Use of15N/14N rations to evaluate the food source of the mysid,Neomysis intermedia Czerniawsky,in a eutrophic lake in Japan

The stable isotope ratios of nitrogen were measured in the mysid,Neomysis intermedia, together with various biogenic materials in a eutrophic lake, Lake Kasumigaura, in Japan throughout a year of 1984/85. The mysid, particulate organic matter (POM, mostly phytoplankton), and zooplankton showed a clear seasonal change in ¹⁵N with high values in spring and fall, but the surface bottom mud did not. A year to year variation as well as seasonal change in ¹⁵N was found in the mysid. The annual averages of ¹⁵N of each material collected in 1984/85 are as follows: surface bottom mud, 6.3 (range: 5.7–6.9); POM, 7.9 (5.8–11.8); large sized mysid, 11.6 (7.7–14.3); zooplankton, 12.5 (10.0–16.4); prawn, 13.2 (9.9–15.4); goby, 15.1 (13.8–16.7). The degree of¹⁵N enrichment by the mysid was determined as 3.2 by the laboratory rearing experiments. The apparent parallel relationship between the POM and the mysid in the temporal patterns of ¹⁵N with about 3 difference suggests the POM (mostly phytoplankton) as a possible food source ofN. intermedia in this lake through the year.     

Analysis of primary food sources and trophic relationships of aquatic animals in a mangrove-fringed estuary,Khung Krabaen Bay (Thailand) using dual stable isotope techniques

Stable carbon (¹³C) and nitrogen (¹⁵N) isotopes were used to elucidate primary food sources and trophic relationships of organisms in Khung Krabaen Bay and adjacent offshore waters. The three separate sampling sites were mangroves, inner bay and offshore. The ¹³C values of mangrove leaves were –28.2 to –29.4, seagrass –10.5, macroalgae –14.9 to –18.2, plankton –20.0 to –21.8, benthic detritus –15.1 to –26.3, invertebrates –16.5 to –26.0, and fishes –13.4 to –26.3. The ¹⁵N values of mangrove leaves were 4.3 to 5.7, seagrass 4.3, macroalgae 2.2 to 4.4, plankton 5.7 to 6.4 , benthic detritus 5.1 to 5.3, invertebrates 7.2 to 12.2 , and fishes 6.3 to 15.9. The primary producers had distinct ¹³C values. The ¹³C values of animals collected from mangroves were more negative than those of animals collected far from shore. The primary carbon sources that support food webs clearly depended on location. The contribution of mangroves to food webs was confined only to mangroves, but a mixture of macroalgae and plankton was a major carbon source for organisms in the inner bay area. Offshore organisms clearly derived their carbon through the planktonic food web. The ¹⁵N values of consumers were enriched by 3–4 relative to their diets. The ¹⁵N data suggests that some of aquatic animals had capacity to change their feeding habits according to places and availability of foods and as a result, individuals of the same species could be assigned to different trophic levels at different places.     

Natural abundance of 15N in tropical plants with emphasis on tree legumes

Natural abundance of ¹⁵N ( ¹⁵N) of leaves harvested from tropical plants in Brazil and Thailand was analyzed. The ¹⁵N values of non-N₂-fixing trees in Brazil were +4.5±1.9, which is lower than those of soil nitrogen (+8.0±2.2). In contrast, mimosa and kudzu had very low ¹⁵N values (–1.4+0.5). The ¹⁵N values of Panicum maximum and leguminous trees, except Leucaena leucocephala, were similar to those of non-N₂-fixing trees, suggesting that the contribution of fixed N in these plants is negligible. The ¹⁵N values of non-N₂-fixing trees in Thailand were +4.9±2.0. Leucaena leucocephala, Sesbania grandiflora, Casuarina spp. and Cycas spp. had low ¹⁵N values, close to the value of atmospheric N₂ (0), pointing to a major contribution of N₂ fixation in these plants. Cassia spp. and Tamarindus indica had high ¹⁵N values, which confirms that these species are non-nodulating legumes. The ¹⁵N values of Acacia spp. and Gliricidia sepium and other potentially nodulating tree legumes were, on average, slightly lower than those of non-N₂-fixing trees, indicating a small contribution of N₂ fixation in these legumes.     

The Use of the [13C]/[12C] Ratio for the Assay of the Microbial Oxidation of Hydrocarbons

The study deals with a comparative analysis of the relative abundances of the carbon isotopes ¹²C and ¹³C in the metabolites and biomass of the Burkholderia sp. BS3702 and Pseudomonas putida BS202-p strains capable of utilizing aliphatic (n-hexadecane) and aromatic (naphthalene) hydrocarbons as sources of carbon and energy. The isotope compositions of the carbon dioxide, biomass, and exometabolites produced during the growth of Burkholderia sp. BS3702 on n-hexadecane (¹³C = –44.6 ± 0.2) were characterized by the values of ¹³C_{CO 2} = –50.2 ± 0.4, ¹³C_biom = –46.6 ± 0.4, and ¹³C_exo = –41.5 ± 0.4, respectively. The isotope compositions of the carbon dioxide, biomass, and exometabolites produced during the growth of the same bacterial strain on naphthalene (¹³C = –21 ± 0.4) were characterized by the isotope effects ¹³C_{CO 2} = –24.1 ± 0.4, ¹³C_biom = –19.2 ± 0.4, and ¹³C_exo = –19.1 ± 0.4, respectively. The possibility of using the isotope composition of metabolic carbon dioxide for the rapid monitoring of the microbial degradation of petroleum hydrocarbons in the environment is discussed.     

Food web structure in the high Arctic Canada Basin: evidence from δ13C and δ15N analysis

The food-web structure of the Arctic deep Canada Basin was investigated in summer 2002 using carbon and nitrogen stable isotope tracers. Overall food-web length of the range of organisms sampled occupied four trophic levels, based on 3.8 trophic level enrichment (¹⁵N range: 5.3–17.7). It was, thus, 0.5–1 trophic levels longer than food webs in both Arctic shelf and temperate deep-sea systems. The food sources, pelagic particulate organic matter (POM) (¹³C=–25.8, ¹⁵N=5.3) and ice POM (¹³C=–26.9, ¹⁵N=4.1), were not significantly different. Organisms of all habitats, ice-associated, pelagic and benthic, covered a large range of ¹⁵N values. In general, ice-associated crustaceans (¹⁵N range 4.6–12.4, mean 6.9) and pelagic species (¹⁵N range 5.9–16.5, mean 11.5) were depleted relative to benthic invertebrates (¹⁵N range 4.6–17.7, mean 13.2). The predominantly herbivorous and predatory sympagic and pelagic species constitute a shorter food chain that is based on fresh material produced in the water column. Many benthic invertebrates were deposit feeders, relying on largely refractory material. However, sufficient fresh phytodetritus appeared to arrive at the seafloor to support some benthic suspension and surface deposit feeders on a low trophic level (e.g., crinoids, cumaceans). The enriched signatures of benthic deposit feeders and predators may be a consequence of low primary production in the high Arctic and the subsequent high degree of reworking of organic material.     

Polar bears make little use of terrestrial food webs: evidence from stable-carbon isotope analysis

Summary The mean stable-carbon isotope ratios (¹³C) for polar bear (Ursus maritimus) tissues (bone collagen –15.7, muscle –17.7, fat –24.7) were close to those of the same tissues from ringed seals (Phoca hispida) (–16.2, –18.1, and –26.1, respectively), which feed exclusively from the marine food chain. The ¹³C values for 4 species of fruits to which polar bears have access when on land in summer ranged from –27.8 to –26.2, typical of terrestrial plants in the Arctic. An animal's ¹³C signature reflects closely the ¹³C signature of it's food. Accordingly, the amount of food that polar bears consume from terrestrial food webs appears negligible, even though some bears spend 1/3 or more of each year on land during the seasons of greatest primary productivity.     

A simple urea leaf-feeding method for the production of 13C and 15N labelled plant material

The use of ¹³C isotope tracer techniques in terrestrial ecology has been restricted by the technical requirements and high costs associated with the production of ¹³C enriched plant material by ¹³CO₂ release in labelling chambers. We describe a novel, simple and relatively inexpensive method for the small-scale production of ¹³C and ¹⁵N labelled plant material. The method is based on foliar feeding of plants with a urea solution (97 atom% ¹³C, 2 atom% ¹⁵N) by daily misting. Maize was grown in a greenhouse in a compost–soil mixture and enclosed in clear polythene bags between urea applications. Final enrichment in 27 d old maize shoots was 211 ¹³C (1.34 atom% ¹³C) and 434 ¹⁵N (0.52 atom% ¹⁵N). Enrichments of hot-water extractable fractions (289 ¹³C, 469 ¹⁵N) were only slightly higher than those observed in plant bulk material, which suggests that daily urea applications ensured fairly uniform labelling of different biochemical fractions and plant tissues. Recovery of applied excess ¹³C and ¹⁵N in plant shoots was 22% and 42%, respectively. Roots were less enriched (21 ¹³C and 277 ¹⁵N), but no attempts were made to recover roots quantitatively.     

Stable isotope and radiocarbon compositions of methane emitted from tropical rice paddies and swamps in Southern Thailand

Stable isotopes (¹³C, D) and radiocarbon weremeasured in methane bubbles emitted from rice paddies and swamps in southernThailand. Methane emitted from the Thai rice paddies was enriched in¹³C (mean ¹³C; –51.5 ±7.1 and–56.5 ± 4.6 for mineral soil and peat soil paddies,respectively)relative to the reported mean value of methane from temperate rice paddies(– 63 ± 5). Large seasonal variation was observed in¹³C(32) in the rice paddies, whereas variationinD was much more smaller (20), indicating that variation in¹³C is due mainly to changes in methane production pathways.Values of ¹³C were lower in swamps (–66.1 ±5.1)than in rice paddies. The calculated contribution of acetate fermentation from¹³C value was greater in rice paddies (mineral soils:62–81%, peat soils: 57–73%) than in swamps (27–42%). Din methane from Thai rice paddies (–324± 7 (n=46)) isrelativelyhigher than those from 14 stations in Japanese rice paddies ranging from–362 ± 5 (Mito: n=2) to –322 ± 8(Okinawa: n=3), due tohigher D in floodwaters. ¹⁴C content in methane produced fromThai rice paddies (127±1 pMC) show higher ¹⁴Cactivity compared with previous work in paddy fields and those from Thai swamps(110±2 pMC).     

Forest- and pasture-derived carbon contributions to carbon stocks and microbial respiration of tropical pasture soils

The clearing of tropical forest for pasture leads to important changes in soil organic carbon (C) stocks and cycling patterns. We used the naturally occurring distribution of¹³C in soil organic matter (SOM) to examine the roles of forest- and pasture-derived organic matter in the carbon balance in the soils of 3- to 81-year-old pastures created following deforestation in the western Brazilian Amazon Basin state of Rondônia. Different ¹³C values of C3 forest-derived C (-28) and C4 pasture-derived C (-13) allowed determination of the origin of total soil C and soil respiration. The ¹³C of total soil increased steadily across ecosystems from -27.8 in the forest to -15.8 in the 81-year-old pasture and indicated a replacement of forest-derived C with pasture-derived C. The ¹³C of respired CO₂ increased more rapidly from -26.5 in the forest to -17 in the 3- to 13-year-old pastures and indicated a faster shift in the origin of more labile SOM. In 3-year-old pasture, soil C derived from pasture grasses made up 69% of respired C but only 17% of total soil C in the top 10 cm. Soils of pastures 5 years old and older had higher total C stocks to 30 cm than the original forest. This occurred because pasture-derived C in soil organic matter increased more rapidly than forest-derived C was lost. The increase of pasture-derived C in soils of young pastures suggests that C inputs derived from pasture grasses play a critical role in development of soil C stocks in addition to fueling microbial respiration. Management practices that promote high grass production will likely result in greater inputs of grass-derived C to pasture soils and will be important for maintaining tropical pasture soil C stocks.     

Trophodynamic linkage between river runoff and coastal fishery yield elucidated by stable isotope data in the Gulf of Lions (NW Mediterranean)

The link between climate-driven river runoff and sole fishery yields observed in the Gulf of Lions (NW Mediterranean) was analysed using carbon- and nitrogen stable isotopes along the flatfish food webs. Off the Rhone River, the main terrestrial (river POM) and marine (seawater POM) sources of carbon differed in ¹³C (–26.11 and –22.36, respectively). Surface sediment and suspended POM in plume water exhibited low ¹³C (–24.38 and –24.70, respectively) that differed more from the seawater POM than from river POM, demonstrating the dominance of terrestrial material in those carbon pools. Benthic invertebrates showed a wide range in ¹⁵N (mean 4.30 to 9.77) and ¹³C (mean –23.81 to –18.47), suggesting different trophic levels, diets and organic sources. Among the macroinvertebrates, the surface (mean ¹³C –23.71) and subsurface (mean ¹³C –23.81) deposit-feeding polychaetes were particularly ¹³C depleted, indicating that their carbon was mainly derived from terrestrial material. In flatfish, ¹⁵N (mean 9.42 to 10.93) and ¹³C (mean –19.95 to –17.69) varied among species, indicating differences in food source and terrestrial POM use. A significant negative correlation was observed between the percentage by weight of polychaetes in the diet and the ¹³C of flatfish white muscle. Solea solea (the main polychaete feeder) had the lowest mean ¹³C, Arnoglossus laterna and Buglossidium luteum (crustacean, mollusc and polychaete feeders) had intermediate values, and Solea impar (mollusc feeder) and Citharus linguatula (crustacean and fish feeder) exhibited the highest ¹³C. Two different benthic food webs were thus identified off the Rhone River, one based on marine planktonic carbon and the other on the terrestrial POM carried by the river. Deposit-feeding polychaetes were responsible for the main transfer of terrestrial POM to upper trophic levels, linking sole population dynamics to river runoff fluctuations.     

Sources of nitrate in rivers draining sixteen watersheds in the northeastern U.S.: Isotopic constraints

The feasibility of using nitrogen and oxygenisotope ratios of nitrate (NO₃ ^–) forelucidating sources and transformations ofriverine nitrate was evaluated in a comparativestudy of 16 watersheds in the northeastern U.S.A. Stream water was sampled repeatedly at theoutlets of the watersheds between January andDecember 1999 for determining concentrations,¹⁵N values, and ¹⁸Ovalues of riverine nitrate.In conjunction with information about land useand nitrogen fluxes,¹⁵N_nitrate and¹⁸O_nitrate values providedmainly information about sources of riverinenitrate. In predominantly forested watersheds,riverine nitrate had mean concentrations ofless than 0.4 mg NO₃ ^–-N L^–1,¹⁵N_nitrate values of lessthan +5, and ¹⁸O_nitratevalues between +12 and +19. This indicatesthat riverine nitrate was almost exclusivelyderived from soil nitrification processes withpotentially minor nitrate contributions fromatmospheric deposition in some catchments. Inwatersheds with significant agricultural andurban land use, concentrations of riverinenitrate were as high as 2.6 mg NO₃ ^–-NL^–1 with ¹⁵N_nitratevalues between +5 and +8 and¹⁸O_nitrate values generallybelow +15. Correlations between nitrateconcentrations, ¹⁵N_nitratevalues, and N fluxes suggest that nitrate inwaste water constituted a major, and nitrate inmanure a minor additional source of riverinenitrate. Atmospheric nitrate deposition ornitrate-containing fertilizers were not asignificant source of riverine nitrate inwatersheds with significant agricultural andurban land use. Although complementary studiesindicate that in-stream denitrification wassignificant in all rivers, the isotopiccomposition of riverine nitrate sampled at theoutlet of the 16 watersheds did not provideevidence for denitrification in the form ofelevated ¹⁵N_nitrate and¹⁸O_nitrate values. Relativelylow isotopic enrichment factors for nitrogenand oxygen during in-stream denitrification andcontinuous admixture of nitrate from theabove-described sources are thought to beresponsible for this finding.     

Selective feeding by larval dytiscids (Coleoptera:Dytiscidae) and effects of fish predation on upper littoral zone macroinvertebrate communities of arctic lakes

Carbon and nitrogen stable isotopic data from the primary producers in mangrove ecosystems are needed to investigate trophic links and biogeochemical cycling. Compared with other mangrove species (e.g. Rhizophora mangle) very few measurements have been conducted on the white mangrove, Laguncularia racemosa. The carbon and nitrogen stable isotopic and elemental compositions of L. racemosa were analyzed and compared from Florida and Belize. ¹³C values of L. racemosa from Florida (mean = –26.4) were slightly higher than those from Twin Cays, Belize (mean = -27.4), which may be due to higher salinity in some parts of the Florida site. There was no difference between the ¹⁵N values from L. racemosa from these two sites (Florida mean = 0.6; Belize mean = 0.3), which are indicative of nitrogen derived from nitrogen fixation in a planktonic marine system. However, higher ¹⁵N values from L. racemosa at Man of War Cay in Belize (11.4 and 12.3), which is fertilized by roosting marine birds (14.0), illustrate that L. racemosa can sensitively reflect alternative nitrogen sources. Although the isotopic data could not distinguish between Avicennia germinans, R. mangle and L. racemosa in Belize the L. racemosa had considerably higher C/N ratios (46.5 – 116.1) compared with the Florida samples (42.2 – 76.0) or the other mangrove species. Unlike some previous findings from R. mangle, substrate characteristics (e.g. salinity, NH₄ ⁺, and H₂S) were not related to the isotopic or elemental composition of L. racemosa. ¹³C, ¹⁵N and C/N were analyzed for ecosystem components from L. racemosa habitats at Twin Cays, including other plants (e.g. R. mangle, A. germinans and seagrass), detritus, microbial mats and sediments. Results from mass-balance calculations show that mangrove detritus composes very little of the sediment, which is principally composed of microbial biomass (80 – 90%). Detritus at some sites is also influenced by sources other than that from L. racemosa, including seagrass leaves.     

Emission of gaseous nitrogen oxides from an extensively managed grassland in NE Bavaria, Germany.

We analysed the stable isotope composition of emitted N₂O in a one-year field experiment (June 1998 to April 1999) in unfertilized controls, and after adding nitrogen by applying slurry or mineral N (calcium ammonium nitrate). Emitted N₂O was analysed every 2–4 weeks, with additional daily sampling for 10 days after each fertilizer application. In supplementary soil incubations, the isotopic composition of N₂O was measured under defined conditions, favouring either denitrification or nitrification. Soil incubated for 48 h under conditions favouring nitrification emitted very little N₂O (0.024 mol g_dw ^–1) and still produced N₂O from denitrification. Under denitrifying incubation conditions, much more N₂O was formed (0.91 mol g_dw ^–1 after 48 h). The isotope ratios of N₂O emitted from denitrification stabilized at ¹⁵N = –40.8 ± 5.7 and ¹⁸O = 2.7 ± 6.3. In the field experiment, the N₂O isotope data showed no clear seasonal trends or treatment effects. Annual means weighted by time and emission rate were ¹⁵N = –8.6 and ¹⁸O = 34.7 after slurry application, ¹⁵N = –4.6 and ¹⁸O = 24.0 after mineral fertilizer application and ¹⁵N = –6.4 and ¹⁸O = 35.6 in the control plots, respectively. So, in all treatments the emitted N₂O was ¹⁵N-depleted compared to ambient air N₂O (¹⁵N = 11.4 ± 11.6, ¹⁸O = 36.9 ± 10.7). Isotope analyses of the emitted N₂O under field conditions per se allowed no unequivocal identification of the main N₂O producing process. However, additional data on soil conditions and from laboratory experiments point to denitrification as the predominant N₂O source. We concluded (1) that the isotope ratios of N₂O emitted from the field soil were not only influenced by the source processes, but also by microbial reduction of N₂O to N₂ and (2) that N₂O emission rates had to exceed 3.4 mol N₂O m^–2 h^–1 to obtain reliable N₂O isotope data.     

Response of soil δ15N and nutrients to eastern red cedar (Juniperus virginiana) encroachment into a relict calcareous prairie

The calcareous prairies of Louisiana have been threatened by the encroachment of woody plants, primarily eastern red cedar (Juniperus virginiana). The restoration and management of these rare plant communities require a thorough understanding of the soils supporting them. The knowledge of whether eastern red cedar encroachment has altered these soils is also of interest. We studied the depth distribution, at contrasting vegetation types (prairie, transition, forest) and landscape positions, of ¹⁵N, total N, organic C, C/N ratio, Ca, Mg, K and pH of three relict prairie-forest associations in north central Louisiana, USA. The effect of vegetation type was significant for soil ¹⁵N and Ca. Plant leaf samples from prairie, transition, and forest showed similar ¹⁵N signals, and mean values ranged between –1.6 and –1.1. The order of soil ¹⁵N enrichment of the 0–10cm depth relative to corresponding leaves was forest soil> transition soil>prairie soil. The forest soil was significantly enriched with ¹⁵N compared with the prairie soil and transition soil. Except for C/N ratio, all the soil properties significantly decreased with depth while ¹⁵N increased with depth. Significant differences in C/N ratio, Ca and Mg were associated with landscape position. The change in soil pH due to woody encroachment was restricted to the 0–10cm depth. The results suggest that the prairie soil was distinctly different from the forest soil and that the vegetation at transition (encroaching woody plants) was altering the surface soil pH towards forest-like conditions.     

Über den Einfluß von Nahrung und Substratsalinität auf Verhalten,Fortpflanzung und Wasserhaushalt von Enchytraeus albidus Henle (Oligochaeta)

Zusammenfassung Enchytraeus albidus aus dem Anwurf mariner Algen an der Kieler Förde (Ostsee) erträgt als Nahrung die folgenden dort vorkommenden Pflanzen (Reihenfolge mit abnehmender Verträglichkeit): Fucus — Grünalgen —Seegras (Zostera) — Rotalgen (Delesseria). Diese Reihenfolge gilt für Nahrungsaufnahme, Fortpflanzungsrate und Überlebensdauer.Mit zunehmender Fäulnis des Nahrungssubstrates steigt die Zahl der Tiere, die aus ihm fliehen. Ihre Anzahl wird außerdem bestimmt durch den Salzgehalt des Substrates: Von 15–45 ist sie proportional der Substratsalinität. Bei 60 ist die Aktivität der Tiere bereits stark eingeschränkt.Bei Fucus-Nahrung ertragen auf Sand gehaltene Tiere eine Salinität von 60–70 länger als 4 Wochen, auf Filtrierpapier dagegen nur 50 für durchschnittlich 1 Woche. Die obere Fortpflanzungsgrenze liegt bei 40 Salzgehalt im Substrat. Bei 5 werden die meisten Kokons abgelegt. Die Sterblichkeit im Kokon ist bei 15 am geringsten. Auf den Substratsalinitäten 0–15 ist die Entwicklungsdauer im Kokon signifikant kürzer als auf Substraten von 30 und 40. Enchytraeus hat sich als Rückwanderer zum Meer mit einer sekundär erweiterten Poikilosmotie an den neuen Lebensraum angepaßt. Er kann eine Binnenkonzentration entsprechend etwa 72 längere Zeit ertragen. Auf niedrigen Salzgehalten besitzt er eine ausgeprägte Hypertonieregulation.

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Summary Enchytraeus albidus was fed with Fucus, green algae, Zostera marina and Delesseria. Judging from absorption of food, rate of reproduction and duration of life, the animals preferred the plants in the sequence given above.As the putrefaction of a Fucus substrate advances, more and more enchytraeids leave it. A changing salinity of the substrate also influences the number of emigrating worms, increasing it from 15–45, but decreasing it towards 60. Fed with Fucus E. albidus tolerates a salinity of 60–70 on sand for more than 4 weeks, on filter paper only 50 for about one week.Reproduction is possible at salinities up to 40. Cocoon production is most frequent at 5. The mortality of young worms within the cocoons is lowest at 15. The incubation period is significantly shorter at salinities of 0–15 than at 30 and 40.As a terrestrial immigrant to the seashore Enchytraeus albidus secondarily enlarged its range of poikilosmosis, tolerating a concentration of 72 in its coelomic fluid for some time. At low salinities it maintains a remarkable degree of hyperosmosis.

     

Insect food preferences analysed using 13C/12C ratios

Summary Stable carbon isotope ratio analysis is a powerful technique in tracing ecosystem carbon flows, especially those between primary and secondary producers. The distinctive ¹³C/¹²C ratios of plant species tend to pass along the food chain with little further fractionation, hence the stable carbon isotope composition of an animal is an important clue to what it has eaten. We compared the stable carbon isotope composition of plants and insects in an old field in Georgia. Of the dominant plants in the old field, 6 were C₄ species and had ¹³C¹ values of-10.9 to 12.9, and 7 were C₃ species with values of-27.3 to-29.1. Insects known to be feeding on only one plant species had ¹³C values within 1 of the isotopic composition of the plant. Wasp larvae parasitizing two insect species had ¹³C values 1.3 and 1.7 higher than that of the food plant. A variety of insects of unknown food habits collected on monospecific and mixed species plant stands in the old field had ¹³C values ranging from-10.1 to-30.0. Two species of leafhopper and a grasshopper had isotopic compositions within the range of C₄ plant values; a tortoise beetle and a lace bug had isotopic compositions within C₃ plant values. Other insects had intermediate ¹³C values, suggesting a mixed diet composed of both C₃ and C₄ plants. The carbon isotopic ratios of field collected insects appears to be a useful qualitative indicator of their feeding preference.     

Trophic relationships in the nearshore zone of Martel Inlet (King George Island,Antarctica): δ<Superscript>13</Superscript>C stable-isotope analysis

Carbon isotopic composition was used to assess the linkage between three different potential sources of energy and the community in the shallow coastal zone of Martel Inlet. Stable ¹³C ratios ranged from –28.7 for the zooplankton plus phytoplankton to –14.4 for the grazer Nacella concinna. Microphytobenthos (–16.7) was considerably more enriched in ¹³C than were suspended particulate matter (SPM) (–25.6) and macroalgal fragments (–23.6 and –21.1), indicating that stable carbon isotope analysis might be used to discern the relative contribution of these sources of primary production. There is a benthic-pelagic coupling between plankton, benthic suspensivores, the ophiuroid Ophionotus victoriae and the icefish Chaenocephalus aceratus. Benthic grazers such as N. concinna, deposit feeders such as Yoldia eightsi and the nematodes showed a tight coupling with the microphytobenthos and the sediment. Some omnivorous/depositivorous polychaetes, echinoids, amphipods and the fish Notothenia coriiceps showed values close to the ratios of the macroalgal fragments. Benthic carnivores and/or scavengers were generally enriched over suspensivores and depleted in relation to microphytobenthos grazers, showing a considerable overlap in ¹³C values throughout the food web, without any clear coupling with the primary sources of organic matter. The trophic web in the shallow zone of high benthic production and under seasonal ice cover in the Antarctic is more complex than it is in shelf areas, where SPM is the main food source. The soft-bottom community in the shallow zone of Martel Inlet is enriched in ¹³C due to the significant input of carbon from the microphytobenthos and macroalgal fragments.     

Carbon and nitrogen isotope ratios in different compartments of a healthy and a declining Picea abies forest in the Fichtelgebirge,NE Bavaria

Summary Natural carbon and nitrogen isotope ratios were measured in different compartments (needles and twigs of different ages and crown positions, litter, understorey vegetation, roots and soils of different horizons) on 5 plots of a healthy and on 8 plots of a declining Norway spruce (Picea abies (L.) Karst.) forest in the Fichtelgebirge (NE Bavaria, Germany), which has recently been described in detail (Oren et al. 1988a; Schulze et al. 1989). The ¹³C values of needles did not differ between sites or change consistently with needle age, but did decrease from the sun-to the shade-crown. This result confirms earlier conclusions from gas exchange measurements that gaseous air pollutants did no long-lasting damage in an area where such damage was expected. Twigs (¹³C between-25.3 and-27.8) were significantly less depleted in ¹³C than needles (¹³C between-27.3 and-29.1), and ¹³C in twigs increased consistently with age. The ¹⁵N values of needles ranged between-2.5 and-4.1 and varied according to stand and age. In young needles ¹⁵N decreased with needle age, but remained constant or increased in needles that were 2 or 3 years old. Needles from the healthy site were more depleted in ¹⁵N than those from the declining site. The difference between sites was greater in old needles than in young ones. This differentiation presumably reflects an earlier onset of nitrogen reallocation in needles of the declining stand. ¹⁵N values in twigs were more negative than in needles (-3.5 to-5.2) and showed age- and stand-dependent trends that were similar to the needles. ¹⁵N values of roots and soil samples increased at both stands with soil depth from-3.5 in the organic layer to +4 in the mineral soil. The ¹⁵N values of roots from the mineral soil were different from those of twigs and needles. Roots from the shallower organic layer had values similar to twigs and needles. Thus, the bulk of the assimilated nitrogen was presumably taken up by the roots from the organic layer. The problem of separation of ammonium or nitrate use by roots from different soil horizons is discussed.     

Stable isotope ecology in the Ituri Forest

The Ituri Forest, Democratic Republic of Congo (formerly Zaire) is an example of a closed canopy forest showing extreme depletion in ¹³C. ¹³C values for plants from the canopy top, from gaps in the canopy, and from the subcanopy average –29.0±1.7, –30.4±0.9, and –34.0±1.5, respectively. The ¹³C of forest mammals show these differences, with the subcanopy browsers (okapi, dwarf antelope) having ¹³C values for tooth enamel much more negative than subcanopy frugivores who derive their food from the canopy top, and from folivores and omnivores living in gap or clearing areas. Nitrogen isotopes in plants from this ecosystem have an average ¹⁵N value of 5.4±1.8 and do not show significant differences at the 95% confidence interval between plants from the canopy top, from gaps in the canopy, and from the subcanopy. The ¹⁸O_SMOW values of surface waters in the study area are between –2.0 and –2.7. The ¹⁸O_PDB for tooth enamel ranged from –3 to +7.     

Origin and fate of dissolved inorganic carbon ininterstitial waters of two freshwater intertidalareas: A case study of the Scheldt Estuary, Belgium

Processes affecting the concentration and isotopiccomposition of dissolved inorganic carbon (DIC) wereinvestigated in pore waters of two freshwaterintertidal areas of the Scheldt Estuary, Belgium. Porewater ¹³C_DIC values from marshes andmudflats varied from –27 to +13.4, these very largevariations reflect the contribution of differentcarbon sources to the DIC pool.In pore waters of the upper mudflat, river water DICand dissolution of calcite contribute to a lesserextent (10% and 16% respectively) to the total DICpool. Results indicate that inorganic carbon added tothe pore water of the mudflats has a ¹³Cvalue of +20.3 in May 1998. These strongly enriched¹³C_DIC values suggest that the majorcontribution (up to three-quarters) to total DIC isCO₂ derived from methanogenesis.In pore waters of the marshes, CO₂ derived fromorganic matter degradation (–27.5) and river DIC(–11.5 to –16.1) are the major sources of inorganiccarbon contribution to the total DIC pool. In porewaters from a marsh site colonised by willow trees,the contribution from CO₂ derived from organicmatter degradation is larger than in pore waters froman area with only reed vegetation. In the latter caseriver water DIC is the major source of pore waterDIC.