Benthic community and food web structure on the continental shelf of the Bay of Biscay (North Eastern Atlantic) revealed by stable isotopes analysis

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Journal of Marine Systems 72 (2008) 17 – 34 www.elsevier.com/locate/jmarsys

Benthic community and food web structure on the continental shelf of the Bay of Biscay (North Eastern Atlantic) revealed by stable isotopes analysis François Le Loc'h a,b,⁎, Christian Hily b , Jacques Grall b,c a

c

UR 070 RAP, Centre de Recherche Halieutique, Institut de Recherche pour le Développement, Avenue Jean Monnet, B.P. 171, 34203 Sète Cedex, France b Laboratoire des Sciences de l'Environnement Marin, UMR 6539 CNRS, Institut Universitaire Européen de la Mer, Université de Bretagne Occidentale, Place Nicolas Copernic, 29280 Plouzané, France Observatoire du Domaine Côtier, FR 2195 CNRS, Institut Universitaire Européen de la Mer, Université de Bretagne Occidentale, Place Nicolas Copernic, 29280 Plouzané, France Received 15 July 2006; received in revised form 12 March 2007; accepted 5 May 2007 Available online 4 December 2007

Abstract The North Bay of Biscay continental shelf is a major French demersal fishery, but little was known on the trophic food web of its benthic communities. In order to determine the benthic trophic web, the objectives of this study are to describe the macro- and megafaunal benthic community structure (species richness, abundance and biomass) and to establish the trophic pathways (food sources and trophic levels) by applying carbon and nitrogen stable isotopic analysis to the main benthic and demersal species (invertebrates and fish). Two distinct benthic communities have been identified: a muddy sand community within the central part of the bay, and an outer Bay of Biscay Ditrupa sand community of higher species richness, abundance and biomass than the muddy sand community. Deposit-feeders, suspension feeders and predators, distributed in three main trophic levels, dominate both communities. Large differences in stable carbon ratio values within the primary consumers provide evidence of two different food sources: i) a pelagic food source made up of recent sedimenting particulate organic matter on which zooplankton and suprabenthos feed and ii) a benthic detrital food source supplying deposit feeders and partly benthic suspension feeders. Differences in isotopic signatures were also observed within the upper trophic levels that allowed estimation of the contribution of each food source component to the diet of the upper consumers. Finally, the use of stable isotopic composition together with the species' feeding strategy allow identification of the main differences between the trophic functioning of the two benthic communities and highlight the importance of the role of detrital pathways in the carbon cycling within the continental shelf benthic trophic web. © 2007 Elsevier B.V. All rights reserved. Keywords: Benthic invertebrates; Demersal fish; Detritus; Food sources; Trophic level; Bay of Biscay

1. Introduction ⁎ Corresponding author. UR 070 RAP, Centre de Recherche Halieutique, Institut de Recherche pour le Développement, Avenue Jean Monnet, B.P. 171, 34203 Sète Cedex, France. Fax: +33 4 99 57 32 95. E-mail address: [email protected] (F. Le Loc'h). 0924-7963/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2007.05.011

Pelagic primary and secondary production, generated in the euphotic zone, supply the continental shelf benthic communities through the sedimentation of

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organic material (Graf, 1992). A fundamental requirement to understand energy pathways through complex benthic food webs from primary production to the higher-level consumers is the knowledge of trophic linkages among organisms (Hobson et al., 2002). The Bay of Biscay continental shelf supports an important demersal and benthic fishery, mainly for European hake (Merluccius merluccius), sole (Solea solea) and Norway lobster (Nephrops norvegicus). These benthic and demersal communities have been recently studied (Poulard et al., 2003; Poulard and Blanchard, 2005), but no study since that of Glémarec (1969) has investigated macro- and megafaunal invertebrate benthic communities although these potentially constitute the main prey stock for target fish species. The reconstruction of marine food webs is largely constrained by methodological difficulties, especially those associated with gut content analyses, and these limitations are particularly evident in the case of small sized invertebrates. Stable isotope analysis provides a powerful tool for the study of the trophic relationships within marine ecosystems (Peterson and Fry, 1987; Michener and Schell, 1994). Indeed, the stable carbon and nitrogen isotopic composition of an animal depends on the stable isotopic composition of its food sources and on isotopic fractionation during the feeding process. Stable nitrogen and carbon isotope ratios (expressed respectively as δ15N and δ13C) are typically enriched from prey to consumers by 3.4‰ for δ15N and 1‰ for δ13C (DeNiro and Epstein, 1981; Minagawa and Wada, 1984). Thus, the large δ15N shift between a consumer and its prey allows δ15N to be a reliable indicator of the trophic position of an organism within the food web relative to the primary producers. However, recent reviews indicate that δ15N fractionation depends on multiple factors, such as the N-content of the food, environmental conditions, taxonomy and even trophic group (Vander Zanden and Rasmussen, 2001; Vanderklift and Ponsard, 2003; Mc Cutchan et al., 2003). Even if the application of a unique trophic enrichment factor for δ15N appears as a simplification of such a complex system, a mean trophic fractionation of 3.4‰ could be widely applicable in aquatic food webs according to Post (2002). The δ13C analysis is useful to elucidate the origin and pathways of organic matter in food webs, as the primary sources may be isotopically distinct thus allowing distinctions to be made between pelagic and benthic contributions (Hobson et al., 2002) and tracking fluxes of particulate matter along the food chain up to the higher trophic levels (Le Loc'h and Hily, 2005). As primary consumers integrate the temporal isotopic

variation of their food sources (Post, 2002), they can be used as an isotopic baseline to calculate the reliance of the upper consumers on benthic food sources (Sherwood and Rose, 2005). Thus isotopic composition provides information on the relative importance of each source within the consumer's diet. Trophic structure and pelagic–benthic coupling are now quite well understood in shallow or intertidal benthic ecosystems (Riera et al., 2002; Vizzini and Mazzola, 2006). In contrast the functioning of benthic invertebrate and fish continental shelf communities and the sources of organic matter for their subsistence have rarely been studied (Pinnegar and Polunin, 2000). The main goal of this study is to assess the trophic origins and pathways within the benthic communities of the Northern Bay of Biscay continental shelf. Our main objectives were i) to describe the macro- and megafaunal benthic community structure (species richness, abundance and biomass), ii) to identify the different trophic levels and the food sources supporting the dominant and commercial species, iii) to link the two approaches in order to understand how benthic species are organized to exploit the trophic sources on the continental shelf soft bottom. 2. Materials and methods 2.1. Study area The study area is located around 47°N and extends between 3 and 5°W on the French continental shelf of the Bay of Biscay (northeast Atlantic Ocean; Fig. 1). A large sedimentary muddy bank known as the “Grande Vasière” characterizes the northern continental shelf of the Bay of Biscay. The Grande Vasière is situated between a depth range of 80 m to 130 m over a distance of 275 km from south to north and 55–75 km from east to west. To the west, hard bottom separates the Grand Vasière from the outer edge of the continental shelf constituted by the Ditrupa sands down to 160 m (Glémarec, 1971). 2.2. Community sampling During the last week of May and the first week of June 2001, five areas (four located within the central part of the Grande Vasière and one on the continental shelf external margin) were sampled on the Bay of Biscay continental shelf (Fig. 1). Within each of these areas, three stations were sampled as zonal-replicates, except for area B where only two stations were visited. The faunal sampling strategy was the same as that of Le

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Fig. 1. Localization of the sampling stations within the north-western part of the continental shelf of the Bay of Biscay. Stations A, B, C and D are located on the Grande Vasière inside the dotted marks, stations E are on the external margin.

Loc'h and Hily (2005). A 2-m beam trawl was used to sample the epibenthos, i.e. invertebrate megafauna (individual mean size N10 mm) and demersal fish (Kaiser et al., 1994; Jennings et al., 1999; Ellis et al., 2000). This proved to be an efficient method for large and rare species and for the integration of small-scale seabed patchiness (Frauenheim et al., 1989). The beam trawl was fitted with a chain mat and a 20-mm mesh liner. During sampling (20 min), warp length was 3 times the water depth, and the distance trawled was measured by onboard differential global positioning system (DGPS). Five benthic macrofauna (1–10 mm) replicate samples per station were collected with a Hamon grab (0.25 m2). A suprabenthic sledge (MACER-GIROQ) was used for suprabenthos collection (0.5 mm net mesh size) and zooplankton was sampled throughout the entire water column using a WP2 zooplankton net (0.2 mm mesh size). Finally, a Reineck corer was used for granulometric determinations. In each of the five zones, particulate organic matter (POM) was collected with a Niskin bottle twice a day during four days in May 2002 at two depths. The first

depth was near the fluorescence maximum (identified using a CTD probe coupled to a fluorimeter) at approximately 30 m depending on the station and the time of day, and the second was close to the bottom at a distance of 1–2 m from the sediment. They were named POM surface layer and POM bottom respectively. 2.3. Community analyses Once on board, faunal samples were washed, sorted on a 1 mm mesh size and preserved in 7% formaldehyde solution. Benthic macro- and megafaunal organisms were identified to species level for most taxonomic groups (except for nematods, nemerteans and phoronids), and counted. Biomass of each taxon was measured as ash-free dry weight (weight loss after combustion at 450 °C for 4 h). 2.4. Stable isotopes Samples were prepared and analysed for stable isotopes following Le Loc'h and Hily (2005). POM

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was collected by filtration of seawater (4 l were filtered for each sample) on precombusted Whatman GF/F filters and then stored at − 20 °C. Subsequently, the filters were exposed to HCl vapor for 4 h in order to remove carbonates before being placed in tin cups (Lorrain et al., 2003). The samples were analysed for isotope ratios using a Finnigan Delta S isotope ratio mass spectrometer coupled to a Carlo Erba NA 2100 Elemental Analyser. Because of the low organic matter content (b2%) and the high carbonate content measured in the sediment, total extraction of the sediment organic matter (SOM) without degradation was not possible; therefore isotopic analysis of the sediment organic matter was not carried out. Species selected for isotopic analysis were those that were dominant in terms of both abundance and biomass, in order to obtain a synthetic image of the trophic structure within each of the communities. Zooplankton and suprabenthos samples were acidified to remove any residual carbonates from cuticles and then rinsed with distilled water (Riera et al., 2000). Pre-treatment acidification is necessary to eliminate carbonates; extended acidification (3 h) may affect δ 15 N values (Pinnegar and Polunin, 1999; Carabel et al., 2006); our samples were acidified with HCl 1.2 N for less than 10 min. It is well known that lipids are depleted in 13 C compared to carbohydrates and proteins (DeNiro and Epstein, 1977; Griffiths, 1991), implying that fatty tissues tend to be isotopically lighter compared to lean ones. Therefore, trophic interpretations based on δ13 C signatures may be cofounded by lipid effects (Wada et al., 1987; Bodin et al., 2007). In order to minimize these effects mega- and macrofaunal (except polychaetes) low-lipid muscle tissue was used for stable isotope analysis. Muscle samples were taken from the dorsal musculature of fish, from the pereiopod 1 of the Brachyura, from the abdomen of the Natantia, Reptantia, Macrura, Stomatopoda and large Mysidacea and from the siphon of bivalves. Polychaete viscera were extracted by dissection and the analyses were carried out on the remaining whole-body, after removal of chetae. After dissection, the tissue samples of every taxa were washed very carefully with distilled water in order to prevent any contamination by sediment carbonates (Kharlmamenko et al., 2001; O'Reilly et al., 2002). All samples were stored frozen individually at − 20 °C before freeze-drying. Each dried sample was then ground to obtain a homogeneous powder, and 1 mg of this powder was weighed in tin capsules for stable carbon and nitrogen isotopic analyses. The 13C/12C and 15N/14N ratios of faunal tissue were determined

by continuous flow isotope ratio mass spectrometry (CF-IRMS) using a Europa Scientific ANCA-NT 20-20 Stable Isotope Analyser together with ANCA-NT Solid/ Liquid Preparation Module. As the samples contained more than 10% nitrogen, the CF-IRMS was operated in dual isotope mode, allowing δ15N and δ13C to be measured in the same sample. Analytical precision (standard deviation, n = 5) was 0.2‰ for both nitrogen and carbon, as estimated from standards analysed together with the samples. Stable isotope ratios were expressed in conventional δ notation as parts per mil (‰) according to the following equation: dX ¼ ½ð R sample=R standardÞ  1  1000

ð1Þ

where X is 13C or 15N and R is the corresponding 13 12 C/ C or 15 N/14N ratio, respectively. As δ15N values provide an indication of the trophic position of a consumer, the following formula was used to estimate trophic level: Trophic Level ¼ d15 Nconsumer  d15 Nmean


POM

ð2Þ

=3:4 þ 1

where 3.4‰ is the assumed 15N trophic enrichment factor according to Minagawa and Wada (1984). In a benthic ecosystem such as the Grande Vasière (depth N 100 m), no primary production can occur on the bottom; consequently, the only organic material available for benthic primary consumers is assumed to be the POM sedimenting from upper water layers and therefore this is designated as the first trophic level.

Table 1 Sediment characteristics of the sampling stations (mean, standard deviation is given in brackets)

Number of stations sampled in the area Water depth (m) Pelitic fraction (b63 µm) (%) Fine and medium sands (63–500 µm) (%) Median grain size (µm) Sediment type

A

B

C

D

E

3

2

3

3

3

98– 103 13.7 (2.4) 80.7 (8.5) 181 (31) FV

102– 110 32.5– 34.1 54.0– 66.0 109– 115 FV–SHV

111– 117 13.4 (3.1) 66.9 (20.0) 210 (39) FV

115– 127 12.8 (1.3) 82.4 (4.3) 172 (13) FV

138– 143 8.4 (1.1) 79.4 (1.8) 192 (8) SFB

Sediment type as defined in Chassé and Glémarec (1976), FV: fine muddy sands, SHV: heterogeneous muddy sands, SFB: biogenic fine sands.

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Table 2 Mean values for abundance, biomass and specific richness (number of species) parameters for the five sampling areas of the North Bay of Biscay A A

B

C

D

E

Stations sampled in the area

3

2

3

3

3

Abundance (ind m− 2) Biomass (mg AFDW m− 2) Specific richness per station Total species richness

263.7 (89.8) 690 (398) 52.3 (12.2) 84

162–213 1117–1163 30–38 50

236.0 (64.2) 1097 (182) 49.7 (14.2) 87

437.3 (116.5) 975 (303) 67.3 (9.1) 117

1027.3 (161.3) 2172 (598) 98.3 (4.5) 156

B A

B

C

D

E

Stations sampled in the area

1

3

2

3

2

Abundance (ind 1000 m− 2) Biomass (g AFDW 1000 m− 2) Specific richness per station

44.7 101.8 14

124.3 (33.3) 103.0 (15.0) 26.3 (4.2)

103.6–199.1 91.5–149.7 20–27

243.0 (57.7) 107.7 (22.9) 47.0 (12.8)

862.0–1550.6 185.0–327.5 84–87

Standard deviations are given between brackets. A) Macrofaunal (1–10 mm) parameters B) megafaunal (N10 mm) parameters.

According to the approach of Vander Zanden and Vadeboncoeur (2002) and Sherwood and Rose (2005), we use δ13C values as an indicator of food origin. The reliance on benthic affinity prey (RBAP) of upper consumer species was estimated using the formula: RBAP ¼ X þ d13 CConsumer = d13 CConsumer  100

II

 d13 CConsumer

I Benthic affinity baseline



I Pelagic affinity baseline

 d13 CConsumer




I Pelagic affinity baseline

ð3Þ

where δ13C Consumer I Benthic affinity baseline and δ13C Consumer 13 I Pelagic affinity baseline are the mean δ C of the benthic and pelagic primary consumer used as baseline. Here, zooplankton is considered as the pelagic primary con-

sumer baseline and the bivalve Nucula sulcata and the gastropod Scaphander lignarius as the benthic primary consumer baseline (see Table 4). δ13C Consumer II is the δ13C of the secondary consumer considered. As primary consumers are used as baseline, RBAP is calculated for consumers with TL higher than 2. Moreover, to take into account the δ13C trophic enrichment along the food wed, we used a trophic fractionation of 1‰ (DeNiro and Epstein, 1978) as described by the equation:
X ¼  TLConsumer II  TLConsumer I Benthic Affinity baseline  1 ð4Þ where TL Consumer I Benthic affinity baseline and TL Consumer II are respectively the Trophic Level of the benthic primary

Fig. 2. Multidimensional scaling ordination of square root transformed abundance data.

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Table 3 Percent dominance in terms of biomass, feeding strategy, spatial localization and taxonomic group for the major biomass species Species

Biomass dominance (%)

Feeding strategy

Spatial localization

Taxonomic group

Brissopsis lyrifera† Dasybranchus gajolae Callianassa subterranea Nephtys caeca Glycera rouxii Terebellides stroemi⁎ Aponuphis fauveli Labidoplax sp.

15.5 11.7 10.4 9.5 2.3 2.1 2.0 2.0

0 0 0 2.5 0.8 0.3 0 0

SSDF SSDF SF P P SDF SF SDF

Infauna Infauna Infauna Infauna Infauna Infauna Epifauna Infauna

1.8 1.4 1.4 1.3 1.3 1.3 1.2 1.1 1.1 1.0 0.9 0.8

0 1.9 0.9 0 0 0 0.1 0.1 0.2 1.3 0 0.9

P P S P SDF SF SDF SDF P SDF P SF

Infauna Infauna Supra-benthic Epifauna Infauna Infauna Infauna Epifauna Epifauna Infauna Epifauna Epifauna

0.8 0.7 0.6 0.5 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0 0 0 0.1 0 0 0 0 0 0 0 0 0 0 0 0

0.2 0 0 0.9 0 1.0 8.6 0.2 2.2 0 0.7 0 0.1 0.3 0 0.1 0.1 0 13.4 12.7 8.8 4.8 4.2 2.2 1.0 0.7 0.7 0.5 0.5 0.5 0.5 0.3 0.2 0.2

P P P P P SF SF P P P SF SF SF G P P P P SSDF SF SSDF SSDF P SSDF P P P P P P P SF P P

Epifauna Epifauna Demersal Infauna Epifauna Epifauna Epifauna Epifauna Infauna Epifauna Epifauna Epifauna Infauna Epifauna Epifauna Epifauna Epifauna Supra-benthic Infauna Epifauna Infauna Infauna Epifauna Epifauna Epifauna Epifauna Epifauna Epifauna Epifauna Epifauna Epifauna Epifauna Epifauna Epifauna

Echinodermata Echinoidea Annelida Polychaeta Crustacea Decapoda Annelida Polychaeta Annelida Polychaeta Annelida Polychaeta Annelida Polychaeta Echinodermata Holothurioidea Crustacea Decapoda Annelida Polychaeta Crustacea Isopoda Crustacean Decapoda Crustacea Decapoda Mollusca Pelecypoda Mollusca Pelecypoda Annelida Polychaeta Crustacea Decapoda Annelida Polychaeta Chordata Osteichtyes Echinodermata Ophiuroidea Chordata Osteichtyes Chordata Osteichtyes Chordata Osteichtyes Annelida Polychaeta Crustacea Decapoda Crustacea Amphipoda

Nephrops norvegicus Lumbrinereis impatiens⁎ Natatolana borealis Munida sarsi Alphaeus glaber Dosinia lupinus Nucula sulcata Owenia fusiformis† Liocarcinus depurator Ampharete grubei Arnoglossus laterna Amphiura filiformis Microchirus variegatus Enchelyopsus cimbrius Merluccius merluccius Goniada sp. Goneplax rhomboides Ampelisca spinipesnpd Bryozoa† Astropecten irregularis Lumbrinereis fragilis Solea vulgaris Turitella communis Magelona alleni Venus ovata Calliostoma granulatum Lesueuriogobius friseii Aponuphis tubicola Callionymus lyra⁎ Crangon allmani⁎ Sipunculus nudus† Ditrupa arietina Spatangus purpureus† Onchodesma steenstupi† Aponuphis bilineata Scaphander lignarius Eulalia sanguinea Arnoglossus imperialis Atelecyclus rotundatus Porania pulvillus Pagurus pridauxi Macropipus tuberculatus Lepidorhombus whiffiagonis Chlamys tigerina Callionymus maculatus Ebalia tuberosa

Grande Vasière

External margin

Echinodermata Asterides Annelida polychaeta Chordata Osteichtyes Mollusca Gastropoda Annelida polychaeta Mollusca Pelecypoda Mollusca Gastropoda Chordata Osteichtyes Annelida polychaeta Chordata Osteichtyes Crustacea Eumalacostraca Sipuncula Sipunculidea Annelida polychaeta Echinodermata Echinoidea Sipuncula Sipunculidea Annelida polychaeta Mollusca Gastropoda Annelida polychaeta Chordata Osteichtyes Crustacea Decapoda Echinodermata Asterides Crustacea Eumalacostraca Crustacea Decapoda Chordata Osteichtyes Mollusca Pelecypoda Chordata Osteichtyes Crustacea Decapoda

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Table 3 (continued ) Species

Anapagurus laevis Lophius piscatorius Total

Biomass dominance (%) Grande Vasière

External margin

0 0 76.4

0.1 0.1 80.4

Feeding strategy

Spatial localization

Taxonomic group

SF P

Epifauna Epifauna

Crustacea Eumalacostraca Chordata Osteichtyes

In bold species analysed for stable isotopes in this study, ⁎species analysed in a previous study (Le Loc'h and Hily, 2005), †species that were associated with a trophic group based on their feeding strategy. The feeding strategy is based on Pearson (1971), Gros and Hamon (1988), Bonsdorff and Pearson (1999): P: predator, SF: suspension feeder, SDF: surface deposit feeder, SSDF: sub-surface deposit feeder, G: grazer.

consumer baseline and secondary consumer (calculated with formula (2)). As they are time integrators of the primary producer isotopic variability, the primary consumers are often used as isotopic baseline (Vander Zanden and Vadeboncoeur, 2002; Post, 2002; Jennings and Warr, 2003). 2.5. Statistical analysis As all POM values follow a normal distribution according to the Shapiro–Wilk test (p b 0.05), a Student test was used to compare the differences in mean δ13C and δ15N between depth and season. In order to identify group of species, a hierarchical cluster analysis (Euclidian distance, Ward's minimum-variance method) was performed on stable nitrogen and carbon values (Davenport and Bax, 2002; Le Loc'h and Hily, 2005; Grall et al., 2006). Multidimensional scaling (MDS) was performed on fourth root transformed abundance data to identify the benthic communities. Similarities were calculated using the Bray–Curtis similarity index. 3. Results 3.1. Characteristics of the sampled stations The depth range was from 98 m (zone A) to 143 m (zone E) from coast to offshore. According to Chassé and Glémarec's (1976) biotope classification, the central Grande Vasière stations belong to fine muddy sands with a pelitic fraction ranging from 12 to 14% and a median grain size around 200 µm, with the exception of area B, with more heterogeneous sediment (pelitic fraction up to 34% and median grain size around 110 µm) (Table 1). Offshore, on the external margin, station E sediments are biogenic fine sands with a similar mean grain size (ca. 192 µm) to that in the Grande Vasière, but a lower pelitic content (b10%) and presence of a calcareous coarse fraction (shell fragments and Ditrupa tubes).

3.2. Communities description 3.2.1. Macrofauna Within the 14 stations sampled, a total of 7538 individuals were identified belonging to 188 species. Within the benthic communities, annelids were the most represented phylum with 81 species, followed by crustaceans (56 species), molluscs (32 species) and echinoderms (12 species). Macrofaunal abundances range from 162 (area B) to 437.3 ind m− 2 (area D) within the central Grande Vasière while they attain up to 1027.3 ind m− 2 for area E (Table 2A). The same pattern is observed for species richness per station with a minimum of 30 species in station B in contrast to a mean of 98.3 species on the external margin. Biomass is also higher on area E (2172 mg AFDW m− 2) in comparison to the Grande Vasière (690 to 1163 mg AFDW m− 2). 3.2.2. Megafauna Molluscs (48 species), crustaceans (36 species), fish (22 species) and echinoderms (13 species) were the dominating phyla among the 128 species collected with the 2-m beam trawl. Megafaunal densities and biomass were much lower than those of the macrofauna, with difference of 1 and 3 orders of magnitude for biomass and abundance respectively (Table 2B). As for macrofauna, megafauna exhibit differences in their distribution with the highest biomass, abundance and species richness on the external margin. 3.2.3. Community Results of the multidimensional scaling (MDS) based on the abundance of macro- and megafauna separated the Grande Vasière central stations from those of the external margin (Fig. 2). Thus, stations A, B, C and D were pooled and can be considered as part of a Grande Vasière central community. Within the 322 taxa identified, the 39 species that contributed most to the biomass accounted for 76.4% and 80.4% of the mean total biomass of the Grande Vasière and the

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Ditrupa sand communities respectively (Table 3). Among these, the deposit feeders Brissopsis lyrifera, Dasybranchus gajolae, Terebellides stroemi, Labidoplax sp., the suspension feeders Callianassa subterra-

nea and Aponuphis fauveli and the predators Nephtys caeca, Glycera rouxii and N. norvegicus represented 57.3% of the Grande Vasière community biomass. The Ditrupa sand community is characterized by the

Table 4 Stable nitrogen and stable carbon isotope values (mean, standard deviation is given in brackets) of the food web of the Grande Vasière and corresponding trophic level (TL) and reliance on benthic affinity prey (RBAP), n correspond to the number of individual isotopic values, with the exception of zooplankton and suprabenthos where n is the number of isotopic values for a pool of individuals Species POM (−30 m) spring 2002 POM (−30 m) end of summer 2002 ⁎ POM bottom spring 2002 POM bottom end of summer 2002 ⁎ Cnidarians Virgularia mirabilis Crustaceans Zooplankton ⁎ Suprabenthos ⁎ Callianassa subterranea Alpheus glaber Munida sarsi Nephrops norvegicus Goneplax rhomboides Liocarcinus depurator Meiosquilla desmaresti Natatolana borealis Echinoderms Holothurian Polychaetes Magelona alleni Aponuphis fauveli Ampharete grubei Aponuphis tubicola Nephtys caeca Dasybranchus gajolae Glycera rouxii Bivalves Chlamys septemradiata Pecten maximus Venus ovata Nucula sulcata Gastropods Calliostoma granulatum Pisces Merluccius merluccius (4–10 cm) Callionymus maculatus Arnoglossus laterna Merluccius merluccius (19–41 cm) Enchelyopsus cimbrius Lesueuriogobius friseii Lepidorhombus whiffiagonis Microchirus variegatus Scyliorhinus canalicula Lophius piscatorius Solea solea The RBAP were not calculated for TL ≤ 2.2. ⁎ From Le Loc'h and Hily (2005).

Code

n

C/N ratio

8 8 8 8

δ15N (‰)

TL

3.57 (0.80) 4.14 (1.04) 5.66 (0.40) 4.23 (0.93)

δ13C (‰)

RBAP

− 23.76 (0.70) − 23.73 (1.08) − 22.95 (0.60) − 22.93 (0.63)

– –

5.02

8.79

2.1

− 18.92

–

5.48 (0.20) 5.59 (0.71) 4.89 (0.24) 3.99 (0.05) 4.04 (0.12) 4.09 (0.08) 4.22 (0.14) 4.15 (0.13) 4.12 6.03

8.17 (0.74) 8.05 (0.59) 8.89 (0.75) 9.81 (0.50) 10.06 (0.70) 10.70 (0.58) 11.54 (0.68) 12.41 (0.62) 12.8 12.81

2.0 1.9 2.2 2.4 2.5 2.7 2.9 3.2 3.3 3.3

− 20.40 (0.82) − 19.93 (1.11) − 16.98 (0.27) − 16.03 (0.41) − 16.23 (0.53) − 16.02 (0.39) − 16.15 (0.29) − 15.99 (0.48) − 16.3 − 18.52

– – – 0.97 0.91 0.91 0.83 0.80 0.70 0.20

1

4.30

11.2

2.9

− 15.6

0.98

Mal Afa Agr Atu Nca Dga Gro

1 1 3 2 5 2 2

5.24 4.91 5.08 (0.65) 4.40 4.37 (0.22) 4.42 4.53

7.6 8.8 9.55 (0.51) 10.88 11.12 (0.95) 11.74 14.26

1.8 2.1 2.4 2.7 2.8 3.0 3.7

− 17.0 − 17.8 − 16.58 (0.69) − 16.93 − 16.21 (0.49) − 16.22 − 15.68

– – 0.86 0.69 0.84 0.71 0.75

Cse Pma Vov Nsu

2 1 2 5

4.24 4.07 4.93 4.57 (0.12)

5.15 5.3 6.85 9.34 (0.44)

1.1 1.1 1.6 2.3

− 16.85 − 16.3 − 17.36 − 16.03 (0.19)

– – – –

Cgr

1

4.09

11.7

3.0

− 17.5

0.50

Mme j Cma Ala Mme a Eci Lfr Lwh Mva Sca Lpi Sso

5 3 3 3 6 7 6 3 3 3 3

4.12 (0.04) 4.12 (0.06) 4.13 (0.04) 4.01 (0.02) 4.02 (0.02) 4.10 (0.05) 4.11 (0.08) 4.19 (0.03) 3.59 (0.26) 4.06 (0.02) 4.19 (0.09)

10.79 (0.57) 11.68 (0.30) 11.73 (0.32) 12.70 (1.32) 12.80 (0.71) 13.11 (0.75) 13.40 (0.78) 13.60 (0.26) 13.65 (0.88) 13.98 (0.66) 15.39 (1.53)

2.7 3.0 3.0 3.3 3.3 3.4 3.5 3.5 3.6 3.7 4.1

− 18.05 (0.35) − 16.41 (0.29) − 17.22 (0.09) − 17.04 (0.64) − 16.63 (0.18) − 16.67 (0.20) − 16.38 (0.38) − 16.32 (0.15) − 16.49 (0.38) − 16.93 (0.70) − 16.23 (0.35)

0.44 0.76 0.57 0.54 0.63 0.60 0.64 0.65 0.61 0.48 0.55

Vmi

2

Zpk Spb Csu Agl Msa Nno Grh Lde Mde Nbo

4 4 5 10 14 21 10 9 1 2

Hol

F. Le Loc'h et al. / Journal of Marine Systems 72 (2008) 17–34

25

3.3. Stable isotopes

seasonal δ15N variability, the δ15N mean POM used in the TL formula (Eq. (2)) is the bottom mean for both seasons (δ15N mean POM = 4.95‰). Only two POM samples were collected at both depths on the Ditrupa sands community at the end of summer. Therefore, the bottom POM δ15N mean was used in the TL equation for this community.

3.3.1. POM Significant differences were detected for the Grande Vasière POM isotopic ratios between depths in spring (p b 0.001 and p b 0.019 for δ15N and δ13C respectively, Student test), while no differences were found at the end of summer (p b 0.431 and p b 0.045 for δ15N and δ13C respectively, Student test) (Table 4). Moreover seasonal δ15N bottom POM variations occurred, with higher values in spring (p b 0.006, Student test), while no significant differences were detected for δ13C bottom POM values between the two seasons (p = 0.468, Student test). Therefore, to take into account the POM

3.3.2. TL and RBAP Forty-three species (32 and 18 species, for the Grande Vasière and the Ditrupa sands communities respectively) and pools of zooplankton and suprabenthos were analysed for stable isotopic ratios of the two benthic communities with 6 species belonging to both communities (Tables 4 and 5). On the North Bay of Biscay continental shelf, invertebrates show a wider range in stable isotope ratios than fish. The stable nitrogen isotope ratios varied considerably among the Grande Vasière benthic community from the suspension feeder Chlamys septemradiata (δ15 N = 5.2‰) to

deposit feeders Sipunculus nudus, Spatangus pupureus and Onchodesma steenstrupi, the suspension feeders Ditrupa arietina and bryozoa, predators Aponuphis bilineata and N. caeca and the grazer S. lignarius (57.2% of the biomass).

Table 5 Stable nitrogen and stable carbon isotope values (mean, standard deviation is given in brackets) of the food web of the Ditrupa sand community and corresponding trophic level (TL) and reliance on benthic affinity prey (RBAP), n correspond to the number of individual isotopic values, with the exception of zooplankton and suprabenthos where n is the number of isotopic values for a pool of individuals Species POM (−30 m) end of summer 2002 POM bottom end of summer 2002 Crustaceans Zooplankton Suprabenthos Anapagurus laevis Macropipus tuberculatus Galathea dispersa Pagurus pridauxi Atelecyclus rotundatus Ebalia tuberosa Echinoderms Holothurie Polychaetes Ditrupa arietina Aponuphis tubicola Aponuphis fauveli Nephtys caeca Goniada sp. Glycera rouxii Bivalves Chlamys tigerina Arcopella balaustina Gastropods Scaphander lignarius Calliostoma granulatum Pisces Arnoglossus imperialis

Code

n

C/N ratio

2 2

δ15N ‰

TL

δ13C ‰

3.40 5.00

1

− 22.00 − 23.25

RBAP

Zpk Spb Ala Mtu Gdi Ppr Aro Atu

1 1 1 3 3 6 3 1

5.74 6.40 5.59 4.28 (0.03) 4.40 (0.08) 4.24 (0.08) 4.13 (0.01) 4.25

9.8 10.0 8.3 10.26 (0.23) 10.57 (0.26) 10.70 (0.21) 11.21 (0.82) 12.5

2.4 2.5 2.0 2.6 2.6 2.7 2.8 3.2

− 21.6 − 21.6 − 16.5 − 16.98 (0.31) − 16.43 (0.47) − 16.06 (0.45) − 16.05 (0.14) − 16.9

– – – 0.74 0.81 0.87 0.84 0.64

Hol

2

4.56

10.29

2.6

− 16.24

0.87

Dar Atu Afa Nca Gon Gro

1 2 1 2 1 1

7.22 5.14 4.88 4.36 4.44 4.50

6.7 8.89 9.4 11.71 12.9 13.7

1.5 2.1 2.3 3.0 3.3 3.6

− 18.0 − 17.83 − 17.9 − 16.68 − 16.2 − 16.8

– – – 0.72 0.74 0.60

Cti Aba

1 1

4.30 4.53

7.9 9.3

1.8 2.3

− 16.7 − 16.6

– –

Sli Cgr

2 2

4.38 4.22

9.80 11.51

2.4 2.9

− 15.53 − 16.73

– 0.72

Aim

3

4.13 (0.01)

11.77 (0.22)

3.0

− 17.43 (0.20)

0.59

The RBAP were not calculated for TL ≤ 2.2.

26

F. Le Loc'h et al. / Journal of Marine Systems 72 (2008) 17–34

the carnivorous flatfish S. solea (δ 15 N = 15.4‰). While the range is narrower within the Ditrupa sand community than in the Grande Vasière community, varying from the suspension feeder D. arietina (δ15 N = 6.7‰) to the predator G. rouxii (δ15 N = 13.7‰). Thus, a continuum of four trophic levels, from POM (TL1) to S. solea (TL 4.1) or G. rouxii (TL 3.6) is detected in both benthic communities. Some TLs derived from bottom POM values seem surprising: some of the benthic suspension feeders such as the Pectinidae, have values inferior to 2, while other primary consumers as zooplankton and suprabenthos have a TL close to 2. Among primary consumers, δ13C values vary greatly, deposit feeders being 13C-enriched in comparison to zooplankton or suprabenthos. This large δ13C difference from pelagic (zooplankton δ13C = − 20.40‰ and − 21.55‰, for Grande Vasière and Ditrupa sands respectively) to benthic primary consumers (N. sulcata δ13C = − 16.03‰ and S. lignarius δ13 C = − 16.58‰) allows their use to estimate the Reliance on Benthic Affinity Prey (RBAP) of the upper consumers as illustrated by formula 3 (Tables 4 and 5). Thus, the

RBAP ranges from 0.20 for the scavenger swimming isopod Natatolana borealis to 0.98 for the deposit feeder Labidoplax sp. For both communities, almost all the benthic and demersal taxa have a RBAP greater than 0.50 with the exception of the M. merluccius juveniles (4–10 cm) (0.46) and the anglerfish L. piscatorius (0.50). 3.3.3. Identification of trophic groups According to the results of the cluster analysis performed on both communities' stable isotopic nitrogen and carbon ratios, species are grouped in eight units (Figs. 3 and 4). Comparison between the clustering results, the TL and the RBAP and the biological and ecological features, provides a useful basis for a description of the different feeding patterns observed. Within the Grande Vasière benthic and demersal community, suspension and deposit feeders have the lowest TL. They are divided into four feeding groups (Fig. 3). Zooplankton and suprabenthos, with TL 2 and δ 13 C around − 20‰, are clustered in the primary consumer, water column suspension feeders group (C1-WCSF). While benthic suspension feeders

Fig. 3. Distribution of carbon and nitrogen stable isotope ratios (mean ± standard deviation) among groups composing the Grande Vasière benthic food web. The groups of taxa (circled) are chosen from the result of the hierarchical cluster analysis, C1-WCSF: primary consumer-water column suspension feeders, C1-BSF: primary consumer-benthic suspension feeders, C1-DF: primary consumer-deposit feeders, C1-int: primary consumer intermediate, C2-BAF: secondary consumer-benthic affinity feeders, C2-PAF: secondary consumer-pelagic affinity feeders, C2-int: secondary consumer intermediate, C3: tertiary consumers, see Table 4 for label code.

F. Le Loc'h et al. / Journal of Marine Systems 72 (2008) 17–34

27

Fig. 4. Distribution of carbon and nitrogen stable isotope ratios (mean ± standard deviation) among groups composing the Ditrupa sand benthic food web. The groups of taxa (encircled) are chosen from the result of the hierarchical cluster analysis, C1-BSF mix: primary consumer-benthic suspension feeders, see Fig. 3 for circled group name and Table 5 for label code.

(C1-BSF) had the lowest TL, especially the Pectinidae (TL 1.1), with δ13 C values around − 17‰. Surface deposit feeders (C1-DF) such as N. sulcata occupied an intermediate TL around 2.5 with δ 13 C values close to those of the benthic suspension feeders. Another intermediate group (TL between 2.1 and 2.7) clusters hake juveniles, the Cnidarian Virgularia mirabilis and the Polychaeta Hyalinoecia fauveli. It is characterized by δ13 C values between − 19 and − 18‰. The upper consumers are also divided into four feeding groups. The δ13C allows discrimination between two groups with a TL close to 3, (1) the predators (N. norvegicus, N. caeca) and the sub-surface deposit feeders (D. gajolae) with a RBAP N 0.75 (C2-BAF) and (2) predator (Arnoglossus laterna), grazer (Calliostoma granulatum) and scavenger (N. borealis) with RBAP b 0.70 (C2-PAF). Most of the fish are clustered in the C2 intermediate group, with a TL around 3.5 and a RBAP between 0.48 and 0.65. Finally, S. solea and G. rouxii are included in the top-predator group (C3). Despite the lower number of species analysed for stable isotopes in the external margin benthic community, the same type of classification as for the Grande Vasière is applied. Eight feeding groups are also detected with the cluster analysis (Fig. 4). As for the Grande

Vasière, zooplankton and suprabenthos are isolated. Two groups of benthic suspension feeders are identified, the first one (C1-BSF) composed of the Pectinidae Chlamys tigerina and the crustacean Anapagurus laevis and the second (C1-BSF mix) with the only polychaete D. arietina. The polychaetes Hyalinoecia tubicola and H. fauveli are included in a primary consumer intermediate group (C1-int) with a TL around 2.2 and a δ13C value of − 17.8‰. The bivalve Arcopella balaustina and the gastropod S. lignarius constitute the primary consumers deposit feeders group (C1-DF). The upper consumers are divided into three groups, the first one clusters carnivorous species with a largely pelagic food source with TLs close to 3 (Arnoglossus imperialis, N. caeca), the second one includes the benthic affinity secondary consumers (C2-BAF) and the third one the upper consumers with TL N 3.2 (G. rouxii, Goniada sp. and Ebalia tuberosa). 3.3.4. Food web structure In order to elucidate the trophic pathways of these complex benthic systems, we have associated the biomass of each species with the trophic group in which they have been included based on stable isotopic analyses (Table 6). A more accurate trophic

28

F. Le Loc'h et al. / Journal of Marine Systems 72 (2008) 17–34

Table 6 Correspondence between biomass structure of the macro and megafaunal benthic communities (percentage) and trophic guilds (trophic level and reliance on benthic affinity prey) identified by stable isotopes analysis (mean ± standard deviation) Trophic guild

Primary consumers Benthic suspension feeders Surface deposit feeders Intermediate Secondary consumers Benthic affinity feeders Pelagic affinity feeders Intermediate Tertiary consumers Top carnivorous Total

Grande Vasière

External margin

Biomass contribution (%)

Trophic level

Reliance on benthic affinity prey

Biomass contribution (%)

Trophic level

Reliance on benthic affinity prey

2.2 16.3 2.5

1.7 (0.1) 2.4 (0.1) 2.4 (0.4)

– 0.91 (0.06) 0.69

14.2 3.7 8.7

1.8 (0.3) 2.4 2.5

– – –

41.5 2.5 7.4

2.9 (0.1) 3.1 (0.2) 3.4 (0.2)

0.84 (0.10) 0.42 (0.20) 0.62 (0.10)

33.6 1.9 4.4

2.8 (0.2) 3.0 3.3

0.91 (0.08) 0.66 0.69

2.6 75.0

3.9

0.65

0.8 67.3

3.6

0.60

Reliance on benthic affinity prey was not calculated for mean trophic level ≤2.5.

structure was also obtained using recent data from a study on the Grande Vasière (Le Loc'h and Hily, 2005). Moreover, species with well known feeding strategy were added to the trophic structure (see Table 3). For example, the urchins B. lyrifera and Spatangus purpureus which represent an important biomass were not analysed here because of their high lipid content which influences the stable isotopic ratios (Pinnegar and Polunin, 1999; Sotiropoulos et al., 2004; Bodin et al., 2007). However since they are sub-surface deposit feeders, their biomass was added to the C2-BAF group which clusters all the sub-surface deposit feeders analysed. Thus 75 and 67.3% of the total benthic macrofaunal biomass of the Grande Vasière and the External Margin community respectively, were taken into account to establish the trophic structure; the species not retained belong mainly to small size macrofauna. 4. Discussion 4.1. Community structure Statistical analysis of the grab and beam trawl data separates two distinct benthic communities: the muddy sand community, dominated by the carnivorous polychaetes N. caeca, G. rouxii, the deposit feeders T. stroemi, D. gajolae and the crustacean C. subterranea on one hand, and the external margin fine sands community, dominated by the suspension feeder D. arietina on the other hand. These two main communities were previously identified by Le Danois (1948) and described by Glémarec (1969), although the present

study is the first to provide quantitative data from these bottoms in terms of abundance and biomass. The Ditrupa sands show the highest values for abundance, biomass as well as for species richness. The presence of shell fragments within the biogenic fine sands of the external margin (i.e. dead Ditrupa tubes) created complex habitat structure. High habitat heterogeneity and complexity are directly related to highest diversity (Le Hir and Hily, 2005) and provide microhabitats for macrofaunal organisms (Mc Coy and Bell, 1991). Thus, in contrast to the homogenous muddy sand habitat of the Grande Vasière, the structurally complex habitat of the external margin could explain the higher biomass and biodiversity observed at this site. 4.2. Food web source Offshore, the continental shelf benthic communities are coupled to pelagic primary and secondary production through the sedimentation of organic material (Rowe, 1971; Hargrave, 1973; Graf, 1992; Marcus and Boero, 1998). The isotopic signatures of the North Bay of Biscay POM are in the same range than those of other continental shelves (South Bay of Biscay: Fontugne and Jouanneau, 1987, George Bank: Fry, 1988, Southeastern Australia: Davenport and Bax, 2002). The North Bay of Biscay benthic communities are supplied by the sedimentation of the pelagic primary production associated with the Loire and Vilaine river plumes (Lampert et al., 2002; Loyer et al., 2006). On the Grande Vasière, at the end of summer, there was no difference between surface layer and bottom POM stable isotopic composition, which indicates that the sedimentation occurs

F. Le Loc'h et al. / Journal of Marine Systems 72 (2008) 17–34

without qualitative transformation. In contrast, in spring, bottom POM stable isotopic ratios are significantly enriched in 13C and 15N in comparison to POM collected in the surface layer. These differences may be explained by different biochemical or/and phytoplankton species composition. Indeed, the North Bay of Biscay continental shelf is characterized by a seasonal succession of phytoplankton communities, associated with depth segregation (Lunven et al., 2005). Stable isotope fractionation varies both within groups and between groups of marine phytoplankton (Needoba et al., 2003) as well as phytoplankton size classes (Bode et al., 2004). In our study site, near the sea surface, POM collected in the chlorophyll a maximum is mainly composed of phytoplankton cells and sedimenting phytodetritus and other organic material originating from recent pelagic production (Lunven et al., 2005). Around 100 m depth, bottom POM is constituted of a mix of resuspension of sedimented organic matter, benthic organic matter secondary production and degradation of organic matter from both of these sources. Despite this, the lowest pigment concentrations were measured in bottom water at the end of summer, the Chlorophyll a/Phaeopigment ratios were the highest, indicating the presence of less degraded phytoplankton at that period (Le Loc'h, 2004). Moreover, due to different physical forcing (tidal currents and wind) resuspension was more active during the spring sampling cruise than at the end of summer leading to the highest sedimentation in the moored bottom sediment trap (Le Loc'h, 2004). Therefore, phytodetritus and other organic material are subject to bacterial degradation and other physico-chemical processes during sinking and resuspension processes, altering isotopic composition (Macko and Estep, 1984; Lehmann et al., 2002). These processes lead to temporal variability of the available organic matter (quality and quantity) which supply the benthic communities. 4.3. Trophic level and reliance on benthic affinity prey According to Minagawa and Wada (1984) and Le Loc'h and Hily (2005), a trophic enrichment factor of 3.4 was used to calculate the TL based on bottom POM. Although this factor can vary according to the taxonomic groups and the feeding types in a given ecosystem (Michener and Schell, 1994; Post, 2002; Vanderklift and Ponsard, 2003), its range of variation remains low (3 to 4‰) and can be widely applicable in aquatic food web studies (Post, 2002). In the Grande Vasière as well as in the Ditrupa sand communities, benthic invertebrates extend over a

29

continuum of almost three different trophic levels. This finding appears to be a general feature of temperate subtidal benthic ecosystems (Davenport and Bax, 2002; Grall et al., 2006; Bodin et al., 2008). The large range of δ13C ratios within primary consumers is also a general trend off continental shelf communities (Hobson et al., 2002; Sherwood and Rose, 2005). Zooplankton and suprabenthos feeding on POM, as attested by their isotopic signature, are depleted in 13C in comparison to the benthic deposit feeders that ingest sedimented organic matter. Because of the degradation and microbial recycling occurring in the bottom nepheloid layer, it has been suggested that during these processes deposited organic matter could be enriched in 13C (Goering et al., 1990; Hobson et al., 1995). δ13C clearly discriminates between different sources of organic matter, allowing tracking of fluxes of matter along the food chain up to the higher trophic levels. Thus, the δ13C range of variation within the primary consumers allows the distinction between pelagic and benthic affinity components (Le Loc'h and Hily, 2005). Since their turnover and lifespan are much longer than those of their prey, primary consumers integrate the temporal isotopic variation of their food sources (Post, 2002). Thus, they can be used as an isotopic baseline to calculate the reliance of the upper consumers on the benthic affinity component (Vander Zanden and Vadeboncoeur, 2002; Sherwood and Rose, 2005). 4.4. Trophic groups The feeding behaviors of different species are important elements in the complex relationships that emerge from stable isotope analyses, especially in benthic invertebrate communities where a wide range of isotopic signatures and feeding modes are present (Davenport and Bax, 2002). The clustering of species in trophic guilds provides synthetic information to understand the trophic structure of the community. In our study, both due to sampling and diversity of feeding modes within the primary consumers, some trophic groups are comprised of only few species or/and individuals. This low number of analyses may limit the significance of some of the clustering. Nevertheless, the large range of the isotopic signatures allows discrimination between the food sources of different species as the variability of isotopic signatures within species is much lower than the variability among species. Thus, suspension feeders are divided into three functional groups according to their δ13C values, revealing a wide diet spectrum. The first group (C1-WCSF, TL 2) clusters zooplankton and suprabenthos (mainly Euphausiids) that

30

F. Le Loc'h et al. / Journal of Marine Systems 72 (2008) 17–34

feed in the water column with δ13C values indicating a diet based on POM. The second group groups polychaetes, bivalves and crustaceans (C1-BSF). It shows highly enriched δ13C signatures close to those measured for the surface-selective deposit feeders (C1-DF), suggesting that some benthic suspension feeders and surface deposit feeders exploit the same food source. Since no primary production can occur on the sediment surface at a mean depth of 100 m on the continental shelf, we must consider that bottom organic matter is a mixture of different components, originating either from recently sedimented pelagic production more or less enriched by the bacterial loop or from benthic detrital secondary production. Thus, the diet of the C1-DF and C1-BSF groups is composed of this mixture of detrital material mainly originating from pelagic sedimented organic matter. The differences between species of C1-DF and C1-BSF groups are mainly attributed to differences in feeding mode and anatomy (i.e. suspension feeders capture particles in suspension in the interface water whereas selective deposit feeders ingest the interface superficial film). Finally, the third suspension feeder group (C1-int) had intermediate δ13C values suggesting a mixed diet based on both POM and detrital organic matter. Surprisingly close δ15N values between POM and some benthic suspension feeders are measured in the Grande Vasière as well as in the Ditrupa sand communities. In particular the Pectinidae bivalves have very low TL values (TL 1.1 for C. septemradiata and Pecten maximus), close to those observed for the polychaete D. arietina (TL 1.5). Similar results were found on continental shelf communities for benthic suspension feeders and POM on the Georges Bank (Fry, 1988) and on the Arctic Canadian Basin (Iken et al., 2005). According to these authors, such low TL cannot be fully explained by fractionation patterns (Iken et al., 2005). Some suspension feeders may not assimilate bulk bottom POM nor bulk detrital organic matter but they may base their diet on particles selected on the basis of size and/or biochemical quality, as revealed by their isotopic composition (Rau et al., 1990; Raikow and Hamilton, 2001). Furthermore, the diets of some suspension feeding bivalves such as mussels or cockles are not only based on suspended organic particles (i.e. phytoplankton, resuspended deposited material, microheterotrophs and bacteria, Kang et al., 1999; Kreeger and Newell, 1996, 2001), but also on dissolved organic carbon (Roditi et al., 2000). These low δ15N values within some benthic suspension feeders reveal the complex pathways at the base of the continental shelf benthic food webs and highlight the capacity of the primary consumers to consume different food sources,

which favor a high species diversity within the suspension feeders. Spatial/vertical segregation of the food quality is also revealed by the three suspension feeder groups: (1) a group that feed on recently produced material in the water column (2) an intermediate group composed of erect epifauna which can feed on both sources and (3) the detrital benthic layer feeders. Such spatial segregation within the benthic suspension feeder groups has already been shown (Le Loc'h and Hily, 2005; Grall et al., 2006) revealing the diversity of the feeding strategies and the specific adaptations to the various food sources available. Therefore, a species trophic position must be carefully considered in energy budget analyses and community functional analyses, as behavioral and anatomical organs features are not sufficient to place a species in a specific trophic group. The third trophic level of the food web (i.e. the secondary consumers: predators) are also divided into three groups based on their RBAP which revealed different feeding strategies in relation to their spatial localization on the bottom (i.e. endo- or epifauna). Indeed, the first group (C2-PAF) is composed of a mix of species with different feeding strategies: from the swimming, opportunistic scavenger N. borealis to the epi-benthic, omnivorous polychaete Aponuphis tubicola and the benthic carnivorous fish A. laterna. All of them are able to capture pelagic prey. In contrast, the second group (C2-BAF) clusters both sub-surface deposit feeders and endo-benthic or burrowing predators that capture their prey at the surface or in the sediment. Sub-surface deposit feeders such as the polychaete D. gajolae ingest detrital organic matter, micro- and meio-fauna (Fauchald and Jumars, 1979) while the predators of this functional group such as the crustacean N. norvegicus capture benthic macrofauna (Cristo, 1998). The last secondary consumer group includes the majority of the benthic and demersal fishes and carnivorous crustaceans and polychaetes. It is considered as an intermediate group (C2-int) because the species which are regrouped have RBAP values intermediate between those of the two others groups and a slightly higher TL. This trophic position highlights the opportunities these predators have to catch either benthic or pelagic prey. Finally, the top predators, the benthic fish S. solea and the polychaete G. rouxii, occupy the fourth TL on the Grande Vasière. These nitrogen isotopic signatures are even higher than those of other marine predators such as dolphins of the Bay of Biscay (Das et al., 2000), but lower than those of the tope shark collected in the adjacent Celtic Sea (Domi et al., 2005).

F. Le Loc'h et al. / Journal of Marine Systems 72 (2008) 17–34

This highest trophic level occupied by a benthic invertebrate such as a polychaete appears as a general feature within benthic continental shelf communities (Davenport and Bax, 2002; Hobson et al., 2002; Sherwood and Rose, 2005). Very similar trophic groups were identified from the Grande Vasière and the Ditrupa sand benthic communities, with only slight differences mainly concerning the suspension feeders. Indeed, the water column suspension feeders sampled on the Ditrupa sand community had a higher TL than on the Grande Vasière. These differences may be partly explained by the hydrological features of each study site. As for the benthic communities, the Grande Vasière and the external margin represent two different hydrological patterns over the Bay of Biscay which influence the rate and the nature of the pelagic biological production (Planque et al., 2004; Puillat et al., 2004). Thus, the isotopic seasonal variability of the primary producers is probably associated with the time lag required for a consumer to reflect the isotopic signature of its diet, (Vizzini and Mazzola, 2003; Matthews and Mazunder, 2005) and may explain the zooplankton TL 2.4 (calculated from POM isotopic signature). Indeed, our POM isotopic sampling did not allow integration of the temporal variability of the POM isotopic composition, in contrast to that of the consumers. 4.5. Food web structure Our results show that macrofaunal benthic primary consumers are the second major trophic group in terms of biomass, with a domination of the surface deposit feeders on the Grande Vasière, while the suspension feeders dominate on the external margin. Taking into account the higher benthic biomass on the external margin, we hypothesize that this different trophic organization between the two benthic communities reveals the existence of an additional trophic input on the external margin both in term of quantity and seasonality. Indeed, on the continental shelf break, internal tidal waves could induce a vertical flux of nutrients towards the surface layers resulting in phytoplankton production (Holligan and Groom, 1986). In the Bay of Biscay, this shelf break, surface primary production has already been detected (Lampert et al., 2002) and higher organic matter sedimentation was measured on the external margin than on the Grande Vasière (Le Loc'h, 2004). Because suspension feeder biomass is 15 times higher on the external margin, we suggest that in addition to sedimentation of the continental shelf primary production, the primary production generated

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at the shelf break supplies the benthic community of the external margin. Furthermore, the Grande Vasière site received its maximum trophic inputs via sedimentation of the pelagic primary production from March to June, while they are minimum in summer when stratification is established and in winter when primary production is weak (Loyer et al., 2006) thus affecting the benthic food web structure of the Grande Vasière (Le Loc'h, 2004). The secondary consumers of benthic affinity, mainly dominated by the sub-surface deposit feeders, occupied the major trophic position within the macrobenthic biomass of both communities. In contrast, the secondary consumers of pelagic affinity feeders are scarce which reveals the importance of the detrital pathways for offshore continental shelf benthic communities. This contrasts with coastal benthic ecosystems where the majority of the benthic biomass is trapped in the filter feeders group supplied by both pelagic and benthic primary production (Grall et al., 2006). Predators of the benthic community feed to an increasing degree in the pelagic zone as they increase in their trophic position. This could be a general trend of the continental shelf benthic and demersal ecosystems and this could be linked to the higher degree of mobility of the top predators. This is well know in the Bay of Biscay for top-predator fish such as monkfish or hake that have a piscivorous diet composed of demersal and pelagic fishes (Pereda and Olaso, 1990; Guichet, 1995), but was not previously shown for benthic invertebrates and for the whole community. Thus it can be concluded that trophic coupling between benthic and pelagic food webs exists either through the consumption of pelagic primary production by benthic suspension feeders and through the ingestion of pelagic affinity prey by benthic top predators. 5. Conclusions This study demonstrated that measurements of stable isotopes (carbon and nitrogen) constitute an additional means of understanding the trophic structure of an ecosystem and tracing its energy flow. In conclusion, our results provide a novel contribution to the knowledge of the role of detrital pathways in the carbon cycling within the benthic trophic web of the continental shelf. Acknowledgments This study was funded by the CNRS, University of Brest and IFREMER. The authors are grateful to the crews of the RV Côtes de la Manche and Thalassa for

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their assistance in collecting samples, to C. Scrimgeour and N. Naulet for running samples, Alphonse Tram, Daniel Guichard, Albert Quentin and Robert Marc. The manuscript was greatly improved by the comments of anonymous reviewers. References Bode, A., Alvarez-Ossorio, M.T., Carrera, P., Lorenzo, J., 2004. Reconstruction of trophic pathways between plankton and the North Iberian sardine (Sardina pilchardus) using stable isotopes. Sci. Mar. 68 (1), 165–178. Bodin, N., Le Loc'h, F., Hily, C., 2007. Effects of lipid removal on carbon and nitrogen stable isotope ratios in crustaceans tissues. J. Exp. Mar. Biol. Ecol. 341, 168–175. Bodin, N., Le Loc'h, F., Caisey, X., Le Guellec, A.-M., Abarnou, A., Loizeau, V., Latrouite, D., 2008. Polychlorinated biphenyls and stable isotopes in the spider crab food web. Env. Poll. 151, 252–261. Bonsdorff, E., Pearson, T., 1999. Variation in the sublittoral macrozoobenthos of the Baltic seas: a functional group approach. Aust. J. Ecol. 24, 312–326. Carabel, S., Godinez-Dominguez, E., Verisimo, P., Fernandez, L., Freire, J., 2006. An assessment of sample processing methods for stable isotope analyses of marine food webs. J. Exp. Mar. Biol. Ecol. 336, 254–261. Chassé, C., Glémarec, M., 1976. Principes généraux de la classification des fonds pour la cartographie sédimentaire. J. Rech. Océanogr. 1 (3), 1–11. Cristo, M., 1998. Feeding ecology of Nephrops norvegicus (Decapoda: Nephropidae). J. Nat. Hist. 32 (10–11), 1493–1498. Das, K., Lepoint, G., Loizeau, V., Debacker, V., Dauby, P., Bouquegneau, J.M., 2000. Tuna and dolphin associations in the North-east Atlantic: evidence of different ecological niches from stable isotope and heavy metal measurements. Mar. Pollut. Bull. 40 (2), 102–109. Davenport, S.R., Bax, N.J., 2002. A trophic study of a marine ecosystem of Southeastern Australia using stable isotopes of carbon and nitrogen. Can. J. Fish. Aquat. Sci. 59, 514–530. DeNiro, M.J., Epstein, S., 1977. Mechanism of carbon isotope fractionation associated with lipid synthesis. Science 197, 261–263. DeNiro, M.J., Epstein, S., 1978. Influence of diet on the distribution of carbon isotopes in animals. Geochim. Cosmochim. Acta 42, 495–506. DeNiro, M.J., Epstein, S., 1981. Influence of the diet on the distribution of nitrogen isotopes in animals. Geochim. Cosmochim. Acta 45, 341–351. Domi, N., Bouquegneau, J.M., Das, K., 2005. Feeding ecology of five commercial shark species of the Celtic Sea through stable isotope and trace metal analysis. Mar. Environ. Res. 60, 551–569. Ellis, J.R., Rogers, S.I., Freeman, S.M., 2000. Demersal assemblages in the Irish Sea, St George's Channel and Bristol Channel. Estuar. Coast. Shelf Sci. 51, 299–315. Fauchald, K., Jumars, P.A., 1979. The diet of worms: a study of polychaete feeding guilds. Oceanogr. Mar. Biol. Ann. Rev. 17, 193–284. Fontugne, M., Jouanneau, J.-M., 1987. Modulation of the particulate organic carbon flux to the ocean by a macrotidal estuary: evidence by measurements of carbone isotopes in organic matter from the Gironde System. Estuar. Coast. Shelf Sci. 24, 377–387. Frauenheim, K., Neumann, V., Theil, H., Turkay, M., 1989. The distribution of the larger epifauna during summer and winter in the North Sea and its suitability for environmental monitoring. Senckenb. Marit. 20, 101–118.

Fry, B., 1988. Food web structure on Georges Bank from stable C, N and S isotopic compositions. Limnol. Oceanogr. 33 (5), 1182–1190. Glémarec, M., 1969. Les peuplements benthiques du plateau continental Nord-Gascogne, Ph.D. Thesis, Paris, 167 pp. Glémarec, M., 1971. L'endofaune du plateau continental Nord-Gascogne étude des facteurs écologiques. Vie et Milieu 22, 93–108. Goering, J., Alexander, V., Haubenstock, N., 1990. Seasonal variability of stable carbon and nitrogen isotope ratios of organisms in a North Pacific Bay. Estuar. Coast. Shelf Sci. 30, 239–260. Graf, G., 1992. Benthic–pelagic coupling: a benthic view. Oceanogr. Mar. Biol. Ann. Rev. 30, 149–190. Grall, J., Le Loc'h, F., Guyonnet, B., Riera, P., 2006. Community structure and food web based on stable isotopes (δ15N and δ13C) analysis of the North Eastern Atlantic maerl bed. J. Exp. Mar. Biol. Ecol. 338, 1–15. Griffiths, H., 1991. Applications of stable isotope technology in physiological ecology. Funct. Ecol. 5 (2), 254–269. Gros, P., Hamon, D., 1988. Typologie biosédimentaire de la baie de Saint-Brieuc (Manche ouest), et estimation de la biomasse des catégories trophiques macrozoobenthiques. Rap. IFREMER, DERO-88-27 EL, Euphorbe. 153 pp. Guichet, R., 1995. The diet of European hake (Merluccius merluccius) in the northern part of the Bay of Biscay. ICES J. Mar. Sci. 52, 21–31. Hargrave, B.T., 1973. Coupling carbon flow through some pelagic and benthic communities. J. Fish. Res. Board Can. 30, 1317–1326. Hobson, K.A., Ambrose, W.G.J., Renaud, P.E., 1995. Sources of primary production, benthic–pelagic coupling, and trophic relationships within the Northeast Water Polynya: insight from δ13C and δ15N analysis. Mar. Ecol. Prog. Ser. 128, 1–10. Hobson, K.A., Fisk, A., Karnovsky, N., Holst, M., Gagnon, J.M., Fortier, M., 2002. A stable isotope (δ13C, δ15N) model for the North Water food web: implications for evaluating trophodynamics and the flow of energy and contaminants. Deep-Sea Res., Part 2 49, 5131–5150. Holligan, P.M., Groom, S.B., 1986. Phytoplankton distributions along the shelf break. Proc. R. Soc. Edinb. 88B, 239–263. Iken, K., Bluhm, B.A., Gradinger, R., 2005. Food web structure in the high Arctic Canada Basin: evidence from δ13C and δ15N analysis. Polar Biol. 28, 238–249. Jennings, S., Warr, K.J., 2003. Environmental correlates of large-scale spatial variation in the δ15N of marine animals. Mar. Biol. 142, 1131–1140. Jennings, S., Lancaster, J., Woolmer, A., Cotter, J., 1999. Distribution, diversity and abundance of epibenthic fauna in the North Sea. J. Mar. Biol. Assoc. U.K. 79, 385–399. Kaiser, M.J., Rogers, S.I., MacCandless, D.T., 1994. Improving quantitative surveys of epibenthic communities using a modified 2 m beam trawl. Mar. Ecol. Prog. Ser. 106, 131–138. Kang, C.K., Sauriau, P.-G., Richard, P., Blanchard, G.F., 1999. Food sources of the infaunal suspension-feeding bivalve Cerastoderma edule in a muddy sandflat of Marennes-Oléron Bay, as determined by analyses of carbon and nitrogen stable isotopes. Mar. Ecol. Prog. Ser. 187, 147–158. Kharlmamenko, V.I., Kiyashko, S.I., Imbs, A.B., Vyshkvartev, D.I., 2001. Identification of food sources of invertebrates from the seegrass Zostera marina community using carbon and sulfur stable isotope ratio and fatty acid analyses. Mar. Ecol. Prog. Ser. 220, 103–117. Kreeger, D.A., Newell, R.I.E., 1996. Ingestion and assimilation of carbon from cellulolytic bacteria and heterotrophic flagellates by the mussels Geukensia demissa and Mytilus edulis (Bivalvia, Mollusca). Aquat. Microb. Ecol. 11, 205–214.

F. Le Loc'h et al. / Journal of Marine Systems 72 (2008) 17–34 Kreeger, D.A., Newell, R.I.E., 2001. Seasonal utilization of different seston carbon sources by the ribbed mussel, Geukensia demissa (Dillwyn) in a mid-Atlantic salt marsh. J. Exp. Mar. Biol. Ecol. 260, 71–91. Lampert, L., Quéguiner, B., Labasque, T., Pichon, A., Lebreton, N., 2002. Spatial variability of phytoplankton composition and biomass on the eastern continental shelf of the Bay of Biscay (North-East Atlantic Ocean). Evidence for a bloom of Emiliana huxleyi (Prymnesiophycea) in spring 1998. Cont. Shelf Res., 22(8), pp. 1225–1247. Le Danois, E., 1948. Les profondeurs de la mer. Payot, Paris. 303 pp. Le Hir, M., Hily, C., 2005. Macrofaunal diversity and habitat structure in intertidal boulder fields. Biodivers. Conserv. 14, 233–250. Lehmann, M.F., Bernasconi, S.M., Barbieri, A., McKenzie, J., 2002. Preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis. Geochim. Cosmochim. Acta 66 (20), 3573–3584. Le Loc'h, F., 2004. Structure, fonctionnement et évolution des communautés benthiques des fonds meubles exploités du plateau continental Nord Gascogne. Ph.D Thesis, Brest, 326 pp. Available online at: http://tel.archives-ouvertes.fr/tel-00009359/. Le Loc'h, F., Hily, C., 2005. Stable carbon and nitrogen isotope analysis of Nephrops norvegicus/Merluccius merluccius fishing grounds in the Bay of Biscay (NE Atlantic). Can. J. Fish. Aquat. Sci. 62, 123–132. Lorrain, A., Savoye, N., Chauvaud, L., Paulet, Y.-M., Naulet, N., 2003. Decarbonatation and preservation method for the analysis of organic C and N contents and stable isotope ratios of low-carbonated suspended particulate matter. Anal. Chim. Acta 49, 125–133. Loyer, S., Lampert, L., Ménesguen, A., Cann, P., Labasque, T., 2006. Seasonal evolution of the nutriment pattern on Biscay Bay continental shelf over the years 1999–2000. Sci. Mar. 70 (1), 31–46. Lunven, M., Guillaud, J.-F., Youénou, A., Crassous, M.-P., Berric, R., Le Gall, E., Kérouel, R., Labry, C., Aminot, A., 2005. Nutriment and phytoplankton distribution in the Loire plume (Bay of Biscay, France) resolved by a new Fine Scale Sampler. Estuar. Coast. Shelf Sci. 65 (1–2), 94–108. Macko, S.A., Estep, M.L.F., 1984. Microbial alteration of stable nitrogen and carbon isotopic compositions of organic matter. Org. Geochem. 6, 787–790. Marcus, N.H., Boero, F., 1998. Minireview: the importance of the benthic–pelagic coupling and the forgotten role of life cycles in coastal aquatic systems. Limnol. Oceanogr. 43 (5), 763–768. Matthews, B., Mazunder, A., 2005. Temporal variation in body composition (C:N) helps explain seasonal patterns of zooplankton δ13C. Freshw. Biol. 50, 502–515. Mc Coy, E.D., Bell, S.S., 1991. Habitat structure and diversification of a complex topic. In: Shapmann, et al. (Ed.), Habitat Structure: The Physical Arrangement of Objects in Space, pp. 3–27. Mc Cutchan, J.H., Lewis Jr., W.M., Kendall, C., McGrath, C.C., 2003. Variation in trophic shift for stable isotope ratios of carbon, nitrogen, and sulfur. Oikos 102, 378–390. Michener, R.H., Schell, D.M., 1994. Stable isotope ratios as tracers in marine and aquatic food web. In: Lajtha, K., Michener, R.H. (Eds.), Stable Isotopes in Ecology and Environmental Science. Blackwell Scientific Publication, pp. 138–157. Minagawa, M., Wada, E., 1984. Stepwise enrichment of 15N along food chains: further evidence and the relation between 15N and animal age. Geochim. Cosmochim. Acta 48, 1135–1140. Needoba, J.A., Waser, N.A., Harrison, P.J., Calvert, S.E., 2003. Nitrogen isotope fractionation in 12 species of marine phyto-

33

plankton during growth on nitrate. Mar. Ecol. Prog. Ser. 255, 81–91. O'Reilly, C.M., Hecky, R.E., Cohen, A.S., Plisnier, P.-D., 2002. Interpreting stable isotopes in food web: recognizing the role of time averaging at different trophic levels. Limnol. Oceanogr. 47 (1), 306–309. Pearson, T.H., 1971. Studies on the ecology of the macrobenthic fauna of lochs Linnhe and Eil, west coast of Scotland. Vie et Milieu 22 (1), 53–91. Pereda, P., Olaso, I., 1990. Feeding of Hake and Monkfish in the nontrawlable area of the shelf of the Cantabrian Sea. ICES C.M. 1990/G, 45, pp. 1–10. Peterson, B.J., Fry, B., 1987. Stable isotopes in ecosystem studies. Ann. Rev. Ecolog. Syst. 18, 293–320. Pinnegar, J.K., Polunin, N.V.C., 2000. Contributions of stable-isotope data to elucidating food webs of mediterranean rocky littoral fishes. Oecologia 122, 399–409. Pinnegar, J.K., Polunin, N.V.C., 1999. Differential fractionation of δ13C and δ15N among fish tissues: implications for the study of trophic interactions. Funct. Ecol. 13, 225–231. Planque, B., Lazure, P., Jégou, A.-M., 2004. Detecting hydrological landscapes over the Bay of Biscay continental shelf in spring. Clim. Res. 28, 41–52. Post, D.M., 2002. Using stable isotopes to estimate trophic position: models, methods, and assumptions. Ecology 83 (3), 703–718. Poulard, J.-C., Blanchard, F., 2005. The impact of climate change on the fish community structure of the eastern continental shelf of the Bay of Biscay. ICES J. Mar. Sci. 62, 1436–1443. Poulard, J.-C., Blanchard, F., Boucher, J., Souissi, S., 2003. Variability in the demersal fish assemblages of the Bay of Biscay during the 1990s. ICES Mar. Sci. Symp. 219, 411–414. Puillat, I., Lazure, P., Jégou, A.-M., Lampert, L., Miller, P.I., 2004. Hydrographical variability on the French continental shelf in the Bay of Biscay, during the 1990s. Cont. Shelf Res. 24, 1143–1163. Raikow, D.F., Hamilton, S.K., 2001. Bivalve diets in a Midwestern U.S. stream: a stable isotope enrichment study. Limnol. Oceanogr. 46 (3), 514–522. Rau, G.H., Teyssie, J.L., Rassoulzadegan, F., Fowler, S.W., 1990. 13 12 C/ C and 15N/14N variations among size-fractionated marine particles: implications for their origin and trophic relationships. Mar. Ecol. Prog. Ser. 59, 33–38. Riera, P., Montagna, P.A., Kalke, R.D., Richard, P., 2000. Utilization of estuarine organic matter during growth and migration by juvenile brown shrimp Penaeus aztecus in a South Texas estuary. Mar. Ecol. Prog. Ser. 199, 205–216. Riera, P., Stal, L.J., Nieuwenhuize, J., 2002. δ13C versus δ15N of cooccurring molluscs within a community dominated by Crassostrea gigas and Crepidula fornicata (Oosterschelds, The Netherlands). Mar. Ecol. Prog. Ser. 240, 291–295. Roditi, H.A., Fisher, N.S., Sanudo-Wihelmy, S.A., 2000. Uptake of dissolved organic carbon and trace elements by zebra mussels. Nature 407, 78–80. Rowe, G.T., 1971. Benthic biomass and surface productivity. In: Costlow Jr., J.D. (Ed.), Fertility of the Sea. Gordon and Breach, New York, pp. 441–454. Sherwood, G.D., Rose, G.A., 2005. Stable isotope analysis of some representative fish and invertebrates of the Newfoundland and Labrador continental shelf food web. Estuar. Coast. Shelf Sci. 63, 537–549. Sotiropoulos, M.A., Tonn, W.M., Wassenaar, L.I., 2004. Effects of lipid extraction on stable carbon and nitrogen isotope analyses

34

F. Le Loc'h et al. / Journal of Marine Systems 72 (2008) 17–34

of fish tissues: potential consequences for food web studies. Ecol. Freshw. Fish 13, 155–160. Vanderklift, M.A., Ponsard, S., 2003. Sources of variation in consumer-diet δ15N enrichment: a meta-analysis. Oecologia 136, 169–182. Vander Zanden, M.J., Rasmussen, J.B., 2001. Variation in δ15N and δ13C trophic fractionation: implications for aquatic food web studies. Limnol. Oceanogr. 46 (8), 2061–2066. Vander Zanden, M.J., Vadeboncoeur, Y., 2002. Fishes as integrators of benthic and pelagic food webs in lakes. Ecology 83 (8), 2152–2161.

Vizzini, S., Mazzola, A., 2003. Seasonal variations in the stable carbon and nitrogen isotope ratios (13C/12C) and (15N/14N) of primary producers and consumers in a western Mediterranean coastal lagoon. Mar. Biol. 142, 1009–1018. Vizzini, S., Mazzola, A., 2006. Sources and transfer of organic matter in food webs of a Mediterranean coastal environment: evidence for spatial variability. Est. Coast. Mar. Sci. 66, 459–467. Wada, E., Minagawa, M., Mizutani, H., Tsuji, T., Imaizumi, R., 1987. Biogeochemical studies on the transport of organic matter along the Otsuchi River Watershed, Japan. Estuar. Coast. Shelf Sci. 25 (3), 321–336.