Oxygen and carbon isotopic composition of fish otoliths

Oxygen and carbon isotopic composition of the aragonite of fish otoliths was measured on 175 specimens comprising 24 different species in 1989 and 1990. All specimens but two came from the northern Adriatic Sea or the northern Tyrrhenian Sea (two freshwater specimens were studied for comparison with the marine fish). The data obtained confirm the results of previous research suggesting the existence of equilibrium conditions between the otolith aragonite and ambient water with respect to ·¹⁸O(CO₃ ^2-) values. Examination of one of the species indicated that the CaCO₃ of otoliths probably accumulates continuously over time, seasonal isotopic changes being clearly visible (from a set of radial spot samples) for both oxygen and carbon isotopes. The apparent isotopic equilibrium with ambient water suggests that the ¹⁸O(H₂O) of the endolymph is equal to that of seawater and considerably different from that of fish body water. In the case of ¹³C(CO₃ ^2-), isotopic equilibrium with dissolved carbon species in seawater is never reached, even though the contribution of metabolic CO₂ is variable among different species and even among different individuals of the same species. This rules out the possibility of using ¹³C(CO₃ ^2-) values obtained from fossil otoliths for paleoenvironmental and paleobiological conclusions.     

Hydrogen, carbon and sulphur isotope ratios in peat: the role of diagenessis and water regimes in reconstruction of past climates

We demonstrate that carbon, hydrogen and sulphur isotope ratios both, in the total peat (¹³C_tp=–25.52 to –28.27, D_tp=–78.67 and –109.24, ³⁴S=4.35 to 19.87), and in cellulose from the peat (¹³C_nc=–25.06 to –27.33 and D_nc=–92.43 to –118.02) are not affected by postdepositional changes. Therefore, the original isotope composition of plants are in general preserved in the peat and represent an archive of the past environmental variations. These can be supported by (i) good correlations between ¹³C_tp and ¹³C_nc, and between D_tp and D_nc, (ii) high horizontal homogeneity of ¹³C_tp and ¹³C_nc in the scale of one peat-bog – the same major factor(s) control(s) C isotopic ratios, (iii) no correlation between organic sulphur concentrations and ³⁴S value –³⁴S results from variations in the water level.This revised version was published online January 2005 with corrections to the title.     

High turnover of inorganic carbon in kelp habitats as a cause of δ13C variability in marine food webs

In a study to assess qualitatively the importance of organic matter derived from kelp production in the Aleutian Islands of subarctic Alaka, replicated samples of autotrophic sources and primary and secondary consumer organisms were sampled for ¹³C among sources, sites, (treatment) islands, and years. Unanticipated variation in the ¹³C of kelps occurred among overtly similar sites at different islands. Variation in the ¹³C of the surface canopy-forming kelp Alaria fistulosa was particularly extreme, ranging from-15.5 to-28.0 compared to the understory kelps, Laminaria spp. A. fistulosa ¹³C varied by as much as 6 to 7 among similar sites at a given island within years, and by as much as 3 to 4 between years at the same sampling site. In serveral cases, ¹³C variation was weakly tracked by some consumer organisms, suggesting that even detritus pathways through the food web can be localized and tightly coupled. Dynamic cycles in the concentration and ¹³C of dissolved inorganic carbon (DIC) and aqueous CO₂ concentration ([CO₂]_aq) were measured at three sites on one island. The ¹³C or organic carbon fixed by A. fistulosa, calculated from diurnal DIC concentration and ¹³C measurements, varied by 15 and varied inversely with [CO₂]_aq concentrations. Local DIC variability, probably resulting from high productivity and decreased turbulence in dense kelp habitats, provides a possible mechanism of variation in kelp ¹³C. The short-term variability in the ¹³C of organic carbon fixed by kelps indicates that sampling methodology and design must assess this potential variation in marine macrophyte ¹³C before making assumptions about the transfer of ¹³C-invariate organic matter to higher trophic levels. On the positive side, a predictable relationship between [CO₂]_aq concentration and kelp ¹³C offers a potentially robust means to assess productivity effects on CO₂ limination in kelps and other complex aquatic macrophyte habitats.     

Precision and accuracy of stable isotope signals in otoliths of pen-reared cod (<Emphasis Type="Italic">Gadus morhua</Emphasis>) when sampled with a high-resolution micromill

A high-resolution sampling technique for stable-isotope measurements in otoliths is now available, but little is known about the precision and accuracy of the results when using this technique. Otolith samples of pen-reared cod (Gadus morhua L.) with well-defined temperature history were obtained using this high-resolution sampling technique. Samples from different-aged cod otoliths were collected in two yearly increments corresponding to the calendar years 1994 and 1995, and the stable-oxygen (¹⁸O) and -carbon (¹³C) isotope compositions of the samples were measured. Otolith ¹⁸O values had a clear seasonal pattern in response to the seasonal water temperature. Otolith material deposited in the same calendar years of different-aged fish showed similar ¹⁸O values. High precision of the temperature estimate using otolith ¹⁸O values was obtained when comparing results from different otoliths. The accuracy of the temperature estimates, relative to measured mean monthly water temperature, was also high, but reduced sampling resolution in the otoliths significantly reduced the accuracy due to attenuation of the ¹⁸O signals. We found that the otolith ¹³C values had a cyclic pattern, roughly in phase with the ¹⁸O values. This was most likely caused by temperature-increased metabolism. Indications of age-specific otolith ¹³C values are also presented.Communicated by M. Kühl, Helsingør     

Stable isotope and biochemical fractionation in the marine pelagic food chain: the jellyfish Pelagia noctiluca and net zooplankton

In our field study we analyzed the C and H isotopic and biochemical (C, N, P, protein, lipid, carbohydrate) composition of the jellyfish Pelagia noctiluca (collected from the Gulf of Trieste in 1985 to 1986) and its presumed diet-net zooplankton. The mean ¹³C (-18.8) and D (-58.4) ratios of P. noctiluca showed enrichment in heavy isotopes relative to net zooplankton (2 for carbon and 30 for hydrogen). Both the jellyfish and net zooplankton were characterized by a linear correlation between ¹³C and D. C. N, P, protein, lipid, and carbohydrate contents of P. noctiluca were low on a dry weight basis as compared to net zooplankton. Significantly lower C:N and C:P ratios were found in jellyfish indicating a greater loss of carbon relative to nitrogen and phosphorus along the passage to a higher trophic level. Isotopic and biochemical evidence indicate that, though collected in nearshore waters, P. noctiluca depended on autochthonous marine organic matter.     

Selective predation on mature male Byblis japonicus (amphipoda: Gammaridea) by the barface cardinalfish,Apogon semilineatus

Ontogenetic change of habitat depths of Nautilus pompilius in the Philippines (Tañon Strait) and Fiji is considered by comparing ¹⁸O/¹⁶O ratios in septa and cameral fluid of live-caught specimens and ambient sea-water. ¹⁸O values of cameral liquid become heavier with decreasing volume within a chamber, which may be due to isotopic fractionation during discharge across the siphuncular wall. All of the seven Philippine and Fiji specimens analyzed show a distinct change in ¹⁸O from light values in the first seven septa to heavier values in the succeeding septa. Two different isotopic temperature scales are obtained for the Fiji and Philippine populations, suggesting a differential vital effect of metabolism between them. Sightly light ¹⁸O values in Septa 1 to 7 and hatching at relatively high temperatures in aquaria both suggest that N. pompilius hatch at the shallowest depths within their inhabitable vertical range. Depths of postembryonic animals in the Philippines and Fiji waters estimated from the isotopic temperature-depth diagrams range from 120 to 160 m and from 440 to 520 m, respectively, both of which correlate well with capture records. More than several tens of small-scale ¹⁸O cycles are detected in the sequence of nacreous layers within the single septum of a submature Philippine specimen. This can be interpreted as reflecting daily vertical migration.     

Oxygen and carbon stable isotopes in the otoliths of wild and laboratory-reared Australian salmon (Arripis trutta)

Australian salmon,Arripis trutta, collected from the east coast of Tasmania, Australia, in 1987, were weighed and measured and their otoliths marked by immersing fish in an oxytetracycline hydrochloride/seawater solution before placement in constant-temperature aquaria. Individual somatic and otolith growth rates were determined for input into mass balance models. Mass balance models were used to determine the oxygen and carbon isotopic composition of otolith material produced during captivity. There was a significant relationship between ¹⁸O measured in the otolith aragonite and ambient temperature (r ² = 0.77). The linear relationship between these data, where ¹⁸O = 6.69 – 0.326 (T, °C), was not significantly different from a relationship indicative of equilibrium deposition of oxygen isotopes in aragonite. Otolith carbon was significantly depleted in¹³C relative to equilibrium deposition, with depletions >6.0 at all temperatures. There was no relationship between ¹³C and temperature. It was estimated that >30% of the otolith carbon was from metabolically derived sources. Significant differences in otolith carbon isotopes among wild juvenile Australian salmon were hypothesised to be attributable to differences in diet. Levels of variability for both oxygen and carbon isotopes in laboratory-maintained and wild fish were similar to that found by other researchers for foraminifera and these results highlight the importance of large sample sizes when estimating environmental temperatures from oxygen isotopes measured in fish otoliths.     

Spatial and temporal shifts in stable isotope values of the bottom-dwelling shrimp<Emphasis Type="Italic"> Nauticaris marionis</Emphasis> at the sub-Antarctic archipelago

Spatial and temporal dynamics of carbon and nitrogen stable isotope signatures of the bottom-dwelling caridean shrimp Nauticaris marionis were measured during April and May between 1984 and 2000 in the vicinity of Marion Island (the Prince Edward Islands, Southern Ocean). There was one trophic-level enrichment in bulk ¹⁵N and ¹³C signatures between small (<20 mm long) and large (>20 mm) specimens of N. marionis, suggesting distinct trophic differentiation among major shrimp size groups. Both ¹⁵N and ¹³C values of N. marionis increased with the depth, reflecting changes in their diet. There were no clear temporal trends in bulk ¹⁵N signatures of N. marionis. However, compound-specific ¹⁵N measurements of amino acids indicated that N. marionis from the inter-island realm occupied the trophic level of second order carnivores, while similarly sized shrimps in the near-shore realm were at the trophic level of first order carnivores. Compound-specific measurements also identified a change in the source of inorganic nitrogen at the base of the food web between the inter-island and near-shore realms. In contrast to the bulk ¹⁵N values, a significant shift in bulk ¹³C values of N. marionis was observed between 1984 and more recent years. This temporal change appears to be linked to changes in the overall productivity of the Prince Edward Island inter-island system, which could be linked to global climate change.Communicated by J.P. Thorpe, Port Erin     

Oxygen- and carbon-isotope analyses of the articulate brachiopodLaqueus californianus: a recorder of environmental changes in the subeuphotic zone

This study is the first systematic attempt to use oxygen and carbon isotopes in modern brachiopods to investigate seasonality and growth rates. A comparison of oxygen-isotope analyses of shells of living specimens ofLaqueus californianus dredged from 80 and 130 m off Santa Catalina Island, California, to available hydrographie data indicates that this articulate brachiopod secretes its shell in or close to oxygen isotopic equilibrium with ambient seawater. Periodic oxygen-isotope depletions appear to result from El Nino events. Unexpectedly low ¹³C values associated with the 1983 El Nino may be explained by increased bacterial activity or organic loading into the Southern California Bight associated with these warm-water pulses. Growth rates determined from annual cycles in ¹⁸O records are variable, but generally average between 2 and 3 mm yr^–1 for mature individuals. Because of the longevity ofL. californianus,¹⁸O profiles provide high-resolution seasonal temperature records spanning one to two decades. Our data suggest that oxygen isotopes in brachiopod shells can be utilized as monitors of environmental change in the subeuphotic zone.     

Spatial variation and ontogenic shifts in the isotopic composition of Mediterranean littoral fishes

Stable carbon and nitrogen isotope signatures have been determined for littoral fishes from three localities in the Balearic Islands (NW Mediterranean) for trophic assessment. Variation in the isotopic composition was examined at multiple levels: the individual organism, individual populations (ontogenetic variation) and at area effects (different islands). Cluster analyses based on ¹³C and ¹⁵N resulted in two very different dendrograms for the fish assemblages from Menorca and Formentera. In both schemes, congeners of the genera Diplodus and Labrus did not cluster together, suggesting that closely related species do not necessarily feed on similar foods. Significant differences in ¹⁵N were detected between locations for Scorpaena porcus, Scorpaena scrofa and Sciaena umbra, and significant differences in ¹³C were detected for Scorpaena porcus and Scorpaena scrofa. Overall there were significant spatial differences in the trophic position of littoral fishes among the three Balearic Islands, and fishes from Palma, Mallorca tended to feed at a higher trophic level than fishes from either Menorca and Formentera. Large-scale geographic comparisons, using earlier data from three sites in western Mallorca and in Corsica revealed that throughout the western Mediterranean generally, the average trophic level of fishes differs remarkably little. However, large discrepancies were observed in ¹⁵N for individual species, notably Scorpaena porcus, D. annularis, D. vulgaris and Serranus scriba. ¹³C composition of fishes did not seem to change greatly among the locations, although marked discrepancies were observed for D. puntazzo and Mullus surmuletus. Given the large-scale patterns we have observed, it is possible that the extent of multichannel omnivory which has been purported to exist in complex aquatic systems, may have been greatly overstated. Regression of ¹⁵N and ¹³C with fish body size revealed clear and significant trends for some species. In particular, ¹⁵N appeared to increase with body length in the macrocarnivorous fishes Scorpaena porcus, Scorpaena scrofa, Serranus scriba and Sciaena umbra. Trends were less apparent in animals which feed primarily on small benthic invertebrates, notably the four Diplodus species (D. vulgaris, D. sargus, D. annularis, D. puntazzo), and the labrids Labrus viridis and L. merula.Communicated by J.P. Thorpe, Port Erin     

Carbon fixation in marine phytoplankton: carboxylase activities and stable carbon-isotope ratios; physiological and paleoclimatological aspects

We measured the activity of three carboxylases: RuBP carboxylase, PEP carboxylase and PEP carboxykinase of marine phytoplankton species in culture and in natural communities. Activities of the three carboxylases were measured simultaneously with stable carbon-isotope ratios. The enzymatic activities have been used to estimate the importance of carboxylation and its impact on the ¹³C:¹²C ratio (expressed as ¹³C). The marine phytoplankton species in culture were Fragilariopsis kerguelensis, Nitzschia turgiduloides, Skeletonema costatum, Phaeodactylum tricornutum, Isochrysis galbana, Dunaliella marina, and Prorocentrum micans, and the field samples were collected from different depths off the coast of Portugal (August/September, 1981). Our results indicate that, as in terrestrial plants, the ¹³C value is a good indicator of the extent of carboxylation. RuBP carboxylase activity was always predominant, whereas the ¹³C value never reached values typical of the C₄ pathway. The carboxylases could be PEP carboxylase (in dinoflagellates) or PEP carboxykinase (in diatoms). carboxylation increased at the end of the exponential growth phase in a diatom culture and with increased biomass in natural samples. We interpret these increases as an adaptative response mechanism to poor environmental conditions, especially to low light intensity.     

Community structure of the benthos in Scottish sea-lochs. III. Further studies on patchiness

Large-scale patchiness in the distribution of the benthic fauna was investigated in Loch Etive (Scottish west coast) by two series of van Veen grab hauls. Each series was taken along a traverse across the width of the loch, with sampling points about 100 m apart. One sample series was taken on sandy mud and the other, at a greater depth, on soft mud. Two approaches in data processing were applied: (1) The variance: mean ratio and the Morisita I tests for significant aggregation were applied to the species abundances in the sample series; (2) 3 measures of sample homogeneity, each involving the calculation of an index of faunal similarity, were applied to the separate samples. The results, however, could not show significant differences between the two series of samples. These findings were compared to results obtained from (a) a previous study, where differences in patchiness between areas were shown for a smaller scale of sampling design in Loch Etive and neighbouring areas, and to which the I method is also applied here; and (b) the results of applying the variance: mean ratio and I tests to data published by Holme (1953). The present results indicate greater aggregation at the present scale of sampling than for the previous, smaller scale of sampling, or for the comparable scale of Holme's sampling. The degree of concordance shown by the values of I for the circular-design sampling to the values of the 3 measures of patchiness applied previously, and to the means of the species abundances, was measured by Spearman's rank correlation coefficient. The results clearly demonstrated the I values, unlike the others measured, to be almost completely independent of the mean. It was concluded that, for comparing the pattern from benthic samples using standard-size bottom samplers, where the mean may vary widely between each set of samples, the I method is probably most useful.     

Habitat and age of the giant squid (<Emphasis Type="Italic">Architeuthis sanctipauli</Emphasis>) inferred from isotopic analyses

The age and habitat of the giant squid, Architeuthis sanctipauli Velain, 1877, were determined based on isotopic analyses of the statoliths of three female specimens captured off Tasmania, Australia, between January and March 1996. Assuming that the aragonite of the statoliths formed in equilibrium with seawater, ¹⁸O analyses indicated that the squid lived at temperatures of 10.5–12.9°C, corresponding to average depths of 125–250 m and maximum depths of 500 m. The capture records indicated that these squid may have occasionally ranged still deeper, to as much as 1000 m. All the statoliths were labeled with bomb ¹⁴C (¹⁴C=+22.9 to +44.6), consistent with the depths inferred from ¹⁸O. A thin section through one of the statoliths revealed 351 growth increments grouped into check-ring structures every 10–16 increments. A model for statolith growth and the pattern of temporal change in ¹⁴C in the water column was used to estimate the ages of the three specimens. These estimates were very sensitive to the choice of depth range over which ¹⁴C values were integrated. Assuming that the capture depths represented the maximum habitat depths of these individuals, the calculations suggested an age of 14 years or less. More refined age estimates require a better understanding of the variation of ¹⁴C and temperature with depth in the areas in which the squids live.Communicated by J.P. Grassle, New Brunswick     

Dependence of consumers on macroalgal (Laminaria solidungula) carbon in an arctic kelp community: δ13C evidence

Stable carbon isotope measurements (¹³C) were used to assess the importance of kelp carbon (-13.6 to-16.5) versus phytoplankton carbon (-25.5 to-26.5) to resident fauna of an isolated kelp bed community on Alaska's north arctic coast from 1979 to 1983. The predominant kelp, Laminaria solidungula, showed some seasonal variation in ¹³C which was correlated with changes in the carbon content of the tissue. Animals that showed the greatest assimilation of kelp carbon (>=50%) included macroalgal herbivores (gastropods and chitons,-16.9 to-18.2), a nonselective suspension feeder (an ascidian,-19.0) and a predatory gastropod (-17.6). Animals that showed the least incorporation of kelp carbon into body tissues (<=7%) included selective suspension-feeders (hydroids, soft corals and bryozoans,-22.8 to-25.1). Sponges, and polychaete, gastropod and crustacean omnivores exhibited an intermediate dependence on kelp carbon (15 to 40%). Within some taxonomic groups, species exhibited a broad range in isotopic composition which was related to differences in feeding strategies. In the polychaete group alone, ¹³C values identified four major feeding habits: deposit-feeders (-18.0), omnivores (-20.4), predators (-22.2) and microalgal herbivores (-23.0). Distinct seasonal changes in the ¹³C values of several animals indicated an increased dependence on kelp carbon during the dark winter period when phytoplankton were absent. Up to 50% of the body carbon of mysid crustaceans, which are key prey species for birds, fishes and marine mammals, was composed of carbon derived from kelp detritus during the ice-covered period.     

Sterols in coral-reef animals

The sterol constituents of both coral and non-coral-reef animals (mollusks, coelenterates, echinoderms) have been investigated, and determined by gas-liquid chromatography and spectral analyses. All coral-reef animals examined contained larger amounts of C₂₈-sterols, such as 24-methylcholesterol, than non-coral-reef animals. The seastar Acanthaster planci contains the same sterol constituents as some coral-reef coelenterates and mollusks, differing only in the position of the double bond in a steroid ring. This observation suggests the transport of dietary ⁵-sterols to ⁷-sterols through the food-chain in A. planci.     

Allometry of gill weights,gill surface areas,and foot biomass δ<Superscript>13</Superscript>C values of the chemoautotroph–bivalve symbiosis<Emphasis Type="Italic"> Solemya velum</Emphasis>

The protobranch bivalve Solemya velum Say, 1822 has large gills, which harbor chemolithoautotrophic bacteria that supply the majority of the clams organic carbon. A substantial portion of the CO₂, O₂, H₂S, and other nutrients necessary for symbiont autotrophy and host heterotrophy are acquired from the environment through the gills, whose large size may be necessary to facilitate the acquisition of sufficient O₂ from S. velums habitat to meet the combined demands of the host and symbionts. Large gills may also result in an oversupply of CO₂, which may in turn be responsible for the isotopically depleted ¹³C values observed in S. velum biomass (–31 to –34). Alternatively, gill hypertrophy may simply be an adaptation to house a large population of symbionts adjacent to their environmental source of dissolved gases and other nutrients. To better understand gill function in this symbiosis, gill weights, gill surface areas, and foot ¹³C values were measured as a function of total weights. S. velum gill weights were found to be a substantial portion of total clam weight, averaging 38% of wet weight, compared to nonsymbiotic protobranch bivalves Yoldia limatula Say, 1831 (5%) and Nucula proxima Say, 1822 (11%). Gill weights are a smaller percentage of total weight in larger individuals; the allometric equation for gill weight (G) as a function of total weight (M) is G=0.26M^0.85. Dry weights scale similarly. Gill surface areas are immense; the average gill surface area measured was 107 cm² g^–1 total soft tissue wet weight, the highest value for any marine invertebrate. Gill surface area (SA) also scales with size (SA=69.8M^0.85). When gill surface areas were calculated with respect to gill wet weights, they did not scale with size. The ¹³C values do not scale with size either, consistent with high rates of CO₂ supply at all sizes. Extraordinarily high rates of CO₂ supply relative to demand are supported by a model for CO₂ delivery based on Ficks law and the allometric relationship between surface areas and total weight, consistent with a role for large gill surface areas in the generation of isotopically depleted tissue ¹³C values.Communicated by J. P. Grassle, New Brunswick     

Isotopic characterization of atmospheric nitrogen inputs as sources of enhanced primary production in coastal Atlantic Ocean waters

Current estimates indicate that atmospheric nitrogen deposition is responsible for 26 to over 70% of new nitrogen (N) input to North Carolina estuaries and coastal waters. Concentrations of N in coastal rainfall events in a 2-yr period (August 1990 to 1992) ranged from 0.7 to 144 M for NO ₃ ^- and 0.5 to 164 M for NH ₄ ⁺ . The ¹⁵N values of the NO ₃ ^- and the NH ₄ ⁺ were determined in 15 rain events. NH₄ ⁺ values averaged-3.13 (range:-12.5 to+3.6), while NO ₃ ^- plus dissolved organic N fractions had an average ¹⁵N of+1.0 (range:-2.0 to+4.7). The uptake of this isotopically light N into particulate N, in parallel with primary productivity and biomass (as chlorophyll a) determinetions, was examined in microcosm and mesocosm bioassays. As phytoplankton productivity and biomass increased with added rainwater N, the ¹⁵N of particulate N decreased. To investigate the effects of significant atmospheric N loading with stable isotope tracers, we measured the ¹⁵N of the>1 m fraction from surrounding coastal waters. Owing to the episodic nature of atmospheric deposition and the great variation in N loading with each event, a simple assessment of the atmospheric contribution was not possible. During a period in which rainfall inputs were significant and frequent (August 1992), ¹⁵N values were several more negative than during periods of drought (Fall 1990). These experiments and observations emphasize the contribution of atmospheric nitrogen deposition to new production in coastal waters.     

Diet and trophic position of Atlantic bluefin tuna (<Emphasis Type="Italic">Thunnus thynnus</Emphasis>) inferred from stable carbon and nitrogen isotope analysis

Stable ¹³C and ¹⁵N isotope analyses of scale, bone, and muscle tissues were used to investigate diet and trophic position of North Atlantic bluefin tuna (Thunnus thynnus Linnaeus) during residency in the northwest Atlantic Ocean off the northeast coast of the United States. Adult bluefin tuna scales collected from fish between June and October 2001 were significantly enriched in ¹³C compared to both muscle and bone across all months, while muscle was significantly enriched in ¹⁵N compared to either bone or scale throughout the same period. In muscle tissue, there was evidence of a shift over the summer from prey with ¹³C values (–17 to –18) that were characteristic of silver hake (Merluccius bilinearis) to species with ¹³C values of –20 to –21 that were similar to Atlantic herring (Clupea harengus) and sandlance (Ammodytes americanus). Depletion of ¹⁵N values in adult scales and bone compared to muscle tissue may be explained by bone and scale samples representing juvenile or life-long feeding habits, isotopic routing, or isotopic differences in amino acid composition of the three tissue types. Adult bluefin tuna were estimated to be feeding at a trophic position similar to pelagic sharks in the northwest Atlantic Ocean, while the trophic positions of yellowfin tuna (Thunnus albacares), albacore tuna (Thunnus alalunga), and juvenile bluefin tuna were indicative of a diet of up to a full trophic position below adult bluefin tuna. The close relationship between the juvenile bluefin ¹⁵N values and those of suspension feeders suggests that nektonic crustaceans or zooplankton may contribute significantly to the diet of bluefin tuna, a food source previously overlooked for this species in the northwest Atlantic Ocean.Communicated by J.P. Grassle, New Brunswick     

Carbon-isotope ratio gradients in western arctic zooplankton

Zooplankton from 87 stations in the Bering, Chukchi and Beaufort Seas sampled in 1985, 1986 and 1987 showed a geographic gradient in stable carbon-isotope ratios (¹³C). The zooplankton most depleted in ¹³C were found in the central and eastern Beaufort Sea and those most enriched were from the Bering and Chukchi Seas. Average ¹³C values ranged from-20.9 to-26.7 for copepods and from-19.4 to-25.1 for euphausiids. Euphausiids show a minimum of 1.0 enrichment relative to copepods throughout the study area. Relative biomasses of the major zooplankton taxa varied significantly across the Alaskan Beaufort Sea in October 1986, with euphausiids dominating in the west and copepods in the east. These differences in taxonomic composition affected the weighted ¹³C values for total zooplankton and may produce an even more pronounced geographic gradient in zooplankton ¹³C than that found within a single taxon. The bowhead whale Balaena mysticetus migrates between wintering areas in the Bering Sea and summering areas in the Beaufort Sea and feeds over this geographic range. The zooplankton ¹³C gradient is the probable source of ¹³C oscillations found along the baleen plates of this planktivore.     

Stable isotopic investigation of physiological and environmental changes recorded in shell carbonate from the giant clam Tridacna maxima

The aragonitic shell of the photosymbiont-bearing bivalve Tridacna maxima contains a record of the physiological and environmental changes the organism has experienced during its lifetime. This record is preserved as chemical and microstructural variations throughout the shell. Stable isotopic analyses of oxygen (¹⁸O/¹⁶O) and carbon (¹³C/¹²C) in shell carbonate were combined with growth increment studies to interpret the shell record of specimens collected from the Rose Atoll (Lat. 14°31S; Long. 168°10W) in April 1982. The seasonal water temperature cycle is recorded in the oxygen isotopic signature of the clams, permitting the recognition of annual cycles in the ¹⁸O profile. The total number of these cycles corresponds to the age of a specimen, while the cycle length is a measure of the yearly growth rate. Large-amplitude cycles, reflecting year-round calcification, characterize the early portion of the growth record. With the onset of sexual maturity and slower growth at an age of approximately ten years, the cycles decrease in amplitude and become more erratic. During this later growth phase calcification is limited to the cooler months of the year, perhaps in response to a re-ordering of energy priorities between growth and gametogenesis. A growth curve developed from the ¹⁸O profile indicates rapid juvenile shell growth followed by slower growth thereafter producing a lifespan of several decades. Carbon isotopic analyses of T. maxima were compared to analyses of the symbiont-barren gastropod Terebra areolata collected from the same locality in April 1984. A 2 depletion in the ¹³C composition of T. maxima shell carbonate is attributed to a symbiontenhanced metabolic rate and an increased flow of isotopically light, respired CO₂ into the carbon pool used in calcification. Such a depletion may prove useful in identifying the presence of photosymbionts in extinct species of fossil mollusks.