A mechanistic model has been developed to characterize and quantify sediment-mixing due to macrobenthic organisms that construct gallery systems. The mixing model is time-and spacedependent and employs ordinary differential equations. It uses (1) biological parameters -the size of the bioturbated zone, rate of biodiffusion and rate of biotransport; (2) physical parametes -output to the water-column coefficient and rate of physical mixing due to local water currents; and (3) biogeochemical parameters -decay rate of the tracer. This gallery-diffusor model is based on a combination of 2 processes: biodiffusion in the sediment layer containing very dense gallery systems, and biotransport in the region of tube bottoms. The performance of this gallery-diffusor model is compared with that of the biodiffusor model classically used to describe mixing of such organisms. Both models are applied to conservative tracer profiles measured in laboratory experiments with the polychaete Nereis diversicolor. Our new model provides mechanisms to describe and explain the tracer-profile shapes observed in sediments. It includes rapid particle transport from the upper layer of the sediment to the tube bottom zone, which is not taken into account with the biodiffusor model but which is of great importance in understanding the processes of organic matter degradation in the sedimentary column. It also makes possible the accurate quantification of the different components of the mixing process of an organism (in this study, the polychaete N. diversicolor). The gallery-diffusor model constitutes 1 of 5 elementary components in a global bioturbation model that allows the study, quantification and prediction of sediment reworking by macrobenthic communities according to their functional group and composition and/or to the specific characteristics of the individual organisms. KEY WORDS: Bioturbation · Nereis diversicolor · Model · Functional groups · MacrobenthosResale or republication not permitted without written consent of the publisher

Vertical distributions and die1 migrations at nineteen stations located in the Ligurian and Tyrrhenian Seas (northwestern Mediterranean) are described in detail and compared for the main species of micronekton and macroplankton. Sampling was performed in April 1994 in the O-700 m water column in most cases, down to 350,450 or 550 m for the coastal or relatively shallow sites. Most of the twenty species showed similar distributions throughout the sampled area, either non-diel-migrating distributions such as the euphausiid Stylocheiron longicorne and the fish Cyclothone braueri, or clear diel-migrant behaviour (e.g. the siphonophore Chelophyes appendiculata and the euphausiid Euphausia krohni). Four species, the siphonophore Lensia conoidea, the pteropods Cavolinia infzexa and Clio pyramidata and the salp Salpa fusiformis, showed heterogeneity between distributions, with different preferential daytime depths. For example, moderate or extensive migration and even slight reverse migration were observed in Lensia conoidea, depending on the station sampled. The differences in vertical distributions did not appear to be related to hydrological conditions (temperature, salinity); b'ut rather to chlorophyll content (and ambient light) or bottom depth of the station, or to mating behaviour of the species. 0 Elsevier, Paris vertical distributioln / macroplankton I micronekton I Mediterranean Sea I Tyrrhenian Sea Resume-. Distributions verticales du macroplancton et du micronecton en mer Ligure et en mer Tyrrhenienne (Mediterranee nord-occidentale). Cette etude presente et compare les distributions verticales et les migrations nyctht-m&ales des principales especes de macroplancton et de micronecton observees en mer Ligure et en mer Tyrrhenienne (MCditerranCe nerd-occidentale). Dix-neuf stations ont CtC Cchantillonnees, en avril 1994, jusqu'a 700 m de profondeur generalement ou jus,qu'a 350,450 ou 550 m pour les zones cot&es ou peu profondes. La majorite des vingt especes Ctudikes montre des distributions similaires sur l'ensemble de la zone, avec la m&me distribution de jour et de nuit (c'est par exemple le cas de 1"euphausiace Stylocheiron longicorne et du Poisson Cyclothone bruueri) ou le meme comportement migratoire (comme pour le siphonophore Chelophyes appendiculata et l'euphausiace Euphausia krohni). Quatre especes, le siphonophore Lensia conoidea, les pteropodes Cavolinia infzexa et Clio pyramia!ata et la salpe Salpa fusiformis, presentent des distributions variables, avec des niveaux bathymttriques preferentiels differents le jour. Ainsi, selon la station Cchantillonnee, on observe chez Lensia conoidea une migration mod&e ou de grande amplitude, ou m&me une leg&e migration inverse. Ces differences de distributions verticales n'apparaissent pas likes aux conditions hydrologiques (temperature, salinite); elles seraient plutiit associees au contenu en chlorophylle (et a la lumiere ambiante) ou a la profondeur du site, OLI aussi au comportement reproducteur de l'espece. 0 Elsevier, Paris

No abstract

No abstract