Differences In δ15N Research Articles

δ13C versus δ15N of co-occurring molluscs within a community dominated by Crassostrea gigas and Crepidula fornicata (Oosterschelde, The Netherlands)

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 240:291-295 (2002) - doi:10.3354/meps240291 δ13C versus δ15N of co-occurring molluscs within a community dominated by Crassostrea gigas and Crepidula fornicata (Oosterschelde, The Netherlands) P. Riera1,2,*, L. J. Stal2, J. Nieuwenhuize2 1Station Biologique de Roscoff, Université Pierre & Marie Curie Paris VI - CNRS, Place Georges-Teissier, BP 74, 29682 Roscoff cedex, France 2Centre for Estuarine and Coastal Ecology, Netherlands Institute of Ecology (NIOO-CEMO), Postbus 140, 4400 AC Yerseke, The Netherlands *E-mail: riera@sb-roscoff.fr ABSTRACT: The aim of this study was to investigate, by use of δ13C and δ15N, the diet of co-occurring intertidal molluscs species within a community dominated by the oyster Crassostrea gigas and its epibiont, the common Atlantic slipper limpet Crepidula fornicata. The results showed that the δ13C versus δ15N values of C. gigas differed from those of the deposit feeders considered but also from the other suspension-feeding molluscs (i.e. Mytilus edulis, Cerastoderma edule and C. fornicata), being more depleted in 13C and in 15N. The results suggest more similarities in the assimilated food sources of C. fornicata, C. edule and M. edulis. In particular, the δ13C and δ15N differences between C. gigas and C. fornicata suggest that these associated suspension feeders in many shellfish culture areas may not necessarily be competitors for food sources when food is not limited. KEY WORDS: Benthic food web · δ13C · δ15N · Molluscs · Crassostrea gigas · Crepidula fornicata · Oosterschelde Full text in pdf format PreviousExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 240. Online publication date: September 12, 2002 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2002 Inter-Research.

Size-related differences in feeding habitat use of adult female loggerhead turtles Caretta caretta around Japan determined by stable isotope analyses and satellite telemetry

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 233:273-281 (2002) - doi:10.3354/meps233273 Size-related differences in feeding habitat use of adult female loggerhead turtles Caretta caretta around Japan determined by stable isotope analyses and satellite telemetry Hideo Hatase1,*, Noriyuki Takai1,**, Yoshimasa Matsuzawa1,***, Wataru Sakamoto1, Kazuyoshi Omuta2, Kiyoshi Goto3, Nobuaki Arai4, Tateki Fujiwara1 1Laboratory of Fisheries and Environmental Oceanography, Graduate School of Agriculture, Kyoto University, Sakyo, Kyoto 606-8502, Japan 2Yakushima Sea Turtle Research Group, 489-8 Nagata, Kamiyaku, Kagoshima 891-4201, Japan 3Minabe Sea Turtle Research Group, 278 Higashiyoshida, Minabe, Wakayama 645-0001, Japan 4Laboratory of Biosphere Informatics, Graduate School of Informatics, Kyoto University, Sakyo, Kyoto 606-8501, Japan *E-mail: hatase@kais.kyoto-u.ac.jp Present addresses: **Chugoku National Industrial Research Institute, 2-2-2 Hiro-Suehiro, Kure, Hiroshima 737-0197, Japan ***Sea Turtle Association of Japan, 5-17-18-302 Nagao-motomachi, Hirakata, Osaka 573-0163, Japan ABSTRACT: Stable isotope analyses and satellite telemetry were used to investigate the relationship between the body size and feeding habitat of adult female loggerhead turtles Caretta caretta around Japan. A total of 149 females from 2 nesting sites, 600 km apart, 1 in Minabe and 1 in Yakushima, was examined by stable isotope analyses of the egg-yolk in 1998 and 1999. Since there were neither significant intra- nor inter-clutch variations in δ13C or δ15N egg-yolks from the same female, the isotope signatures of a single egg-yolk from any clutch in a nesting season could be used to represent those of its nesting female. Both isotopic values increased as the body size of the nesting female increased, irrespective of nesting site. Comparisons between isotopic values of loggerhead egg-yolks and prey items revealed size-related differences among both feeding habits (planktonic or benthic) and habitat areas (pelagic or neritic) of female loggerheads. In 1999, 5 females nesting in Minabe were tracked by satellite. After the nesting season, 2 females with low isotopic values migrated to the pelagic Pacific along the Kuroshio Current, whereas 3 with high isotopic values migrated to the neritic East China Sea. The former females were smaller than the latter. These results were consistent with inferences from stable isotope analyses and previous findings, indicating that the body size, habitat area, and feeding habit of female loggerheads around Japan are closely related. In addition, there were no significant differences in δ13C or δ15N between recruits and remigrants, implying that female loggerheads do not change their feeding habitats during the reproductive stage. We postulate that female habitat selection as a function of body size is closely related to the recruitment and settlement in immature loggerheads, which return to Japan after developmental migrations in the North Pacific. KEY WORDS: Caretta caretta · Adult female · Body size · Feeding habitat · Stable isotope analyses (δ13C, δ15N) · Satellite telemetry · Western North Pacific Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 233. Online publication date: May 21, 2002 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2002 Inter-Research.

REGULAR PAPERS Unusual stable isotope fractionation patterns observed for fish host—parasite trophic relationships

Trophic relationships between four taxa of fish parasite (Cestoda, Nematoda, Isopoda, Copepoda) and their hosts (Gasterosteus aculeatus, Merlangius merlangus, Boops boops, Platichthys flesus) were investigated using stable isotopes of carbon and nitrogen. δ15N differences between parasite and host were unlike those conventionally observed among consumers and their diets. In no case were parasites enriched in 15N with respect to the host organism; endoparasites were significantly and consistently depleted. In no case were parasites enriched in 15N with respect to the host organism; endoparasites were significantly and consistently depleted.

Responses of consumers and food resources to a high magnitude, unpredicted flood in the upper Mississippi River basin

AbstractThe Mississippi and Missouri Rivers experienced flooding in 1993 that fell outside the annual predictable flood period of spring and early summer. Flooding began in late June, peaked in late July (25 232 m3/s on the upper Mississippi and 21 240 m3/s on the Missouri) and remained at or near flood stage into October 1993. This study was performed to determine if disturbance by an unpredicted flood event would alter trophic dynamics of river–floodplain systems by creating shifts in the composition of organic matter available to consumers. The Ohio River, which did not flood during the same period, was examined for comparison. Stable isotopic ratios of carbon and nitrogen from samples collected in 1993 and 1994 were used to characterize potential food sources and determine linkages between food sources and invertebrate and fish consumers. Pairwise contrasts, performed separately for each river, indicated there were few interannual differences in δ13C and δ15N of organic matter sources and consumers. Between sample period (flood year versus normal water year) trends in both flooded rivers were similar to between‐year trends observed for the Ohio River. Trophic structure of the Mississippi and Ohio Rivers was similar in both years, with fine and ultra‐fine transported organic matter and dissolved organic matter representing the major sources of organic matter. Overlapping isotopic signatures in the Missouri River made tracking of sources through the consumers difficult, but similarities in δ13C and δ15N between years indicated trophic structure did not change in response to the flood. The results suggest that consumers continued to rely on sources of organic matter that would be used in the absence of the unpredicted 1993 flood. It is proposed that trophic structure did not change in response to flooding in the Mississippi and Missouri Rivers because both rivers exhibited the same trends observed in the Ohio River. Copyright © 2001 John Wiley & Sons, Ltd.

D15N isotope studies of life history and trophic position of Fundulus heteroclitus and Menidia menidia

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 214:299-305 (2001) - doi:10.3354/meps214299 d15N isotope studies of life history and trophic position of Fundulus heteroclitus and Menidia menidia Martin P. A. Griffin1, Ivan Valiela2,* 1State of Wisconsin, Department of Natural Resources, 101 S. Webster St., PO Box 7921, Madison, Wisconsin 53707-7921, USA 2Boston University Marine Program, Marine Biological Laboratory, Woods Hole, Massachusetts 02543, USA *Corresponding author. E-mail: mpagriffin@yahoo.com ABSTRACT: We used δ15N isotopic signatures to follow the trophic position as well as the life history of 2 of the most abundant fish species of the Waquoit Bay estuarine system: common killifish Fundulus heteroclitus and Atlantic silverside Menidia menidia. Juvenile fish bore isotopic signatures specific to the estuary in which they had been growing. The fish changed in δ15N as they grew and had significant, estuary-specific differences in δ15N signatures. F. heteroclitus increased in length by 3 to 4 cm as they increased in δ15N by up to 4”. The magnitude of the shift in δ15N is consistent with movement of the fish ~1 trophic step up the food web. F. heteroclitus remained within their native estuaries year-round, and retained the isotopic signatures of their prey in each estuary. M. menidia moving offshore during the winter months lose estuary-specific differences in δ15N, and appear to feed at a higher trophic level than fish residing in the estuaries. Evidence from gut content analysis, coupled with results of the stable isotopic measurement, closely concurs with the life history characteristics of these 2 well-known fish species, and provides additional insight into life history habits and feeding. KEY WORDS: Stable isotopes · Food webs · Trophic level · Life history · Migration · Nitrogen loading Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 214. Online publication date: April 26, 2001 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2001 Inter-Research.

Feeding relationships in Mediterranean bathyal assemblages elucidated by carbon and nitrogen stable-isotope data

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 220:13-23 (2001) - doi:10.3354/meps220013 Feeding relationships in Mediterranean bathyal assemblages elucidated by stable nitrogen and carbon isotope data N. V. C. Polunin1,*, B. Morales-Nin2, W. E. Pawsey1, J. E. Cartes3, J. K. Pinnegar1, J. Moranta2 1Department of Marine Sciences & Coastal Management, University of Newcastle, Newcastle upon Tyne NE1 7RU, United Kingdom 2Consejo Superior de Investigaciones Cientificas-Universitat de les Illes Balears, Institut Mediterrani d¹Estudis Avançats, Miguel Marqes 21, 07190 Esporles, Mallorca, Spain 3Consejo Superior de Investigaciones Cientificas, Instituto de Ciencias del Mar, Paseo Juan de Borbón s/n, 08039 Barcelona, Spain *E-mail: n.polunin@ncl.ac.uk ABSTRACT: Fishes and invertebrates were collected from the continental slope southwest of the Balearic Islands, Mediterranean, in October 1996 and May 1998 from 3 faunally-distinct depth zones: 200 to 800 m (upper slope), 800 to 1425 m (middle slope) and 1425 to 1800 m (lower slope). Differences in δ15N and δ13C between October 1996 and May 1998 were observed only in the crustaceans Aristeus antennatus and Nematocarcinus exilis. There were few differences among depths in δ15N or δ13C of benthic fishes or crustaceans: the shrimps A. antennatus and Acanthephyra eximia showed higher δ15N values at greater depth, and δ13C values of the fishes Alepocephalus rostratus (Alepocephalidae) and Coelorhynchus coelorhynchus (Macrouridae) also tended to increase with increasing depth. Differences in plankton and suprabenthos were also observed among zones (at depth, greater δ15N values of gelatinous plankton, calanoid copepods and peracarids were recorded, along with smaller δ13C values of gelatinous plankton and peracarids). These are attributed to a microbially-mediated increase in δ15N and a reduction in δ13C of the particulate material remaining after preferential utilization of labile organic matter in the stable and relatively high (13°C) temperature of the Mediterranean water column. Strong positive correlations between δ15N and δ13C data for plankton, fishes and crustaceans indicated a single primary source material for these communities, which is attributed to marine snow. KEY WORDS: Deep-sea · Mesozooplankton · Fishes · Crustaceans · Suprabenthos · Mediterranean · Bathyal Full text in pdf format PreviousNextExport citation Mail this link - Contents Mailing Lists - RSS Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 220. Online publication date: September 27, 2001 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2001 Inter-Research.

WHAT CAN STABLE ISOTOPES (δ15N AND δ13C) TELL ABOUT THE FOOD WEB OF SOIL MACRO-INVERTEBRATES?

Animals reflect the stable isotope ratios (δ15N and δ13C) of their diet, with a slight enrichment in 15N that allows the use of δ15N as a trophic-level indicator. Stable isotope contents were measured for the litter, soil, and macro-invertebrates of three temperate deciduous forest sites, in spring, summer, and autumn, to study the trophic structure of this community. No distinct trophic structure could be derived from measurements of δ13C. In contrast, when the δ15N values of all animal species were grouped together, the hypothesis of an isotopically similar diet was rejected. Therefore, the community spreads over more than one trophic level and was subdivided into detritivores and predators. The potential detritivore food sources in the forest litter and soil showed a variety of isotopic ratios. Despite this fact, the variance of the isotopic ratios of the detritivorous species was not larger than what could be expected from interspecific variability of trophic isotopic enrichment alone. This was also the case for the predators in most of the sample sets. However, in some cases this variance was significantly larger, due to a small number of species with high δ15N values. The δ15N values of the detritivores indicated that the mean δ15N value of their food was close to that measured for the superficial litter layers. The difference in δ15N between detritivores and predators was highly significant and never significantly different from the value expected for one trophic transfer (3.4‰), although often slightly higher. Most of the litter macro-invertebrate community we studied can therefore be described as belonging to two trophic levels, one feeding on the superficial litter layers (or on soil fractions that have a similar δ15N value), and a second trophic level feeding on the first, with some indication of intraguild predation among the predators. Between-site differences of up to 7‰ were found for δ15N in the litter, and the δ15N values of the whole animal community were shifted in accordance with the local value of the litter δ15N. Therefore, the trophic structure must be studied in relation to the local isotopic content of the litter. Seasonal differences in isotopic ratios of the litter or animal samples were neither large nor consistent. These findings indicate similar trophic structure of the communities at the three sites and during the three sampling periods.

Correlations between foliar δ15N and nitrogen concentrations may indicate plant-mycorrhizal interactions.

Nitrogen isotope measurements may provide insights into changing interactions among plants, mycorrhizal fungi, and soil processes across environmental gradients. Here, we report changes in δ15N signatures due to shifts in species composition and nitrogen (N) dynamics. These changes were assessed by measuring fine root biomass, net N mineralization, and N concentrations and δ15N of foliage, fine roots, soil, and mineral N across six sites representing different post-deglaciation ages at Glacier Bay, Alaska. Foliar δ15N varied widely, between 0 and -2‰ for nitrogen-fixing species, between 0 and -7‰ for deciduous non-fixing species, and between 0 and -11‰ for coniferous species. Relatively constant δ15N values for ammonium and generally low levels of soil nitrate suggested that differences in ammonium or nitrate use were not important influences on plant δ15N differences among species at individual sites. In fact, the largest variation among plant δ15N values were observed at the youngest and oldest sites, where soil nitrate concentrations were low. Low mineral N concentrations and low N mineralization at these sites indicated low N availability. The most plausible mechanism to explain low δ15N values in plant foliage was a large isotopic fractionation during transfer of nitrogen from mycorrhizal fungi to plants. Except for N-fixing plants, the foliar δ15N signatures of individual species were generally lower at sites of low N availability, suggesting either an increased fraction of N obtained from mycorrhizal uptake (f), or a reduced proportion of mycorrhizal N transferred to vegetation (T r). Foliar and fine root nitrogen concentrations were also lower at these sites. Foliar N concentrations were significantly correlated with δ15N in foliage of Populus, Salix, Picea, and Tsuga heterophylla, and also in fine roots. The correlation between δ15N and N concentration may reflect strong underlying relationships among N availability, the relative allocation of carbon to mycorrhizal fungi, and shifts in either f or T r.

Differential fractionation of δ13C and δ15N among fish tissues: implications for the study of trophic interactions

1. Stable isotopes of nitrogen and carbon are widely used in the evaluation and understanding of trophic interactions, and especially so in aquatic food‐webs. However, differences among tissues which occur in consumers such as fish may confound resolution of these ecological relationships, and greatly affect dietary compositions calculated through mass‐balance equations. 2.δ13C and δ15N were determined for homogenized whole fry (≈ 2·7 g) and particular tissues (white muscle, red muscle, liver and heart) of juveniles (≈ 20·6 g) of Rainbow Trout Oncorhynchus mykiss. 3. Differences in δ13C and δ15N were observed among tissues; removal of lipid resulted in these tissues becoming statistically indistinguishable but more variable in δ13C, particularly in tissues with higher lipid content. Differences in δ15N among tissues may be related to the relative composition of ‘essential’ and ‘non‐essential’ amino acids. 5. Acidification, which is used to reduce variability caused by the presence of inorganic carbonate, had a significant effect on δ15N, while having a non‐significant effect on the δ13C. This was the case for whole ground fish‐fry, which contained 22·6% inorganic carbonates in the form of bones and scales, and for individual fish tissues which contained only very small amounts of inorganic carbonate. 6. White muscle was found to be less variable in δ13C and δ15N than all other tissues, and is probably the best tissue for use in ecological work. Red muscle, which is often closely associated with white muscle, is more variable in δ13C and may constitute a source of significant error in source material identification and dietary overlap.

Spatial variability in Mussels used to assess base level nitrogen isotope ratio in freshwater ecosystems

Freshwater mussels have been used to establish base level nitrogen isotope ratio values (δ15N) used in trophic position and food web studies in freshwater ecosystems. In this study, we assess the variability introduced when using unionid mussels in this manner by investigating the spatial variation in δ15N values in mussels from different locations in shallow freshwater ponds, and also differences in δ15N in different tissue types in the mussels. Results from the analysis of adductor, foot and a section of the mantle tissues from mussels across all ponds showed that adductor tissue was consistently enriched by about 1‰ versus the mantle and foot. The foot showed the least variability which, coupled with ease in obtaining consistent samples, led us to select this tissue type for subsequent analysis. The six ponds included in the study had average mussel δ15N values ranging from 4.9‰ to 11.9‰. Four of the six ponds showed no significant within pond differences between δ15N values from mussels collected at different sampling sites. The range of mussel δ15N values obtained from within ponds showed that using data from a single mussel to assess baseline δ15N values would result in the introduction of a maximum error of 0.56 of a trophic level to subsequent trophic position calculations.

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.

Vascular plant 15N natural abundance in heath and forest tundra ecosystems is closely correlated with presence and type of mycorrhizal fungi in roots.

In this study we show that the natural abundance of the nitrogen isotope 15, δ15N, of plants in heath tundra and at the tundra-forest ecocline is closely correlated with the presence and type of mycorrhizal association in the plant roots. A total of 56 vascular plant species, 7 moss species, 2 lichens and 6 species of fungi from four heath and forest tundra sites in Greenland, Siberia and Sweden were analysed for δ15N and N concentration. Roots of vascular plants were examined for mycorrhizal colonization, and the soil organic matter was analysed for δ15N, N concentration and soil inorganic, dissolved organic and microbial N. No arbuscular mycorrhizal (AM) colonizations were found although potential host plants were present in all sites. The dominant species were either ectomycorrhizal (ECM) or ericoid mycorrhizal (ERI). The δ15N of ECM or ERI plants was 3.5-7.7‰ lower than that of non-mycorrhizal (NON) species in three of the four sites. This corresponds to the results in our earlier study of mycorrhiza and plant δ15N which was limited to one heath and one fellfield in N Sweden. Hence, our data suggest that the δ15N pattern: NON/AM plants > ECM plants ≥ ERI plants is a general phenomenon in ecosystems with nutrient-deficient organogenic soils. In the fourth site, a␣birch forest with a lush herb/shrub understorey, the differences between functional groups were considerably smaller, and only the ERI species differed (by 1.1‰) from the NON species. Plants of all functional groups from this site had nearly twice the leaf N concentration as that found in the same species at the other three sites. It is likely that low inorganic N availability is a prerequisite for strong δ15N separation among functional groups. Both ECM roots and fruitbodies were 15N enriched compared to leaves which suggests that the difference in δ15N between plants with different kinds of mycorrhiza could be due to isotopic fractionation at the␣fungal-plant interface. However, differences in δ15N between soil N forms absorbed by the plants could also contribute to the wide differences in plant δ15N found in most heath and forest tundra ecosystems. We hypothesize that during microbial immobilization of soil ammonium the microbial N pool could become 15N-depleted and the remaining, plant-available soil ammonium 15N-enriched. The latter could be a main source of N for NON/AM plants which usually have high δ15N. In contrast, amino acids and other soil organic N compounds presumably are 15N-depleted, similar to plant litter, and ECM and ERI plants with high uptake of these N forms hence have low leaf δ15N. Further indications come from the δ15N of mosses and lichens which was similar to that of ECM plants. Tundra cryptogams (and ECM and ERI plants) have previously been shown to have higher uptake of amino acid than ammonium N; their low δ15N might therefore reflect the δ15N of free amino acids in the soil. The concentration of dissolved organic N was 3-16 times higher than that of inorganic N in the sites. Organic nitrogen could be an important N source for ECM and, in particular, ERI plants in heath and forest tundra ecosystems with low release rate of inorganic N from the soil organic matter.

THE STABLE NITROGEN ISOTOPE RATIO AS A MARKER OF FOOD-WEB INTERACTIONS AND FISH MIGRATION

We used stable nitrogen isotopes to describe the pelagic food-web structure of three coastal Baltic Sea areas, each of which was sampled twice. Two of the areas were influenced by 15N-rich nutrient discharges from a sewage treatment plant. Analyses were made of particulate organic matter (<35 μm, mainly phytoplankton), zooplankton, mysids (Mysis mixta and M. relicta), sprat (Sprattus sprattus), smelt (Osmerus eperlanus), four size classes of herring (Clupea harengus), and pikeperch (Stizostedion lucioperca). Discharges from the sewage treatment plant significantly increased δ15N values in the whole food web, from phytoplankton to piscivorous fish. Based on nitrogen isotopic compositions, consistent trophic food-web structures were observed on both occasions and in all three areas. The results indicate that zooplankton and mysids may have more complex diets than assumed before. Apparent trophic fractionation, i.e., differences in δ15N between a consumer and its assumed food, averaged 2.4‰ with a standard e...

The Stable Nitrogen Isotope Ratio as a Marker of Food-Web Interactions and Fish Migration

We used stable nitrogen isotopes to describe the pelagic food-web structure of three coastal Baltic Sea areas, each of which was sampled twice. Two of the areas were influenced by 15N-rich nutrient discharges from a sewage treatment plant. Analyses were made of particulate organic matter (<35 μm, mainly phytoplankton), zooplankton, mysids (Mysis mixta and M. relicta), sprat (Sprattus sprattus), smelt (Osmerus eperlanus), four size classes of herring (Clupea harengus), and pikeperch (Stizostedion lucioperca). Discharges from the sewage treatment plant significantly increased δ15N values in the whole food web, from phytoplankton to piscivorous fish. Based on nitrogen isotopic compositions, consistent trophic food-web structures were observed on both occasions and in all three areas. The results indicate that zooplankton and mysids may have more complex diets than assumed before. Apparent trophic fractionation, i.e., differences in δ15N between a consumer and its assumed food, averaged 2.4‰ with a standard error of ±0.5‰. Differences between areas in fish δ15N show young-of-the-year herring, sprat, smelt, and pikeperch to be relatively non-migratory.

Comparison of the difference and delta 15Nitrogen approaches for evaluating liquid urea ammonium nitrate utilization by maize

Isotopic nitrogen (N) research techniques may be required in watershed studies to determine the impact of landscape position on fertilizer efficiency and the soil supplying power. However, traditional approaches using I5N labeled fertilizer may not be suitable when farmer equipment is used. The δ15N natural abundance isotopic approach has been used to evaluate N cycling in watersheds. The objectives of this study were to measure the precision of the δ15N measurement by the Europa 20–20 ratio mass spectrometer (Europa Scientific Ltd, UK), and to compare the difference and δ15N approaches for measuring fertilizer use by maize (Zea mays). A replicated field study containing two different N rates (0 and 15.7 g N m‐2) were used for the study. Maize samples were collected at the 8th‐leaf, silking, and plant maturity in 1992 and 1993. Samples were dried (80°C), ground (<1‐mm), weighed (stover 12 mg and grain 3 mg), and analyzed fortotal N and δ15N. Fertilizer utilization at the three growth stages was determined using the natural abundance δ15N and nonisotopic difference (fertilizer‐control) techniques. During the study, the Europa 20–20 ratio mass spectrometer (Europa Scientific Ltd, UK) analyzed over a 100 samples a day and had consumable costs of less than $2.00 per sample. The standard deviations of the mean were less than 0.11 and 0.21 %o in 51 and 77% of the stover samples, respectively. In 1992, grain yields were not influenced by N fertilizer additions, while in 1993 grain yields were increased by N fertilizer. The difference method estimated that in 1992, 16% of the N fertilizer was utilized by the crop, while the natural abundance δ15N approach estimated that 36% of the fertilizer N was used by the crop. Differences between calculated values by the two techniques resulted from the difference method calculating net fertilizer use, while the δ15N approach calculated fertilizer contained in the plant. Because the δ15N approach estimates fertilizer use, this approach can be used to calculate soil N contained in the plant. In watershed studies, this information may provide the tools needed to evaluate N use in responsive and nonresponsive sites within a field. This research shows that the δ15N method compliments the difference method, can be used to measure actual fertilizer use when farmer equipment is used, and that the Europa 20–20 ratio mass spectro‐meter (Europa Scientific Ltd, UK) has acceptable precision for the δ15N natural abundance approach.

Preliminary studies of the impact of excreted N on cycling and uptake of N in pasture systems using natural abundance stable isotopic discrimination

The possibility of using natural abundance techniques to determine N transformations and flows after deposition of cattle dung has been examined. These preliminary results showed that δ15N in dung was greater than in plants growing in association with particular pats. This, and other observational information, indicated that dung pats of different ages were being examined. There were significant variations in plant δ15N signatures within and between species grown in association and away from the dung. It was probable that variation in plant δ15N was brought about by changes in soil mineral N pools after transfer of N derived from the dung. This resulted in different δ15N signatures in Trifolium repens (because of changes in N utilization from soil or atmospheric pools), in Lolium perenne (because of changes in δ15N in soil mineral N), but not in Ranunculus repens (because the majority of active roots were outside the range of immediate influence of the deposited dung). The differences in δ15N allowed the development of hypotheses for changes in soil N pools and the acquisition of N by plants from soil, dung or atmospheric sources.

Water and nitrogen dynamics in an arid woodland.

Arid environments are characterized by spatial and temporal variation in water and nitrogen availability. differences in δ15N and δD of four co-occurring species reveal contrasting patterns of plant resource acquisition in response to this variation. Mineralization potential and nitrogen concentration of surface soils associated with plant canopies were greater than inter-canopy locations, and values decreased with increasing depth in both locations. Mineralization potential and nitrogen concentration were both negatively correlated with soil δ15N. The spatial variation in soil δ15N caused corresponding changes in plant δ15N such that plant δ15N values were negatively correlated with nitrogen concentration of surface soils. Plants occurring on soils with relatively high nitrogen concentrations had lower δ15N, and higher leaf nitrogen concentrations, than plants occurring on soils with relatively low nitrogen concentrations. Two general temporal patterns of water and nitrogen use were apparent. Three species (Juniperus, Pinus andArtemisia) relied on the episodic availability of water and nitrogen at the soil surface. δ15N values did not vary through the year, while xylem pressure potentials and stem-water δD values fluctuated with changes in soil moisture at the soil surface. In contrast,Chrysothamnus switched to a more stable water and nitrogen source during drought. δ15N values ofChrysothamnus increased throughout the year, while xylem pressure potentials and stem-water δD values remained constant. The contrasting patterns of resource acquisition have important implications for community stability following disturbance. Disturbance can cause a decrease in nitrogen concentration at the soil surface, and so plants that rely on surface water and nitrogen may be more susceptible than those that switch to more stable water and nitrogen sources at depth during drougnt.

The use of natural abundance of nitrogen isotopes in plant physiology and ecology

ABSTRACTNitrogen stable isotope (15N, 14N) natural abundance has been much less used than carbon isotopes (13C, 12C) in plant physiology and ecology. Analytical problems, the lower fractional abundance of 15N than of 13C in the biosphere, the greater complexity of the N cycle relative to the C cycle, and smaller expressed discriminations in nature, are contributing factors. The major N pools, globally, have different isotope signatures: atmospheric N2 is 15N‐depleted relative to organic N (including sedimentary N), a situation resulting from a greater expressed discrimination in the organic N to N2 (via denitrification) reaction than of diazotrophy during accumulation of the reduced N. Essentially all of the enzymes except nitrogenase which transform N compounds show discrimination against 15N, although for glutamine synthetase, and the amination of 2‐oxoglutarate and pyruvate, this is only seen in terms of NH4+ rather than the true substrate, NH3. Discrimination is expressed in various N interconversions within plants, leading to substantial differences in δ15N (up to 12‰) among N compounds and macroscopic plant parts. N isotope fractionation during assimilation of exogenous combined N is often much lower than that expected from studies of isolated enzymes due to processes which show very little discrimination, such as limitation by transport through aqueous solution and membranes. Application of 15N/14N discrimination studies to plant ecology have concentrated largely on distinguishing diazotrophy from N supplied from combined N, based on the lower 15N/14N in diazotrophs due to the higher 15N/14N of combined N sources not being offset by fractionation during uptake. While potentially very useful, a number of pitfalls are discussed in its ecological use in both terrestrial and aquatic systems. N isotope discrimination is also useful in tracking N through food webs, and hence, back to combined N sources for plants.

Rapid, storm-induced changes in the natural abundance of 15N in a planktonic ecosystem, Chesapeake Bay, USA

Samples of dissolved inorganic nitrogen (DIN), particulate nitrogen (PN) and two species of Zooplankton were collected during two north-south transects of the Chesapeake Bay in the autumn of 1984 (27–28 September and 3–5 October). During the first transect, the natural abundance of 15N (δ15N) in the major dissolved and planktonic pools of nitrogen suggested that the δ15N of PN was largely determined by isotopic fractionation during uptake of NH4+ by phytoplankton. Averaged over the transect as a whole, the δ15N of the herbivorous calanoid copepod Acartia tonsa was 4.1‰ higher than that of the PN, while the δ15N of the carnivorous ctenophore Mnemiopsis leidyi was 6.4‰ higher than that of the PN. In the interval between the two transects, storm-induced mixing of the water column resulted in the injection of NH4+ into the surface layer of the bay. This perturbation in the estuarine nitrogen cycle resulted in marked changes in the δ15N of the major dissolved and planktonic pools of nitrogen in the bay. In combination with ancillary physical, chemical and biological data, these changes in δ15N provided estimates of the isotopic fractionation factor for NH4+ uptake by phytoplankton (α = 1.0065 −1.0080) as well as the turnover time of nitrogen in Acartia tonsa (6.0–9.6 days). Despite the changes in δ15N observed during this cruise, the relative distribution of 15N between trophic levels was preserved: during the second transect, the difference in δ15N between Acartia tonsa and PN was 3.6‰ and the difference in δ15N between Mnemiopsis leidyi and PN was 7.3‰. These results demonstrate that the natural abundance of 15N can change dramatically on a time scale of days and that time-series studies of the natural abundance of 15N can be a useful complement to studies using tracer additions of 15N to document nitrogen transformations in planktonic ecosystems.

Appraisal of the Nitrogen-15 Natural-Abundance Method for Quantifying Dinitrogen Fixation

Several investigators have questioned the use of the 15N natural-abundance method of estimating N2 fixation because of variability in soil δ15N and small differences between the δ15N of soil N and atmospheric N. Investigations were conducted to compare the 15N natural-abundance and 15N-isotope-dilution methods for estimating N2 fixation of field-grown pea (Pisum sativum L.) and lentil (Lens culinaris Medik.). Spatial variability was assessed at three sites by determining the δ15N of non-N2-fixing plants. Seasonal variation in δ15N for spring and winter wheat (Triticum aestivum L.), flax (Linum usitatissimum L.), barley (Hordeum vulgare L.), rape (Brassica napus L.) and lentil was determined at one site. Comparisons between δ15N and 15N-enriched isotope-dilution methods for estimating N2 fixation by lentil were conducted at several sites over a 3-yr period. Variability in δ15N of the reference plant was site dependent: the δ15N ranged from 2.8 to 9.3 at the first site, 3.4 to 8.8 at the second site, and 3.5 to 6.2 at the third site. The average δ15N of four of the five non-N2-fixing plants increased from 5.4 at 42 d after planting to 6.9 at the final harvest. The fifth non-N2-fixing plant, rape, accumulated most of its N during the first 42 d after planting, and its δ15N value declined from 8.1 at 42 d after planting to 7.3 at the final harvest. The δ15N values for lentil were similar at 42 and 63 d after planting to the δ15N values of the four reference plants, but did not increase after 63 d. Estimates of N2 fixation were not significantly different in 18 out of 21 comparisons; in two comparisons the δ15N method and in one comparison the 15N-enriched method provided higher estimates of N2 fixation. Overall, both methods appeared to provide equally reliable estimates of N2 fixation for lentil.