Structure, fonctionnement, évolution des communautés benthiques des fonds meubles exploités du plateau continental Nord Gascogne

Dans le double cadre de la conservation des écosystèmes soumis à l'exploitation et du développement durable d'une pêcherie, cette étude intégrée d'écologie benthique concerne la structure, le fonctionnement et l'évolution des communautés benthiques du plateau continental Nord Gascogne. Deux communautés distinctes ont été mises en évidence : une première dans la partie centrale de la Grande Vasière et une autre au niveau de la marge externe. Les divergences structurelles au sein de ces communautés traduisent un fonctionnement différent de ces deux écosystèmes contraints par des apports trophiques, des relations biotiques et des régimes de perturbations très différents. Des différences remarquables, notamment sur l'épifaune, ont été détectées en relation avec les perturbations engendrées par le passage des engins traînants. La comparaison des communautés à 35 ans d'intervalle révèle une évolution à long terme des entités biosédimentaires. La structure du réseau d'interactions trophiques a été précisée, en termes de niveaux trophiques et d'origine de la nourriture. Les apports trophiques par sédimentation ont été quantifiés. Aux échelles spatio-temporelles considérées, ils montrent une forte variabilité traduisant le rôle majeur des forçages physiques. Les données acquises au cours de cette étude ont été utilisées pour développer un modèle de réseau trophique à l'état stable de l'écosystème benthodémersal de la Grande Vasière. A l'échelle annuelle, ce modèle révèle que la majeure partie des apprts trophiques par sédimentation n'est pas consommée, que la pression de prédation exercée sur les consommateurs primaires benthiques est forte et que l'incidence des rejets de pêche dans le fonctionnement trophique de cet écosystème est faible. La définition de stratégies de gestion des pêcheries en terme de préservation des écosystèmes soumis à l'exploitation et de développement durable de ces activités doit prendre en compte l'ampleur et le rythme de ces modifications.In the double context of the conservation of ecosystems exposed to exploitation and in the sustainable use of a fishery, this integrated benthic ecology study concerns the structure, functioning and evolution of the benthic communities of the North Bay of Biscay continental shelf. Two distinct communities were separated : the first one in the central part of the Grande Vasière and the second one offshore, on the external margin. The structural divergences within these communities reflect a different functioning of these two ecosystems forced by different trophic inputs, biotic relationships and disturbance sources. Remarkable differences, mainly on the epifauna, were detected in relation to the disturbances generated by the bottom-trawling gears. The comparison of the benthic communities separated by 35 years revealed a long term evolution of the biosedimentary units. The interactions within the trophic webs have been specified, in terms of trophic levels and food sources. The trophic inputs from the pelagic sedimentation were quantified. At the different spatio-temporal scales considered, they showed a strong variability highlighting the major role of the physical forcing. The data collected in this study was used to develop a steady state trophic web model of the benthic-demersal ecosystem of the central part of the Grande Vasière. At the annual scale, this model reveals that the major part of the particle sedimentation imports is not consumed, the predation pressure exerted on the benthic primary consumers is important and that the consequences of the fishing discards in the trophic webs are weak. The definition of fisheries management strategy in term of preservation of the exploited ecosytems and sustainable use of these activities must take into account the amplitude width and rates of these modifications.BORDEAUX1-Lab.Océanogr.Arcacho (330092101) / SudocBREST-BU Droit-Sciences-Sports (290192103) / SudocPLOUZANE-Bibl.La Pérouse (290195209) / SudocSudocFranceF

Effect of lipid removal on carbon and nitrogen stable isotope ratios in crustacean tissues

The analysis of tissue's naturally occurring stable carbon and nitrogen isotope ratios is a useful tool to delineate trophic relationships. However, the interpretation of delta C-13 and delta N-15 is complicated by the influence of multiple factors such as the tissuespecific lipid content. The aim of this work was to evaluate the effects of lipid extraction on delta C-13 and delta N-15 compositions in muscle, hepatopancreas and gonads of a marine decapod crustacean, the spider crab Maja brachydacryla. Samples were analyzed for stable isotopes before and after lipid removal, using a derived Soxhlet extraction method. Differences in delta C-13 and delta N-15 were measured among tissues before and after treatment. Lipid extraction of muscle did not have a significant effect on either delta C-13 or delta N-15. By contrast, ecologically significant shifts forbothearbon and nitrogen stable isotopes ratios (+2.9 +/- 0.8 parts per thousand for delta C-13, and +1.2 +/- 0.7 parts per thousand for delta N-15) were noticed in the hepatopancreas. In regard to gonads, lipid extraction led to a shift only on delta C-13 (+1.3 +/- 0.3 parts per thousand). Finally, the derived Soxhlet extraction method removed the lipid influence for delta C-13, and had an effect on delta N-15 composition for lipid-rich samples. We recommend this treatment for carbon stable isotope studies on decapod crustacean lipid-rich tissues

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

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

Study of the ichthyofauna diet in the Ichkeul Lake (Tunisia)

In order to define the structure of trophic network of Ichkeul Lake in Tunisia, diet of main teleost fish species was determined during two seasons, wet and dry. A total of 491 stomachs from 16 teleost species was analyzed. The analysis revealed spatial variability (East under marine influence and West with more continental effect) and temporal one (wet and dry seasons). Based on stomach contents, nine trophic groups were revealed, including eight monospecific groups. The main prey are mudflat snails (Hydrobia ventrosa, H. acuta), bivalves (Abra alba, Cerastoderma glaucum), amphipods and isopods, as well as seagrass (Potamogeton pectinatus, Ruppia cirrhosa) and water brackish algae (Ulva spp. and Chaetomorpha spp., Cladophora sp., Polysiphonia sp.)

Community structure and food web based on stable isotopes (delta N-15 and delta C-13) analysis of a North Eastern Atlantic maerl bed

Maerl beds are highly biodiverse biogenic substrata that have been receiving increasing attention in the last decade. Although maerl beds represent important nursery areas for commercial fishes and molluscs, little is known on the trophic web of their communities. Community structure parameters of maerl bed of the Bay of Brest (species richness, abundance, biomass and dominating species) were studied in parallel with the carbon and nitrogen isotopic composition of their main benthic species (macrofaunal, and megafaunal organisms) in order to assess the trophic levels and differences in the potential food sources of maerl inhabitants. The major potential sources of energy were identified to originate either from epiphytic macroalgae and microphytobenthos both growing on maerl thalli, together with sedimenting (sedimentary) particulate organic matter (POM) originating from the water column. The majority of the macro- and megafaunal organisms investigated were filter feeders, selective-deposit feeders and predators/scavengers. Filter feeders fall into three different groups representing different trophic pathways (i) sponges feeding directly on POM (water column filter feeders 1), (ii) ascidians and holothurians feeding on POM and probably captured pelagic preys (water column filter feeders 11), and (iii) filter feeding molluscs and crustaceans were hypothesised to feed on microphytobenthos or on decaying sedimented POM (Interface filter feeders). Selective deposit feeders were also divided into two subgroups. Carnivores were also distinguished between those with scavenging habits and true predators. Coupling of the trophic levels observed with the community biomass structure revealed that most of the benthic biomass derives its food from detritic sedimented POM and/or microphytobenthos, with interface filter feeders (23% of the biomass), selective deposit feeders (12%). Carnivores made up to 14% of the total biomass. Generally stable isotopes ratio mean values overlap and cover a large range within feeding types, indicating a strong overlap in food sources and a high degree of complexity of the food web presumably due to the diversity of the potential food sources. (c) 2006 Elsevier B.V. All rights reserved

Small-scale spatial and temporal interactions among benthic crustaceans and one fish species in the Bay of Biscay

In the summer of 2004, a video survey was carried out in the northern part of the central mud bank (Grande Vasiere) of the Bay of Biscay to study the small scale relationship between the dominant crustacean megafauna Nephrops norvegicus, Munida rugosa and Goneplax rhomboides and juvenile hake (Merluccius merluccius). Using a towed body, high-resolution videos were recorded in six sampling sites. Statistical modelling using generalised additive models (GAM) revealed variations in activity patterns for two species. More N. norvegicus were observed outside their burrows at dawn and somewhat at dusk (no observations during night) while G. rhomboides was less observed in the morning. In addition, reduced spatial overlap between G. rhomboides and N. norvegicus suggested reduced competition for food but also space as both are burrowing species. The observed temporal and spatial activity patterns may contribute to regulating assemblage structure as competing species may be actively foraging at different times and locations thus reducing direct competition

Spatio-temporal isotopic signatures (delta C-13 and delta N-15) reveal that two sympatric West African mullet species do not feed on the same basal production sources

Potential trophic competition between two sympatric mullet species, Mugil cephalus and Mugil curema, was explored in the hypersaline estuary of the Saloum Delta (Senegal) using C-13 and N-15 composition of muscle tissues. Between species, N-15 compositions were similar, suggesting a similar trophic level, while the difference in C-13 compositions indicated that these species did not feed from exactly the same basal production sources or at least not in the same proportions. This result provides the first evidence of isotopic niche segregation between two limno-benthophageous species belonging to the geographically widespread, and often locally abundant, Mugilidae family