The aim of this work was to identify and compare, using nitrogen and carbon stable isotope data, the food sources supporting consumer communities in a Mediterranean seagrass bed (Gulf of Calvi, Corsica) with those in an adjacent epilithic alga-dominated community. Isotopic data for consumers are not significantly dierent in the two communities. Particulate matter and algal material (seagrass epi¯ora and dominant epilithic macroalgae) appear to be the main food sources in both communities. Generally, the d 13 C of animals suggests that the seagrass Posidonia oceanica (L.) Delile represents only a minor component of their diet or of the diet of their prey, but the occurrence of a mixed diet is not excluded. P. oceanica dominates the diet of only of few species, among which holothurians appear as key components in the cycling of seagrass material.

Associations of tunas and dolphins in the wild are quite frequent events and the question arises how predators requiring similar diet in the same habitat share their environmental resources. As isotopic composition of an animal is related to that of its preys, stable isotope ( 13 C/ 12 C and 15 N/ 14 N) analyses were performed in three predator species from the North-east Atlantic: the striped dolphin, Stenella coeruleoalba, the common dolphin Delphinus delphis and the albacore tuna, Thunnus alalunga, and compared to their previously described stomach content. Heavy metals (Cd, Zn, Cu and Fe) are mainly transferred through the diet and so, have been determined in the tissues of the animals. Tuna muscles display higher d 15 N than in common and striped dolphins (mean: 11.4 vs. 10.3½ and 10.4½, respectively) which re¯ects their higher trophic level nutrition. Higher d 13 C are found in common ()18.4½) and striped dolphin ()18.1½) muscles than in albacore tuna ()19.3½) probably in relation with its migratory pattern. The most striking feature is the presence of two levels of cadmium concentrations in the livers of the tunas (32 mg kg À1 dry weight (DW) vs. 5 mg kg À1 DW). These two groups also di er by their iron concentrations and their d 15 N and d 13 C liver values. These results suggest that in the Biscay Bay, tunas occupy two di erent ecological niches probably based on di erent squid inputs in their diet. Ó

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The discrepancy between the ecological significance of amphipods in the Antarctic and our poor knowledge of their ecofunctional role calls for a more detailed investigation of their trophic status in this ecosystem. A total of 12 amphipod species from suspension feeder to scavenger have been considered in this study. Our objective was to investigate whether the combination of fatty-acid and stable-isotope signatures into a 2-dimensional trophic biomarker assay would increase accuracy in the identification of Antarctic benthic amphipod trophic position. Amphipod isotopic averages ranged from-29.3 ‰ (δ 13 C) and 4.1 ‰ (δ 15 N) for the suspension feeder Ampelisca richardsoni to-21.7 ‰ (δ 13 C) and 11.9 ‰ (δ 15 N) for the high predator Iphimediella sp. Cluster analysis of the fattyacid composition separated the amphipod species into 4 trophic groups: suspension feeders, macroherbivores, omnivores and scavengers. The suspension feeder was isolated due to an important proportion of 18:4(n-3), a fatty-acid biomarker of phytoplankton. Macro-herbivores were found to rely heavily on macroalgal carbon, containing a high percentage of arachidonic acid (20:4(n-6)). Scavenger amphipods revealed a unique fatty-acid composition dominated by 1 single fatty acid, 18:1(n-9), probably the result of a very intensive de novo biosynthesis to cope with starvation periods. Our data emphasise the need to combine different types of information to be able to draw the right conclusions regarding trophic ecology. Indeed, in some cases, the exclusive use of 1 type of tracing method, fatty acids or stable isotopes, would have resulted in misleading/false conclusions in the trophic classification of amphipods. Therefore, a 2-dimensional biomarker assay is a useful tool to elucidate the trophic positions of benthic amphipods.

In Revellata Bay (Gulf of Calvi, Corsica, France), the sparid fish Surpa salpa L. is the main macro-consumer of Posidonia oceanica (L.) DELILE leaf. Stomach contents were analysed and "C/"C isotopic ratios were measured in fish muscle and potential food sources (algae, P. oceanica leaf and its epiphytes) to determine their relative contribution to the fish diet. S. salpa has an age-related mixed diet: juveniles are plankton feeders, young, sub-adults and adults are herbivorous, and, the older the individuals. the higher the relative contribution of P. oceanica to the diet. Our results and former studies of carbon stocks and fluxes in the P. oceanica bed of Revellata Bay have enabled an estimation of the general impact of S. salpu grazing on infralittoral communities: the studied species consumes 24 g C . m -' . a ~ I from P. oceanica leaf, 4.8 from epiphytes and 13 from epilithic algae. The fish net production and biomass turnover in that zone have beenestimated to be 1.2 g C . m -2 . a-' and 1.5 a-I, respectively.

Stable isotopes of carbon and nitrogen are increasingly used in marine ecosystems, for ecological and environmental studies. Here, we examine some applications of stable isotopes as ecological integrators or tracers in seagrass ecosystem studies. We focus on both the use of natural isotope abundance as food web integrators or environmental tracers and on the use of stable isotopes as experimental tools. As ecosystem integrators, stable isotopes have helped to elucidate the general structure of trophic webs in temperate, Mediterranean and tropical seagrass ecosystems. As environmental tracers, stable isotopes have proven their utility in sewage impact measuring and mapping. However, to make such environmental studies more comprehensible, future works on understanding of basic reasons for variations of N and C stable isotopes in seagrasses should be encouraged. At least, as experimental tracers, stable isotopes allow the study of many aspects of N and C cycles at the scale of a plant or at the scale of the seagrass ecosystem.

The uptake of nitrate and ammonium by the roots and leaves of Posidonia oceanica were determined between February 1997 and June 1999 by in situ experiments using the isotope 15 of nitrogen ( 15 N) as a tracer in a nutrient-poor coastal zone of the NW Mediterranean Sea (Revellata Bay, Corsica). Nitrate and ammonium leaf uptakes are recorded at 0.05 and 0.1 µM respectively. The high variability observed cannot be explained solely by the variation of the substrate concentrations in the water column. For leaves, mean specific uptake rates were 47 ± 45 and 43 ± 64 µg N g N -1 h -1 . Nitrate and ammonium leaf uptake fluxes (g N m -2 yr -1 ) seem to have the same importance on an annual basis. Nitrate uptake occurs mainly in winter and early spring, when nitrate concentrations in the water column are highest. The uptake of N, and mainly of ammonium, is significant throughout the year with maxima at the beginning of spring, but it is insufficient to meet the annual N requirement of the plant. Posidonia root biomass was very high and corresponded to high specific N uptake rates by the roots. Ammonium was incorporated by the roots 6 times faster than nitrate. In the sediment, this uptake capacity is limited by the nutrient diffusion rate, and the root uptake is therefore insufficient to meet the N requirements of the plant. In fact, P. oceanica of Revellata Bay have a complex N budget involving uptake and recycling processes and allowing the plants to meet their N requirements in one of the most nutrient-poor areas of the NW Mediterranean Sea. We calculated that leaf and root would contribute to 40 and 60% of the annual N uptake, respectively, and 60% of the annual N requirement of the plant.KEY WORDS: Seagrass · Nitrogen uptake · 15 N tracer · NW MediterraneanResale or republication not permitted without written consent of the publisher