Spatial and temporal variations in food web structure from newly-opened habitat at hydrothermal vents

To highlight the spatio-temporal variability of the food web structure of hydrothermal vent fauna from newly-opened habitat, a series of Titanium Ring for Alvinellid Colonization devices (TRACs) was deployed at TICA site on the East Pacific Rise in 2006. This experiment was conducted for periods of 4 days, 13 days and one month and deployments were aligned along a gradient from the basaltic bottom to the vent openings. δ¹³C values of colonists revealed a narrower range of carbon sources in proximity to vent openings in Alvinella pompejana habitat than in Tevnia jerichonana habitat, separated by a distance of four meters. This was possibly due to a spatial change in available food sources with a possible higher contribution of particulate organic matter (POM) to the siboglinid habitat compared to a higher contribution of microbial primary producers such as Epsilonproteobacteria in the alvinellid habitat. Temporal variability was also observed during experimentation in the form of a shift in either δ¹³C and/or δ¹⁵N values for A. pompejana, Lepetodrilus elevatus, dirivultid copepods and polynoid polychaetes within a one-month window showing first of all, fast tissues turnover and secondly, a possible switch in feeding strategy or food sources. Lepidonotopodium riftense and Branchinotogluma sandersi may have to alternate between detritivorous and predatory feeding strategies. In addition, through the analysis of stable isotope composition of A. pompejana and its episymbionts, we provided evidence that these attached bacteria formed part of the worms’ diet during the course of these colonization experiments.     

Environmental correlates of large-scale spatial variation in the δ13C of marine animals

Carbon stable isotopes can be used to trace the sources of energy supporting food chains and to estimate the contribution of different sources to a consumer's diet. However, the δ¹³C signature of a consumer is not sufficient to infer source without an appropriate isotopic baseline, because there is no way to determine if differences in consumer δ¹³C reflect source changes or baseline variation. Describing isotopic baselines is a considerable challenge when applying stable isotope techniques at large spatial scales and/or to interconnected food chains in open marine environments. One approach is to use filter-feeding consumers to integrate the high frequency and small-scale variation in the isotopic signature of phytoplankton and provide a surrogate baseline, but it can be difficult to sample a single consumer species at large spatial scales owing to rarity and/or discontinuous distribution. Here, we use the isotopic signature of a widely distributed filter-feeder (the queen scallop Aequipecten opercularis) in the north-eastern Atlantic to develop a model linking base δ¹³C to environmental variables. Remarkably, a single variable model based on bottom temperature has good predictive power and predicts scallop δ¹³C with mean error of only 0.6‰ (3%). When the model was used to predict an isotopic baseline in parts of the overall study region where scallop were not consistently sampled, the model accounted for 76% and 79% of the large-scale spatial variability (10¹–10⁴ km) of the δ¹³C of two fish species (dab Limanda limanda and whiting Merlangus merlangius) and 44% of the δ¹³C variability in a mixed fish community. The results show that source studies would be significantly biased if a single baseline were applied to food webs at larger scales. Further, when baseline δ¹³C cannot be directly measured, a calculated baseline value can eliminate a large proportion of the unexplained variation in δ¹³C at higher trophic levels.     

Trophic relationships along a bathymetric gradient at the deep-sea observatory HAUSGARTEN

Deep-seafloor communities, especially those from the ice-covered Arctic, are subject to severe food limitation as the amount of particulate organic matter (POM) from the surface is attenuated with increasing depth. Here, we use naturally occurring stable isotope tracers (δ¹⁵N) to broaden our rudimentary knowledge of food web structure and the response of benthic organisms to decreasing food supplies along the bathymetric transect (～1300–5600 m water depth) of the deep-sea observatory HAUSGARTEN. Encompassing five trophic levels, the HAUSGARTEN food web is among the longest indicating continuous recycling of organic material typical of food-limited deep-sea ecosystems. The δ¹⁵N signatures ranged from 3.0‰ for Foraminifera to 21.4‰ (±0.4) for starfish (Poraniomorpha tumida). The majority of organisms occupied the second and third trophic level. Demersal fish fed at the third trophic level, consistent with results from stomach contents analysis. There were significant differences in the δ¹⁵N signatures of different functional groups with highest δ¹⁵N values in predators/scavengers (13.2±0.2‰) followed by suspension feeders (11.2±0.2‰) and deposit feeders (10.2±0.3‰). Depth (=increasing food limitation) affected functional groups in different ways. While the isotopic signatures of predators/scavengers did not change, those of suspension feeders increased with depth, and the reverse was found for deposit feeders. In contrast to the results of other studies, the δ¹⁵N signatures in POM samples obtained below 800 m did not vary significantly with depth indicating that changes in δ¹⁵N values are unlikely to be responsible for the depth-related δ¹⁵N signature changes observed for benthic consumers. However, the δ¹⁵N signatures of sediments decreased with increasing depth, which also explains the decrease found for deposit feeders. Suspension feeders may rely increasingly on particles trickling down the HAUSGARTEN slope and carrying higher δ¹⁵N signatures than the decreasing POM supplies, which elevates the δ¹⁵N value of their tissues. Our results imply that a depth-stratified approach should be taken to avoid a misinterpretation of data obtained at different depths.     

Nekton migration and feeding location in a coastal area – A stable isotope approach

Stable isotope analysis was used to investigate nekton movements and feeding location in a coastal area adjacent to a major European river, the Tagus, Portugal. Particulate organic matter isotopic signatures presented a gradient from the river towards the sea. Phytoplankton, zooplankton, polychaetes and the crab, Polybius henslowii, provided evidence of the incorporation of terrestrial organic matter into the lower levels of the food web, reflecting local isotopic signatures. Two fish species reflected the coastal isotopic gradient in δ¹³C, Diplodus vulgaris and Arnoglossus imperialis and the latter also presented isotopic differerences among the sites for δ¹⁵N. Alloteuthis subulata, Trisopterus luscus and Callionymus lyra were isotopicaly distinct among sites for δ¹⁵N. An increase of δ¹⁵N with length was detected for T. luscus and C. lyra, possibly showing ontogenic trophic level changes. Since A. subulata did not present differences in length and still showed isotopic distinction for δ¹⁵N, among areas, it was concluded that local biogeochemical factors may also have an influence. Diplodus bellottii and Dicologlossa cuneata did not reflect any isotopic signature reflecting their wide migration and feeding across the coastal area. Central isotopic ranges, defined as the site mean values for δ¹³C and δ¹⁵N ± 1‰ were determined for each species and site and those deviating from these were considered transient individuals. Central isotopic ranges accounted for 87% of A. imperialis, 80% of A. subulata, 77% of T. luscus, 67% of C. lyra and 50% of D. vulgaris. The number of individuals within each central isotopic range was surprisingly high for an open coastal area and comparable to those of more structured environments.     

Nursery fidelity,food web interactions and primary sources of nutrition of the juveniles of Solea solea and S. senegalensis in the Tagus estuary (Portugal): A stable isotope approach

Stable carbon and nitrogen isotopes were used to assess site fidelity of Solea solea and Solea senegalensis juveniles, to investigate food web interactions and to determine the dominant nutrient pathways in two nursery areas in the Tagus estuary, Portugal. Samples of water from the main sources and from the nursery areas and respective saltmarsh creeks were collected for isotope analysis, as well as sediment, benthic microalgae, saltmarsh halophytes, S. solea, S. senegalensis and its main prey, Nereis diversicolor, Scrobicularia plana and Corophium spp. While site fidelity was high in 0-group juveniles, it was lower for 1-group juveniles, possibly due to an increase in mobility and energy demands with increasing size. Analysis of the food web revealed a complex net of relations. Particulate organic matter from the freshwater sources, from each nursery's waters and saltmarsh creeks presented similar isotopic composition. Sediment isotopic composition and saltmarsh halophytes also did not differentiate the two areas. All components of the food web from the benthic microalgae upwards were isotopically different between the nursery areas. These components were always more enriched in δ¹³C and δ¹⁵N at the lower nursery area than at the nursery located upstream, appearing as if there were two parallel trophic chains with little trophic interaction between each other. A mixture of carbon and nitrogen sources is probably being incorporated into the food web. The lower nursery area is more dependent upon an isotopically enriched energy pathway, composed of marine particulate organic matter, marine benthic microalgae and detritus of the C₄ saltmarsh halophyte Spartina maritima. The two nursery areas present a different level of dependence upon the freshwater and marine energy pathways, due to hydrological features, which should be taken into account for S. solea and S. senegalensis fisheries and habitat management.     

Food web structure of two Mediterranean lagoons under varying degree of eutrophication

The food web structure and functioning of two north-western Mediterranean lagoons exhibiting contrasting degrees of eutrophication and marine influences were compared through δ¹³C and δ¹⁵N analysis of major potential food sources and consumers. The Lapalme Lagoon is well preserved and has kept a natural and temporary connection with the open sea. Conversely, the Canet Lagoon is heavily eutrophicated and its water exchange with the open sea has been artificially reduced. In Lapalme, all potential food sources and consumers exhibited δ¹⁵N values indicative of pristine coastal areas. Suspended particulate organic matter (POM) and sediment organic matter (SOM) pools seemed to constitute the main food sources of most primary consumers. Both primary producers and all consumers were much more ¹⁵N-enriched (by ∼ 10‰) and more ¹³C-depleted in Canet than in Lapalme. This reflected: (1) the assimilation of important amounts of anthropogenic nitrogen in the food web, and (2) a marked and uniform influence of ¹³C-depleted allochtonous sources of carbon. Based on the mean δ¹⁵N of primary consumers, we found rather similar food web lengths in both lagoons with top consumers at trophic levels 3.6 and 4.0 in Canet and Lapalme, respectively. However, the eutrophication of the Canet Lagoon resulted in a simplification of the food web structure (i.e., a single trophic pathway from a ¹⁵N-enriched fraction of the SOM pool to top predators) compared to what was observed in Lapalme Lagoon where additional ¹³C-enriched food sources played a significant trophic role. Moreover, some consumers of Canet tended to exploit primary producers to a larger extent (and thus to exhibit lower trophic levels) than in Lapalme.     

Origin and decomposition of sinking particulate organic matter in the deep water column inferred from the vertical distributions of its δN, δ and δ

Sinking particles were analyzed for their nitrogen isotopic ratio δ¹⁵N) of total particulate nitrogen (PN), stable carbon isotopic ratio (δ¹³C) and radioactive isotopic ratio (δ¹⁴C) of total particulate organic carbon (POC), at three different latitudinal (temperate, subpolar and equatorial) and geomorphological (trench, proximal abyssal plain and distal abyssal plain) sites in the western North Pacific Ocean using year-long time series sediment trap systems, to clarify the common vertical trends of the isotopic signals in deep water columns. Although the δ¹⁵N and δ¹³C values of sinking particulate organic matter (POM) were partly affected by the resuspension of sedimentary POM from the sea floor, especially in the trench, the changes in δ¹⁵N and δ¹³C values owing to the resuspension could be corrected by calculation of the isotopic mass balance from δ¹⁴C of sinking POC. After this correction, common downward decreasing trends in δ¹⁵N and δ¹³C values were obtained in the deep water columns, irrespective of the latitudes and depths. These coincidental isotopic signals between δ¹⁵N and δ¹³C values provide new constraints for the decomposition process of sinking POM, such as the preferential degradation of ¹⁵N- and ¹³C-rich compounds and the successive re-formation of the sinking particles by higher trophic level organisms in the deep water column.     

Pelagic nitrogen dynamics in the Vietnamese upwelling area according to stable nitrogen and carbon isotope data

Upwelling and nitrogen (N) fixation provide new N for primary production off southern central Vietnam. Here we evaluate the roles of both N sources for zooplankton nutrition by comparing δ¹⁵N and δ¹³C values in nitrate, particulate organic matter (POM), and six net-plankton size fractions from monsoon and intermonsoon seasons. The δ¹³C values in POM and the net-plankton size fractions differed by 2–4‰ at any time. We assume that plankton from the POM filters was dominated by nano-and picoplankton as opposed to micro- and mesoplankton in the net-samples. The implications of this are discussed in terms of size differential pathways of C and N in the planktonic food web. We used δ¹⁵N to estimate the differences in N nutrition between the actual upwelling region and the oligotrophic area further offshore. The δ¹⁵N values of the net-plankton size fractions were depleted in δ¹⁵N by ca. 2‰ outside compared to inside the upwelling area during the monsoon season. We attribute these patterns to the additional utilization of N derived from N fixation. The concomitant findings of high N fixation rates reported earlier and low subthermocline nitrate (nitrate_sub) values of 2.9–3.6‰ support this conclusion. Net-plankton δ¹⁵N values increased with size, pointing to the dominance of higher trophic levels in the larger size fractions. According to a two source mixing model N fixation may have provided up to 13% of the N demand in higher trophic levels.     

Redefining the trophic importance of seagrasses for fauna in tropical Indo-Pacific meadows

Fauna species living in seagrass meadows depend on different food sources, with seagrasses often being marginally important for higher trophic levels. To determine the food web of a mixed-species tropical seagrass meadow in Sulawesi, Indonesia, we analyzed the stable isotope (δ¹³C and δ¹⁵N) signatures of primary producers, particulate organic matter (POM) and fauna species. In addition invertebrates, both infauna and macrobenthic, and fish densities were examined to identify the important species in the meadow. The aims of this study were to identify the main food sources of fauna species by comparing isotopic signatures of different primary producers and fauna, and to estimate qualitatively the importance of seagrass material in the food web. Phytoplankton and water column POM were the most depleted primary food sources for δ¹³C (range −23.1 to −19.6‰), but no fauna species depended only on these sources for carbon. Epiphytes and Sargassum sp. had intermediate δ¹³C values (−14.2 to −11.9‰). Sea urchins, gastropods and certain fish species were the main species assimilating this material. Seagrasses and sedimentary POM had the least depleted values (−11.5 to −5.7‰). Between the five seagrass species significant differences in δ¹³C were measured. The small species Halophila ovalis and Halodule uninervis were most depleted, the largest species Enhalus acoroides was least depleted, while Thalassia hemprichii and Cymodocea rotundata had intermediate values. Fourteen fauna species, accounting for 10% of the total fauna density, were shown to assimilate predominantly (>50%) seagrass material, either directly or indirectly by feeding on seagrass consumers. These species ranged from amphipods up to the benthic top predator Taeniura lymma. Besides these species, about half of the 55 fauna species analyzed had δ¹³C values higher than the least depleted non-seagrass source, indicating they depended at least partly for their food on seagrass material. This study shows that seagrass material is consumed by a large number of fauna species and is important for a large portion of the food web in tropical seagrass meadows.     

Trophic ecology and vertical patterns of carbon and nitrogen stable isotopes in zooplankton from oxygen minimum zone regions

The unique physical and biogeochemical characteristics of oxygen minimum zones (OMZs) influence plankton ecology, including zooplankton trophic webs. Using carbon and nitrogen stable isotopes, this study examined zooplankton trophic webs in the Eastern Tropical North Pacific (ETNP) OMZ. δ¹³C values were used to indicate zooplankton food sources, and δ¹⁵N values were used to indicate zooplankton trophic position and nitrogen cycle pathways. Vertically stratified MOCNESS net tows collected zooplankton from 0 to 1000 m at two stations along a north-south transect in the ETNP during 2007 and 2008, the Tehuantepec Bowl and the Costa Rica Dome. Zooplankton samples were separated into four size fractions for stable isotope analyses. Particulate organic matter (POM), assumed to represent a primary food source for zooplankton, was collected with McLane large volume in situ pumps.The isotopic composition and trophic ecology of the ETNP zooplankton community had distinct spatial and vertical patterns influenced by OMZ structure. The most pronounced vertical isotope gradients occurred near the upper and lower OMZ oxyclines. Material with lower δ¹³C values was apparently produced in the upper oxycline, possibly by chemoautotrophic microbes, and was subsequently consumed by zooplankton. Between-station differences in δ¹⁵N values suggested that different nitrogen cycle processes were dominant at the two locations, which influenced the isotopic characteristics of the zooplankton community. A strong depth gradient in zooplankton δ¹⁵N values in the lower oxycline suggested an increase in trophic cycling just below the core of the OMZ. Shallow POM (0–110 m) was likely the most important food source for mixed layer, upper oxycline, and OMZ core zooplankton, while deep POM was an important food source for most lower oxycline zooplankton (except for samples dominated by the seasonally migrating copepod Eucalanus inermis). There was no consistent isotopic progression among the four zooplankton size classes for these bulk mixed assemblage samples, implying overlapping trophic webs within the total size range considered.     

Utilization of carbon sources in a northern Brazilian mangrove ecosystem

Carbon and nitrogen stable isotope ratios (¹³C and ¹⁵N) and trophic level (TL) estimates based on stomach content analysis and published data were used to assess the contribution of autotrophic sources to 55 consumers in an intertidal mangrove creek of the Curuçá estuary, northern Brazil. Primary producers showed δ¹³C signatures ranging between −29.2 and −19.5‰ and δ¹⁵N from 3.0 to 6.3‰. The wide range of the isotopic composition of carbon of consumers (−28.6 to −17.1‰) indicated that different autotrophic sources are important in the intertidal mangrove food webs. Food web segregation structures the ecosystem into three relatively distinct food webs: (i) mangrove food web, where vascular plants contribute directly or indirectly via POM to the most ¹³C-depleted consumers (e.g. Ucides cordatus and zooplanktivorous food chains); (ii) algal food web, where benthic algae are eaten directly by consumers (e.g. Uca maracoani, mullets, polychaetes, several fishes); (iii) mixed food web where the consumers use the carbon from different primary sources (mainly benthivorous fishes). An IsoError mixing model was used to determine the contributions of primary sources to consumers, based on δ¹³C values. Model outputs were very sensitive to the magnitude of trophic isotope fractionation and to the variability in ¹³C data. Nevertheless, the simplification of the system by a priori aggregation of primary producers allowed interpretable results for several taxa, revealing the segregation into different food webs.     

The trophic significance of the invasive seaweed Sargassum muticum in sandy beaches

Native and exotic seaweeds frequently lie on the beach and sustain part of the benthic food web. However, the role of exotic seaweeds as food sources for beach consumers has been poorly studied. We studied the temporal and spatial variability in the trophic significance of the invasive brown seaweed Sargassum muticum on sandy beaches. We measured the stable isotopes (δ¹³C and δ¹⁵N) in the tissues of S. muticum and of invertebrate consumers and estimated the dietary biomass proportion of S. muticum during four sampling dates at two beaches and heights on the shore. Samples were collected from eight pitfall traps placed at a distance of 2 m from each other. Detrital macroalgae and seagrasses were also collected by hand within an area of 30 cm around each pitfall trap. We measured the spatial and temporal variability in the isotope composition of the beach consumers and of S. muticum using different models of analyses of variance. We then calculated the biomass proportion of S. muticum to the animal diet with a two-isotopic mixing model. The invasive alga S. muticum seemed to be one of the main food sources for the amphipod Talitrus saltator and, to a less extent, for the isopod Tylos europaeus. The importance of S. muticum was however temporally variable and decreased during spring (in March and May), probably due to the availability of native macrophytes. The supply of invasive wrack to beach food webs thus deserves more attention if we want to understand their role in influencing food web dynamics.     

Ontogenetic shifts in feeding habits of orangemouth weakfish (Cynoscion xanthulus): From estuarine benthic feeder to marine nektivore top predator

Orangemouth weakfish is a highly important commercial and game fish species in the Tropical Eastern Pacific. However, studies assessing changes in its trophic ecology relevant to size, habitat (estuarine or marine), sex, and season are non-existent. We assessed its feeding habits and trophic position (TP) according to the aforementioned factors. Its dietary breadth was composed of 33 types of prey. The feeding habits and isotopic values (δ¹⁵N and δ¹³C) registered differences between sizes and habitat, indicating ontogenetic changes in habitat as well as TP, as small organisms (TP 3.1) inhabited the estuarine area, feeding on benthic and demersal organisms, whilst large individuals (TP 4.1) were found in the marine environment, preying on nektonic fauna. No differences were found according to sex or season. The isotopic niche showed that the different sizes of this species are generalist consumers with a clear separation between them and between habitats, as estuarine juvenile and preadult organisms have a larger isotopic niche than adult marine organisms, being an opportunist tertiary estuarine consumers during their early stages, and a top marine predator during the adult phase.     

Community structure in mussel beds at Logatchev hydrothermal vents and a comparison of macrofaunal species richness on slow- and fast-spreading mid-ocean ridges

Species lists for vent fields on the Mid‐Atlantic Ridge (MAR) from 14°N to 38°N suggest that there is a northern (>27°N), shallow (<2000 m) fauna and a southern (<27°N), deeper (>3000 m) endemic vent fauna, but little is known about how community structure varies along the ridge axis. In this study, quantitative samples of macrofaunal invertebrates associated with mussels (Bathymodiolus puteoserpentis) were collected at Logatchev (14°45′N), the southern‐most explored vent field on the MAR. Community structure (including species composition, species richness, diversity, and relative species abundances) in mussel beds at Logatchev was compared with that of Snake Pit (23°22′N) and Lucky Strike (37°17′N) mussel beds. The most striking feature of the Logatchev mussel‐bed macrofaunal invertebrate community was the tremendous abundance (up to 2390 individuals per liter of mussel‐volume sampled) and biomass of the ophiuroid, Ophioctenella acies. Logatchev and Snake Pit mussel beds share >50% of their associated macrofaunal species; these two sites share only 20–25% of their macrofaunal species with Lucky Strike. Species–effort curves and univariate measures of diversity (H′, J′) do not support the claim that diversity of vent organisms on the MAR is highest at Logatchev, at least when one assesses this within a habitat type. Multivariate analysis readily differentiates the species‐abundance characteristics of Logatchev, Snake Pit, and Lucky Strike mussel‐bed macrofaunas. The relationship between sea‐floor spreading rate and diversity was explored through comparison of species richness in mussel‐bed habitats on slow‐spreading (MAR), fast‐spreading [northern East Pacific Rise (EPR)], and ultra‐fast‐spreading (southern EPR) mid‐ocean ridges. Species richness was greater in samples from the faster‐spreading ridge axes, where vents are more closely spaced but shorter lived, than on slow‐spreading centers, where vents are further apart but longer lived.     

Spatial structures of hydrothermal vents and vent-associated megafauna in the back-arc basin system of the Okinawa Trough,western Pacific

Spatial patterns and morphology of hydrothermal vents and the occurrence of vent-associated megafauna were investigated in the back-arc basin system of the Okinawa Trough, western Pacific. Amongst hydrothermal vent fields located on the rising slopes to the NE and S of the basin, the Iheya North area has been subjected to a series of intensive diving surveys. Hydrothermal vents demonstrated concentrated patterns of distribution on different spatial scales. In Iheya North, the majority of vents occurred in lines parallel to the NE to W/SW alignment of the spreading axis, within a radius of ca. 200 m around the most active sulphide structure with the highest recorded fluid temperature of over 300°C. The morphology of hydrothermal vents varied greatly from an incipient flat rock with crevices to a 20- to 30-m-tall, multi-flanged structure with concomitant variation in the distribution and abundance of vent-associated megafauna, particularly a galatheid Shinkaia crosnieri and Bathymodiolus mussels. Comparison of active and inactive vents revealed that the spatial extent of Shinkaia ‘aggregation’ (a group of individuals with short nearest-neighbour distances) effectively defined a habitat unit of this species, and active hydrothermal chimneys contained more of these units, leading to greater spatial occupancy by Shinkaia. Given the temporally unstable nature of vent structures as recognised by repeated surveys, vent assemblages are thought to be closely dictated by the spatio-temporal dynamics of vents in the Okinawa Trough back-arc system.     

基于稳定性同位素的普里兹湾南极大磷虾不同发育阶段食性转变分析

As one of the most common and dominant species in the Southern Ocean, Antarctic krill(Euphausia superba)play a significant role in food web structure and the process of energy flow. The diet of Antarctic krill in the Prydz Bay during austral summer of 2012/2013 was investigated and the ontogenetic shift in krill diet was evaluated using the stable isotope method. The nitrogen stable isotope values(δ~(15) N) of adults((2.78±0.58)‰) were much higher than those of juveniles((1.69±0.70)‰), whereas the carbon stable isotope values(δ~(13) C) of adults(–(28.26±1.08)‰) were slightly lower than those of juveniles(–(27.48±1.35)‰). Particulate organic matter(POM)from 0, 25, and 50 m depth combined(0/25/50 m) represented phytoplankton food items. The results showed that phytoplankton food items in surface water and mesozooplankton were two essential food items for Antarctic krill in the Prydz Bay during summer. POM(0/25/50 m) contributes 56%–69% and 26%–34% to the diet of juvenile and adult krill, respectively, whereas mesozooplankton composes 13%–34% and 58%–71% of the diet of juvenile and adult krill, respectively. Thus, an ontogenetic diet shift from POM(0/25/50 m), which consists mainly of phytoplankton, to a higher trophic level diet containing mesozooplankton, was detected. The capacity for adults to consume more zooplankton food items may minimize their food competition with juveniles, which rely mostly on phytoplankton food items. This suggests "diet shift with ontogeny" which may somehow help krill keep their dietary energy budget balanced and well adapted to the Antarctic marine ecosystem as a dominant species.     

Trophic ecology of the rocky shore community associated with the Ascophyllum nodosum zone (Roscoff,France): A δC vs δN investigation

This study aimed to characterize the structure and functioning of the benthic food web associated with the Ascophyllum nodosum zone of the rocky shore of Roscoff by using δ¹³C and δ¹⁵N. Several characteristics of the trophic ecology of the invertebrates associated with this mid-littoral habitat and which belong to different functional groups (e.g., grazers, filter-feeders, predators and omnivores) were highlighted. In particular, the filter feeder species (including mostly sponges) used macroalgae-derived organic matter as a substantial food requirement. The results also pointed out an important stable isotopes variability for strict coexisting primary consumers which: (1) is directly related to the high δ¹⁵N range of the food sources; (2) makes it impossible to establish a unique trophic level scale based on δ¹⁵N values, as previously done in coastal environments; and (3) points out the existence of major co-occurring trophic pathways which characterise the Ascophyllum nodosum habitat.     

Trophic significance of the kelp Laminaria digitata (Lamour.) for the associated food web: a between-sites comparison

This study aimed at establishing the trophic significance of the kelp Laminaria digitata for consumers inhabiting two rocky shores of Northern Brittany (France), displaying contrasted ecological conditions. The general trophic structure did not vary between these two sites, with a wide diversity of filter-feeders and predators, and only 14% of the species sampled belonging to the grazers' trophic group. The diversity of food sources fueling the food web appeared also similar. The food webs comprised four trophic levels and the prevalence of omnivory appeared relatively low compared to previous studies in the same area. Conversely, to the food web structure, which did not differ, the biochemical composition of L. digitata differed between the two sites, and was correlated to a larger diversity of grazers feeding on this kelp in sheltered conditions. This indicated that the spatial variability occurring in the nutritive value of L. digitata is likely to deeply affect the functioning of kelp-associated food webs. The contribution of L. digitata-derived organic matter to the diet of filter-feeders inhabiting these two environments was assessed using the mixing model Isosource, which showed the higher contribution of kelp matter in sheltered conditions. These results highlight the spatial variability that may occur in the functioning of kelp-associated food webs. Moreover, this suggests that hydrodynamics is likely to control the availability of kelp-derived organic matter to local filter-feeders, probably through an increase of detritus export in exposed areas.