Eric Thiébaut

Spatio-temporal persistence of the Abra alba : Pectinaria koreni muddy-fine sand community of the eastern Bay of Seine

The spatial distribution and temporal variations of the Abra alba-Pectinaria koreni community from the eastern part of the Bay of Seine were investigated through four winter surveys (1986, 1987, 1988 and 1991). A grid of 40–67 stations was sampled each year using a Hamon grab (0·25 m -2 ) for macrofauna collection and sediment analysis. The fauna was dominated by polychaetes and in a lower part by molluscs and echinoderms. Although species richness with ~100 species collected during each survey was high, total density and biomass (±SE) resulted principally from about ten dominant species, and varied between 846·9 ±163·5 and 1135·3 ±186·7 ind m -2 and 23·52 ±4·41 and 27·48 ±4·45 g m -2 respectively. The dominant species were patchily distributed and exhibited weak spatio-temporal fluctuations except the two bivalves A. alba and Cultellus pellucidus . Factorial correspondence analysis (FCA) of the distribution of main species among stations and automatic hierarchical classification identified five faunal assemblages which were relatively stable in space and time. The temporal stability of the community could be generated by processes of larval retention near parental population, and sediment stabilization induced by the low abundance of deposit feeders and the high densities of the tube-dwelling polychaete Owenia fusiformis . While the sediment variables were a poor indicator of the spatial structure of the community, the salinity gradient off the Seine Estuary and post-settlement processes (e.g. food limitation and postlarval drifting) have been proposed to explain it. Comparison with an earlier survey (i.e. 1971) suggested a degree of long-term persistence of the community structure in qualitative and quantitative terms.

Spatial variation of metal bioaccumulation in the hydrothermal vent mussel Bathymodiolus azoricus

The variability of the bioaccumulation of metals (Ag, Cd, Cu, Fe, Mn and Zn) was extensively studied in the mussel Bathymodiolus azoricus from five hydrothermal vent sites inside three main vent fields of increasing depth along the Mid-Atlantic Ridge: Menez Gwen, Lucky Strike and Rainbow. Metal bioaccumulation varied greatly between vent fields and even between sites inside a vent field with B. azoricus showing a great capacity to accumulate metals. The bioaccumulation of these metals also varied significantly among tissues. The main target was the gills where metals were mainly associated with soluble compounds whereas in the digestive gland they were mainly associated with insoluble compounds. Storage of metals under insoluble forms in B. azoricus seems to be a major pathway for the detoxification of both essential and non-essential metals. Mussels from the studied fields can be discriminated following their metallic load but the segregation relies partially on the composition of the metal-enriched fluids.

Spring abundance and distribution of the ctenophore Pleurobrachia pileus in the Seine estuary: advective transport and diel vertical migration

Several sets of plankton and hyperbenthos samples were collected from the Seine estuary in late May-early June of 1991, 1992 and 1994, and the abundance, size-frequency, horizontal and vertical distribution, and short-term fluctuations of Pleurobrachia pileus (O.F. Müller, 1776) were determined. In the Seine estuary, P. pileus was more abundant than reported for any other European shallow waters. Its depth-averaged density was 33 individuals m-3 in 1991 and 32 individuals m-3 in 1992, with a maximum density of 808 individuals m-3. The ctenophores displayed passive tidal advection and active diel vertical migration. In the outer estuary, high abundances were observed around low tide, and diel vertical migration was very pronounced. However, many individuals remained aggregated near the bottom at all times. In spring, the distribution of P. pileus coincided with the 33 and 150‰ S isohalines, with high abundances on the marine side of the estuary. It is suggested that aggregation of ctenophores near the bottom and diel vertical migration play an important role in the retention of P. pileus in the estuary. Two schemes of P. pileus life cycle in the Bay of Seine are proposed.

Does the genetic structure of Pectinaria koreni (Polychaeta: Pectinariidae) conform to a source–sink metapopulation model at the scale of the Baie de Seine?

Abstract. According to a recent demographic survey, the population structure of Pectinaria koreni might fit a source–sink metapopulation model at least at a regional scale. Spatial and temporal genetic structure of the populations present in the Baie de Seine (eastern Baie de Seine and Baie des Veys) was assessed using four highly polymorphic microsatellite loci which have revealed strong intra-locality genetic diversity. In the eastern Baie de Seine, both temporal (1994–1996) and spatial (1994) genetic differentiation were relatively low but significantly different from zero despite a 15-day dispersing larval stage. Such structures may be explained by the settlement of larvae from different gene pools and differing recruitment histories among sites within the eastern Baie de Seine. At a larger scale, similar levels of spatial differentiation were observed in 1999 between the eastern Baie de Seine and the Baie des Veys. The lack of any significant differences in gene diversity and allelic richness rules out a source–sink functioning at the scale of our study. The present paper provides further knowledge on the population dynamics of a univoltine species and its persistence in a highly dispersive environment via a shifting spatial mosaic.

Infaunal predation regulates benthic recruitment : an experimental study of the influence of the predator Nephtys hombergii (Savigny) on recruits of Nereis diversicolor (O.F. Müller)

Abstract Field and laboratory experiments were carried out to examine predatory activity effects of the infaunal polychaete Nephtys hombergii on the recruitment of Nereis diversicolor . Using N. diversicolor juveniles as prey, we tested the effects of (1) the temperature variations on predatory activity and (2) the predator density ( d =114 and 342 ind. m −2 ) on prey mortality, on their growth and their escape behaviour. In the field, N. diversicolor alone ( d =13750 ind. m −2 ), and N. hombergii ( d =114 and 342 ind. m −2 ) and N. diversicolor juveniles together were enclosed in 15-cm dia. PVC cylinders during 12 weeks, from 7 March to 30 May 1995, in a mudflat of the Rance Estuary. In the presence of the predator, the biomass of N. diversicolor juveniles declined 8–12× more than in control cylinders, whereas their individual weight was increased. Although it had little effect on the biomass of prey, predator density regulated the consumption by N. hombergii . Predation activity was minimal, but effective, at a temperature of 7°C, increased between 9 and 11°C, and became constant above 11°C. In the laboratory, gradual or sudden temperature variations had little effect on predatory activity of N. hombergii , presumably because they were already physiologically active. Reduction in the abundance of infauna may be due to prey emigration as well as predation: beyond their mortality, N. hombergii induced, proportional to its density, the emigration of juveniles. The experimental results suggest that infaunal predation, by regulating recruit densities, should be considered in addition to the interactions between adults as a strong force structuring soft-bottom communities.

Role of physico-chemical environment on gastropod assemblages at hydrothermal vents on the East Pacific Rise (13 degrees N/EPR)

Deep-sea hydrothermal vents display extreme and highly variable environmental conditions that are expected to be among the most important factors structuring associated benthic populations and communities. We tested this assumption, focusing on the distribution of gastropods, as well as on the demographic population structure and reproductive biology of one dominant gastropod species in zones characterized by alvinellid polychaetes and vestimentiferan tubeworms. A total of 14 biological samples from both types of habitats were collected at three sites on the East Pacific Rise 13°N vent field in May 2002. At all vents except one, the physico-chemical environment was described in two steps: (1) pH, total sulphide and reduced iron concentrations have been measured in situ in Alvinella habitats and correlations to temperature were assessed at the scale of each sampled vent; and (2) assuming the consistency of these relationships within a single edifice, ranges of physico-chemical factors were estimated for each biological sample from the corresponding fine scale temperature measurements. A total of 11 gastropod species were identified from all samples and 2 main faunal assemblages were distinguished: one dominated by Lepetodrilus elevatus in the alvinellid zone as well as in the vestimentiferan zone, and one dominated by the peltospirids Nodopelta heminoda, N. subnoda and Peltospira operculata confined to the alvinellid zone. Peltospirid gastropods were dominant over lepetodrilid gastropods in the more acidic, sulphide-richer, and hotter environments. Although this pattern could be related to specific physiological tolerances to temperature and sulphide toxicity, the weak correlation between community structure and physico-chemical variables suggests that additional factors are also involved. Particularly, the low species richness and the overwhelming dominance of L. elevatus in one faunal assemblage suggest that this species may outcompete peltospirids and greatly affect community structure. This hypothesis is supported by large differences in the demographic structure and reproductive biology of L. elevatus between the 2 faunal assemblages.

A model study on variations in larval supply: are populations of the polychaete Owenia fusiformis in the English Channel open or closed?

Abstract. The polychaete Owenia fusiformis is one of the most ecologically important species in the muddy fine sand sediments in the English Channel where it is distributed in geographically separated populations. A vertically averaged Lagrangian hydrodynamic model integrating tidal residual currents and wind-induced currents was used to drive an advection–diffusion model for investigating the variability of larval transport in order to assess the self-seeding capabilities and the degree of connectivity between local populations. Three different types of environmental forcing (i.e. tidal forcing alone, tidal forcing coupled with either NE winds or SW winds) were applied to 19 distinct populations. Without wind influence, self-seeding is the principal mechanism involved in the renewal of most populations. However, larval retention ranged from under 1% up to 81% in relation to the adult habitat size and the mean velocity of tidal residual currents. Wind forcing had a strong influence on larval dispersal patterns by modifying the origin and densities of settlers as well as the degree of connectivity between populations. As a consequence, larval supply from distant populations generally exceeded local supply and the inter-annual variability of wind forcing induced large year-to-year variations in larval settlement rates. Larval exchanges occurred mainly between neighbouring populations and three groups of interconnected local populations were thereby identified. Within each group, settlement patterns were related to inter-annual variations in the direction and magnitude of larval exchanges.

Investigating isotopic functional indices to reveal changes in the structure and functioning of benthic communities

With the use of stable isotopes, new concepts have emerged based on the idea that the ecological niche can be approximated by the isotopic niche defined as a δ-space area with isotopic δ values as coordinates. This study aims to (i) redefine functional indices originally based on quantitative biological traits of species and demonstrate the ecological significance of newly defined isotopic functional indices (IFI) in a δ-isotopic space, (ii) compare IFI using biomass data with existing unweighted isotopic indices using only isotopic compositions. Using a community-wide approach, we tested IFI using isotopic compositions of a large set of associated species from two marine benthic communities widely reported in coastal shallow waters: the common Amphiura filiformis muddy-sand community and the engineered Haploops nirae sandy-mud community. Biomass and isotopic composition (13C and 15N) of all species were measured during four seasons. IFI were calculated in the isotopic space defined by the two communities, and variations were analysed: (i) isotopic functional richness indices measure the overall extent of the community trophic niche. They are higher in the Haploops community due to a higher diversity in food sources but also to longer food chains. (ii) isotopic functional evenness indices quantify the regularity in species distribution and the density in species packing. They showed that the biomass is concentrated at the edges of the food web in the Haploops community, outside the isotopic range of the main food source. (iii) isotopic functional divergence indices quantify the degree to which species distribution maximizes the divergence. They showed a larger utilization of secondary food sources in the Haploops community. The IFI variations responded according to expectations overall, based on the extensive knowledge of those communities. Results highlighted that IFI weighted with species biomass provide new insights into how the structure of energy accumulation as biomass between species is likely to underpin community structure and the interplay between structural components of richness, diversity and evenness of biomass distribution.

Do hydrodynamic factors affect the recruitment of marine invertebrates in a macrotidal area

For marine benthic invertebrates exhibiting complex life cycles, changes in populations’ distribution and abundance are governed by a large variety of physical, chemical and biological processes. From field observations in the Bay of Seine and laboratory experiments conducted since 1987 on the polychaete Pectinaria koreni, the present study highlights the relative importance of hydrodynamical and biological factors which affect individuals within both the planktonic and benthic phases at different scales of space and time in a macrotidal area. Pectinaria koreni is one of the main macrofaunal component of the Abra alba muddy fine sand community of the eastern Bay of Seine. Despite a highly advective and diffusive environment, a relative larval retention near adult population was reported due to some local hydrodynamics features (e.g. tidal residual circulation, Seine river plume front) and the interaction between the vertical current structure and the larval vertical migration. Although larval retention could be disrupted by wind induced currents, multiple spawning events over the reproductive period increase the likelihood that at least one larval cohort ensures a high recruitment during the life-span. Following a massive settlement whatever the sediment grain size, the newly settled larvae exhibited a high immediate decrease of their densities as a result of postlarval mortality and migration. Postlarval drifting was induced by a combination of physical factors (i.e. tidal currents and swell) and postlarval behaviour in response to sediment texture and adult/settlers interactions. According to the hydrodynamics of the bay, this process may generate a postlarval transport from offshore bottoms to coastal suitable habitats and counteract the demographic effects of larval dispersal. A conceptual model of factors governing the recruitment and population maintenance of Pectinaria koreni is proposed and discussed in comparison with results obtained on another polychaete, Owenia fusiformis, in the same area.

Characteristics of the Ascophyllum nodosum stands and their associated diversity along the coast of Brittany, France

The present study aimed at estimating the characteristics of the Ascophyllum nodosum stands along the coast of Brittany, France. Although both an ecologically and economically important macroalga on sheltered rocky shores of the North Atlantic, no study has simultaneously dealt with the variability of the densities, lengths and biomasses of A. nodosum together with a description of its associated algal and animal diversity. There were significant differences in mean lengths and variations in the length–population structures between sites. However, the biomasses and densities showed no significant differences. The biomasses are amongst the highest ones estimated over the entire species distribution. The algal and animal assemblages were typical of A. nodosum zones, but only the identity composition of the algal communities seemed to reflect site differences in environmental forces. The biomasses measured in the present study should help improve future macroalgae biomass and metabolism estimates on regional or global scales. Finally, the data will provide a reference state for future studies on the responses of fucoid canopies to environmental changes.