual signal of an adult’s diets and foraging habitats. In contrast, the stable isotope values of adult and chick blood are reflective of their diets during the breeding season. We found that sex-specific dietary segregation in adults breeding in Antarctica was notable during the breeding season (blood samples) but absent when examined across the entire year (feather samples). In addition, blood stable isotope values differed between chicks and adults, indicating that adults provision their offspring with a relatively higher amount of penguin and seal prey that what they consume themselves. This finding confirms previous work that suggests that chicks are preferentially fed with prey of presumably higher nutritional value such as carrion. Finally, based on isotopic differences between major oceanographic zones in the Southern Ocean, our data indicate population-specific differences in foraging distribution, with Antarctic populations move seasonally between Antarctic and subantarctic zones, while Patagonian populations likely forage in subtropical waters and in continental shelf habitats year-round.     

Using carbon and nitrogen isotopic values of body feathers to infer inter- and intra-individual variations of seabird feeding ecology during moult

To determine whether stable isotope measurements of body feathers can be used to investigate the isotopic niche of moulting (inter-nesting) adult seabirds, we examined the stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopic composition of body feathers of breeding wandering albatrosses (Diomedea exulans) from Crozet Islands, southern Indian Ocean. First we showed that the isotopic composition of body feathers was not significantly different from that of wing feathers, being thus a safe alternative to flight feathers whose collection impairs the birds’ flying ability. Second, we looked at the variances in δ¹³C and δ¹⁵N values resulting from the isotopic measurement of a single feather, four different feathers, and a pool of four feathers per bird, to delineate the best isotopic analytical procedure. A two-step protocol is proposed that allows investigating both the intra- and inter-individual components of the niche width of the species. In a first step, isotopic measurements on a single feather per bird are used to define isotopic specialist from isotopic generalist populations. In a second step and for generalist populations only, measurements on additional (three) feathers per bird are used to delineate type A from type B isotopic generalists (Bearhop et al. in J Anim Ecol 73:1007–1012, 2004). Third, from a biological point of view, our data showed different moulting isotopic niches for adult males and females, and also within female wandering albatrosses. Since the isotopic composition of body feathers in this species reflects that of wing feathers, our results suggest that, after validation, body feathers have the potential for investigating the foraging ecology of other Procellariiforms and seabirds during the poorly known inter-nesting period.     

Moulting patterns drive within-individual variations of stable isotopes and mercury in seabird body feathers: implications for monitoring of the marine environment

One major limitation in the use of body feathers of seabirds as a monitoring tool of the trophic structure and contamination levels of marine ecosystems is the degree of heterogeneity in feather chemical composition within individuals. Here, we tested the hypothesis that moulting patterns drive body feather heterogeneity, with synchronous moult minimizing within-individual variations, in contrast to asynchronous feather growth. Chicks of white-chinned petrels Procellaria aequinoctialis (representative of bird chicks) and adults of king penguins Aptenodytes patagonicus (representative of adult penguins) that moult their body feathers synchronously showed very low within-individual variations in their feather δ¹³C and δ¹⁵N values and mercury (Hg) concentrations. By contrast, body feathers of adults of Antarctic prions Pachyptila desolata (representative of adult seabirds with asynchronous feather growth during a protracted moult) presented much higher within-individual variances for the three parameters. These findings have three important implications for birds presenting a synchronous body moult. (1) They suggest that all body feathers from the same individual have identical δ¹³C and δ¹⁵N values and Hg content. (2) They predict negligible within-individual variations in the body feather values of other useful stable isotopes, such as δ²H and δ³⁴S, as well as in the concentrations of other compounds that are deposited in the keratin structure. (3) Analysis of one or any number of pooled body feathers is equally representative of the individual. In conclusion, we recommend that long-term routine monitoring investigations focus on birds presenting synchronous rather than asynchronous moult of body feathers both in marine and terrestrial environments. This means targeting chicks rather than adults and, for seabirds, penguins rather than adults of flying species.     

Comparative foraging ecology and ecological niche of a superabundant tropical seabird: the sooty tern Sterna fuscata in the southwest Indian Ocean

Over 6-million pairs of sooty terns Sterna fuscata breed once a year in the southwest Indian Ocean, mostly on three islands of the Mozambique Channel (Europa, Juan de Nova and Glorieuses) and in the Seychelles region. Seasonal reproduction in either winter or summer is the dominant strategy in the area, but non-seasonal reproduction also occurred in some places like at Glorieuses Archipelago. The feeding ecology of the sooty tern was investigated during the breeding seasons to determine whether terns showed significant differences in their trophic ecology between locations. Regurgitations were analyzed to describe the diet of individuals when breeding, and stable isotopes and mercury concentrations were used to temporally integrate over the medium-term of the trophic ecology of both adults and chicks. Overall, the diet was composed of fish, flying squid and fish larvae in different proportions. At Europa and Aride in the Seychelles, where winter reproduction occurs, large epipelagic prey like flying fish or squid dominated the diet. At Juan de Nova, sooty terns reproduce in summer and rely mostly on fish larvae. At Glorieuses (non-seasonal breeding), the diet was intermediate with fish larvae and flying squid being important prey items. The stable-carbon and nitrogen isotope values in blood confirm the differences observed in dietary analysis, and demonstrate different feeding strategies between colonies. δ¹³C values of feathers showed spatial segregation between birds from the Mozambique Channel and the Seychelles region. Terns from the Seychelles had also higher δ¹⁵N values. Feather δ¹³C values also suggest a significant shift from summer to wintering habitat for birds from Juan de Nova. This study emphasizes the high phenotypic plasticity of the species, which may explain its numerical dominance in all tropical waters of the World’s Ocean.     

Diet of the social groups of long-finned pilot whales (<Emphasis Type="Italic">Globicephala melas</Emphasis>) in the Strait of Gibraltar

The Strait of Gibraltar is inhabited throughout the year by a group of pilot whales (Globicephala melas), but their spatial distribution varies between Summer and Autumn. In this paper, we have used carbon (¹³C/¹²C) and nitrogen (¹⁵N/¹⁴N) stable isotope signatures to investigate the differences in diet amongst seasons, sex and stable social units. Skin samples were collected from 56 individually photo-identified pilot whales during Autumn 2005 and Summer 2006. These individuals were genetically sexed and their isotopic signature determined. The level of inter-individual association both within and between stable social units were compared to Euclidean distances between individual isotopes signatures. No differences in either δ¹⁵N or δ¹³C were found according to the sex of individuals, but significant seasonal differences were found in δ¹⁵N, although not in the δ¹³C values. This suggests that pilot whales are resident year round in the Strait, a finding supported by independent photo-identification. The variation in δ¹⁵N could reflect a shift in pilot whale diet through the year, with pilot whales feeding at a higher trophic level in Autumn compared to Summer. This could also represent a change in the diet of pilot whale prey species. The δ¹³C values were significantly different amongst the four stable social units sampled and individual δ¹³C values were significantly related to the level of inter-individual association, while no relationship was found for δ¹⁵N. These results suggest that within the same general area (i.e. the Strait of Gibraltar), there is some level of specialisation in habitat or prey choice between pilot whales social units.     

Longitudinal and vertical trends in stable isotope signatures (δ13C and δ15N) of omnivorous and carnivorous copepods across the South Atlantic Ocean

Stable isotope (SI) ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) were measured in omnivorous and carnivorous deep-sea copepods of the families Euchaetidae and Aetideidae across the Atlantic sector of the Southern Ocean to establish their trophic positions. Due to high and variable C/N ratios related to differences in lipid content, δ¹³C was corrected using a lipid-normalisation model. δ¹⁵N signals ranged from 3.0–6.9‰ in mesopelagic species to 7.0–9.5‰ in bathypelagic congeners. Among the carnivorous Paraeuchaeta species, the epi- to mesopelagic species Paraeuchaeta antarctica had lower δ¹⁵N values than the mesopelagic P. rasa and bathypelagic P. barbata. The same trend was observed among omnivorous Aetideidae, but was not significant. In the most abundant species P. antarctica, individuals from the western Atlantic had higher δ¹³C and δ¹⁵N values than specimens at the eastern stations. These longitudinal changes in δ¹³C and δ¹⁵N values were attributed to regional differences in hydrography and sea surface temperature (SST), in particular related to a northward extension of the Antarctic Polar Front (APF) at the easternmost stations. The results indicate that even in a mesopelagic carnivorous species, the changes in surface stable isotope signatures are pronounced.     

Inter-colony movements,at-sea behaviour and foraging in an immature seabird: results from GPS-PPT tracking,radio-tracking and stable isotope analysis

Seabird populations contain large numbers of immatures––in some instances comprising >50% of the fully grown adults in the population. These birds are significant components of marine food webs and may contribute to compensatory recruitment and dispersal, but remain severely understudied. Here, we use GPS-PTTs, radio-tracking and analysis of stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes to investigate the movements and foraging ecology of immature seabirds. Our study focussed on immature northern gannets Morus bassanus aged 2–4 attending non-breeding aggregations alongside a large breeding colony. GPS-PTT tracking of five birds revealed that immatures have the ability to disperse widely during the breeding season, with some individuals potentially prospecting at other colonies. Overall, however, immatures were faithful to the colony of capture. During returns to the focal colony, immatures acted as central place foragers, conducted looping and commuting flights, and analysis of the variance in first-passage time revealed evidence of area-restricted search (ARS) behaviour. In addition, stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope analyses indicate that immatures were isotopically segregated from breeders. Our findings provide insights into the foraging, prospecting and dispersal behaviour of immature seabirds, which may have important implications for understanding seabird ecology and conservation.     

Two distinct forms of the chitin-degrading enzyme N-acetyl-β-d-glucosaminidase in the Antarctic krill: specialists in digestion and moult

In the Antarctic krill Euphausia superba two forms of the chitinolytic enzyme N-acetyl-β-d-glucosaminidase (NAGase, EC 3.2.1.52) have been described, previously identified as NAGase B and NAGase C. Here, we demonstrate the organ-specific distribution and physiological relevance of both forms using a polyclonal antibody preparation which allows them to be distinguished immunologically. While NAGase B was localized in the integument and displayed a pattern of activity related to the moult cycle, the activity of NAGase C was independent of the moult cycle and was predominantly found in the gastrointestinal tract. Accordingly, NAGase B played a significant role in chitin degradation during the krill's moult, whereas NAGase C participated in the digestion of chitin-containing dietary components. Chromatographic elution profiles of isolated organs confirmed the immunological results by displaying characteristic organ-specific patterns in NAGase activity. The molecular characteristics of the moulting form, NAGase B, may further indicate a vesicular transport of moulting enzymes from the epidermis into the ecdysial space. Based on our results we develop a hypothesis explaining the concurrent processes of simultaneous chitin degradation and chitin synthesis occurring during moult. Received: 30 December 1998 / Accepted: 23 April 1999     

Importance of freshwater flow in terrestrial–aquatic energetic connectivity in intermittently connected estuaries of tropical Australia

δ¹³C was used to identify seasonal variations in the importance of autochthonous and allochthonous sources of productivity for fish communities in intermittently connected estuarine areas of Australia’s dry tropics. A total of 224 fish from 38 species were collected from six intermittently connected estuarine pools, three in central Queensland (two dominated by C₃ forest and one by C₄ pasture) and three in north Queensland (one dominated by C₃ and two by C₄ vegetation). Samples were collected before and after the wet season. Fish collected in the two forested areas in central Queensland had the lowest δ¹³C, suggesting a greater incorporation of C₃ terrestrial material. A seasonal variation in δ¹³C was also detected for these areas, with mean δ¹³C varying from −20 to −23‰ from the pre- to the post-wet season, indicating a greater incorporation of terrestrial carbon after the wet season. Negative seasonal shifts in fish δ¹³C were also present at the pasture site, suggesting a greater dependence on carbon of riparian vegetation (C₃ Juncus sp.) in the post-wet season. In north Queensland, terrestrial carbon seemed to be incorporated by fish in the two C₄ areas, as δ¹³C of most species shifted towards slightly heavier values in the post-wet season. A two-source, one-isotope mixing model also indicated a greater incorporation of carbon of terrestrial origin in the post-wet season. However, no seasonal differences in δ¹³C were detected for fish from the forested area of north Queensland. Overall, hydrologic connectivity seemed to be a key factor in regulating the ultimate sources of carbon in these areas. It is therefore important to preserve the surrounding habitats and to maintain the hydrologic regimes as close to natural conditions as possible, for the conservation of the ecological functioning of these areas.     

Distribution of foraging habitats of male loggerhead turtles (Caretta caretta) as revealed by stable isotopes and satellite telemetry

Most studies on the foraging ecology of loggerhead turtles (Caretta caretta) have focused on adult females and juveniles. Little is known about the foraging patterns of adult male loggerheads. We analyzed tissues for carbon and nitrogen stable isotopes (δ¹³C and δ¹⁵N) from 29 adult male loggerheads tracked with satellite transmitters from one breeding area in Florida, USA, to evaluate their foraging habitats in the Northwest Atlantic (NWA). Our study revealed large variations in δ¹³C and δ¹⁵N and a correlation between both δ¹³C and δ¹⁵N and the latitude to which the loggerheads traveled after the mating season, thus reflecting a geographic pattern in the isotopic signatures. Variation in δ¹³C and δ¹⁵N can be explained by differences in food web baseline isotopic signatures rather than differences in loggerhead trophic levels. Stable isotope analysis may help elucidate residency and migration patterns and identify foraging sea turtle subpopulations in the NWA due to the isotopically distinct habitats used by these highly migratory organisms.     

Isotopic niches of emperor and Adélie penguins in Adélie Land,Antarctica

The emperor and Adélie penguins are the only two species of penguins that co-occur at high-Antarctic latitudes. We first measured and compared their isotopic niches on the same year in Adélie Land in spring, when the two species co-exist. Emperor and Adélie penguins segregated by their blood isotopic signatures, with adult δ¹³C values (−24.5 ± 0.2 and −25.4 ± 0.2‰, respectively) suggesting that emperor penguins foraged in more neritic waters than Adélie penguins in spring. At that time, difference in their δ¹⁵N values (4.1‰, 12.0 ± 0.4 vs. 7.9 ± 0.1‰) encompassed more than one trophic level, indicating that emperor penguins preyed mainly upon fish (and squids), while Adélie penguins fed exclusively on euphausiids. Second, we compared the food of breeding adults and chicks. The isotopic signatures of adults and chicks of emperor penguins were not statistically different, but δ¹⁵N value of Adélie penguin chicks was higher than that of adults (10.2 ± 0.8 vs. 9.0 ± 0.2‰). The difference showed that adult Adélie penguins captured higher trophic level prey, i.e. higher-quality food, for their chicks. Third, the isotopic signatures of Adélie penguins breeding in Adélie Land showed that adults fed on Antarctic krill in oceanic waters in spring and shifted to neritic waters in summer where they preyed upon ice krill for themselves and upon fish and euphausiids for their chicks. A comparison of isotopic niches revealed large overlaps in both blood δ¹³C and δ¹⁵N values within the community of Antarctic seabirds and pinnipeds. The continuum in δ¹⁵N values nevertheless encompassed more than one trophic level (5.2‰) from Adélie penguin and crabeater seal to the Weddell seal. Such a broad continuum emphasizes the fact that all Antarctic seabirds and marine mammals feed on varying proportions of a few crustacean (euphausiids) and fish (Antarctic silverfish) species that dominate the intermediate trophic levels of the pelagic neritic and oceanic ecosystems.     

Carbon and nitrogen isotopic composition of the fauna from the Broken Spur hydrothermal vent field

 The carbon and nitrogen isotopic composition of 18 faunistic groups collected during the 39th cruise of the R.V. “Akademik Mstislav Keldysh” in September 1996 at the Broken Spur vent field, MAR, was studied. The trophic structure of the Broken Spur vent community is considered. All age stages of the shrimp Rimicaris exoculata living 5 m below the main aggregations at black smokers show higher δ¹⁵N and more depleted δ¹³C values than the same stages inhabiting the black smokers themselves. The shrimps R. exoculata and Chorocaris chacei demonstrate ontogenetic changes in δ¹³C (the former also in δ¹⁵N), with smaller individuals showing higher δ¹⁵N and more depleted δ¹³C values than larger shrimps. Benthopelagic and benthic components of the vent community differ significantly in δ¹³C and δ¹⁵N, the benthic fauna being less dependent upon chemosynthetic production. Received: 30 March 1999 / Accepted: 28 September 1999     

Utilization of nitrogen derived from seabird guano by terrestrial and marine plants at St. Paul, Pribilof Islands, Bering Sea, Alaska

Dense, conspicuous colonies of seabirds and pinnipeds breed on ocean islands throughout the world. Such colonies have been shown to have local impacts on prey populations, but whether or not they affect nutrient cycling has been debated. We determined the natural abundance levels of the stable isotopes (C and N) of primary producers, seabirds and other consumers at and near St. Paul Island, Pribilof Islands, Bering Sea, in summer 1993. Marine primary producers (phytoplankton, as particulate organic matter, and kelp) collected near seabird colonies were ca. 6.5‰ enriched in both ¹⁵N and ¹³C relative to those collected further from shore. Terrestrial plants collected near the seabird colonies were enriched in ¹⁵N (δ¹⁵N ca. 22‰) compared with conspecifics collected away from the colonies (δ¹⁵N ca. 11‰). The trend towards higher δ¹⁵N values in marine and terrestrial plants near bird colonies is consistent with their uptake of ornithogenic N. This ¹⁵N-enrichment of plants using ornithogenic N can be attributed to a combination of two processes: trophic enrichment, and volatilization of ammonia produced during degradation of terrestrially deposited guano. Seabird breeding colonies at St. Paul Island appear to be significant sources of recycled nitrogen for terrestrial plants in the vicinity of colonies and for phytoplankton in the nearshore zone. Received: 4 July 1997 / Accepted: 20 November 1997     

Moult in relation to some aspects of reproduction and growth in swarms of Antarctic krill,Euphausia superba

In a 14-d period 38 swarms of Antarctic krill,Euphausia superba Dana, were sampled in an area 55.5 x 55.5 km to the southwest of Elephant Island, at the north of the South Shetland Islands. Moult stage, maturity stage, and size of ca. 100 krill from each swarm were measured. Each of the characteristics varied greatly between swarms. Moulting krill were found in most swarms, but in one swarm all of the krill were just about to moult. Ways in which moulting may act as a possible sorting mechanism are discussed. Data for all the analysed krill (ca. 3000 specimens) were used to investigate the interdependence of moult rate, sexual maturation and growth. While all immature krill moulted at approximately the saine rate in the study, there were significant differences in the moulting rates of mature male and female krill. Gravid female krill continued to moult, although less frequently than mature males. As a consequence males had to attach spermatophores to females after each moult. It is likely that variation in moult rate of females and males was related to the energy expenditure required for ovary development in females and spermatophore production and searching behaviour in males. Spawning and moulting were only partly coupled in gravid females. Spawning appeared to take place predominantly during Moult Stage D₂. There was no evidence of intermoult growth by intersegmental dilation.     

Contrasting C and N isotope ratios from sperm whale skin and squid between the Gulf of Mexico and Gulf of California: effect of habitat

To investigate feeding variation between populations of an apex oceanic predator, stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) have been compared in skin of female and immature male sperm whales (Physeter macrocephalus) from the northern Gulf of Mexico (GoM) and the Gulf of California (GC). Whale sexes were determined genetically. The δ¹³C and δ¹⁵N values from squid muscle were used from the GC, and from inshore and offshore sites in the GoM. We documented contrastingly lower δ¹³C and δ¹⁵N from whales and squid of the GoM compared with those from the GC. While this difference may be associated with variation in trophic position, geographic variation in biochemical cycling influenced significantly the contrasting isotope values between gulfs. Within the northern GoM, the highly distinct δ¹⁵N values of neritic squid versus mesopelagic squid provide further evidence of habitat specificity in δ¹⁵N.     

A three-isotope approach to disentangling the diet of a generalist consumer: the yellow-legged gull in northwest Spain

The widespread omnivory of consumers and the trophic complexity of marine ecosystems make it difficult to infer the feeding ecology of species. The use of stable isotopic analysis plays a crucial role in elucidating trophic interactions. Here we analysed δ¹⁵N, δ¹³C and δ³⁴S in chick feathers, and we used a Bayesian triple-isotope mixing model to reconstruct the diet of a generalist predator, the yellow-legged gull (Larus michahellis) that breeds in the coastal upwelling area off northwest mainland Spain. The mixing model indicated that although chicks from all colonies were fed with a high percentage of fish, there are geographical differences in their diets. While chicks from northern colonies consume higher percentages of earthworms, refuse constitutes a more important source in the diet of chicks from western colonies. The three-isotope mixing model revealed a heterogeneity in foraging habitats that would not have been apparent if only two stable isotopes had been analysed. Moreover, our work highlights the potential of adding δ³⁴S for distinguishing not only between terrestrial and marine prey, but also between different marine species such as fish, crabs and mussels.     

Spatial and temporal variation in the elemental and stable isotopic content of the seagrasses <Emphasis Type="Italic">Posidonia oceanica</Emphasis> and <Emphasis Type="Italic">Cymodocea nodosa</Emphasis> from the Illes Balears,Spain

Morphology, elemental content and isotopic composition of leaves of the seagrasses Posidonia oceanica and Cymodocea nodosa were highly variable across the Illes Balears, a Spanish archipelago in the western Mediterranean, and varied seasonally at one site in the study area. The data presented in this paper generally expand the reported ranges of nitrogen, phosphorus, iron and arsenic content and δ¹³C and δ¹⁵N for these species. Nitrogen and phosphorus content of P. oceanica leaves also showed significant seasonal variability; on an annual basis, P. oceanica leaves averaged 1.55% N and 0.14% P at this monitoring site. Both N and P were more concentrated in the leaves in winter than in summer, with winter maxima of 1.76% N and 0.17% P and summer minima of 1.34% N and 0.11% P. There was no significant annual pattern observed in the δ¹³C of P. oceanica leaves, but there was a repeated 0.6‰ seasonal fluctuation in δ¹⁵N. Mean annual δ¹⁵N was 4.0‰; δ¹⁵N was lowest in May and it increased through the summer and autumn to a maximum in November. Over the geographic range of our study area, there were interspecific differences in the carbon, nitrogen and phosphorus content of the two species. Posidonia oceanica N:P ratios were distributed around the critical value of 30:1 while the ratios for C. nodosa were lower than this value, suggesting P. oceanica we collected was not consistently limited by N or P while C. nodosa tended toward nitrogen limitation. Nutrient content was significantly correlated to morphological indicators of plant vigor. Fe content of P. oceanica leaves varied by a factor of 5×, with a minimum of 31.1 μg g⁻¹ and a maximum of 167.7 μg g⁻¹. Arsenic was present in much lower tissue concentrations than Fe, but the As concentrations were more variable; the maximum concentration of 1.60 μg g⁻¹ was eight times as high as the minimum of 0.20 μg g⁻¹. There were interspecific differences in δ¹³C of the two species; C. nodosa was consistently more enriched (δ¹³C = −7.8 ± 1.7‰) than P. oceanica (−13.2 ± 1.2‰). The δ¹³C of both species decreased significantly with increasing water depth. Depth related and regional variability in the δ¹³C and δ¹⁵N of both species were marked, suggesting that caution needs to be exercised when applying stable isotopes in food web analyses.     

Growth of the scyllarid lobsters Ibacus peronii and I. chacei

The growth rates of the morphologically similar scyllarid lobsters Ibacus peronii (Leach, 1815) and I. chacei (Brown and Holthuis, 1998) are described using data from a tag/recapture study and from tagged lobsters kept in captivity. Within particular size classes, we found no differences in moult increments between male and female I. peronii nor between male and female I. chacei. Small individuals of both species always had larger moult increments than larger individuals. For I. peronii, females moulted more frequently than males, and smaller size classes moulted more frequently than larger size classes. Female I. peronii therefore grew more quickly than males and reached their estimated size at sexual maturity (51 mm carapace length) after ∼2 yr. Moulting of I. peronii was seasonal, with most lobsters (96.3%) moulting between October and January. We found no differences in growth rates of I. peronii at two locations along the east coast of Australia: Coffs Harbour in New South Wales (30°18′S; 153°08′E), and Lakes Entrance in Victoria (37°53′S; 148°00′E). For I. chacei, we found no differences in the frequency of moulting between males and females and, because we also found no differences in the moult increments between males and females, the growth rates of both sexes were the same. Received: 14 August 1999 / Accepted: 20 January 2000     

Differences in δ13C and δ15N values between feathers and blood of seabird chicks: implications for non-invasive isotopic investigations

Blood and feathers are the most targeted tissues for isotopic investigations in avian ecology, primarily because they can be easily and non-destructively sampled on live individuals. Comparing blood and feather isotopic ratios can provide valuable information on dietary shifts, trophic specialization and migration patterns, but it requires a good knowledge of the isotopic differences between the two tissues. Here, δ¹³C and δ¹⁵N values of whole blood (in blood cells of a few species) and simultaneously grown body feathers were measured in seabird chicks to quantify the tissue-related isotopic differences. Seabirds include 27 populations of 22 wild species that were sampled in 2000–2008, and a review of the literature added 8 groups (including adult birds) to the analysis. The use of a large data set that overall encompasses wide δ¹³C and δ¹⁵N ranges allowed us to depict for the first time accurate relationships between blood and feather isotopic ratios across avian taxa. Blood was impoverished in ¹³C and generally in ¹⁵N compared with feathers. Both mean δ¹³C and δ¹⁵N values of feathers and blood were highly positively and linearly related [feather δ¹³C = 0.972 (±0.020) blood δ¹³C + 0.962 (±0.414), and feather δ¹⁵N = 1.014 (±0.056) blood + 0.447 (±0.665), respectively; both P < 0.0001]. The regressions should be applied to mathematically correct feather or whole blood δ¹³C and δ¹⁵N values when comparing isotopic ratios within and between ecological studies on birds.