Fine-scale movement and assimilation of carbon in saltmarsh and mangrove habitat by resident animals

Despite theories of large-scale movement and assimilation of carbon in estuaries, recent evidence suggests that in some estuaries much more limited exchange occurs. We measured the fine-scale movement and assimilation of carbon by resident macroinvertebrates between adjacent saltmarsh and mangrove habitats in an Australian estuary using ¹³C analysis of animals at different distances into adjacent patches of habitat. ¹³C values of crabs (Parasesarma erythrodactyla –15.7 ± 0.1, Australoplax tridentata –14.7 ± 0.1) and slugs (Onchidina australis –16.2 ± 0.3) in saltmarsh closely matched that of the salt couch grass Sporobolus virginicus (–15.5 ± 0.1). In mangroves, ¹³C values of crabs (P. erythrodactyla –22.0 ± 0.2, A. tridentata –19.2 ± 0.3) and slugs (–19.7 ± 0.3) were enriched relative to those of mangroves (–27.9 ± 0.2) but were more similar to those of microphytobenthos (–23.7 ± 0.3). The ¹³C values of animals across the saltmarsh-mangrove interface fitted a sigmoidal curve, with a transition zone of rapidly changing values at the saltmarsh-mangrove boundary. The width of this transition indicated that the movement and assimilation of carbon is limited to between 5 and 7 m. The ¹³C values of crabs and slugs, especially those in saltmarsh habitat, clearly indicate that the movement and assimilation of carbon between adjacent saltmarsh and mangrove habitat is restricted to just a few metres, although some contribution from unmeasured sources elsewhere in the estuary is possible. Such evidence demonstrating the extent of carbon movement and assimilation by animals in estuarine habitats is useful in determining the spatial arrangement of habitats needed in marine protected areas to capture food web processes.     

Fine-scale movement and assimilation of carbon in saltmarsh and mangrove habitat by resident animals

Despite theories of large-scale movement and assimilation of carbon in estuaries, recent evidence suggests that in some estuaries much more limited exchange occurs. We measured the fine-scale movement and assimilation of carbon by resident macroinvertebrates between adjacent saltmarsh and mangrove habitats in an Australian estuary using δ¹³C analysis of animals at different distances into adjacent patches of habitat. δ¹³C values of crabs (Parasesarma erythrodactyla –15.7 ± 0.1‰, Australoplax tridentata –14.7 ± 0.1‰) and slugs (Onchidina australis –16.2 ± 0.3‰) in saltmarsh closely matched that of the salt couch grass Sporobolus virginicus (–15.5 ± 0.1‰). In mangroves, δ¹³C values of crabs (P. erythrodactyla –22.0 ± 0.2‰, A. tridentata –19.2 ± 0.3‰) and slugs (–19.7 ± 0.3‰) were enriched relative to those of mangroves (–27.9 ± 0.2‰) but were more similar to those of microphytobenthos (–23.7 ± 0.3‰). The δ¹³C values of animals across the saltmarsh-mangrove interface fitted a sigmoidal curve, with a transition zone of rapidly changing values at the saltmarsh-mangrove boundary. The width of this transition indicated that the movement and assimilation of carbon is limited to between 5 and 7 m. The δ¹³C values of crabs and slugs, especially those in saltmarsh habitat, clearly indicate that the movement and assimilation of carbon between adjacent saltmarsh and mangrove habitat is restricted to just a few metres, although some contribution from unmeasured sources elsewhere in the estuary is possible. Such evidence demonstrating the extent of carbon movement and assimilation by animals in estuarine habitats is useful in determining the spatial arrangement of habitats needed in marine protected areas to capture food web processes.     

Trophic relationships of juvenile blue crabs (Callinectes sapidus) in estuarine habitats

Salt marshes and shallow-water macroalgal beds are known to provide nursery habitat for many species of fish and invertebrates. The role of these habitats as refuge from predation is well established, but the degree to which indigenous primary production within the nursery provides food for growth and development of estuarine species remains unresolved. In this study, we tested the hypothesis that juvenile blue crabs depend on indigenous primary production, directly or indirectly, during their entire stay within the nursery. To test this hypothesis, we conducted isotopic studies and stomach content analyses of juveniles from habitats near the mouth of Delaware Bay and from an adjacent lagoonal estuary (ca. 39.5° N, 75.1° W). Primary producers, marsh detritus, various life-history stages of blue crabs and potential prey species were sampled in the main estuary and in an adjacent marsh during the summer and early fall of two consecutive years. Newly settled juveniles (<15 mm carapace width) from the marsh were about 1.8‰ lighter in carbon (−17.2‰) relative to larger juveniles from the marsh (15–30 mm carapace width) and appeared to have retained a carbon isotopic signature indicative of the phytoplankton-based food web associated with larval stages. However, the signature of juveniles changed as a function of size. Large juveniles and crabs >60 mm were enriched in δ¹³C (−14.7 ± 0.1‰) compared to small crabs, suggesting a gradual shift in diet from a planktonic to a detritus-based food web with increasing size. As with crabs from Delaware Bay, the δ¹³C signature of juvenile crabs sampled from macroalgal beds in the lagoonal estuary (Rehoboth Bay) changed as a function of size. Also, δ¹³C ratios of crabs varied among the various species of macroalgae. The δ¹⁵N composition of primary producers in the marsh and main estuary also was reflected in the δ¹⁵N values of crabs and other benthic consumers in the respective habitats. Results of stomach-content analysis in this study were consistent with isotope data. Observed changes in prey preferences were related to changes in size of juvenile crabs and also differed among habitats. Gut content analyses of the three size classes of juveniles in macroalgal beds from Rehoboth Bay indicated that the crabs depend heavily on various amphipod species that occur on the seaweeds. These amphipods graze directly on the macroalgae and are among the most abundant invertebrates in the macroalgal beds. This implies a direct trophic relationship between the juvenile crabs and the macroalgae. In summary, our study provides strong evidence that the value of nursery areas such as salt marshes and macroalgal beds goes beyond that of providing refuge from predation, and that species using these nurseries (e.g. juvenile blue crabs) are ultimately dependent on primary production originating in benthic plants indigenous to the nursery.     

Food web structure in exotic and native mangroves: a Hawaii–Puerto Rico comparison

Plant invasions can fundamentally alter detrital inputs and the structure of detritus-based food webs. We examined the detrital pathways in mangrove food webs in native (Puerto Rican) and introduced (Hawaiian) Rhizophora mangle forests using a dual isotope approach and a mixing model. Based on trophic-level fractionation of 0–1‰ for δ ¹³C and 2–3‰ for δ ¹⁵N, among the invertebrates, only nematodes, oligochaetes, and nereid polychaetes from native mangroves exhibited stable isotopes consistent with a mangrove-derived diet. Certain fauna, in particular tubificid oligochaetes, had δ ¹³C values consistent with the consumption of mangrove leaves, but they were depleted in ¹⁵N, suggesting their primary nitrogen source was low in ¹⁵N, and was possibly N₂-fixing bacteria. In introduced mangroves, all feeding groups appeared to rely heavily on non-mangrove sources, especially phytoplankton inputs. Mixing model results and discriminant analysis showed clear separation of introduced and native mangrove sites based on differential food source utilization within feeding groups, with stronger and more diverse use of benthic foods observed in native forests. Observed differences between native and invasive mangrove food webs may be due to Hawaiian detritivores being poorly adapted to utilizing the tannin-rich, nitrogen-poor mangrove detritus. In addition, differential utilization of mangrove detritus between native and introduced mangroves may be a consequence of forest age. We postulate that increasing mangrove forest age may promote diversification of bacterial food webs important in N and S cycling. Our results also suggest a potentially important role for sulfur bacteria in supporting the most abundant infaunal consumers, nematodes, in the most mature systems. Electronic supplementary material The online version of this article (doi:) contains supplementary material that is available to authorized users.     

Spatial patterns of foliar stable carbon isotope compositions of C<Subscript>3</Subscript> plant species in the Loess Plateau of China

The spatial pattern of foliar stable carbon isotope compositions (δ¹³C) of dominant species and their relationships with environmental factors in seven sites, Yangling, Yongshou, Tongchuan, Fuxian, Ansai, Mizhi and Shenmu, standing from south to north in the Loess Plateau of China, was studied. The results showed that in the 121 C₃ plant samples collected from the Loess Plateau, the foliar δ¹³C value ranged from −22.66‰ to −30.70‰, averaging −27.04‰. The foliar δ¹³C value varied significantly (P<0.01) among the seven sites, and the average δ¹³C value increased by about 1.69‰ from Yangling in the south to Shenmu in the north as climatic drought increased. There was a significant difference in foliar δ¹³C value among three life-forms categorized from all the plant samples in the Loess Plateau (P<0.001). The trees (−26.74‰) and shrubs (−26.68‰) had similar mean δ¹³C values, both significantly (P<0.05) higher than the mean δ¹³C value of herbages (−27.69‰). It was shown that the trees and shrubs had higher WUEs and employed more conservative water-use patterns to survive drier habitats in the Loess Plateau. Of all the C₃ species in the Loess Plateau, the foliar δ¹³C values were significantly and negatively correlated with the mean annual rainfall (P<0.001) and mean annual temperature (P<0.05), while being significantly and positively correlated with the latitude (P<0.001) and the annual solar radiation (P<0.01). In general, the foliar δ¹³C values increased as the latitude and solar radiation increased and the rainfall and temperature decreased. The annual rainfall as the main influencing factor could explain 13.3% of the spatial variations in foliar δ¹³C value. A 100 mm increment in annual rainfall would result in a decrease by 0.88‰ in foliar δ¹³C values.     

A food web analysis of the juvenile blue crab, Callinectes sapidus, using stable isotopes in whole animals and individual amino acids

The stable isotope compositions (C and N) of plants and animals of a marsh dominated by Spartina alterniflora in the Delaware Estuary were determined. The study focused on the juvenile stage of the Atlantic blue crab, Callinectes sapidus, and the importance of marsh-derived diets in supporting growth during this stage. Laboratory growth experiments and field data indicated that early juvenile blue crabs living in the Delaware Bay habitat fed primarily on zooplankton, while marsh-dwelling crabs, which were enriched in ¹³C relative to bay juveniles, utilized marsh-derived carbon for growth. In laboratory experiments, the degree to which juvenile blue crabs isotopically fractionated dietary nitrogen, as well as the growth rate, depended on the protein quality of the diet. The range of δ¹³C of amino acids in laboratory-reared crabs and their diets was almost 20‰, similar to the isotopic range of amino acids of other organisms. In laboratory studies, the δ¹³C of nonessential and essential amino acids in the diet were compared to those in juvenile crabs. Isotopic fractionation at the molecular level depended on diet quality and the crabs' physiological requirements. Comparison of whole-animal isotope data with individual amino acid C isotope measurements of wild juvenile blue crabs from the bay and marsh suggested a different source of total dietary carbon, yet a shared protein component, such as zooplankton. Received: 1 July 1998 / Accepted: 15 March 1999     

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.     

Carbon isotope ratio and photosynthetic activity of phytoplankton in the eutrophic Mikawa Bay,Japan

Primary production by phytoplankton in the eutrophic Mikawa Bay, Japan, was studied by simultaneous measurements of natural carbon isotope ratio (δ ¹³C) and short-term carbon uptake rates (¹³C tracer study) of size-fractionated nannoplankton (<10 μm) and net plankton (>10 μm) samples. Short-term photosynthetic rates, which represent the physiological state of algae, were variable regardless of standing stock sizes. Theδ ¹³C values of particulate organic carbon (POC) in June and July displayed horizontal variations for both the net plankton fraction (−19.8 to −12.7‰) and the nannoplankton fraction (−22.0 to −12.8‰). For both fractions, low concentrations of POC had more negativeδ ¹³C values (−22 to −18‰). Highδ ¹³C values for the net plankton were found when POC concentrations were much higher, due to red tide. This suggests that the increase in algal standing crop for the net plankton fraction resulted from accelerated photosynthetic activity. However the nannoplankton fractions with higher POC values have relatively lowδ ¹³C values.     

Spatial variability in Florida Bay particulate organic matter composition: combining flow cytometry with stable isotope analyses

Long-term management plans for restoration of natural flow conditions through the Everglades increase the importance of understanding potential nutrient impacts of increased freshwater delivery on Florida Bay biogeochemistry. Planktonic communities respond quickly to changes in water quality, thus spatial variability in community composition and relationships to nutrient parameters must be understood in order to evaluate future downstream impacts of modifications to Everglades hydrology. Here we present initial results combining flow cytometry analyses of phytoplankton and bacterial populations (0.1–50 μm size fraction) with measurements of δ¹³C and δ¹⁵N composition and dissolved inorganic nutrient concentrations to explore proxies for planktonic species assemblage compositions and nutrient cycling. Particulate organic material in the 0.1–50 μm size fraction was collected from five stations in Northeastern and Western Florida Bay to characterize spatial variability in species assemblage and stable isotopic composition. A dense bloom of the picocyanobacterium, Synechococcus elongatus, was observed at Western Florida Bay sites. Smaller Synechococcus sp. were present at Northeast sites in much lower abundance. Bacteria and detrital particles were also more abundant at Western Florida Bay stations than in the northeast region. The highest abundance of detritus occurred at Trout Creek, which receives freshwater discharge from the Everglades through Taylor Slough. In terms of nutrient availability and stable isotopic values, the S. elongatus population in the Western bay corresponded to low DIN (0.5 μM NH ₄ ⁺ ; 0.2 μM NO ₃ ⁻ ) concentrations and depleted δ¹⁵N signatures ranging from +0.3 to +0.8‰, suggesting that the bloom supported high productivity levels through N₂-fixation. δ¹⁵N values from the Northeast bay were more enriched (+2.0 to +3.0‰), characteristic of N-recycling. δ¹³C values were similar for all marine Florida Bay stations, ranging from −17.6 to −14.4‰, however were more depleted at the mangrove ecotone station (−25.5 to −22.3‰). The difference in the isotopic values reflects differences in carbon sources. These findings imply that variations in resource availability and nutrient sources exert significant control over planktonic community composition, which is reflected by stable isotopic signatures.     

Interspecific variability of δ13C among trees in rainforests of French Guiana: functional groups and canopy integration

The interspecific variability of sunlit leaf carbon isotope composition (δ¹³C), an indicator of leaf intrinsic water-use efficiency (WUE, CO₂ assimilation rate/leaf conductance for water vapour), was investigated in canopy trees of three lowland rainforest stands in French Guiana, differing in floristic composition and in soil drainage characteristics, but subjected to similar climatic conditions. We sampled leaves with a rifle from 406 trees in total, representing 102 species. Eighteen species were common to the three stands. Mean species δ¹³C varied over a 6.0‰ range within each stand, corresponding to WUE varying over about a threefold range. Species occurring in at least two stands displayed remarkably stable δ¹³C values, suggesting a close genetic control of species δ¹³C. Marked differences in species δ¹³C values were found with respect to: (1) the leaf phenology pattern (average δ¹³C=–29.7‰ and –31.0‰ in deciduous-leaved and evergreen-leaved species, respectively), and (2) different types of shade tolerance defined by features reflecting the plasticity of growth dynamics with respect to contrasting light conditions. Heliophilic species exhibited more negative δ¹³C values (average δ¹³C=–30.5‰) (i.e. lower WUE) than hemitolerant species (–29.3‰). However, tolerant species (–31.4‰) displayed even more negative δ¹³C values than heliophilic ones. We could not provide a straightforward ecophysiological interpretation of this result. The negative relationship found between species δ¹³C and midday leaf water potential (Ψ_wm) suggests that low δ¹³C is associated with high whole tree leaf specific hydraulic conductance. Canopy carbon isotope discrimination (Δ _A ) calculated from the basal area-weighed integral of the species δ¹³C values was similar in the three stands (average Δ _A =23.1‰), despite differences in stand species composition and soil drainage type, reflecting the similar proportions of the three different shade-tolerance types among stands. Received: 30 November 1999 / Accepted: 23 March 2000     

Isotopic tracking of foraging and long-distance migration in northeastern Pacific pinnipeds

We investigated the impact of foraging location (nearshore vs offshore) and foraging latitude (high vs middle) on the carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope compositions of bone collagen of northern fur seals (Callorhinus ursinus), harbor seals (Phoca vitulina), California sea lions (Zalophus californianus), and northern elephant seals (Mirounga angustirostris). Nearshore-foraging harbor seals from California had δ¹³C values 2.0‰ higher than female northern elephant seals foraging offshore at similar latitudes. Likewise, nearshore-foraging harbor seals from Alaska had values 1.7‰ higher than male northern fur seals, which forage offshore at high latitudes. Middle-latitude pinnipeds foraging in either the nearshore or offshore were ¹³C enriched by ∼1.0‰ over similar populations from high latitudes. Male northern elephant seals migrate between middle and high latitudes, but they had δ¹³C values similar to high-latitude, nearshore foragers. Female northern fur seal δ¹³C values were intermediate between those of high- and middle-latitude offshore foragers, reflecting their migration between high- and middle-latitude waters. The δ¹³C values of California sea lions were intermediate between nearshore- and offshore-foraging pinnipeds at middle latitudes, yet there was no observational support for the suggestion that they use offshore food webs. We suggest that their “intermediate” values reflect migration between highly productive and less-productive, nearshore ecosystems on the Pacific coasts of California and Mexico. The relative uniformity among all of these pinnipeds in δ¹⁵N values, which are strongly sensitive to trophic level, reveals that the carbon isotope patterns result from differences in the δ¹³C of organic carbon at the base of the food web, rather than differences in trophic structure, among these regions. Finally, the magnitude and direction of the observed nearshore-offshore and high-to middle-latitude differences in δ¹³C values suggest that these gradients may chiefly reflect differences in rates and magnitudes of phytoplankton production as well as the δ¹³C value of inorganic carbon available for photosynthesis, rather than the input of ¹³C-enriched macroalgal carbon to nearshore food webs. Received: 8 September 1998 / Accepted: 24 February 1999     

The δ13C changes in four plant species of the Loess Plateau over the last 70 years

The relative abundance of carbon isotope (δ¹³C) was measured in four C₃ species (Sophora viccifolia, Quercus liaotungensis, Ostryopsis davidiana and Zizyphus jujuba var. spinosa) of the Loess Plateau in China from the 1930’s to 2002. The results showed that the δ¹³C values in the four species varied from −25.05‰ to −29.75‰ with their a average at −27.04‰. A decrease in the δ¹³C value with time was found in all the four species, which indicating that the water use efficiencies (WUEs) of all the measured species declined during 70 years. However, the decrease in δ¹³C value differed among the four species with its significant decreases measured in two of the species, Sophora viciifolia and Quercus liaotungensis, its relatively significant decrease found in Ostryopsis davidiana, and its slight decrease appearing in Zizyphus jujuba var. spinosa. in the δ¹³C values in the four species decreased by 14.65 ‰, 14.46‰, 11.99‰ and 2.44‰, respectively. The different species were shown to have different sensitivities to climatic change, and Zizyphus jujuba var. spinosa was found to be the most drought-tolerant species of the four, which had a high WUE.     

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.     

Contributions of Different Organic Carbon Sources to <Emphasis Type="Italic">Daphnia</Emphasis> in the Pelagic Foodweb of a Small Polyhumic Lake: Results from Mesocosm DI<Superscript>13</Superscript>C-Additions

Abstract Freshwater ecosystems derive organic carbon from both allochthonous and autochthonous sources. We studied the relative contributions of different carbon sources to zooplankton in a small, polyhumic, steeply stratified lake, using six replicate surface-to-sediment enclosures established during summer and autumn 2004. We added ¹³C-enriched bicarbonate to the epilimnion of half the enclosures for three weeks during each season and monitored carbon stable isotope ratios of DIC, DOC, POC and Daphnia, along with physical, chemical and biological variables. During summer, ¹³C-enriched DIC (δ¹³C up to 44 ± 7.2‰) was soon taken up by phytoplankton (δ¹³C up to −5.1 ± 13.6‰) and was transmitted to Daphnia (δ¹³C up to −1.7 ± 7.2‰), demonstrating consumption of phytoplankton. In contrast, during autumn, ¹³C-enriched DIC (δ¹³C up to 56.3 ± 9.8‰) was not transmitted to Daphnia, whose δ¹³C became progressively lower (δ¹³C down to −45.6 ± 3.3‰) concomitant with decreasing methane concentration. Outputs from a model suggested phytoplankton contributed 64–84% of Daphnia diet during summer, whereas a calculated pelagic carbon mass balance indicated only 30–40% could have come from phytoplankton. Although autumn primary production was negligible, zooplankton biomass persisted at the summer level. The model suggested methanotrophic bacteria contributed 64–87% of Daphnia diet during autumn, although the calculated carbon mass balance indicated a contribution of 37–112%. Thus methanotrophic bacteria could supply virtually all the carbon requirement of Daphnia during autumn in this lake. The strongly ¹³C-depleted Daphnia values, together with the outputs from the models and the calculated carbon mass balance showed that methanotrophic bacteria can be a greater carbon source for Daphnia in lakes than previously suspected.     

Carbon nutrition of mature green orchid <Emphasis Type="Italic">Serapias strictiflora</Emphasis> and its mycorrhizal fungus <Emphasis Type="Italic">Epulorhiza</Emphasis> sp.

We studied the nutritional modes of the orchid Serapias strictiflora and its mycorrhizal fungus Epulorhiza sp. using the differences in carbon isotopic composition (δ¹³C) of C₃ orchid and C₄ maize tissues. We found that if cultivated in substrate lacking any organic compounds, the mycorrhizal extraradical mycelia (δ¹³C = −26.3 ± 0.2 ‰) developed well, despite being fully dependent on nutrition from orchid roots (δ¹³C = −28.6 ± 0.1 ‰). If the mycorrhizal fungus had additional access to and colonized decaying maize roots (δ¹³C = −14.6 ± 0.1 ‰), its isotopic composition (δ¹³C = −21.6 ± 0.4 ‰) reflected a mixture of biotrophy and saprotrophy. No statistically significant differences in δ¹³C of new storage tubers were found between Epulorhiza-associated orchids with (δ¹³C = -28.2 ± 0.1 ‰) and without access to maize roots (δ¹³C = −28.6 ± 0.2 ‰). We conclude that autotrophy is the predominant nutritional mode of mature S. strictiflora plants and that they supply their mycorrhizal fungus with substantial amount of carbon (69 ± 3 % of the fungus demand), even if the fungus feeds saprotrophically.     

Piscivorous birds as top predators and fishery competitors in the lagoon ecosystem

Trophic patterns of omnivorous freshwater shrimps, Exopalaemon modestus and Macrobrachium nipponensis, were investigated in two shallow eutrophic lakes by using stable isotope analysis. δ¹⁵N and δ¹³C of M. nipponensis and E. modestus increased with increasing body weight, which might be attributed to larger individuals ingesting organisms that feed higher up the food chain and/or increased assimilation of benthic food items with enriched isotopic signatures. Of the freshwater shrimps occurring in the studied lakes, those from Lake Taihu had significantly elevated δ¹⁵N and δ¹³C values (4.3‰ and 1.8‰, respectively) compared with those from the less eutrophic Lake Chaohu, indicating that the isotopic signature might partially reflect the trophic states of their habitats. Mixing model results suggested that the benthic food web provides the primary carbon source for both shrimp species, and that E. modestus assimilated relatively more pelagic food sources than M. nipponensis in these lakes. Handling editor: S. Wellekens     

Contributions of stable-isotope data to elucidating food webs of Mediterranean rocky littoral fishes

The food webs of rocky infra-littoral ecosystems in the Mediterranean have been little studied. In this investigation stable isotopes and dietary data were compared in an attempt to describe features of the food webs concerned. δ¹³C and δ¹⁵N were determined for plants, invertebrates and fishes from the Bay of Calvi, Corsica. Dietary data were derived from the literature. δ¹³C of plants ranged from –8.59‰ to –33.74‰, of benthic invertebrates from –17.0‰ to –20.52‰, of planktonic invertebrates from –20.08‰ to –22.34‰ and of fishes from –16.27‰ to –19.59‰. δ¹⁵N was generally greater at higher trophic levels. δ¹⁵N of plants was 0.95–2.92‰, of benthic invertebrates 1.69–6.54‰, of planktonic invertebrates 3.51–6.82‰ and of fishes 4.63–9.77‰. ¹³C enrichment tended to be associated with benthic food chains and ¹³C depletion with planktonic chains. Stable-isotope data suggested more varied diets for many species than implied by gut-contents data. Omnivory and trophic plasticity were widespread, and many consumers fed lower down the food chain than previous studies had suggested. Both stable-isotope and gut-contents analysis resolved differences between fishes feeding on planktonic and benthic prey and indicated that the herbivorous fish Sarpa salpa fed on a diet substantially different from that of other fishes. Zooplankton were important in the diets of several consumers (both primary and secondary), as was plankton derived detritus. One species of fish previously identified as planktivorous was shown to feed largely on benthic organisms, whilst several species of benthic invertebrates may feed on plankton-derived detritus. Although herbivores seemed to obtain most of their C from macroalgae, δ¹⁵N data suggested that many of these animals supplemented their intake of N, although gut-contents analysis did not provide evidence for such uptake. The isotopic data have elucidated several features of the food web which we would not otherwise have detected. Received: 26 April 1999 / Accepted: 24 September 1999     

Discrimination between12C and13C by marine plants

Summary The natural abundance¹³C/¹²C ratios (as δ¹³C) of organic matter of marine macroalgae from Fife and Angus (East Scotland) were measured for comparison with the species' ability to use CO₂ and HCO ₃ ^- for photosynthesis, as deduced from previously published pH-drift measurements. There was a clear difference in δ¹³C values for species able or unable to use HCO ₃ ^- . Six species of Chlorophyta, 12 species of Phaeophyta and 8 species of Rhodophyta that the pH-drift data suggested could use HCO ₃ ^- had δ¹³C values in the range -8.81‰ to -22.55‰. A further 6 species of Rhodophyta which the pH-drift data suggested could only use CO₂ had δ¹³C values in the range -29.90‰ to-34.51‰. One of these six species (Lomentaria articulata) is intertidal; the other five are subtidal and so have no access to atmospheric CO₂ to complicate the analysis. For these species, calculations based on the measured δ¹³C of the algae, the δ¹³C of CO₂ in seawater, and the known¹³C/¹²C discrimination of CO₂ diffusion and RUBISCO carboxylation suggest that only 15–21% of the limitation to photosynthesisin situ results from CO₂ diffusion from the bulk medium to the plastids; the remaining 79–85% is associated with carboxylation reactions (and, via feedback effects, down-stream processes). This analysis has been extended for one of these five species,Delesseria sanguinea, by incorporating data onin situ specific growth rates, respiratory rates measured in the laboratory, and applying Fick's law of diffusion to calculate a boundary layer thickness of 17–24 μm. This value is reasonable for aDelesseria sanguinea frondin situ. For HCO ₃ ^- -using marine macroalgae the range of δ¹³C values measured can be accommodated by a CO₂ efflux from algal cells which range from 0.306 of the gross HCO ₃ ^- influx forEnteromorpha intestinalis (δ¹³C=-8.81‰) in a rockpool to 0.787 forChondrus crispus (δ¹³C=-22.55‰). The relatively high computed CO₂ efflux for those HCO ₃ ^- -users with the more negative δ¹³C values implies a relatively high photon cost of C assimilation; the observed photon costs can be accommodated by assuming coupled, energy-independent inorganic carbon influx and efflux. The observed δ¹³C values are also interpreted in terms of water movement regimes and obtaining CO₂ from the atmosphere. Published δ¹³C values for freshwater macrophytes were compared with the ability of the species to use CO₂ and HCO ₃ ^- and again there was an apparent separation in δ¹³C values for these two groups. δ¹³C values obtained for marine macroalgae for which no pH-drift data are available permit predictions, as yet untested, as to whether they use predominantly CO₂ or HCO ₃ ^-     

Variation in the δ<Superscript>13</Superscript>C of Two Mangrove Plants is Correlated with Stomatal Response to Salinity

To better understand the relationship between salinity and the carbon stable isotope composition (expressed as δ¹³C) of mangrove plants and to test whether the patterns of variation in δ¹³C of mangrove plants differ from those of nonhalophytes as response to salinity, the effect of salinity on leaf δ¹³C in two dominant mangrove species, Aegiceras corniculatum and Kandelia candel, was studied. Furthermore, to determine whether the variation in δ¹³C of mangrove species is adjusted by stomatal conductance, K. candel was selected as an example, and leaf gas-exchange characteristics of the seedlings were measured. It was observed that both mangrove species had a lower leaf δ¹³C under their optimum salinity (1.50% for Ae. corniculatum and 2.00% for K. candel). This variation in δ¹³C of mangrove plants was attributable largely to stomatal adjustment as for nonhalophytes in which a strong correlation between δ¹³C and relevant photosynthetic properties is observed. This result suggests that the different response pattern in δ¹³C was a consequence of the variation in stomata in relation to the different tolerance to salinity. The optimum salinity inferred by leaf δ¹³C provides a feasible method for comparing salt tolerance between mangrove plants belonging to different species, which is useful for mangrove restoration.     

Influence of stand structure on carbon-13 of vegetation, soils, and canopy air within deciduous and evergreen forests in Utah, United States

Carbon isotope ratios (δ¹³C) were studied in evergreen and deciduous forest ecosystems in semi-arid Utah (Pinus contorta, Populus tremuloides, Acer negundo and Acer grandidentatum). Measurements were taken in four to five stands of each forest ecosystem differing in overstory leaf area index (LAI) during two consecutive growing seasons. The δ¹³C_leaf (and carbon isotope discrimination) of understory vegetation in the evergreen stands (LAI 1.5–2.2) did not differ among canopies with increasing LAI, whereas understory in the deciduous stands (LAI 1.5–4.5) exhibited strongly decreasing δ¹³C_leaf values (increasing carbon isotope discrimination) with increasing LAI. The δ¹³C values of needles and leaves at the top of the canopy were relatively constant over the entire LAI range, indicating no change in intrinsic water-use efficiency with overstory LAI. In all canopies, δ¹³C_leaf decreased with decreasing height above the forest floor, primarily due to physiological changes affecting c _i/c _a (> 60%) and to a minor extent due to δ¹³C of canopy air (< 40%). This intra-canopy depletion of δ¹³C_leaf was lowest in the open stand (1‰) and greatest in the denser stands (4.5‰). Although overstory δ¹³C_leaf did not change with canopy LAI, δ¹³C of soil organic carbon increased with increasing LAI in Pinus contorta and Populus tremuloides ecosystems. In addition, δ¹³C of decomposing organic carbon became increasingly enriched over time (by 1.7–2.9‰) for all deciduous and evergreen dry temperate forests. The δ¹³C_canopy of CO₂ in canopy air varied temporally and spatially in all forest stands. Vertical canopy gradients of δ¹³C_canopy, and [CO₂]_canopy were larger in the deciduous Populus tremuloides than in the evergreen Pinu contorta stands of similar LAI. In a very wet and cool year, ecosystem discrimination (Δ_e) was similar for both deciduous Populus tremulodies (18.0 ± 0.7‰) and evergreen Pinus contorta (18.3 ± 0.9‰) stands. Gradients of δ¹³C_canopy and [CO₂]_canopy were larger in denser Acer spp. stands than those in the open stand. However, ¹³C enrichment above and photosynthetic draw-down of [CO₂]_canopy below tropospheric baseline values were larger in the open than in the dense stands, due to the presence of a vigorous understory vegetation. Seasonal patterns of the relationship δ¹³C_canopy versus 1/[CO₂]_canopy were strongly influenced by precipitation and air temperature during the growing season. Estimates of Δ_e for Acer spp. did not show a significant effect of stand structure, and averaged 16.8 ± 0.5‰ in 1933 and 17.4 ± 0.7‰ in 1994. However, Δ_e varied seasonally with small fluctuations for the open stand (2‰), but more pronounced changes for the dense stand (5‰). Received: 15 April 1996 / Accepted: 19 October 1996