Fueling phocids: Divergent exploitation of primary energy sources and parallel ontogenetic diet switches among three species of subarctic seals

Determining how marine predators partition resources is hindered by the difficulty in obtaining information on diet and distribution. Stable isotopes (SI) of carbon (¹³C/¹²C, δ¹³C) and nitrogen (¹⁵N/¹⁴N, δ¹⁵N) provide a two‐dimensional estimate of the dietary space of consumers; an animal's isotopic composition is directly influenced by what they consume and where they feed. Harp (Pagophilus groenlandicus) and hooded (Cystophora cristata) seals are abundant phocid species found in the North Atlantic. We measured and contrasted SI values between seals sampled at nearshore and offshore sites to test for effects of sampling location, sex, age‐class, and body size to gain insight into how these species partition space and prey resources. In addition we contrasted previously published results for gray seals (Halichoerus grypus). Isotope values differed significantly by age class and location in harp and hooded seals. We found significant differences in SI values (mean δ¹³C and δ¹⁵N ± SE) between all species. Hooded seals, a continental shelf‐edge, deep‐diving species, exhibited low SI values (juveniles: ?20.9‰ ± 0.03‰, 13.36‰ ± 0.05‰; adults: ?20.41‰ ± 0.03‰, 14.81‰ ± 0.04‰) characteristic of feeding on meso‐ to bathypelagic prey. Harp seals, which dive to moderate depths primarily on the shelf had intermediate SI values (juveniles: ?20.53‰ ± 0.01‰, 13.91‰ ± 0.01‰; adults: ?20.13‰ ± 0.01‰, 14.96‰ ± 0.01‰) characteristic of feeding on epipelagic prey, whereas gray seals, which feed on or near the sea floor in shallow shelf waters, had high SI values (juveniles: ?19.74‰ ± 0.04‰, 17.51‰ ± 0.05‰; adults: ?18.86‰ ± 0.01‰, 17.23‰ ± 0.02‰) characteristic of feeding on demersal prey. In all species, δ¹³C values increased with body size and age in the same manner, indicating that seals exploit or forage in deeper habitats as they get larger and older. We hypothesize that the consistent ontogenetic shift in foraging niche, despite large differences between species in their diving behavior, geographic range and habitat use, not only reflects increased access to different prey due to increased diving capacity, but a progressive adjustment to balance energy budgets by reducing foraging costs.     

Differences in δ13C and δ15N stable isotopes in the pearly razorfish Xyrichtys novacula related to the sex,location and spawning period

In the present study, Xyrichtys novacula (Labridae) were sampled at five locations around the islands of Ibiza and Formentera (western Mediterranean Sea). Isotopic signatures of δ¹³C, δ¹⁵N and the C:N ratio were analysed in relation to locality, sex and size differences. δ¹³C and δ¹⁵N partitioning was also studied in the reproductive spawning period. There were significant differences in the δ¹³C signature between localities for both sexes, but not for δ¹⁵N. Sex differences were also found with a mean ±s.e . value of ?17·38 ± 0·06‰δ¹³C and 8·36 ± 0·05‰δ¹⁵N for females and ?17·17 ± 0·07‰δ¹³C and 8·80 ± 0·06‰δ¹⁵N for males. Increasing total length in both sexes was positively correlated with δ¹⁵N enrichment and a significant positive linear regression was established for both variables. During the reproductive spawning period, there were changes in δ¹³C fractioning with enrichment in postspawning females and males (with respect to prespawning and spawning periods) and δ¹⁵N impoverishment in postspawning females (with respect to prespawning and spawning periods). Xyrichtys novacula uses local food sources, as confirmed by δ¹³C and δ¹⁵N, and females and males use different food sources, thus avoiding intraspecific competition. This was confirmed by δ¹⁵N enrichment as size increased. Spawning leads to special requirements for gonad maturation, which is reflected in the isotopic signatures for both sexes.     

Annual variability in dentin δ15N and δ13C reveal sex differences in weaning age and feeding habits in Risso's dolphins (Grampus griseus)

Teeth of odontocetes accumulate annual dentinal growth layer groups (GLGs) that record isotope ratios, which reflect the time of their synthesis. Collectively, they provide lifetime records of individual feeding patterns from which life history traits can be inferred. We subsampled the prenatal dentin and postnatal GLGs in Risso's dolphins (Grampus griseus) (n = 65) that stranded or were collected as bycatch in Taiwan (1994–2014) and analyzed them for δ¹⁵N and δ¹³C. Age‐specific δ¹⁵N and δ¹³C values were corrected for effects of calendar year, stranding site, C/N, and sex. δ¹⁵N values were higher in prenatal layers (14.94‰ ± 0.74‰) than in adult female GLGs (12.58‰ ± 0.20‰), suggesting fetal enrichment during gestation. Decreasing δ¹⁵N values in early GLGs suggested changes in dietary protein sources during transition to complete weaning. Weaning age was earlier in males (1.09 yr) than in females (1.81 yr). Significant differences in δ¹⁵N values between weaned males and females suggest potential sexual segregation in feeding habits. δ¹³C values increased from the prenatal to the 4th GLG by ~1.0‰, indicative of a diet shift from ¹³C‐depleted milk to prey items. Our results provide novel insights into the sex‐specific ontogenetic changes in feeding patterns and some life history traits of Risso's dolphins.     

Trophic fractionation of carbon and nitrogen stable isotopes in Chironomus riparius reared on food of aquatic and terrestrial origin

1. Trophic fractionation was studied in short‐term laboratory feeding experiments with larvae of the deposit‐feeding midge Chironomus riparius. Larvae were fed food of terrestrial (oats, peat) and aquatic origin (Spirulina, Tetraphyll^®). 2. By analysing both whole larvae and isolated gut contents we were able to distinguish between the isotopic signature of recently ingested food and that of assimilated carbon and nitrogen in body tissue. Additionally we studied the effects of microbial conditioning, i.e. the colonisation and growth on food particles of microbes, on the isotopic signal of food resources. 3. Nitrogen fractionation for the different food types ranged from 0.67‰ to 2.68‰ between consumer and diet and showed that isotopic fractionation can be much lower than the value of 3.4‰ that is commonly assumed. 4. Microbial degradation of food particles resulted in an approximate doubling of the δ¹⁵N in 8 days, from 6.24 ± 0.05‰ to 11.36 ± 0.56‰. Values for δ¹³C increased only marginally, from ?20.66 ± 0.11‰ to ?20.34 ± 0.12‰. These results show that microbial conditioning of food may affect dietary isotope signatures (in particular N) and, unless accounted for, could introduce an error in measures of trophic fractionation. Microbial conditioning could well account for some of the variation in fractionation reported in the literature.     

Foraging segregation by sex and age class in the Guadalupe fur seal from Guadalupe Island,Mexico

A proper assessment of the foraging habits of the Guadalupe fur seal (GFS; Arctocephalus townsendi) is a priority to better understand its recovery, in which the potential for intraspecific competition for prey and space resources is expected to lead to segregation. This study aimed to determine the foraging habits of different sex and age classes. A total of 146 GFS fur samples was collected at Guadalupe Island, Mexico (2014–2020) for stable isotopes (δ¹³C and δ¹⁵N) analysis. Isotopic areas were created (SIBER package in R). Significant isotopic differences were observed between classes. Male (3.6‰²) and female (3.0‰²) juveniles had the largest isotopic areas due to a greater foraging dispersion. Adult females showed the lowest mean δ¹⁵N value (16.1‰ ± 0.5‰) due to foraging trips that are mostly performed towards high latitudes. Except for pups, adult and subadult males presented the highest mean δ¹⁵N (17.4‰ ± 0.4‰) and δ¹³C values (−17.0‰ ± 0.8‰) due to a possible higher trophic level and coastal foraging habits, whereas pups presented the highest mean δ¹⁵N value (17.6‰ ± 0.3‰) because of lactation, which reflects their mothers δ¹⁵N signal plus their own enrichment. Our findings suggest a segregation explained by differences in life history, energy requirements, and a possible strategy to avoid competition.     

Temporal variability in stable isotope ratios of C and N in the vibrissa of captive and wild adult South American sea lions Otaria byronia: More than just diet shifts

We analyzed the δ¹³C and δ¹⁵N values in the vibrissae of captive adult breeding South American sea lions (Otaria byronia) fed at a constant diet and then used this information to analyze the change in stable isotope values along the vibrissae from wild individuals. The overall diet‐to‐vibrissa discrimination factor of the captive animals was 3.0‰ ± 0.1‰ for δ¹³C and 3.6‰ ± 0.1‰ for δ¹⁵N, but the stable isotope ratios fluctuated periodically despite constant diet. The δ¹³C and δ¹⁵N values of the captive male declined at the end of the breeding season, whereas the δ¹³C values of the female increased during the central part of pregnancy and the δ¹⁵N values peaked during lactation. The δ¹³C and δ¹⁵N values of adult wild specimens also fluctuated periodically and vibrissae growth rate (0.15 mm/d in both sexes) was slightly lower than in captivity (0.17 mm/d), assuming an annual periodicity for oscillations. Similarities in the amplitude of the cycles of captive and wild males suggested that fasting was probably the main source of periodic variability in the δ¹⁵N of wild males, whereas pregnancy and lactation were probably the main source of periodic variability for the δ¹³C of wild females.     

Stable isotope composition and parasitic infections of harbor seal young‐of‐the‐year used as prey‐based diet indicators

The stable isotope composition (δ¹³C and δ¹⁵N values) of harbor seals (Phoca vitulina) is influenced by their diet. Young‐of‐the‐year during lactation and postweaning fast are expected be enriched in ¹⁵N compared to foraging seals. We studied the temporal variation of stable isotope composition of young‐of‐the‐year and adults to determine from which point in time the young‐of‐the‐year tissues (i.e., muscles and vibrissae) are influenced by independent foraging only. These results were compared with the development of trophically transmitted parasitic infections. The δ¹⁵N values in young‐of‐the‐year muscles decreased from June (20.3‰ ± 0.5‰) to October (18.5‰ ± 0.4‰), while those of foraging seals were all year long below 19.2‰. This decrease coincides with the increase of parasitic infections in young‐of‐the‐year, reflecting a shift to fish diet. Together these results suggest that the muscles of the young‐of‐the‐year older than 5–6 mo reflect independent foraging and that they can therefore be used in community diet studies. The nursing signal in vibrissae was unclear, as the δ¹⁵N values of young‐of‐the‐year were stable over time, whereas those of adults varied seasonally. However, δ¹⁵N values of nursing pups were significantly higher than those of adults in May and June, maybe due to their reliance on milk.     

Characteristics of stable isotope signature of diet in tissues of captive Japanese macaques as revealed by controlled feeding

We determined the magnitude of isotopic fractionation of carbon and nitrogen stable isotope ratios (as enrichment factors, Δδ¹³C and Δδ¹⁵N, respectively) between the tissues and diets of captive Japanese macaques (Macaca fuscata) using a controlled feeding experiment, to provide basic data for reconstructing their feeding habits. The Δδ¹³C and Δδ¹⁵N values, respectively, were 0.9 ± 0.2 ‰ (mean ± standard deviation, SD) and 3.0 ± 0.3 ‰ for whole blood, 1.3 ± 0.2 ‰ and 4.3 ± 0.3 ‰ for plasma, and 0.8 ± 0.2 ‰ and 3.0 ± 0.2 ‰ for red blood cells. However, the Δδ¹³C and Δδ¹⁵N values for hair were 2.8 ± 0.3 ‰ and 3.4 ± 0.2 ‰, respectively. No difference was detected in the δ¹³C and δ¹⁵N values of hair sampled from different parts of the body. We investigated the effects of diet on δ¹³C in growing hair by alternating the diet of the macaques each month between two diets that differed markedly in δ¹³C. Hair regrown after shaving repeatedly recorded the δ¹³C of the diet consumed during the time of hair growth. On the other hand, hair naturally grown during the diet-change experiment did not show a clear pattern. One possible reason is that the hair had grown abnormally under unnatural indoor conditions and showed complicated isotope signatures. To reconstruct the long-term feeding history of Japanese macaques, we need to further clarify the relationships between the stable isotope signature of diet and various body tissues.     

Natural 15N abundance of epiphytes depends on the position within the forest canopy: source signals and isotope fractionation

The natural ¹⁵N abundance (δ¹⁵N) of epiphytes and its N sources were studied in the canopy of a lowland rainforest in Costa Rica. Vascular and non‐vascular epiphytes and canopy soils were collected from four canopy zones and analysed for N contents and δ¹⁵N signals. In addition, the N concentrations and δ¹⁵N signatures of bulk precipitation, throughfall and stemflow were measured during the wet and the dry season. The δ¹⁵N values of epiphyte leaves decreased significantly from the lower zones (means of ?3·9 and ?4·3‰) to the upper zones (means of ?5·4 and ?6·1‰) of the canopy. In contrast, δ¹⁵N signatures of canopy soils (average ?0·3‰) differed little between the zones. Bulk deposition was enriched in ¹⁵N (+4·3‰) compared to all other potential N sources and was higher than throughfall and stemflow (+0·5 to ?1·3‰). δ¹⁵N values of atmospheric deposition were inversely related to those of the epiphyte leaves, whereas N isotopic composition of canopy soils did not vary significantly. Consequently, it is concluded that the variations in foliar N isotope composition of epiphytes were not simply caused by utilization of isotopically different N sources, but by different ¹⁵N discrimination during N acquisition.     

Australian mulga ecosystems –13C and 15N natural abundances of biota components and their ecophysiological significance

Samples of recently produced shoot material collected in winter/spring from common plant species of mulga vegetation in eastern and Western Australia were assayed for ¹³C and ¹⁵N natural abundance. ¹³C analyses showed only three of the 88 test species to exhibit C₄ metabolism and only one of seven succulent species to be in CAM mode. Non-succulent winter ephemeral C₃ species showed significantly lower mean δ¹³C values (– 28·0‰) than corresponding C₃-type herbaceous perennials, woody shrubs or trees (– 26·9, – 25·7 and – 26·2‰, respectively), suggesting lower water stress and poorer water use efficiency in carbon acquisition by the former than latter groups of taxa. Corresponding values for δ¹⁵N of the above growth and life forms lay within the range 7·5–15·5‰. δ¹⁵N of soil NH₄⁺ (mean 19·6‰) at a soft mulga site in Western Australia was considerably higher than that of NO₃^– (4·3‰). Shoot dry matter of Acacia spp. exhibited mean δ¹⁵N values (9·10 ± 0·6‰) identical to those of 37 companion non-N₂-fixing woody shrubs and trees (9·06 ± 0·5‰). These data, with no evidence of nodulation, suggested little or no input of fixed N₂ by the legumes in question. However, two acacias and two papilionoid legumes from a dune of wind-blown, heavily leached sand bordering a lake in mulga in Western Australia recorded δ¹⁵N values in the range 2·0–3·0‰ versus 6·4–10·7‰ for associated non-N₂-fixing taxa. These differences in δ¹⁵N, and prolific nodulation of the legumes, indicated symbiotic inputs of fixed N in this unusual situation. δ¹⁵N signals of lichens, termites, ants and grasshoppers from mulga of Western Australia provided evidence of N₂ fixation in certain termite colonies and by a cyanobacteria-containing species of lichen. Data are discussed in relation to earlier evidence of nitrophily and water availability constraints on nitrate utilization by mulga vegetation.     

Trophic discrimination factor of nitrogen isotopes within amino acids in the dobsonfly Protohermes grandis (Megaloptera: Corydalidae) larvae in a controlled feeding experiment

The trophic discrimination factor (TDF) of nitrogen isotopes (¹⁵N/¹⁴N) within amino acids, between a stream‐dwelling dobsonfly larva (Protohermes grandis: Megaloptera; Corydalidae) and its diet (chironomid larvae), was determined in controlled feeding experiments. Last‐instar larvae of P. grandis were collected from the Yozawa‐gawa River, central Japan, and reared in the laboratory. After fed to satiation for 1 month, one group of larvae was each fed one living chironomid larva per day for 4 weeks, while a second group was starved for 8 weeks. The larvae were harvested at intervals and the nitrogen isotopic composition of glutamic acid (δ¹⁵N_Glu) and phenylalanine (δ¹⁵N_Phe) were determined to calculate TDF. The mean TDF of satiated and starved larvae were 7.1‰ ± 0.5‰ (n = 3) and 7.3‰ ± 0.5‰ (n = 5), respectively. Thus, the TDF for P. grandis larvae in this study was similar to that reported for other arthropods (approximately 7‰) and was independent of satiation or starvation. A previous study of wild P. grandis larvae, based on the δ¹⁵N_Glu and δ¹⁵N_Phe values, estimated its trophic position (TP) as approximately 2.0 ± 0.1 (n = 5), a low value close to that of algivores, although they are generally characterized as carnivores (usually accepted as TP ≥ 3). The TDF for P. grandis larvae suggests that their low TPs in nature were caused by incorporation of vascular plant‐derived amino acids (with a different δ¹⁵N profile from that of algae) and not by an unusually low TDF or by the effects of the satiation/starvation on amino acid metabolism.     

Isotopic niche differs between seal and fish‐eating killer whales (Orcinus orca) in northern Norway

Ecological diversity has been reported for killer whales (Orcinus orca) throughout the North Atlantic but patterns of prey specialization have remained poorly understood. We quantify interindividual dietary variations in killer whales (n = 38) sampled throughout the year in 2017–2018 in northern Norway using stable isotopic nitrogen (δ¹⁵N: ¹⁵N/¹⁴N) and carbon (δ¹³C: ¹³C/¹²C) ratios. A Gaussian mixture model assigned sampled individuals to three differentiated clusters, characterized by disparate nonoverlapping isotopic niches, that were consistent with predatory field observations: seal‐eaters, herring‐eaters, and lumpfish‐eaters. Seal‐eaters showed higher δ¹⁵N values (mean ± SD: 12.6 ± 0.3‰, range = 12.3–13.2‰, n = 10) compared to herring‐eaters (mean ± SD: 11.7 ± 0.2‰, range = 11.4–11.9‰, n = 19) and lumpfish‐eaters (mean ± SD: 11.6 ± 0.2‰, range = 11.3–11.9, n = 9). Elevated δ¹⁵N values for seal‐eaters, regardless of sampling season, confirmed feeding at high trophic levels throughout the year. However, a wide isotopic niche and low measured δ¹⁵N values in the seal‐eaters, compared to that of whales that would eat solely seals (δ_N‐measured = 12.6 vs. δ_N‐expected = 15.5), indicated a diverse diet that includes both fish and mammal prey. A narrow niche for killer whales sampled at herring and lumpfish seasonal grounds supported seasonal prey specialization reflective of local peaks in prey abundance for the two fish‐eating groups. Our results, thus, show differences in prey specialization within this killer whale population in Norway and that the episodic observations of killer whales feeding on prey other than fish are a consistent behavior, as reflected in different isotopic niches between seal and fish‐eating individuals.     

Isotopic enrichment in Rhizoprionodon porosus (Poey, 1861) and Carcharhinus porosus (Ranzani, 1840) embryos

Isotopic values of two Caribbean sharpnose shark Rhizoprionodon porosus litters (Poey, 1861) with two and three embryos and one litter of 11 smalltail shark Carcharhinus porosus embryos showed enriched ¹⁵N and ¹³C compared to their mothers. In R. porosus, embryonic isotope values were 3.06 ± 0.07‰ and 0.69 ± 0.15‰ greater than their mothers' for δ¹⁵N and δ¹³C, respectively, whereas in C. porosus, δ¹⁵N and δ¹³C were 1.79 ± 0.09‰ and 1.31 ± 0.17‰ greater in embryos than their mothers.     

Effect of salinity on growth,biochemical composition,and lipid productivity of Nannochloropsis oculata CS 179

Effect of salinity (15, 25, 35, 45, and 55‰) on growth, biochemical composition, and lipid productivity of Nannochloropsis oculata CS 179 was investigated under controlled cultivation in a 19‐day study. The results demonstrate that the dry biomass of N. oculata was the highest at a salinity of 25‰ among the treatments in the first 10‐day cultivation (P<0.05). During days 14–19 (stage III), the dry biomass productivity was the highest at a salinity of 35‰ (P<0.05). The algae had the highest chlorophyll a content (26.47 mg g^?1) at 25‰ in stage I, and it decreased continuously at stage III. Protein content (as% of dry biomass) of algae reached the highest value of 42.25 ± 2.10% at 15‰, and the lipid content was the highest of 32.11 ± 1.30% of dry biomass at 25‰. However, the lipid productivity of these algae was the highest at 35‰ (64.71 mg L^?1 d^?1; P<0.001). C16 series content was the highest among the total fatty acid methyl esters (FAME), and eicosapentaenoic acid C20:5n‐3 (EPA) content was high at the low salinity. Fatty acid profiles of N. oculata varied significantly under different salinities.     

Testing the efficiency of three 15N-labeled nitrogen compounds for indirect labeling of grasshoppers via plants in the field

In field studies of plant–insect herbivore interactions it is often difficult to establish which herbivore has fed on a particular plant. We investigated the suitability of three different ¹⁵N‐labeled nitrogen compounds (ammonium, nitrate, and glycine) for indirect marking of three grasshopper species [Omocestus viridulus (L.), Chorthippus parallelus (Zett.), and Chorthippus biguttulus (L.) (Orthoptera: Acrididae)] through labeling their food plants in the field. In two short‐term experiments grassland plots of 1 m² were separately labeled with either one of the different nitrogen compounds. Grasshoppers were caged on three food‐plant species [Dactylis glomerata L., Holcus lanatus L. (Poaceae), and Trifolium repens L. (Fabaceae)] present in these plots for 72 h. Significantly enriched δ¹⁵N values in grasshoppers were found in all plant/grasshopper combinations. Enrichment in grasshoppers was positively correlated with the enrichment of plants and labeling with nitrate resulted in highest ¹⁵N enrichment. In a long‐term experiment, individuals of C. biguttulus were placed in a cage covering an area of 1 m² for 37 days, with sampling of grasshoppers at regular intervals. δ¹⁵N values of the grasshopper and a common food plant, D. glomerata, increased steadily over time, up to 40‐fold by the end of the experiment. Our results demonstrate that ¹⁵N‐labeling of plants is an appropriate tool for the investigation of insect–plant interactions under natural conditions.     

Foraging ecology and niche overlap in pygmy (Kogia breviceps) and dwarf (Kogia sima) sperm whales from waters of the U.S. mid‐Atlantic coast

A complementary approach of stomach content and stable isotope analyses was used to characterize the foraging ecology and evaluate niche overlap between pygmy (Kogia breviceps) and dwarf (K. sima) sperm whales stranded on the U.S. mid‐Atlantic coast between 1998 and 2011. Food habits analysis demonstrated both species were primarily teuthophagous, with 35 species of cephalopods, and 2 species of mesopelagic fishes represented in their overall diets. Pianka's Index of niche overlap suggested high overlap between whale diets (O_n = 0.92), with squids from the families Histioteuthidae, Cranchidae, and Ommastrephidae serving as primary prey. Pygmy sperm whales consumed slightly larger prey sizes (mean mantle length [ML] = 10.8 cm) than dwarf sperm whales (mean ML = 7.8 cm). Mean prey sizes consumed by pygmy sperm whales increased with growth, but showed no trend in dwarf sperm whales. Significant differences were not detected in δ¹⁵N and δ¹³C values of muscle tissues from pygmy (10.8‰ ± 0.5‰, ?17.1‰ ± 0.6‰), and dwarf sperm whales (10.7‰ ± 0.5‰, ?17.0‰ ± 0.4‰), respectively. Isotopic niche widths also did not differ significantly and dietary overlap was high between the two species. Results suggest the feeding ecologies of the pygmy and dwarf sperm whales are similar and both species occupy equivalent trophic niches in the region.     

Role of six European tree species and land‐use legacy for nitrogen and water budgets in forests

Water and nutrient fluxes for single stands of different tree species have been reported in numerous studies, but comparative studies of nutrient and hydrological budgets of common European deciduous tree species are rare. Annual fluxes of water and inorganic nitrogen (N) were established in a 30‐year‐old common garden design with stands of common ash (Fraxinus excelsior), European beech (Fagus sylvatica L.), pedunculate oak (Quercus robur), small‐leaved lime (Tilia cordata Mill.), sycamore maple (Acer pseudoplatanus) and Norway spruce (Picea abies [L.] Karst.) replicated at two sites in Denmark, Mattrup and Vallø during 2 years. Mean annual percolation below the root zone (mm yr^?1±SE, n=4) ranked in the following order: maple (351±38)>lime (284±32), oak (271±25), beech (257±30), ash (307±69)? spruce (75±24). There were few significant tree species effects on N fluxes. However, the annual mean N throughfall flux (kg N ha^?1 yr^?1±SE, n=4) for spruce (28±2) was significantly larger than for maple (12±1), beech (11±1) and oak (9±1) stands but not different from that of lime (15±3). Ash had a low mean annual inorganic N throughfall deposition of 9.1 kg ha^?1, but was only present at Mattrup. Annual mean of inorganic N leaching (kg ha^?1 yr^?1±SE, n=4) did not differ significantly between species despite of contrasting tree species mean values; beech (25±9)>oak (16±10), spruce (15±8), lime (14±8)? maple (1.9±1), ash (2.0±1). The two sites had similar throughfall N fluxes, whereas the annual leaching of N was significantly higher at Mattrup than at Vallø. Accordingly, the sites differed in soil properties in relation to rates and dynamics of N cycling. We conclude that tree species affect the N cycle differently but the legacy of land use exerted a dominant control on the N cycle within the short‐term perspective (30 years) of these stands.     

Taxon-specific variation in the stable isotopic signatures (δ13C and δ15N) of lake phytoplankton

1. The variability in the stable isotope signatures of carbon and nitrogen (δ¹³C and δ¹⁵N) in different phytoplankton taxa was studied in one mesotrophic and three eutrophic lakes in south‐west Finland. The lakes were sampled on nine to 16 occasions over 2–4 years and most of the time were dominated by cyanobacteria and diatoms. A total of 151 taxon‐specific subsamples covering 18 different phytoplankton taxa could be isolated by filtration through a series of sieves and by flotation/sedimentation, followed by microscopical identification and screening for purity. 2. Substantial and systematic differences between phytoplankton taxa, seasons and lakes were observed for both δ¹³C and δ¹⁵N. The values of δ¹³C ranged from ?34.4‰ to ?5.9‰ and were lowest in chrysophytes (?34.4‰ to ?31.3‰) and diatoms (?30.6‰ to ?26.6‰). Cyanobacteria were most variable (?32.4‰ to ?5.9‰), including particularly high values in the nostocalean cyanobacterium Gloeotrichia echinulata (?14.4‰ to ?5.9‰). For δ¹³C, the taxon‐specific amplitude of temporal changes within a lake was usually <1–8‰ (<1–4‰ for microalgae alone and <1–8‰ for cyanobacteria alone), whereas the amplitude among taxa within a water sample was up to 31‰. 3. The values of δ¹⁵N ranged from ?2.1‰ to 12.8‰ and were high in chrysophytes, dinophytes and diatoms, but low in the nitrogen‐fixing cyanobacteria Anabaena spp., Aphanizomenon spp. and G. echinulata (?2.1‰ to 1.6‰). Chroococcalean cyanobacteria ranged from ?1.4‰ to 8.9‰. For δ¹⁵N, the taxon‐specific amplitude of temporal changes within a lake was 2–6‰, (2–6‰ for microalgae alone and 2–4‰ for cyanobacteria alone) and the amplitude among taxa within a water sample was up to 11‰. 4. The isotopic signatures of phytoplankton changed systematically with their physical and chemical environment, most notably with the concentrations of nutrients, but correlations were non‐systematic and site‐specific. 5. The substantial variability in the isotopic signatures of phytoplankton among taxa, seasons and lakes complicates the interpretation of isotopic signatures in lacustrine food webs. However, taxon‐specific values and seasonal patterns showed some consistency among years and may eventually be predictable.     

Identifying critical habitat of the endangered vaquita (Phocoena sinus) with regional δ13C and δ15N isoscapes of the Upper Gulf of California,Mexico

The vaquita (Phocoena sinus) is the world's most endangered cetacean and has experienced a 60% reduction in the size of its population in the past decade. Knowledge of its basic ecology is essential for developing successful management plans to protect and conserve this species. In this study, we identified vaquita foraging areas by creating an isoscape of the Upper Gulf of California (UGC) based on sediment and zooplankton carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope values. Our results confirm that this species is confined to the western region of the UGC, which is characterized by relatively high δ¹⁵N values (sediments: 10.2‰ ± 2.0‰, zooplankton: 15.8‰ ± 1.3‰), higher sea surface temperatures (～16°C–25°C), higher concentrations of silt in sediments, and the highest turbidity. In contrast, the eastern region of the UGC had relatively low sediment (7.7‰ ± 2.4‰) and zooplankton (14.6‰ ± 1.0‰) δ¹⁵N values, and the highest concentrations of sand in sediments. Our approach is an effective use of marine isoscapes over a small spatial scale (<200 km) to identify the environmental characteristics that define the critical habitat for an extremely endangered marine mammal.     

DIFFERENCES IN FORAGING LOCATION OF MEXICAN AND CALIFORNIA ELEPHANT SEALS: EVIDENCE FROM STABLE ISOTOPES IN PUPS

Female northern elephant seals, Mirounga angustirostris, from Año Nuevo (AN) in central California feed offshore in mid‐latitude waters (40°–55°N). Migratory patterns and foraging locations of seals from Mexico are unknown. Rookeries on San Benitos (SB) islands in Baja California Sur, Mexico, are ～1,170 km south of AN. Although the colonies are similar in size, seals from SB begin breeding earlier and have an earlier breeding birthing peak than seals from AN. To determine if the foraging location of seals from Mexico was similar to that of seals from California, we measured δ¹³C and δ¹⁵N values in the hair of 48 suckling pups at SB and 37 from AN, assuming that their isotopic signatures reflected those of mothers' milk, their exclusive diet. The mean δ¹³C and δ¹⁵N values for SB pups (?16.1‰± 0.9‰ and 17.7‰± 0.9‰, respectively) were significantly higher than those for AN pups (?17.6‰± 0.4‰ and 15.6‰± 1.0‰, respectively). From data on environmental isotope gradients and known behavior of SB and AN populations, we hypothesize that the isotope differences are due to females in the SB colony foraging ～8° south of seals from AN. This hypothesis can be tested by deployment of satellite tags on adult females from the SB colony.