Trophic position of deep-sea fish—Assessment through fatty acid and stable isotope analyses

To investigate the trophic ecology of two of the dominant families of deep-sea fish (Macrouridae and Moridae) fatty acid and stable isotope analyses were applied to liver and muscle samples of five abundant species from the NE Atlantic. In conjunction with stomach content data these methods made it possible to identify differences in feeding strategies between the five study species as well as variation in feeding in relation to increasing depth and body size. Biomarkers identified strong similarities between Coryphaenoides armatus and Antimora rostrata though differences were found associating C. armatus more with the benthic food web whereas A. rostrata showed stronger links to the pelagic food web. While Lepidion eques was classified as a species linking benthic and benthopelagic food webs, both fatty acid and stable isotope data suggested that Coryphaenoides guentheri fed on an exclusively benthic diet. Coryphaenoides rupestris on the other hand were largely dependent on a copepod-based food web. Ontogenetic changes in feeding were found for both A. rostrata and C. armatus with the indication of a switch from active predation to scavenging occurring with increasing body size. Biomarkers also reflected the seasonal influx from the photic zone though changes were species-specific and probably reflected the variation in prey availability and abundance in response to these inputs. Our findings have thus demonstrated that the combined use of these biomarkers can elucidate trophic specialisations in situations where conventional methods alone previously provided insufficient data.     

Food web structure of the coastal area adjacent to the Tagus estuary revealed by stable isotope analysis

The identification of energy sources, pathways and trophic linkages among organisms is crucial for the understanding of food web dynamics. Stable isotopes were used to identify the trophic level of food web components and track the incorporation of organic matter of different origins in the coastal ecosystem adjacent to the Tagus estuary. It was shown that the river Tagus is a major source of organic carbon to this system. Also, the wide difference in δ¹³C among the primary consumers allowed the identification of the pelagic and the benthic energy pathways. The maximum trophic level observed was 2.4 for Sepia officinalis. This value is indicative of a short food web. It was concluded that the diet of the upper trophic level species relies directly on the lower food web levels to a considerable extent, instead of relying mostly on intermediate trophic level species. Moreover, the δ¹⁵N values of primary consumers were very close to that of particulate organic matter, probably due to poorly known processes occurring at the basis of the food web. This lowers the trophic length of the whole food web. Reliance on benthic affinity prey was high for all upper trophic level secondary consumers.     

A preliminary investigation of the fish food web in the Gironde estuary,France, using dietary and stable isotope analyses

Carbon and Nitrogen stable isotopes and stomach contents analyses were used to investigate an estuarine fish food web and identify the contribution of these two methods to the knowledge and understanding of the food web's structure and its functioning. The nine most abundant fish species during the warm period in the Gironde estuary (southwest France, Europe) are examined. Observation of the stomach contents reflects a variety of feeding modes between fish species that consume a diverse assortment of prey, with limited dietary overlap. Nevertheless, when regarding the whole fish community, few prey species dominate the stomach contents. Nitrogen isotope ratios indicate a high intraspecific variability inducing an interspecific covering of the signatures. However, a tendency to δ¹⁵N enrichment according to the trophic position of the species studied was observed. Fish assemblages show a trend towards enrichment of their carbon isotopic signatures from the upper estuary (−20.8 ± 1.8‰) towards the lower estuary (−18.3 ± 1.6‰). But whatever the capture zone considered, most of the individual δ¹³C values for each fish analysed are comprised between −22 and −16‰. Only few specimens, belonging to migratory amphihaline species, have significantly lighter values.     

Benthic-pelagic coupling in sewage-affected marine ecosystems

Coupling between degraded benthic habitats and populations of epibenthic and pelagic organisms is evident around major sewage outfalls in the Southern California Bight. The distribution of bottom-feeding fishes appears to be affected, but sampling has not been intensive enough to rigorously define patterns for more than a few species. Some evidence suggests that changes in fish abundance may be related to changes in the distribution and abundance of benthic prey; those species (e.g. Dover sole) that can adapt their diets to take advantage of dense populations of opportunistic polychaetes may be found in large numbers around outfalls. Some benthic crustacean feeders are at a disadvantage. Sewage effluent is an allochthnous source of organic matter to the food web and Dover sole and ridgeback shrimp both show shifts in stable isotope ratios consistent with the thesis that sewage-derived organic matter in sediments is trophically transferred to epibenthic organisms. Constructing a carbon budget based on the δ¹³C shifts in these species from area of a major outfall relative to a distant control station suggests that there is an in situ source of isotopically very light carbon in outfall sediments, perhaps from CO₂ fixation by chemoautotrophs and biological methane oxidation. Benthic-pelagic coupling is also evident in the accumulation of chlorinated hydrocarbons by fish and other organisms. At the two major Los Angeles area outfalls log-transformed liver-to-sediment concentration ratios of chlorinated benzenes, which are accumulated to moderate levels in fish, show a linear relationship to the log of K_ow (the octanol-to-water partioning coefficient) of these compounds. Chlorinated hydrocarbons with greater lipid solubility, e.g. PCBs and p,p-DDE, are concentrated to very high levels. Studies of food webs using Cs/K as a measure of trophic level suggest that trophic-step magnification is concentrating these compounds in higher trophic levels.     

New insights on the trophic ecology of bathyal communities from the methane seep area off Concepción,Chile (~36° S)

Studies of the trophic structure in methane‐seep habitats provide insight into the ecological function of deep‐sea ecosystems. Methane seep biota on the Chilean margin likely represent a novel biogeographic province; however, little is known about the ecology of the seep fauna and particularly their trophic support. The present study, using natural abundance stable isotopes, reveals a complex trophic structure among heterotrophic consumers, with four trophic levels supported by a diversity of food sources at a methane seep area off Concepción, Chile (~36° S). Although methanotrophy, thiotrophy and phototrophy are all identified as carbon fixation mechanisms fueling the food web within this area, most of the analysed species (87.5%) incorporate carbon derived from photosynthesis and a smaller number (12%) use carbon derived from chemosynthesis. Methane‐derived carbon (MDC) incorporation was documented in 22 taxa, including sipunculids, gastropods, polychaetes and echinoderms. In addition, wide trophic niches were detected in suspension‐feeding and deposit‐feeding taxa, possibly associated with the use of organic matter in different stages of degradation (e.g. from fresh to refractory). Estimates of Bayesian standard ellipses area (SEA_B) reveal different isotopic niche breadth in the predator fishes, the Patagonian toothfish Dissostichus eleginoides and the combtooth dogfish Centroscyllium nigrum, suggesting generalist versus specialist feeding behaviors, respectively. Top predators in the ecosystem were the Patagonian toothfish D. eleginoides and the dusky cat shark, Bythaelurus canescens. The blue hake Antimora rostrata also provides a trophic link between the benthic and pelagic systems, with a diet based primarily on pelagic‐derived carrion. These findings can inform accurate ecosystem models, which are critical for effective management and conservation of methane seep and adjacent deep‐sea habitats in the Southeastern Pacific.     

Pathways of organic matter through food webs of diverse habitats in the regulated Nakdong River estuary (Korea)

The benthic macroinvertebrates of the Nakdong River estuary were sampled at three different habitats: two salt marsh (Scirpus triqueter and Phragmites australis) beds and a bare intertidal flat. Fishes were sampled in the main channel. The trophic importance of marsh vascular plants, microphytobenthos, and riverine and channel particulate organic matter to macroinvertebrate and fish production was studied using stable carbon and nitrogen isotope tracers. There was a dramatic change in coverage of macrophytes (salt marshes and seagrass) after the construction of an estuarine barrage in 1987 in the Nakdong River estuary, with the S. triqueter bed increasing, the P. australis bed decreasing, and Zostera marina habitats being nearly lost. Although the invertebrate δ¹³C were within a narrower range than those of the primary producers, the values varied considerably among consumers in these habitats. However, the isotope signatures of consumers showed similarities among different habitats. Cluster analysis based on their isotopic similarity suggested that the isotope variability among species was related more to functional feeding groups than to habitats or taxonomic groups. While δ¹³C values of suspension feeders were close to that of the channel POM (mainly phytoplankton), other benthic feeders and predators had δ¹³C similar to that of microphytobenthos. Isotopic mixing model estimates suggest that algal sources, including microphytobenthos and phytoplankton, play an important role in supporting the benthic food web. Despite the huge productivity of emergent salt marshes, the contribution of the marsh-derived organic matter to the estuarine food webs appears to be limited to some nutrition for some invertebrates just within marsh habitats, with little on the bare intertidal flats or in the channel fish communities. Isotope signatures of the channel fishes also confirm that algal sources are important in supporting fish nutrition. Our findings suggest that benthic and pelagic microalgae made a large contribution to consumer diets, while marsh plants may not have a large role in supporting food webs in this estuarine system.     

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.     

Diet composition of Bathylagus euryops (Osmeriformes: Bathylagidae) along the northern Mid-Atlantic Ridge

The northern Mid-Atlantic Ridge, from Iceland to the Azores (MAR), is the largest topographical feature in the Atlantic Ocean. Despite its size, few studies have described dietary patterns of pelagic fishes along the MAR. MAR-ECO, a Census of Marine Life field project, aimed to describe the food web structure of abundant fish species along the ridge through a series of research expeditions to the MAR. Among the midwater fishes sampled during the MAR-ECO project, Bathylagus euryops (Osmeriformes: Bathylagidae) was the biomass-dominant pelagic species and ranked third in total abundance. In this paper, we describe the dietary composition of B. euryops along the MAR. Overall, copepods represented the dominant prey group consumed by B. euryops. Multivariate analyses, including a cluster analysis and a canonical correspondence analysis, revealed that fish size significantly influenced the diet of B. euryops with ostracods representing the most important prey group at small sizes (<95 mm) and decapod shrimp and calanoid copepods becoming more important with increasing fish size. Due to the high abundance and biomass observed along the MAR combined with its role as a link for energy transfer between zooplankton and higher trophic level predators, B. euryops appears to be an ecologically important species in the oceanic food web of the North Atlantic Ocean.     

Carbon flows in the benthic food web at the deep-sea observatory HAUSGARTEN (Fram Strait)

The HAUSGARTEN observatory is located in the eastern Fram Strait (Arctic Ocean) and used as long-term monitoring site to follow changes in the Arctic benthic ecosystem. Linear inverse modelling was applied to decipher carbon flows among the compartments of the benthic food web at the central HAUSGARTEN station (2500 m) based on an empirical data set consisting of data on biomass, prokaryote production, total carbon deposition and community respiration. The model resolved 99 carbon flows among 4 abiotic and 10 biotic compartments, ranging from prokaryotes up to megafauna. Total carbon input was 3.78±0.31 mmol C m⁻² d⁻¹, which is a comparatively small fraction of total primary production in the area. The community respiration of 3.26±0.20 mmol C m⁻² d⁻¹ is dominated by prokaryotes (93%) and has lower contributions from surface-deposit feeding macro- (1.7%) and suspension feeding megafauna (1.9%), whereas contributions from nematode and other macro- and megabenthic compartments were limited to <1%. The high prokaryotic contribution to carbon processing suggests that functioning of the benthic food web at the central HAUSGARTEN station is comparable to abyssal plain sediments that are characterised by strong energy limitation. Faunal diet compositions suggest that labile detritus is important for deposit-feeding nematodes (24% of their diet) and surface-deposit feeding macrofauna (∼44%), but that semi-labile detritus is more important in the diets of deposit-feeding macro- and megafauna. Dependency indices on these food sources were also calculated as these integrate direct (i.e. direct grazing and predator–prey interactions) and indirect (i.e. longer loops in the food web) pathways in the food web. Projected sea-ice retreats for the Arctic Ocean typically anticipate a decrease in the labile detritus flux to the already food-limited benthic food web. The dependency indices indicate that faunal compartments depend similarly on labile and semi-labile detritus, which suggests that the benthic biota may be more sensitive to changes in labile detritus inputs than when assessed from diet composition alone. Species-specific responses to different types of labile detritus inputs, e.g. pelagic algae versus sympagic algae, however, are presently unknown and are needed to assess the vulnerability of individual components of the benthic food web.     

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.     

Trophic structure of vermetid reef community: High trophic diversity at small spatial scales

Stable isotopes were used to investigate contributions of autochthonous (i.e. benthic: epilithon and macroalgae) and allochthonous (i.e. pelagic: phytoplankton) organic matter sources to the diet of suspension-feeders, grazers and predators associated to small reef-pools (cuvettes) created by the reef-building species Dendropoma petraeum in the north-western coast of Sicily (Italy). Contributions of potential food sources were calculated using Bayesian mixing-models and integrated to a multivariate approach to highlight the diversity of C and N pathways within Dendropoma cuvettes. Both pelagic and benthic organic matter sources were exploited by benthic consumers, although clear differences were revealed in the various species depending on their feeding strategy. Three different trophic pathways were identified: one based mainly on phytoplankton, one based mainly on macroalgae and a third one mainly on epilithon. Suspension-feeders seemed to rely mainly on allochthonous organic matter sources, while grazers showed a wider diet spectrum. Predators revealed a high specialization in each of the three food chains and showed a distinct reliance on organic matter originated from benthic or pelagic sources. Stable isotopes evidenced here a marked differentiation of the trophic niche within the cuvette-associated community, which allows minimizing competition in very space-limited conditions.     

Spatial analysis of the trophic interactions between two juvenile fish species and their preys along a coastal–estuarine gradient

Coastal and estuarine systems provide nursery grounds for many marine fish species. Their productivity has been correlated with terrigeneous inputs entering the coastal–estuarine benthic food web, thereby favouring the establishment of fish juveniles. Studies in these ecosystems often describe the nursery as a single large habitat without verifying nor considering the presence of contiguous habitats. Our study aimed at identifying different habitats based on macrozoobenthic communities and morpho-sedimentary characteristics and assessing the trophic interactions between fish juveniles and their benthic preys within these habitats. It included 43 sampling sites covering 5 habitats in which we described taxonomically and quantitatively the invertebrates and fish communities with stable isotopes and gut contents. It suggested that the benthic common sole Solea solea displayed feeding plasticity at the population level, separating the juveniles (G0) from the older fish (G1) into different “feeding sub-populations”. Size-based feeding plasticity was also observable in the spatial occupancy of that species in the studied bay. The demersal pouting, Trisopterus luscus, equally used the different habitats but displayed low feeding plasticity across and inside each habitat. Stable isotopes proved to be powerful tools to study the spatial distribution of trophic interactions in complex ecosystems like the bay of Vilaine and to define optimal habitats for fish that use the coastal–estuarine ecosystem as nursery grounds.     

Trophic functioning of the St. Lucia estuarine lake during a drought phase assessed using stable isotopes

The St. Lucia Estuary is Africa’s largest estuarine system and is currently experiencing the stress of prolonged freshwater deprivation, manifested by extremely low water levels and hypersalinity. These unprecedented conditions have raised questions regarding the trophic functioning of the ecosystem. Despite the substantial amount of research previously undertaken within this system, no studies of food web structure and function have yet been documented. This study therefore aimed to examine the food web structure of the St. Lucia estuary system through the use of carbon and nitrogen stable isotope analysis. Analysis of carbon isotope ratios indicates that benthic carbon sources are most utilised at sites with low water levels and generally higher salinity (Catalina Bay, Charter’s Creek). Conversely, the estuarine region of the mouth and Narrows, with its elevated water levels and lower salinity, still sustains a viable pelagic food web. Analysis of δ¹⁵N ratios indicates that the number of trophic transfers (food chain length) might be related to water levels. Overall, the study provides a greater understanding of the ecological processes of this complex estuarine lake, which may allow for future comparisons of trophic functioning under drought and normal/wet conditions to be made.     

Feeding activity of the copepod Acartia hongi on phytoplankton and micro-zooplankton in Gyeonggi Bay,Yellow Sea

To improve our understanding of the trophic link between micro-zooplankton and copepods in Gyeonggi Bay, Yellow Sea, the diet composition, ingestion rates, and prey selectivity of Acartia hongi, known as the most abundant and widespread copepod species, was estimated by conducting in situ bottle incubation throughout the different seasons. The results showed that A. hongi preferentially grazed on ciliate and heterotrophic dinoflagellate of a size ranging from 20 to 100 μm rather than phytoplankton. Although micro-zooplankton comprised only an average 13.7% of the total carbon available in the natural prey pool, micro-zooplankton accounted for >70% of the total carbon ration ingested by A. hongi throughout the year, except for winter diatom blooming periods when A. hongi obtained about 60% of its carbon ration from phytoplankton. Our results demonstrated that A. hongi modified their diet composition and feeding rates in response to change in composition and size of prey available to them, and that A. hongi preferentially ingested micro-zooplankton over phytoplankton. Feeding activity of A. hongi could therefore affect the species composition and size structure of natural plankton communities in this study area, particularly the micro-zooplankton. Strongly selective feeding and high grazing pressure by A. hongi on micro-zooplankton shows the role of trophic coupling between copepods and the microbial food web in the pelagic ecosystem of Gyeonggi Bay.     

Food sources of benthic animals on intertidal and subtidal bottoms in inner Ariake Sound,southern Japan,determined by stable isotopes

To evaluate the relative importance of possible food sources, including riverine particulate organic matter, reeds, benthic microalgae, seaweeds, cultured laver (Porphyra) and coastal phytoplankton, for commercial bivalves and co-occurring benthic animals, 73 macrofaunal species were collected from intertidal and subtidal soft bottoms in the inner part of Ariake Sound, Kyushu, southern Japan, and their isotopic compositions were analyzed. The results revealed that (1) both intertidal and subtidal food webs were constituted of 3 trophic levels, (2) suspension-feeding bivalves utilize a mixture of benthic microalgae and coastal phytoplankton, and omnivores and carnivores incorporate benthic microalgae and phytoplankton through their intermediate prey, and (3) 3 bivalves (Scapharca kagoshimensis, Modiolus metcalfei and Atrina lischkeana) inhabiting both intertidal and subtidal bottoms showed similar seasonal fluctuations, suggesting no difference in the diet composition among the species and between the 2 habitats. We conclude that a large biomass of benthic microalgae which was approximately equal to that of phytoplankton and the strong tidal currents that would resuspend benthic microalgae and transport them to subtidal bottom areas account for the benthic microalgal and phytoplankton based trophic structure in the inner part of Ariake Sound.     

Spatial and temporal characteristics of trophic structure of the southern Huanghai Sea fish communities

The spatial and temporal characteristics of trophic structure of fish communities in the southern Huanghai Sea were examined based on the data sampled from bottom trawl surveys conducted during the autumn of 2000 and the spring of 2001. Hierarchical agglomerative cluster method and bootstrap randomization were used to identify significant trophic groups for each fish assemblage in the southern Huanghai Sea. A total of six major trophic groups were identified within this system, which classified predators based upon location in the water column or prey size （ i. e. , benthic to pelagic predators or fish to small invertebrate prey predators）. The similarity level used to identify significant trophic groups in each assemblage ranged from 24% to 34%. Although planktivores were the dominant trophic group in each assemblage （60% - 79% ）, there were spatial and temporal variations in the trophic structure, which reflected the differences in the abundance and availability of dominant preys. Simplified food webs were constructed to evaluate the most important trophic relationships between the dominant prey taxa and the fishes in each assemblage within this system. Although there were some differences in the key prey species among different food webs, pelagic prey items （mainly euphausiids and copepods） represent the most important energetic link between primary producers and higher trophic level predators. The trophic level for most fishes was between 3 and d, and the weighted mean trophic level for each assemblage ranged from 3.3 to 3.4. Compared with previous study in the mid-1980s, there was an obvious downward trend in the trophic level for most fish species, which resulted mainly from the fluctuation in key prey species in the Huanghai Sea. The decrease in the importance of Japanese anchovy seems to be offset by other abundant prey species such as Euphausia pacifica and copepods （ mainly Calanus sinicus ） .     

应用碳、氮稳定同位素研究北部湾带鱼(Trichiurus lepturus)食性及营养级

采用胃含物分析法及碳、氮稳定同位素法,对北部湾带鱼的饵料组成、营养级和摄食习性随肛长的变化进行研究。结果表明,北部湾带鱼的食物主要由中上层鱼类、头足类、底栖甲壳类以及浮游动物等43种饵料生物组成,以质量百分比为指标,优势饵料为蓝圆鲹、裘氏小沙丁鱼、少鳞犀鳕和尖吻小公鱼等鱼类以及中国枪乌贼(6.07%)等头足类。δ13C值随着生物栖息水层的深度而增加,饵料生物中的中上层鱼类蓝圆鲹和底栖鱼类粗纹分别具有最低和最高值。根据食物质量比例及δ15N计算的北部湾带鱼营养级平均值和周年平均值均为3.7。带鱼营养级在各月份之间周年变化及随肛长增加差异不显著。真空冷冻干燥与恒温烘干进行样品预处理δ13C差异不显著,而δ15N则差异极显著。     

Predatory pelagic fishes of the Bijagós Archipelago (Guinea-Bissau) show high overlap in diets dominated by sardinella

Knowledge of trophic interactions between the key components of marine communities is required to understand food-web dynamics and develop ecosystem-based management approaches. In West Africa, where fisheries sustain the livelihoods of a significant part of the human population, this understanding is even more urgent, especially in the face of rapidly expanding fisheries and some stock collapses in the region. We studied the feeding ecology of the Crevalle jack Caranx hippos, West African Spanish mackerel Scomberomorus tritor and Guinean barracuda Sphyraena afra in the Bijagós Archipelago, Guinea-Bissau. These are the most abundant pelagic predatory teleost fishes in the area, but little is known about their ecology despite being species with commercial and recreational value, and they likely also play an important role in various African coastal ecosystems. Our findings show a high degree of dietary overlap among these three predator species, despite some degree of segregation by prey size and type. All three predators depend on Sardinella maderensis as the most important prey, which together with other members of the Clupeidae represented 47–96% of the ingested prey items. There was little difference in the diets of the predators between the dry and rainy seasons. These novel findings suggest a ‘wasp-waist’-structured ecosystem in the Bijagós Archipelago in which S. maderensis is the central small-sized pelagic fish species, and stress the need for an ecosystem-based approach to fisheries management in the region, with precautionary measures taken to avoid the overexploitation of clupeids.     

Spatial organization of food webs along habitat gradients at deep-sea hydrothermal vents on Axial Volcano,Northeast Pacific

Deep-sea hydrothermal vents are characterized by steep spatial gradients and high temporal variability in habitat conditions. This leads to the organization of species distribution along spatial habitat gradients, which may constrain food resource utilization and food web structure. We conducted a stable-isotope-based study to test the hypothesis that food resource utilization is constrained by spatial habitat variability at diffuse hydrothermal vents on Axial Volcano, Northeast Pacific. Our study included the ten most biomass-prominent species and considered the temporal change in food web structure at recently created vent sites during three consecutive years. We related species average stable isotopic composition to their position between the center and the periphery of vent sites, using previously published data. Species spread widely along the δ¹³C axis, and showed a small variability in δ¹⁵N. This indicates that most species partition food resources between isotopically different carbon sources, and that they are not organized along predator–prey trophic chains. Particulate organic matter (POM) stable isotopic composition from a concomitant study corresponds to the signature of the expected diet for most organisms. Species average δ¹³C was significantly correlated to their relative position between the center and the periphery of vent sites. We relate this spatial variability in species isotopic composition to variability in the isotopic signature of both dissolved inorganic carbon (DIC) and POM. This spatial isotopic signal of consumers reveals the spatial structuring of food (POM) production and its consumption by the fauna. Accrual of species during the development of diffuse sites increased the inter-specific spread in δ¹³C, but did not increase the range in δ¹⁵N. Our results show that the spatial organization of species distribution results in a fragmented food web where species partition POM food resources according to their position in space. Shaping of species distribution by habitat gradients therefore constrains food web structure and the occurrence of predator–prey and competitive interactions.