Trophic relationships in a tropical stream food web assessed by stable isotope analysis

1. Stable isotope analysis, coupled with dietary data from the literature, was used to investigate trophic patterns of freshwater fauna in a tropical stream food web (Guadeloupe, French West Indies).
2. Primary producers (biofilm, algae and plant detritus of terrestrial origin) showed distinct δ¹³C signatures, which allowed for a powerful discrimination of carbon sources. Both autochthonous (¹³C-enriched signatures) and allochthonous (¹³C-depleted signatures) resources enter the food web. The migrating behaviour of fishes and shrimps between marine and freshwater during their life cycles can be followed by carbon isotopes. Here, shrimp δ¹³C signatures were shown to shift from −16‰ (for juveniles under marine influence) to −24.7‰ (for adults in freshwater habitats). For resident species, δ¹³C values partly reflected the species' habitat preferences along the river continuum : species living in river mouths were ¹³C-enriched in comparison with those collected upstream.
3. Nitrogen isotopic ratios were also discriminating and defined three main trophic guilds among consumers. The δ¹⁵N values of herbivores/detritivores were 5.0–8.4‰, omnivores 8.8–10.2‰ and carnivores 11–12.7‰.
4. Mixing model equations were employed to calculate the possible range of contribution made by respective food sources to the diet of each species. The results revealed the importance of omnivorous species and the dependence of riverine biota on terrestrial subsidies, such as leaf detritus and fruits. Finally, the abundance of shrimps and their feeding habits placed in relief their key role in tropical freshwater food webs. Isotopic analysis provides a useful tool for assessing animal feeding patterns.     

Mother Knows Best: Occurrence and Associations of Resighted Humpback Whales Suggest Maternally Derived Fidelity to a Southern Hemisphere Coastal Feeding Ground

Site fidelity is common among migratory cetaceans, including humpback whales (Megaptera novaeangliae). In the Northern Hemisphere it has been found that fidelity to humpback whale feeding grounds is transferred maternally but this has never been shown for the species in the Southern Hemisphere. We examined this in a unique feeding area off west South Africa using resighting data of 68 individually identified humpback whales by means of photographic (tail flukes and dorsal fins) and/or molecular methods (microsatellite genotyping) over an 18 year span. We found short-term association patterns and recurrent visits typical of other feeding grounds. Males and females had different seasonality of attendance. Significant female-dominated presence corresponded to timing of an expected influx of females on their southward migration from the breeding ground: firstly non-nursing (possibly pregnant) females in mid-spring, and mothers and calves in mid-to late summer. The potential benefit of this mid-latitude feeding area for females is illustrated by a record of a cow with known age of at least 23 years that produced calves in three consecutive years, each of which survived to at least six months of age: the first record of successful post-partum ovulation for this species in the Southern Hemisphere. We recorded association of a weaned calf with its mother, and a recurring association between a non-lactating female and male over more than two years. Moreover, three animals first identified as calves returned to the same area in subsequent years, sometimes on the same day as their mothers. This, together with numerous Parent-Offspring relations detected genetically among and between resighted and non-resighted whales is strongly suggestive of maternally derived site fidelity at a small spatial scale by a small sub-population of humpback whales.     

Nitrogen and carbon isotope ratios in termites: an indicator of trophic habit along the gradient from wood-feeding to soil-feeding

1. Nitrogen and carbon stable-isotope ratios (δ¹⁵N and δ¹³C) of body tissues, mound/nest materials and dietary substrates were determined in termite species with differing trophic habits, sampled from the Mbalmayo Forest Reserve, southern Cameroon.
2. δ¹⁵N of termite tissues was enriched gradually along a spectrum of species representing a trophic gradient from wood- to soil-feeding. Species that could be identified from their general biology and from gut content analysis as feeding on well-rotted wood or as wood/soil interface feeders showed δ¹⁵N intermediate between sound-wood-feeders and soil-feeders. It is proposed that δ¹⁵N is therefore a possible indicator of the functional position of species in the humification process. Differences in δ¹³C were also observed between wood-feeding and soil-feeding forms.
3. High values of δ¹⁵N in soil-feeding termites suggest that nitrogen fixation is of little importance in these species.
4. A wide range of isotope effects (the difference in isotope ratios between termites and their diet) was observed for both nitrogen (Δδ¹⁵N = –1.6 to + 8.8‰) and carbon (Δδ¹³C = –2.2 to + 3.0‰), which suggests a diversity of nutrient acquisition mechanisms within termites and diverse relationships between termites and their intestinal micro-organisms.     

δ13C of wood in growth-rings indicates cambial activity of drought-stressed trees of Eucalyptus globulus

1. Growth, density and δ¹³C of wood and leaf area were measured in two adjacent stands of 6 year-old Eucalyptus globulus growing in the 600–700 mm year^–1 rainfall region of south-western Australia. Study sites were identical except for differences in the availability of water owing to physical properties of soil profiles and location of sites within the landscape.
2. Abundance of ¹³C (expressed as δ¹³C) in wood of trees growing on the drought-prone site (– 24·8‰±1·4) was greater than in other trees (– 25·8‰±1·2, P <0·001) throughout the 6 years and, with further development, the δ¹³C signatures of wood may become useful indices of drought-susceptibility in plantations within a few years of establishment. The seasonal pattern of δ¹³C of wood appeared to reflect seasonal variation in water availability and duration of cambial activity.
3. Basic density of wood of trees growing on the more drought-prone site (496±14·0 kg m^–3) was reduced compared to other trees (554±5·3 kg m^–3, P <0·001). δ¹³C of wood across boundaries of growth-rings suggested that drought stopped cambial activity resulting in less production of late wood and less dense wood.
4. The stand growing on the drought-prone site had reduced growth, wood yield and leaf area but identical specific leaf area. Annual growth was correlated with the previous season's rainfall. Together, these results suggested that within the same evaporative climate, drought reduces growth primarily by reducing leaf area and that there is a lag between onset of drought and reduced productivity.     

Stable isotopes of carbon and nitrogen as indicators of diet and trophic structure of the fish community in a shallow hypereutrophic lake

Stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes were employed to elucidate energy flows and trophic interactions in Lake Apopka, a hypereutrophic lake in central Florida, U.S.A. Isotope compositions of lake biota ranged from −27·1 to −3·0‰ for δ¹³C, and from 3·7 to 13·9‰ for δ¹⁵N. The food web was based primarily on plankton production with diatoms, Microcystis and zooplankton dominating the diet of fish. Carbon isotope evidence showed that pico- and nano-phytoplankton were not a direct carbon source for fish, but were important to zooplankton. δ¹⁵N mass balance estimates indicated that planktivorous fish obtained 48–85% of their diets from zooplankton. The ∼3‰ range of δ¹⁵N in gizzard shad reflected increasing dependence on zooplankton as fish grew whereas the positive relationship between total length and δ¹⁵N of largemouth bass reflected increasing predation on larger planktivorous fish with growth. The broad ranges of δ¹³C (−25·9 to −9·5‰) and δ¹⁵N (5·8 to 14·4‰) of blue tilapia were indicators of diet diversity. Two presumed omnivores (brown bullhead and white catfish) and piscivores (black crappie, largemouth bass and Florida gar) were found to depend on planktivorous fish. However, stable isotope data revealed no trophic links between blue tilapia, an abundant fish in the near-shore area, and piscivores.     

Trophic level and overlap of sea lions (Zalophus californianus) in the Gulf of California, Mexico

Stable isotope and scat analyses were used in concert to determine trophic level and dietary overlap among California sea lions from different rookeries in the Gulf of California. Isotopic analysis of the fur of sea lion pups revealed differences in δ¹⁵N and δ¹³C values among rookeries during the breeding season. Mean δ¹⁵N and δ¹³C values varied from 20.2‰ to 22.4‰ and from −15.4‰ to −14.0‰, respectively. The pattern of differences among rookeries was similar between years in most cases. Isotopic variations among rookeries were associated with differences in prey consumption. There was a significant correlation between δ¹⁵N value and trophic level, as determined by scat analysis. Joint application of isotopic and scat analyses allowed us to identify how the feeding habits of sea lions vary with location. Our results suggest the presence of spatial structure in available prey as well as the localized use of prey by sea lions across the Gulf of California.     

Stable isotope profiles of partially migratory salmonid populations in Atlantic rivers of Patagonia

In the present study, profiles of stable isotope composition were characterized for two species with partially migratory populations in rivers along the latitudinal gradient of Patagonia, brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss . The effects of factors ( e.g. ontogeny of fishes, location, species and fasting) that may influence the stable isotope analysis (SIA) were evaluated, as was SIA evaluated as a tool to assign individual fish to their corresponding ecotype. Anadromous fishes exhibited enriched δ¹⁵N (15·2 ± 1·0‰; mean ± s . d .) and δ¹³C (−19·2 ± 1·3‰) relative to resident fishes'δ¹⁵N (8·8 ± 1·1‰) and δ¹³C (−23·2 ± 2·5‰). For both species, the difference in δ¹⁵N was larger between resident (range 6·8–10·7‰) and anadromous (range 14·3–17·8‰) fishes than that in δ¹³C. Values of δ¹³C, while not as dramatically contrasting in rainbow trout, provided a powerful anadromy marker for brown trout in the region. Increases were found in both δ¹⁵N and δ¹³C during the spawning migration of anadromous rainbow trout, most likely due to fasting. Differences in stable isotopes between location, size and species were found, suggesting different stable isotopes base levels in freshwater environments and different trophic levels and feeding location of anadromous populations. The SIA was demonstrated as a powerful tool for ecotype discrimination in Patagonian Rivers, overriding any effect of sampling location, size or species.     

ESTIMATION OF PREY DENSITIES REQUIRED BY WESTERN NORTH ATLANTIC RIGHT WHALES

The apparent lack of significant recovery of western North Atlantic right whale populations may be related to the availability of prey in exploitable densities. We have attempted to estimate the concentration of zooplankton required by right whales to obtain a net energetic benefit over the long term, using estimated values for body weight, metabolic rate, assimilation efficiency, time spent feeding, mouth size and swimming speed. The estimated range of required energy densities is 7.57 to 2,394 kcal m⁻³ (3.17 × 10⁴ to 1.00 × 10⁷ joule m⁻³). These values are from one to more than three orders of magnitude greater than the densest concentration sampled in the vicinity of right whale aggregations in the Great South Channel. Right whales must seek out and exploit extremely dense patches of prey organisms in order to feed efficiently. The presence of such dense patches of zooplankton is likely to be a primary characteristic of spring, summer and fall right whale habitats.     

Extensive carbon isotopic heterogeneity among methane seep microbiota

To assess and study the heterogeneity of δ¹³C values for seep microorganisms of the Eel River Basin, we studied two principally different sample sets: sediments from push cores and artificial surfaces colonized over a 14 month in situ incubation. In a single sediment core, the δ¹³C compositions of methane seep-associated microorganisms were measured and the relative activity of several metabolisms was determined using radiotracers. We observed a large range of archaeal δ¹³C values (> 50‰) in this microbial community. The δ¹³C of ANME-1 rods ranged from −24‰ to −87‰. The δ¹³C of ANME-2 sarcina ranged from −18‰ to −75‰. Initial measurements of shell aggregates were as heavy as −19.5‰ with none observed to be lighter than −57‰. Subsequent measurements on shell aggregates trended lighter reaching values as ¹³C-depleted as −73‰. The observed isotopic trends found for mixed aggregates were similar to those found for shell aggregates in that the initial measurements were often enriched and the subsequent analyses were more ¹³C-depleted (with values as light as −56‰). The isotopic heterogeneity and trends observed within taxonomic groups suggest that ANME-1 and ANME-2 sarcina are capable of both methanogenesis and methanotrophy. In situ microbial growth was investigated by incubating a series of slides and silicon (Si) wafers for 14 months in seep sediment. The experiment showed ubiquitous growth of bacterial filaments (mean δ¹³C = −38 ± 3‰), suggesting that this bacterial morphotype was capable of rapid colonization and growth.     

Stable isotope analysis of macroinvertebrates and their food sources in a glacier stream

1. Food sources and trophic structure of the macroinvertebrate community along a longitudinal gradient were examined in a glacier stream of the Swiss Alps (Val Roseg). Analysis of multiple stable isotopes (δ¹³C and δ¹⁵N) and measurement of C : N ratios were used to differentiate between allochthonous and autochthonous organic matter.
2. Although isotopic signatures of algae varied widely among sites and dates, it was possible to discriminate between allochthonous and autochthonous food sources using a site-specific approach.
3. Dominant food sources of herbivorous invertebrates in all main channel sites were epilithic diatoms and the filamentous gold alga Hydrurus foetidus . Allochthonous organic matter was of some importance only in a groundwater-fed stream close to the floodplain margin.
4. Seasonal changes in the δ¹³C signature of the macroinvertebrates corresponded with seasonal changes in δ¹³C of the gold alga H. foetidus . This indicated that the energy base remains autochthonous throughout the year.
5. Because of limited food sources, feeding plasticity of the invertebrate community was high. Both grazers and shredders fed predominantly on algae, whereas gatherer-collectors seemed to be omnivorous.
6. The overall enrichment of δ¹⁵N was 2.25‰ ( r ²=0.99) per trophic level. On a gradient from the glacier site to a downstream forested site trophic enrichment was constant but variation in δ¹⁵N within trophic levels decreased.     

δ 13C of CO2 respired in the dark in relation to δ13C of leaf carbohydrates in Phaseolus vulgaris L. under progressive drought

The variations in δ ¹³C in both leaf carbohydrates (starch and sucrose) and CO₂ respired in the dark from the cotyledonary leaves of Phaseolus vulgaris L. were investigated during a progressive drought. As expected, sucrose and starch became heavier (enriched in ¹³C) with decreasing stomatal conductance and decreasing p _i/ p _a during the first half (15 d) of the dehydration cycle. Thereafter, when stomata remained closed and leaf net photosynthesis was near zero, the tendency was reversed: the carbohydrates became lighter (depleted in ¹³C). This may be explained by increased p _i/ p _a but other possible explanations are also discussed. Interestingly, the variations in δ ¹³C of CO₂ respired in the dark were correlated with those of sucrose for both well-watered and dehydrated plants. A linear relationship was obtained between δ ¹³C of CO₂ respired in the dark and sucrose, respired CO₂ always being enriched in ¹³C compared with sucrose by ≈ 6‰. The whole leaf organic matter was depleted in ¹³C compared with leaf carbohydrates by at least 1‰. These results suggest that: (i) a discrimination by ≈ 6‰ occurs during dark respiration processes releasing ¹³C-enriched CO₂; and that (ii) this leads to ¹³C depletion in the remaining leaf material.     

Stable isotope evidence of trophic interactions between introduced brook trout Salvelinus fontinalis and native brown trout Salmo trutta in a mountain stream of south-west France

The potential trophic impact of introduced brook trout Salvelinus fontinalis on native brown trout Salmo trutta in a mountain stream (south-west France) was investigated using stable isotope analysis (SIA). The isotopic signatures (δ¹³C and δ¹⁵N) of S. fontinalis were similar regardless of the absence or presence of S. trutta , and SIA mixing models revealed that S. fontinalis diet consisted mainly of terrestrial invertebrates. Conversely, a significant shift in S. trutta isotopic signatures (depletion of 1·6‰δ¹³C and enrichment of 0·6‰δ¹⁵N) was observed in sympatry with S. fontinalis ; this may be due to a dietary shift towards terrestrial invertebrates. Contrary to an expected dietary divergence in sympatry, an elevated level of dietary overlap was observed between the non-native and native salmonids when in co-occurrence. This dietary convergence is more likely to be due to behavioural interactions than to variations in food availability or fish displacements.     

Stable carbon and nitrogen ratios in the sand smelt from a Mediterranean coastal area: feeding habits and effect of season and size

Enriched δ¹³C values (annual average – 11.2‰) suggest that in the study site (Stagnone di Marsala, Italy) the sand smelt Atherina boyeri exploited benthic prey throughout the year. In particular, Mysidacea and Isopoda seemed to dominate the diet. Stomach content data from the literature was in partial agreement with the present isotopic analysis suggesting that, while Mysidacea and Isopoda are exploited by A. boyeri , Copepoda Harpacticoida and Amphipoda are the dominant food items of the sand smelt diet. Several hypotheses are proposed to explain such partially different results from the two techniques. Both season and size influenced the δ¹³C and δ¹⁵N values of A. boyeri . Although throughout the sampling year, A. boyeri showed very enriched δ¹³C and δ¹⁵N values consistent with a diet based on benthic resources, it showed significant seasonal differences in both nitrogen and carbon isotope ratios. Such a finding suggests that the food preferences of A. boyeri undergo small seasonal differences, probably as a consequence of prey availability. Size also affected the isotopic composition of the sand-smelt indicating that this species shows ontogenetic diet shifts in the study site. Compared with other Mediterranean trophic data of the sand smelt these results substantiate the great adaptability of A. boyeri to the local environmental resources and its trophic plasticity.     

Stable isotopes indicate the extent of freshwater feeding by cormorants Phalacrocorax carbo shot at inland fisheries in England

1. The numbers of cormorants Phalacrocorax carbo feeding at English freshwater fisheries during winter have increased rapidly over the last 20 years, causing concern among fishery managers and anglers.
2. In order to assess the extent of freshwater feeding, stable isotope ratios of carbon and nitrogen (δ¹³C and δ¹⁵N) in feathers of wild cormorants from inland freshwater fisheries were compared with those in the feathers of piscivorous birds with marine diets (captive 'marine-fed' cormorants, free-ranging shags Phalacrocorax aristotelis ) and freshwater diets (juvenile goosanders Mergus merganser ).
3. Isotope signatures of feathers represent the diet at the time of growth. Feathers grown at different times of the year were taken from wild cormorants; each feather type therefore represented the diet over a different temporal scale.
4. Isotopic analyses of feathers indicated that, when shot, nearly all of the cormorants had been feeding entirely on freshwater prey. The mean δ¹³C value of primary feathers growing when birds were shot was –22·2‰, indicative of an entirely freshwater diet.
5. The move to freshwater habitats from coastal breeding grounds occurred over several months, but once established cormorants appear to have fed at freshwater sites throughout the autumn and winter.
6. The suitability of using a two-source isotopic mixing model in order to quantify the extent of freshwater feeding in piscivorous birds is discussed.
7. Although the results indicate long-term residency and feeding in freshwater systems, they do not indicate whether birds were feeding regularly at the sites at which they were shot, or the composition of the diet. It is recommended that further studies using telemetry and multiple isotope analyses be carried out in order to address these issues.     

Natural 13C and 15N abundance of field-collected fungi and their ecological implications

The natural abundance of ¹³C and ¹⁵N was measured in basidiocarps of at least 115 species in 88 genera of ectomycorrhizal, wood-decomposing and litter-decomposing fungi from Japan and Malaysia. The natural abundance of ¹³C and ¹⁵N was also measured in leaves, litter, soil and wood from three different sites. ¹⁵N and ¹³C were enriched in ectomycorrhizal and wood-decomposing fungi, respectively, relative to their substrates. Ectomycorrhizal and wood-decomposing fungi could be distinguished on the basis of their δ¹³C and δ¹⁵N signatures. Although there was high variability in the isotopic composition of fungi, the following isotope- enrichment factors (ε, mean±SD) of the fungi relative to substrates were observed:
ε_{ectomycorrhizal fungi/litter} = 6.1±0.4‰¹⁵N
ε_{ectomycorrhizal fungi/wood} = 1.4±0.8‰¹³C
ε_{wood-decomposing fungi/wood} = −0.6±0.7‰¹⁵N
ε_{wood-decomposing fungi/wood} = 3.5±0.9‰¹³C
The basis of isotope fractionation in C metabolism from wood to wood-decomposing fungus is discussed.     

Stable isotope and pen feeding trial studies confirm the value of horseshoe crab Limulus polyphemus eggs to spring migrant shorebirds in Delaware Bay

We used stable isotope (SI) methods in combination with pen feeding trials to determine the importance of eggs of the Atlantic horseshoe crab Limulus polyphemus to migratory fattening of red knots Calidris canutus rufa and ruddy turnstones Arenaria interpres morinella during spring stopover in Delaware Bay. By manifesting measurable fractionation (ca +3‰) and rapid turnover, blood plasma δ¹⁵ nitrogen proved a functional marker for SI diet tracking during the short 3-week stopover. Blood samples from free-ranging knots (3 data sets) and turnstones (1 data set) produced similar convergence of plasma δ¹⁵N signatures with increasing body mass that indicated highly similar diets. Asymptotes deviated slightly (0.3‰ to 0.7‰) from that of captive shorebirds fed a diet of only crab eggs during stopover, thus confirming a strong crab egg-shorebird linkage. The plasma δ¹⁵N crab-egg diet asymptote was enriched ca +4.5‰ and therefore readily discriminated from that of either blue mussels Mytilus edulis or coquina clams Donax variabilis , the most likely alternative prey of knots in Delaware Bay. Crab eggs were highly palatable to captive knots and turnstones which achieved rates of mass gain (3–11 g/d) comparable to that of free-ranging birds. Peak consumption rates during hyperphagic events were 23,940 and 19,360 eggs/bird/d, respectively. The empirical conversions of eggs consumed to body mass gained (5,017 eggs/g for knots and 4,320 eggs/g for turnstones) indicate the large quantities of crab eggs required for the maintenance of these shorebird populations during stopover.     

Traffic exposure increases natural 15N and heavy metal concentrations in mosses

Mosses have been used as biomonitors of atmospheric pollution for some years, but few studies have been carried out on the effect of NO_x emissions from traffic on moss tissue N. Eight species of moss (102 samples) growing on walls or roofs next to roads exposed to different traffic densities were collected from urban and rural sites in the UK. The shoots were sampled for total N, their stable isotope ¹⁵N/¹⁴N content (δ¹⁵N) and heavy metal content (Pb, Zn). There was a lack of correlation between tissue total N and traffic exposure, but a very good correlation between traffic exposure and tissue δ¹⁵N. Plants collected near motorways or busy urban roads had δ¹⁵N values ranging between +6 and −1‰, while in rural areas with hardly any traffic these ranged from −2 to −12‰. In a separate survey of mosses, the average δ¹⁵N of shoots from busy roadsides in London was +3.66‰, whereas from samples collected from farm buildings near poultry or cattle pens it was −7.8‰. This indicates that the two main atmospheric N sources, NO_x and NH_x, have different δ¹⁵N signatures, the former tending to be positive and the latter negative. Tissue concentrations of both Pb and Zn show a strong positive correlation with traffic exposure, with Zn in particular being greater than Pb. The results are discussed with regard to the use of moss tissue Zn as a means for monitoring or mapping pollution from vehicles, and of δ¹⁵N as an aid to distinguish between urban (NO_x) and rural (NH_x) forms of N pollution.     

Tracing carbon and oxygen isotope signals from newly assimilated sugars in the leaves to the tree-ring archive

The analysis of δ ¹³C and δ ¹⁸O in tree-ring archives offers retrospective insights into environmental conditions and ecophysiological processes. While photosynthetic carbon isotope discrimination and evaporative oxygen isotope enrichment are well understood, we lack information on how the isotope signal is altered by downstream metabolic processes.
In Pinus sylvestris , we traced the isotopic signals from their origin in the leaf water ( δ ¹⁸O) or the newly assimilated carbon ( δ ¹³C), via phloem sugars to the tree-ring, over a time-scale that ranges from hours to a growing season.
Seasonally, variable ¹³C enrichment of sugars related to phloem loading and transport did lead to uncoupling between δ ¹³C in the tree-ring, and the c _i/ c _a ratio at the leaf level. In contrast, the oxygen isotope signal was transferred from the leaf water to the tree-ring with an expected enrichment of 27‰, with time-lags of approximately 2 weeks and with a 40% exchange between organic oxygen and xylem water oxygen during cellulose synthesis.
This integrated overview of the fate of carbon and oxygen isotope signals within the model tree species P. sylvestris provides a novel physiological basis for the interpretation of δ ¹³C and δ ¹⁸O in tree-ring ecology.     

Stable isotope evidence (δ13C, δ18O) for winter feeding on seaweed by Neolithic sheep of Scotland

The antiquity of the use of seaweed to feed domestic animals was investigated through carbon ( δ ¹³C) and oxygen ( δ ¹⁸O) isotope analysis of tooth enamel bioapatite. The analysis was performed on sheep and cattle teeth from two Neolithic sites in Orkney (Scotland). At the Knap of Howar, c . 3600 bc , carbon isotopes reflect grazing on terrestrial plants throughout the year for both sheep and cattle, with no contribution of seaweed to their diet. At the Holm of Papa Westray North (HPWN), c . 3000 bc , significant contribution of seaweed to the sheep diet during winter is indicated by bioapatite δ ¹³C values as high as −5.7‰, far outside of the range of values expected for the feeding on terrestrial C₃ plants, and δ ¹⁸O values higher than expected during winter, possibly caused by ingestion of oceanic water with seaweed. Ingestion of seaweed by sheep at HPWN might have been necessitated by severe reduction of pastures during winter. Results suggest that sheep ingested fresh seaweed rather than dry fodder, perhaps directly on the shore as sheep do nowadays on North Ronaldsay. A significant difference between the two populations is the exclusive reliance on seaweed by the North Ronaldsay sheep, which have developed physiological adaptations to this diet. Contribution of seaweed to the sheep winter diet at HPWN might have been a first step towards this adaptation.     

Carbon-Isotope Ratios and Photosynthetic Pathways in the Neotropical Family Rapateaceae

Abstract: The Rapateaceae is a small, mainly Neotropical family of terrestrial or occasionally epiphytic herbs that grow on mesic, nutrient-poor sites. Some recent studies suggest that the Rapateaceae may be closely related to the Bromeliaceae, one of the major families containing CAM plants. To investigate the photosynthetic pathway in Rapateaceae, the plant carbon-isotope ratio (δ¹³C) was determined for samples from dried herbarium specimens for 85 of the approximately 100 species in the family. The δ¹³C values ranged from - 37.7 to - 19.8 ‰. Most Rapateaceae showed δ¹³C values typical of C₃ plants. However, six species ( Kunhardtia rhodantha Maguire, Marahuacaea schomburgkii (Maguire) Maguire, Saxofridericia compressa Maguire, Stegolepis grandis Maguire, St. guianensis Klotzsch ex Körn. and St. squarrosa Maguire) showed δ¹³C values less negative than - 23 ‰, i.e., at the higher end of the range for C₃ plants and at the lower end of the distribution for plants exhibiting CAM. The δ¹³C values became significantly less negative with increasing altitude (regression analysis indicating a change from about - 30.7 ‰ at sea level to - 22.5 ‰ at 2500 m). Although other environmental factors and the type of tissue analysed may also influence δ¹³C values, these results suggest that some Rapateaceae may be capable of performing CAM. Further studies, including measurements of diel gas exchange patterns and leaf organic-acid fluctuations, would be needed to demonstrate CAM in Rapateaceae unequivocally, but living material of many of these enigmatic plants is difficult to obtain.