Carole Vallet

Drifting in post-larvae and juveniles in an Abra alba (wood) community of the eastern part of the Bay of Seine (English Channel)

Abstract Recent evidence has shown that some intertidal benthic species may re-enter the water column after larval settlement and undergo active and/or passive redistribution but there are no comparable studies of subtidal benthoplanktonic species with an isolated distribution in a megatidal sea. The present study was therefore conducted in June 1992 at a site located in the Abra alba (Wood) community of the eastern part of the Bay of Seine. The purpose of this work was to determine the species involved in post-settlement transport, to assess the number and the developmental stages of the resuspended organisms and to investigate the processes controlling the abundance of drifting post-larvae or juveniles. Under megatidal conditions drifting was quantitatively important in at least eight fine-sediment dominant species. Their abundance fluctuations in the near-bottom water stratum correlated with the bed shear stress (τ) caused by the tidal current. Swell may induce a doubling of the intensity of τ. However, species differed in their responses to hydrodynamic variables. Some species were most abundant at flood tide ( Abra alba , Tellina fabula Gamelin and Natica alderi Forbes), ebb tide ( Cultellus pellucidus Pennant, Nassarius reticulatus L.) or both ( Mysella bidentata Montagu, Owenia fusiformis delle Chiaje, Pectinaria koreni Malmgren). The vertical distribution of the resuspended benthos varied with species, individual size and hydrodynamic conditions. The results are discussed in relation to the eco-ethological characteristics of the species and the consequences of such drifting mechanisms on population dynamics.

The near-bottom layer as an ecological boundary in marine ecosystems: diversity, taxonomic composition and community definitions

The near-bottom layer of the ocean represents a boundary between two oceanic biotopes (pelagial and benthal), and as a result, the animal populations living in this habitat belong to various diverse ecological groups. There is a profusion of terms to designate the organisms which live near the sea bottom, both in relation to their behaviour and to boundary-layer hydrodynamics. Do the fauna living above the sea bottom form a true community? Should the fauna in this habitat be considered as a true community or a mixed assemblage comprised of benthic and pelagic organism? Between 1988 and 1996, more than 500 suprabenthic hauls were taken with a modified Macer-GIROQ sledge at 15 sites in the English Channel and the Seine Estuary (5–70 m), at 13 sites on the southern edge of the Cap Ferret Canyon (Bay of Biscay, 350– 1100 m), and at 8 sites on the Atlantic seamounts south of the Azores (260–2235 m). This intensive sampling permitted the collection of more than several hundred species and will serve to facilitate discussion concerning the biodiversity of the fauna collected near the sea bottom. This paper proposes that in the estuary, the near-bottom layer is colonized by a mixed assemblage of both pelagic and benthic organisms, while in the coastal and in the bathyal zones, the response to the gradual extinction of light and the decreasing benthic resuspension at near-bottom depths leads to an ecocline.

Main characteristics of the boundary layer macrofauna in the English Channel

Four hundred and thirty two suprabenthic hauls with a modified Macer-GIROQ sledge were taken over the period 1988–1996. Fifteen sites within the English Channel were sampled from the Roscoff and Plymouth waters to the Dover Strait, including the Bay of Morlaix, Bay of Saint Brieuc, the Bay of Seine and the Seine Estuary. The main characteristics of the Benthic Boundary Layer (BBL) macrofauna in the English Channel were elucidated from these data. BBL macrofauna was found over the whole water column sampled (10–150 m). Firstly, the number of species collected was very high (up to 116 species for five hauls) except in the Seine estuary. Numerically, mysids were the dominant group of organisms collected with the sledge, followed by amphipods, cumaceans, isopods and decapods. Abundance and biomass of the fauna were very high in the Seine estuary, especially in the mesohaline and oligohaline zones (maximum >216000 ind. 100 m-3; >200 g AFDW 100 m-3) where the specialised mysids Neomysis integer and Mesopodopsis slabberi occurred in very high abundance. At other sites, the abundance and biomass were moderate (100–10000 ind. 100 m-3; 15–2000 mg AFDW 100 m-3). Abundance reached its maximum when mysids or swarms of amphipods of the genus Apherusa were collected, and during the reproduction of the amphipods of the genus Ampelisca, as in June in the Bay of Morlaix. The number of species, abundance and biomass showed seasonal changes in all sites with, in general, lowest values in winter or spring, and highest values in autumn. For the open sea sites, two main geographical gradients were identified: i) in spring, a higher number of species in the western part of the Channel compared with the eastern part of the Channel and, in autumn, a higher number of species in the eastern part of the Channel compared with the western part, and ii) an increasing density gradient from west to east at spring and a decreasing gradient from west to east during the autumn. The suprabenthic fauna of the Channel can be considered as a unique community including three kinds of species: (i) good swimmer species which were common at all the time and all sites: e.g. Anchialina agilis, Gastrosaccus spp., Haplostylus spp. and Apherusa spp., (ii) species present at all sites only at night: e.g. Atylus vedlomensis, Megaluropus agilis, Melphipidella macra, Orchomenella nana, Pseudocuma longicornis, Schistomysis ornata and Synchelidium maculatum, and iii) species collected at one or a limited number of sites only at night: e.g. Ampelisca spp. at Pierre Noire and Diastylis spp. at site F. The Seine estuary supports a typical suprabenthic estuarine community. Elements of the suprabenthic fauna also showed significant daily rhythms which took several forms depending on the species under consideration.

Lipid characterization of Pleuragramma antarcticum (Nothoteniidae) larvae off East Antarctica (139°E–145.10°E) during summer

Pleuragramma antarcticum (Antarctic silverfish) larvae are a key component of the neritic assemblages in the Antarctic coastal waters and can be considered as an indicator of the future changes that may occur in this area. Lipid class and fatty acid composition was studied to assess the nutritional status and evaluate the type of dominant trophic interactions of P. antarcticum larvae collected between Terre Adélie and the Mertz Glacier Tongue (139°E–145.10°E) during summer 2007. P. antarcticum larvae exhibit moderate lipid levels (11.9–15.0% dry weight). Lipid class analyses showed a similar pattern over the study area consisting mainly of polar lipids (61–75% of total lipids). During their first summer, larvae started to accumulate small amount of lipid reserves in the form of triacylglycerols (10–16% of total lipids). Polar lipids were dominated by phosphatidylcholine (55–59%) followed by phosphatidylethanolamine (19–21%). Fatty acid signature of triacylglycerols indicates (1) a dominance of copepod of the Oithona type in the trophic pattern of P. antarcticum larvae and (2) a significant contribution of phytoplankton. In the same way, the analysis of gut content shows that 70% of larvae fed on various assemblages of phytoplankton and zooplankton (mainly copepods) and 30% of larvae fed exclusively on phytoplankton. Although a carnivorous diet is commonly described, our results suggest that P. antarcticum larvae showed an opportunistic feeding strategy (i.e. high degree of omnivory) and that dietary energy seems to be mainly directed towards fast growth rather than energy storage for periods of starvation.