When animal tissues are prepared for stable isotope ratio analysis, they may or may not be treated with acid prior to analysis to remove carbonates and are loaded into tin or silver weigh boats for quantitative combustion. The effects of these methodological variations are poorly known. The effects of various preservation methods on isotopic compositions are also poorly known. We tested the effects of four preservation methods, (i) formalin, (ii) formalin followed by a transfer to ethanol (formalin/EtOH), (iii) saturated mercuric chloride solution, and (iv) freezing/freezedrying, on the carbon and nitrogen isotopic composition of the muscle tissue of juvenile winter flounder (Pleuronectes americanus) and the tails (including exoskeleton) of mud shrimp (Crangon septemspinosa). Freezing and freeze-drying were the only preservation methods that did not affect stable isotope ratios of carbon and nitrogen. Formalin, formalin/EtOH, and saturated mercuric chloride solution produced significant increases in δ 15 N values (0.5-1.4‰) and decreases in δ 13 C values (0.6-2.3‰) compared with frozen samples. There was also an increase in the variability of δ 15 N and (or) δ 13 C values. We also tested the effects of acidification by comparing samples that were acidified either by fuming with concentrated HCl or by the direct application of 1 N HCl containing 1.0% platinum chloride (a combustion catalyst) to unacidified samples. Neither concentrated HCl fumes nor HCl/platinum chloride had a significant effect on the δ 15 N or δ 13 C values of either species compared with unacidified samples. Therefore, acidification may be unnecessary in the preparation of some marine animals. Finally, we compared the effects of two types of sample boats: tin and silver. We found no significant effect of boat material on the δ 15 N or δ 13 C values of either species.Résumé : Quand les tissus d'animaux sont préparés pour analyse des ratios d'isotopes stables, ils peuvent ou non être traités à l'acide avant analyse pour qu'en soient retirés les carbonates, et sont placés dans des nacelles de fer-blanc ou d'argent en vue de la combustion quantitative. Les effets de ces variations méthodologiques sont peu connus. Les effets des diverses méthodes de préservation sur les compositions isotopiques sont aussi mal connus. Nous avons testé les effets de quatre méthodes de préservation, (i) formol, (ii) bain de formol suivi d'un transfert dans l'éthanol, (iii) solution saturée de chlorure mercurique et (iv) congélation/lyophilisation, sur la composition isotopique en carbone et en azote du tissu musculaire de plies rouges (Pleuronectes americanus) juvéniles et les queues (incluant l'exosquelette) de crevette grise de sable (Crangon septemspinosa). La congélation et la lyophilisation étaient les seules méthodes de préservation qui n'ont pas influé sur les ratios des isotopes stables de carbone et d'azote. Le formol, le bain de formol suivi d'un transfert dans l'éthanol et la solution saturée de chlorure mercurique ont entraîné une hausse significative des v...

Stable-isotope ratios of carbon ( 13 C/ 12 C) and nitrogen ( 15 N/ 14 N) are widely used in the analysis of animal diets. However, using these stable-isotope ratios to infer dietary changes depends on precise knowledge of turnover rates of carbon and nitrogen. In the present study, carbon and nitrogen turnover rates were determined for recently settled juvenile winter flounder Pseudopleuronectes americanus in the laboratory using naturally occurring stable isotopes as dietary tracers. Flounder were reared at 13°C on a diet of rotifers Brachionus plicatilis of known isotopic composition from the time that the larvae began feeding until they reached metamorphosis and began to settle to the benthic habitat. At settlement, the fish were assigned to 1 of 2 temperature treatments (13 and 18°C). A subset of fish at each temperature was maintained on rotifers to serve as controls. The remaining fish were switched to a diet of brine shrimp Artemia sp. (known to be isotopically distinct from rotifers) and then sampled systematically over a 16 d period. Temperature had a significant effect on both carbon and nitrogen turnover rates. At 13°C, the half-life of carbon was 4.1 d (± 0.6), and of nitrogen, 3.9 d (± 0.7). At 18°C, the half-life of carbon was 2.2 d (± 0.3), and of nitrogen, 3.1 d (± 0.3). The change in isotopic composition closely followed predictions based entirely on the production of new tissue. KEY WORDS: Stable isotopes · Diet shifts · Pseudopleuronectes americanus · Winter-flounder habitat · TemperatureResale or republication not permitted without written consent of the publisher

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We conducted a study to determine the trophic pathways leading to juvenile fish in 2 mesohaline tidal marshes bordering Delaware Bay. The relative roles of the major primary producers in supplying energy, ultimately, to the mummichog Fundulus heteroclitus were assessed by measuring the stable isotopic compositions of juveniles (21 to 56 mm total length, TL; most of which were young-of-the-year) and those of macrophyte vegetation, phytoplankton, and benthlc microalgae at each site. We collected samples of primary producers and F. heteroclitus, the dominant fish species in this and other marshes along the east coast of the USA, in June and August 1997, at 2 study sites (upstream and downstream) within Mad Horse Creek (a Spartina alterniflora-dominated site) and Alloway Creek (a Phragmites australis-dominated site), for a total of 4 study sites. Our results indicate that F. heteroclitus production is based on a mixture of primary producers, but the mixture depends on the relative abundance of macrophytes. In S. alterniflora-dominated marshes, C and S isotope ratios indicate that F. heteroclitus production is supported by S. alterniflora production (ca 39%, presumably via detritus), whde in P. australis-dominated marshes, secondary production is based upon P. australis (73%). To our knowledge, this finding provides the first evidence that P. australis may contribute to aquatic food webs in tidal marshes. Benthic microalgae also contribute to the food chain that leads to E heteroclitus in both marsh types, while phytoplankton may be of lesser importance. Benthic microalgal biomass was lower in the P. australis-dominated system, consistent with a greater effect of shading in P. australis-versus S. alterniflora-based creek systems. Based on the difference in nitrogen isotope values between F. heteroclitus and the primary producers, the trophic level of E heteroclitus appears to be similar in the 2 marsh types, despite the differing vegetation types. In summary, the relative roles of the primary producers in supplying energy to F. heteroclitus varies locally and, in particular, with respect to the type of marsh macrophyte vegetation.

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