Land–Ocean Coupling of Carbon and Nitrogen Fluxes on Sandy Beaches

Rivers link oceans with the land, creating global hot spots of carbon processing in coastal seas. Coastlines around the world are dominated by sandy beaches, but beaches are unusual in that they are thought to rely almost exclusively on marine imports for food. No significant connections to terrestrial production having been demonstrated. By contrast, we isotopically traced carbon and nitrogen pathways leading to clams (Donax deltoides) on beaches. Clams from areas influenced by river plumes had significantly different isotope signatures (δ¹³C: −18.5 to −20.2‰; δ¹⁵N: 8.3–10.0‰) compared with clams remote from plumes (δ¹³C: −17.5 to −19.5‰; δ¹⁵N: 7.6–8.7‰), showing that terrestrial carbon and sewage, both delivered in river plumes, penetrate beach food webs. This is a novel mechanism of trophic subsidy in marine intertidal systems, linking the world’s largest shore ecosystem to continental watersheds. The same clams also carry pollution signatures of sewage discharged into rivers, demonstrating that coastal rivers connect ecosystems in unexpected ways and transfer contaminants across the land–ocean boundary. The links we demonstrate between terrigenous matter and the largest of all marine intertidal ecosystems are significant given the immense social, cultural, and economic values of beaches to humans and the predicted consequences of altered river discharge to coastal seas caused by global climate change.     

Effects of nutritional restriction on nitrogen and carbon stable isotopes in growing seabirds

When using stable isotopes as dietary tracers it is essential to consider effects of nutritional state on isotopic fractionation. While starvation is known to induce enrichment of ¹⁵N in body tissues, effects of moderate food restriction on isotope signatures have rarely been tested. We conducted two experiments to investigate effects of a 50–55% reduction in food intake on δ¹⁵N and δ¹³C values in blood cells and whole blood of tufted puffin chicks, a species that exhibits a variety of adaptive responses to nutritional deficits. We found that blood from puffin chicks fed ad libitum became enriched in ¹⁵N and ¹³C compared to food-restricted chicks. Our results show that ¹⁵N enrichment is not always associated with food deprivation and argue effects of growth on diet–tissue fractionation of nitrogen stable isotopes (Δ¹⁵N) need to be considered in stable isotope studies. The decrease in δ¹³C of whole blood and blood cells in restricted birds is likely due to incorporation of carbon from ¹³C-depleted lipids into proteins. Effects of nutritional restriction on δ¹⁵N and δ¹³C values were relatively small in both experiments (δ¹⁵N: 0.77 and 0.41‰, δ¹³C: 0.20 and 0.25‰) compared to effects of ecological processes, indicating physiological effects do not preclude the use of carbon and nitrogen stable isotopes in studies of seabird ecology. Nevertheless, our results demonstrate that physiological processes affect nitrogen and carbon stable isotopes in growing birds and we caution isotope ecologists to consider these effects to avoid drawing spurious conclusions.     

Sulfur,carbon, and nitrogen stable isotope ratios in soft tissues and trophic relationships of fish from the near-shore waters of the peter the great bay in the Sea of Japan

Stable isotope ratios of sulfur (³⁴S/³²S), carbon (¹³C/¹²C), and nitrogen (¹⁵N/¹⁴N) were analyzed in the soft tissues of 12 common species of fish from the near-shore waters of the Peter the Great Bay in the Sea of Japan. The average δ¹³C values of individual species varied from −20.7‰ for planktivorous fish to −16.8‰ for benthivorous fish, reflecting the growing relative contribution of benthic primary producers to fish nutrition. The majority of the various species representatives studied can be assigned to one trophic level, as indicated by their narrow range of δ¹⁵N values (9.9 to 12.6‰). Large interspecific variations were found in the sulfur stable isotope ratios of fish (the mean δ³⁴S values ranged from 11.2 to 19.5‰). This is the result of the different contributions to fish nutrition of infaunal invertebrates that are depleted in ³⁴S due to the microbial food chain of the bottom sediments.     

Diet: tissue stable isotope fractionation of carbon and nitrogen in blood plasma and whole blood of male reindeer Rangifer tarandus

Estimates of diet derived from stable isotope analyses are sensitive to the accuracy of corrections made for diet-tissue fractionation. In particular, diet-tissue fractionation in reindeer Rangifer tarandus may be expected to differ significantly from the generic values often used in stable isotope dietary calculations, given the known values obtained from other ungulates and the complex digestive system and nutrient recycling characteristic of the species. We fed domestic reindeer a homogenous artificial diet of known isotopic value in order to directly determine diet-tissue isotopic fractionation of carbon and nitrogen, the main elements used in stable isotope dietary analyses. Diet-tissue fractionation values for blood plasma were +3.5 ± 0.1‰ (δ¹³C) and +4.2 ± 0.3‰ (δ¹⁵N). Diet-tissue fractionation values for whole blood were +3.7 ± 0.2‰ (δ¹³C) and +2.5 ± 0.3‰ (δ¹⁵N). Metabolic turnover rates were clearly sufficient for complete tissue replacement over the period of artificial feeding for blood plasma, but may not have been so for whole blood, especially for δ¹⁵N. Our values, except for whole blood δ¹⁵N, differ considerably from the generic values often used in dietary studies and interspecific comparisons of trophic niche. The results underline the importance of obtaining as specific as possible fractionation values for the species, tissue, and in some cases sex and physiological status of animals under examination, and the potential problems associated with assuming ‘generic’ fractionation values when comparing species, especially where digestive processes are dissimilar.     

Challenges using stable isotopes for estimating trophic levels in marine amphipods

In marine food web studies, stable isotopes of nitrogen (δ¹⁵N) and carbon (δ¹³C) are widely used to estimate organisms’ trophic levels (TL) and carbon sources, respectively. For smaller organisms, whole specimens are commonly analyzed. However, this “bulk method” involves several pitfalls since different tissues may fractionate stable isotopes differently. We compared the δ¹⁵N and δ¹³C values of exoskeleton versus soft tissue, in relation to whole specimens, of three common Arctic amphipods in Svalbard waters: the benthic Anonyx nugax, the sympagic (ice-associated) Gammarus wilkitzkii and the pelagic Themisto libellula. The δ¹⁵N values of the exoskeletons were significantly lower than those of the soft tissues for A. nugax (10.5 ± 0.7‰ vs. 15.7 ± 0.7‰), G. wilkitzkii (3.3 ± 0.3‰ vs. 8.3 ± 0.4‰) and T. libellula (8.6 ± 0.3‰ vs.10.8 ± 0.3‰). The differences in δ¹³C values between exoskeletons and soft tissues were insignificant, except for A. nugax (−21.2 ± 0.2‰ vs. −20.3 ± 0.2‰, respectively). The δ¹⁵N values of whole organisms were between those of the exoskeletons and the soft tissues, being similarly enriched in ¹⁵N as the exoskeletons (except G. wilkitzkii) and depleted in ¹⁵N by 1.2–3.7‰ compared to the soft tissues. The δ¹⁵N-derived TLs of the soft tissues agreed best with the known feeding preferences of the three amphipods, which suggest a potential underestimation of 0.5–1.0 TL when stable isotope analyses are performed on whole crustaceans with thick exoskeletons. The insignificant or small differences in δ¹³C values among exoskeletons, soft tissues and whole specimens, however, suggest low probability for misinterpretations of crustaceans’ primary carbon source in marine ecosystems with distinctly different δ¹³C-carbon sources.     

Isotope ratios and concentrations of sulfur and nitrogen in needles and soils of Picea abies stands as influenced by atmospheric deposition of sulfur and nitrogen compounds

Concentrations and natural isotope abundance of total sulfur and nitrogen as well as sulfate and nitrate concentrations were measured in needles of different age classes and in soil samples of different horizons from a healthy and a declining Norway spruce (Picea abies (L.) Karst.) forest in the Fichtelgebirge (NE Bavaria, Germany), in order to study the fate of atmospheric depositions of sulfur and nitrogen compounds. The mean δ¹⁵N of the needles ranged between −3.7 and −2.1 ‰ and for δ³⁴S a range between −0.4 and +0.9 ‰ was observed. δ³⁴S and sulfur concentrations in the needles of both stands increased continuously with needle age and thus, were closely correlated. The δ¹⁵N values of the needles showed an initial decrease followed by an increase with needle age. The healthy stand showed more negative δ¹⁵N values in old needles than the declining stand. Nitrogen concentrations decreased with needle age. For soil samples at both sites the mean δ¹⁵N and δ³⁴S values increased from −3 ‰ (δ¹⁵N) or +0.9 ‰ (δ³⁴S) in the uppermost organic layer to about +4 ‰ (δ¹⁵N) or +4.5 ‰ (δ³⁴S) in the mineral soil. This depth-dependent increase in abundance of ¹⁵N and ³⁴S was accompanied by a decrease in total nitrogen and sulfur concentrations in the soil. δ¹⁵N values and nitrogen concentrations were closely correlated (slope −0.0061 ‰ δ¹⁵N per μmol eq N g_dw ⁻¹), and δ³⁴S values were linearly correlated with sulfur concentrations (slope −0.0576 ‰ δ³⁴S per μmol eq S g_dw ⁻¹). It follows that in the same soil samples sulfur concentrations were linearly correlated with the nitrogen concentrations (slope 0.0527), and δ³⁴S values were linearly correlated with δ¹⁵N values (slope 0.459). A correlation of the sulfur and nitrogen isotope abundances on a Δ basis (which considers the different relative frequencies of ¹⁵N and ³⁴S), however, revealed an isotope fractionation that was higher by a factor of 5 for sulfur than for nitrogen (slope 5.292). These correlations indicate a long term synchronous mineralization of organic nitrogen and sulfur compounds in the soil accompanied by element-specific isotope fractionations. Based on different sulfur isotope abundance of the soil (δ³⁴S=0.9 ‰ for total sulfur of the organic layer was assumed to be equivalent to about −1.0 ‰ for soil sulfate) and of the atmospheric SO₂ deposition (δ³⁴S=2.0 ‰ at the healthy site and 2.3 ‰ at the declining site) the contribution of atmospheric SO₂ to total sulfur of the needles was estimated. This contribution increased from about 20 % in current-year needles to more than 50 % in 3-year-old needles. The proportion of sulfur from atmospheric deposition was equivalent to the age dependent sulfate accumulation in the needles. In contrast to the accumulation of atmospheric sulfur compounds nitrogen compounds from atmospheric deposition were metabolized and were used for growth. The implications of both responses to atmospheric deposition are discussed.     

Establishing pathways of energy flow for insect predators using stable isotope ratios: field and laboratory evidence

 Quantifying pathways of energy transfer between plants, pests, and beneficial insects is a necessary step toward maintaining pest stable agroecosystems in the absence of chemical subsidies. A diet switching experiment utilizing a predatory ladybird beetle, Hippodamia variegata (Goeze), evaluated the use of naturally occurring stable C and N isotopes as an economically feasible and safe method for quantifying pathways of energy flow within agroecosystems. Stable isotope values of the ladybird beetle Coleomegilla maculata lengi (Timberlake) collected from an agroecosystem were used to estimate the relative amount of C and N derived from agricultural plants and incorporated by ladybird beetles based on mass balance equations. At the beginning of the diet-switching experiment δ¹³C and δ¹⁵N values of H. variegata (–12.0‰ and 6.3‰, respectively) differed by –0.2‰ and 2.9‰ from the aphids that were provided exclusively as their diet. These data are consistent with previous estimates of trophic level isotope effects. After switching the diet of H. variegata to an alternative food, isotope values of H. variegata gradually shifted toward expected values for individuals fed this diet (–22.9‰ and 8.8‰ for δ¹³C and δ¹⁵N values, respectively). Isotope values of another ladybird beetle, C. maculata, collected from the field indicated that in May, alfalfa and maize (pollen) obtained in the previous year contributed 32% and 68% of the C or N to the diets of these individuals and in August, 52%, 6%, and 42% of the C or N assimilated by these insects was derived from alfalfa, wheat, and maize, respectively. These data are consistent with expectations based on the relative abundance of C. maculata in various crops during the season. The field and laboratory data are a clear indication that isotope values are sensitive to dietary changes on a relatively short time scale (days) and provide a strong basis for the use stable C and N isotope to trace energy flow patterns of these beneficial organisms within agroecosystems. Received: 17 November 1995 / Accepted: 20 June 1996     

Evaluating the utility of stable isotope analyses of archived freshwater sample materials

We evaluated the potential utility of stable isotope analysis of tissues commonly archived by aquatic biologists. Previous studies with chemically preserved samples have shown contradictory results, which present an obstacle for the use of archived sample materials. We tested the effects of ethanol and formalin preservation on zooplankton and of ethanol on benthic macroinvertebrate δ¹³C and δ¹⁵N values. We found that neither formalin nor ethanol had a significant effect on δ¹³C and δ¹⁵N values of preserved zooplankton. Nor did ethanol significantly affect δ¹³C or δ¹⁵N values of macroinvertebrates. However, ethanol preservation slightly, but significantly decreased C:N ratios of both zooplankton and macroinvertebrates, probably reflecting some extraction of lipids. Overall, the effects of preservatives on δ¹³C and δ¹⁵N values that we observed were minor. We also compared δ¹³C and δ¹⁵N values analysed from roach scales and perch operculum bones with those analysed from muscle tissue. Decalcification of scales and operculum bones only slightly improved our comparison to muscle tissue δ¹³C and δ¹⁵N values. Decalcified scales had slightly higher δ¹³C and lower δ¹⁵N values. Similarly, decalcified operculum bones showed slightly increased δ¹³C and decreased δ¹⁵N values to those for fish muscle. Our results confirm that scales and operculum bones can provide a suitable proxy for fish muscle in isotope studies with minor correction. We conclude that various archived sample materials can indeed be used with confidence for historical reconstructions of freshwater food webs by stable isotope analysis. Handling editor: K. Martens     

Sediment organic matter in mountain lakes of north-western Slovenia and its stable isotopic signatures: records of natural and anthropogenic impacts

Sediment organic matter (OM) and its stable carbon and nitrogen isotopes were studied in 12 Slovenian mountain lakes in the Julian Alps. The lakes have different catchment areas and display a range of trophic states. Surface sediment atomic C/N ratios ranged from 8.4 to 13.2. Based on these C/N ratios, we concluded that autochthonous OM dominates in these lakes and constitutes approximately 65–92% of the total OM. Higher contributions of autochthonous OM sources were observed in lakes above the tree line. Relatively constant C/N ratios in the deeper sediments suggest that degradation processes are most intense in the upper few centimetres of the sediments and/or that remaining OM is relatively resistant to further degradation. Surface sediment δ¹³C and δ¹⁵N values ranged from −36.1 to −14.1‰ and from −5.2 to +1.1‰, respectively. In sediment cores from seven lakes, higher δ¹³C and lower δ¹⁵N values characterize oligotrophic lakes situated above the tree line, whereas the reverse is true for eutrophic lakes below the tree line that are also exposed to more anthropogenic impact. Carbon and nitrogen biogeochemical cycling differs considerably among the lakes. Stratigraphic shifts in carbon, total nitrogen, C/N ratios and stable C and N isotopes in cores record changes in inputs, and hence water column processes, as well as alterations in loading to the lakes. The stratigraphic variations are also the result of post-depositional diagenetic changes in the upper few centimetres of sediment. All the lakes show impacts from recent increases in atmospheric deposition of dissolved inorganic nitrogen. Application of sediment OM analysis thus proved to be useful to reconstruct paleoecological changes in sensitive mountain lake ecosystems that are either natural and/or anthropogenically derived.     

Carbon and nitrogen isotope composition of vegetation on King George Island,maritime Antarctic

We report abundance of ¹³C and ¹⁵N contents in terrestrial plants (mosses, lichens, liverworts, algae and grasses) from the area of Barton Peninsula (King George Island, maritime Antarctic). The investigated plants show a wide range of δ¹³C and δ¹⁵N values between −29.0 and −20.0‰ and between −15.3 and 22.8‰, respectively. The King George Island terrestrial plants show species specificity of both carbon and nitrogen isotope compositions, probably due to differences in plant physiology and biochemistry, related to their sources and in part to water availability. Carbon isotope compositions of Antarctic terrestrial plants are typical of the C₃ photosynthetic pathway. Lichens are characterized by the widest carbon isotope range, from −29.0 to −20.0‰. However, the average δ¹³C value of lichens is the highest (−23.6 ± 2.8‰) among King George Island plants, followed by grasses (−25.6 ± 1.7‰), mosses (−25.9 ± 1.6‰), liverworts (−26.3 ± 0.5‰) and algae (−26.3 ± 1.2‰), partly related to habitats controlled by water availability. The δ¹⁵N values of moss samples range widest (−9.0 to 22.8‰, with an average of 4.6 ± 6.6‰). Lichens are on the average most depleted in ¹⁵N (mean = −7.4 ± 6.4‰), whereas algae are most enriched in ¹⁵N (10.0 ± 3.3‰). The broad range of nitrogen isotope compositions suggest that the N source for these Antarctic terrestrial plants is spatially much variable, with the local presence of seabird colonies being particularly significant.     

Stable Isotope Ratios as a Tool for Assessing Changes in Carbon and Nutrient Sources in Antarctic Terrestrial Ecosystems

Samples of an angiosperm species, nine lichen species and a terrestrial alga, were collected from a variety of Antarctic terrestrial habitats, and were analysed for C and N stable isotope composition. Collections were made along natural gradients, the marine gradient, running from the sea coast inland and the moisture gradient, determined by melt water and precipitation runoff, and running towards the sea coast. Considerable variation in stable isotope ratios was found; δ¹³C values ranged between −16 and −32‰ and δ¹⁵N values between −23 and +23‰ The variation in stable carbon isotope ratios could be attributed in part to species specific differences, but differences in water availability also played a role, as was shown for the terrestrial alga Prasiola crispa and the lichen species Usnea antarctica. The differences in the isotope ratios of nitrogen could be retraced to the origin of nitrogen: marine or terrestrial. The nitrogen stable isotope ratios were influenced by both the marine gradient from the sea inland and the melt water and precipitation flow running in the opposite direction, towards the sea. This was shown for the lichen species Turgidosculum complicatulum and the angiosperm species Deschampsia antarctica. The variation in the C and N stable isotope ratios can be used to determine sources and pathways of N and changes in the water availability in Antarctic terrestrial ecosystems. Contrary to earlier reports the use of stable N isotope ratios is possible in this case because of the relative simplicity of the structure of the Antarctic terrestrial ecosystems.     

δ<Superscript>13</Superscript>C and δ<Superscript>15</Superscript>N in tissue of coral polyps and epilithic algae inhabiting damaged coral colonies under the influence of different light intensities

Studies were performed of the carbon and nitrogen stable isotope (δ¹³C and δ¹⁵N) composition (δ¹³C and δ¹⁵N) of the corals Porites cylindrica and P. lutea (5 years after damaging the colonies by the bleaching events) and of epilithic algae settled onto damaged areas of coral colonies. Coral polyps and three epilithic algal communities (‘red algal turf, green algal turf and red calcified crusts’) were sampled along the boundary between communities of coral polyps and algal colonizers from differently illuminated habitats from 2 to 90% of incident surface photosynthetically active radiation (PAR₀). It was found that communities with a predominance of red algae significantly differed from communities with a predominance of green algae in δ¹³C but not in δ¹⁵N values. An influence of habitat irradiance was found only for communities of coral polyps for δ¹³C and δ¹⁵N values: under bright light (70–90% PAR₀) polyp tissues of both coral species were significantly enriched in heavy carbon isotopes and insignificantly in nitrogen isotopes (δ¹³C values difference ~4‰) relative to tissues of corals under lower light 15–50% PAR₀. On the basis of these results we assumed that differences in light intensities in the habitat ranging from 15 to 90% PAR₀ do not influence on accessibility of the main carbon and nitrogen sources for corals and algae, and exchange by these elements between organisms. We also assumed that the relative enrichment in the heavy carbon isotopes of coral tissues in high light is a result of decreased isotope fractionation (or the absence of fractionation in photosynthesis of their zooxanthellae).     

Trophic ecology of the invasive argentine ant: spatio-temporal variation in resource assimilation and isotopic enrichment

Studies of food webs often employ stable isotopic approaches to infer trophic position and interaction strength without consideration of spatio-temporal variation in resource assimilation by constituent species. Using results from laboratory diet manipulations and monthly sampling of field populations, we illustrate how nitrogen isotopes may be used to quantify spatio-temporal variation in resource assimilation in ants. First, we determined nitrogen enrichment using a controlled laboratory experiment with the invasive Argentine ant (Linepithema humile). After 12 weeks, worker δ¹⁵N values from colonies fed an animal-based diet had δ¹⁵N values that were 5.51% greater compared to colonies fed a plant-based diet. The shift in δ¹⁵N values in response to the experimental diet occurred within 10 weeks. We next reared Argentine ant colonies with or without access to honeydew-producing aphids and found that after 8 weeks workers from colonies without access to aphids had δ¹⁵N values that were 6.31% larger compared to colonies with access to honeydew. Second, we sampled field populations over a 1-year period to quantify spatio-temporal variability in isotopic ratios of L. humile and those of a common native ant (Solenopsis xyloni). Samples from free-living colonies revealed that fluctuations in δ¹⁵N were 1.6–2.4‰ for L. humile and 1.8–2.9‰ for S. xyloni. Variation was also detected among L. humile castes: time averaged means of δ¹⁵N varied from 1.2 to 2.5‰ depending on the site, with δ¹⁵N values for queens ≥ workers > brood. The estimated trophic positions of L. humile and S. xyloni were similar within a site; however, trophic position for each species differed significantly at larger spatial scales. While stable isotopes are clearly useful for examining the trophic ecology of arthropod communities, our results suggest that caution is warranted when making ecological interpretations when stable isotope collections come from single time periods or life stages.     

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. Received: 14 December 1998 / Accepted: 16 August 1999     

Preservation effects on stable isotope ratios and consequences for the reconstruction of energetic pathways

Stable isotope analysis provides a powerful tool for describing the energetic pathways in a variety of ecosystems. However, isotope ratios of animal tissues can be altered by preservation methods, potentially leading to biased estimates of energy pathways when they are not taken into account. Here, we investigated the direct preservation effects of formalin, ethanol, NaCl, and drying on the δ¹³C and δ¹⁵N of fish muscle tissues, as well as the ultimate effects on the reconstruction of the energy pathways. All preservation methods, except drying, had significant impacts on δ¹³C and δ¹⁵N values. The effects of preservation appear to be highly taxa-specific and no significant time-dependent variations in nearly 2-year duration of preservation. δ¹³C and δ¹⁵N values were generally changed dramatically within the early stage of the preservation process and became stable over a relatively long-term preservation. Using an isotopic balance mixing model, the isotope-based food web reconstruction reveals that, without preservation correction, the importance of the pelagic energetic pathways for the fishes could be misestimated, except for the drying preservation. These results highlight that preservation can bias the interpretation of food web reconstruction results.     

Foliar and fungal <Superscript>15</Superscript> N:<Superscript>14</Superscript> N ratios reflect development of mycorrhizae and nitrogen supply during primary succession: testing analytical models

Nitrogen isotopes (¹⁵N/¹⁴N ratios, expressed as δ¹⁵N values) are useful markers of the mycorrhizal role in plant nitrogen supply because discrimination against ¹⁵N during creation of transfer compounds within mycorrhizal fungi decreases the ¹⁵N/¹⁴N in plants (low δ¹⁵N) and increases the ¹⁵N/¹⁴N of the fungi (high δ¹⁵N). Analytical models of ¹⁵N distribution would be helpful in interpreting δ¹⁵N patterns in fungi and plants. To compare different analytical models, we measured nitrogen isotope patterns in soils, saprotrophic fungi, ectomycorrhizal fungi, and plants with different mycorrhizal habits on a glacier foreland exposed during the last 100 years of glacial retreat and on adjacent non-glaciated terrain. Since plants during early primary succession may have only limited access to propagules of mycorrhizal fungi, we hypothesized that mycorrhizal plants would initially be similar to nonmycorrhizal plants in δ¹⁵N and then decrease, if mycorrhizal colonization were an important factor influencing plant δ¹⁵N. As hypothesized, plants with different mycorrhizal habits initially showed similar δ¹⁵N values (−4 to −6‰ relative to the standard of atmospheric N₂ at 0‰), corresponding to low mycorrhizal colonization in all plant species and an absence of ectomycorrhizal sporocarps. In later successional stages where ectomycorrhizal sporocarps were present, most ectomycorrhizal and ericoid mycorrhizal plants declined by 5–6‰ in δ¹⁵N, suggesting transfer of ¹⁵N-depleted N from fungi to plants. The values recorded (−8 to −11‰) are among the lowest yet observed in vascular plants. In contrast, the δ¹⁵N of nonmycorrhizal plants and arbuscular mycorrhizal plants declined only slightly or not at all. On the forefront, most ectomycorrhizal and saprotrophic fungi were similar in δ¹⁵N (−1 to −3‰), but the host-specific ectomycorrhizal fungus Cortinarius tenebricus had values of up to 7‰. Plants, fungi and soil were at least 4‰ higher in δ¹⁵N from the mature site than in recently exposed sites. On both the forefront and the mature site, host-specific ectomycorrhizal fungi had higher δ¹⁵N values than ectomycorrhizal fungi with a broad host range. From these isotopic patterns, we conclude:(1) large enrichments in ¹⁵N of many ectomycorrhizal fungi relative to co-occurring ectomycorrhizal plants are best explained by treating the plant-fungal-soil system as a closed system with a discrimination against ¹⁵N of 8–10‰ during transfer from fungi to plants, (2) based on models of ¹⁵N mass balance, ericoid and ectomycorrhizal fungi retain up to two-thirds of the N in the plant-mycorrhizal system under the N-limited conditions at forefront sites, (3) sporocarps are probably enriched in ¹⁵N by an additional 3‰ relative to available nitrogen, and (4) host-specific ectomycorrhizal fungi may transfer more N to plant hosts than non-host-specific ectomycorrhizal fungi. Our study confirms that nitrogen isotopes are a powerful tool for probing nitrogen dynamics between mycorrhizal fungi and associated plants.     

The carbon and nitrogen isotopic values of particulate organic material from the Florida Keys: a temporal and spatial study

The δ¹⁵N and δ¹³C values of particulate organic material (POM) were analyzed from 35 sites in the Florida Keys over the time interval 2000 to 2002. The sites within the study area were delineated into nine transects stretching from Key West to Key Largo. Each transect consisted of three to five sites extending from close to the Keys to the edge of the reef tract. The POM had mean δ¹⁵N and δ¹³C values of +3.6‰ (σ = ±3.2‰) and −19.9‰ (σ = ±0.6‰) respectively. Over the study period there were no statistically significant changes in δ¹⁵N, δ¹³C, or C:N. For the majority of the sampling dates, the δ¹³C values showed a distinct inshore (δ¹³C = −18.3‰, σ = ±1.0‰) to offshore gradient (δ¹³C = −21.4, σ = ±0.9‰). In contrast, the δ¹⁵N values showed no consistent patterns related to the distance from land. The more positive δ¹³C values of the nearshore samples suggest that the source of the carbon and the nitrogen in the POM in the nearshore was mainly derived from the degradation of seagrass detritus and not from the input of anthropogenically derived material from the Florida Keys. In contrast, the POM on the outer reef was dominated by marine plankton. As mineralization and nitrification of the organic nitrogen pool are major contributors to the dissolved inorganic nitrogen in the water column, it is unlikely that variations in the δ¹⁵N of the algae and other benthic organisms reported in the Florida Keys are related to the input of sewage.     

Isotopic enrichment in herbivorous insects: a comparative field-based study of variation

Researchers will be able to use stable isotope analysis to study community structure in an efficient way, without a need for extensive calibrations, if isotopic enrichment values are consistent, or if variation in enrichment values can be predicted. In this study, we generated an experimental data set of δ¹⁵N and δ¹³C enrichment means for 22 terrestrial herbivorous arthropods feeding on 18 different host plants. Mean enrichments observed across a single trophic transfer (plants to herbivores) were −0.53±0.26‰ for δ¹³C (range: −3.47‰ to 1.89‰) and 1.88±0.37‰ for δ¹⁵N (range: −0.20‰ to 6.59‰). The mean δ¹³C enrichment was significantly lower than that reported in recent literature surveys, whereas the mean δ¹⁵N enrichment was not significantly different. The experimental data set provided no support for recent hypotheses advanced to explain variation in enrichment values, including the proposed roles for consumer feeding mode, development type, and diet C:N ratio. A larger data set, formed by combining our experimental data with data from the literature, did suggest possible roles for feeding mode, nitrogen recycling, herbivore life stage, and host plant type. Our results indicate that species enrichment values are variable even in this relatively narrow defined group of organisms and that our ability to predict enrichment values of terrestrial herbivorous arthropods based on physiological, ecological, or taxonomic traits is low. The primary implications are that (1) mean enrichment may have to be measured empirically for each trophic link of interest, rather than relying on estimates from a broad survey of animal taxa and (2) the advantage of using stable isotope analysis to probe animal communities that are recalcitrant to other modes of study will be somewhat diminished as a consequence.Electronic Supplementary Material Supplementary material is available for this article at     

Influence of substratum on the variability of benthic biofilm stable isotope signatures: implications for energy flow to a primary consumer

Benthic biofilms have been identified using stable isotope analysis (SIA) as an important resource supporting many freshwater food webs. However, biofilm δ¹³C signatures are highly variable in freshwaters, which may hamper our understanding of energy flow through food webs in these systems. There has been little consideration of the influence that substratum may have on biofilm δ¹³C signature variability and energy flows to primary consumers. We investigated the effect of organic and inorganic substrata on biofilm dynamics by examining: (1) temporal variability of biofilm stable isotope (δ¹³C, δ¹⁵N) signatures on allochthonous leaf-litter (Eucalyptus camaldulensis) and cobble substrata over 12 months in a lowland river in south-eastern Australia; and (2) the effect of substrata on biofilm energy flows to a grazer snail, Physa acuta (Gastropoda: Physidae), using SIA and ecological stoichiometry in a laboratory experiment. The temporal study indicated that cobble biofilm varied significantly in δ¹³C signature during the 12 months (up to 11‰), whereas the δ¹³C signature of leaf biofilm was less variable (less than 2‰). In contrast, biofilm δ¹⁵N signatures varied temporally on both cobble (2.6‰) and leaf (1‰) substrata. This suggests that leaf biofilm was more reliant on leaf tissue for carbon and therefore less limited by carbon supply than cobble biofilm whereas for nitrogen biofilm on both substrata was reliant on external sources. In the laboratory experiment, snails fed leaf biofilm reflected more of an allochthonous δ¹³C signature than cobble biofilm fed snails, suggesting assimilation of leaf carbon via the heterotrophic microbial community within the biofilm. Snails grew largest on cobble biofilm, which had lower C:N ratios than leaf biofilm. Our results demonstrate that the type of substratum can influence the temporal variability of biofilm δ¹³C signatures and energy flow to primary consumers.     

Mechanism for the small-scale movement of carbon among estuarine habitats: organic matter transfer not crab movement

In theory, carbon is highly mobile in aquatic systems. Recent evidence from carbon stable isotopes of crabs (Parasesarma erythrodactyla and Australoplax tridentata), however, shows that in subtropical Australian waters, measurable carbon movement between adjacent mangrove and saltmarsh habitats is limited to no more than a few metres. We tested whether the pattern in crab δ¹³C values across mangrove and saltmarsh habitats was explained by crab movement, or the movement of particulate organic matter. We estimated crab movement in a mark–recapture program using an array of pitfall traps on 13 transects (a total of 65 traps) covering an area of 600 m² across the interface of these two habitats. Over a 19-day period, the majority of crabs (91% for P. erythrodactyla, 93% for A. tridentata) moved <2 m from the place of initial capture. Crab movement cannot, therefore, explain the patterns in δ¹³C values of crabs. δ¹³C values of detritus collected at 2-m intervals across the same habitat interface fitted a sigmoidal curve of a similar form to that fitting the δ¹³C values of crabs. δ¹³C values of detritus were 2–4‰ more depleted in saltmarsh (−18.5±0.6‰), and 4–7‰ more depleted in mangroves (−25.9±0.1‰) than δ¹³C values of crabs recorded previously in each habitat. Assimilation by crabs of very small detrital fragments or microphytobenthos, more enriched in ¹³C, may explain the disparity in δ¹³C values. Nevertheless, the pattern in δ¹³C values of detritus suggests that crabs obtain their carbon from up to several metres away, but without themselves foraging more then a metre or so from their burrow. Such detailed measurements of carbon movement in estuaries provide a spatially explicit understanding of the functioning of food webs in saltmarsh and mangrove habitats.