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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

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Several long-term studies on Southern Ocean seabirds and seals have suggested a possible link between major declines in breeding performance and El Nin¬ o Southern Oscillation events. We report that the breeding performances and body condition of the blue petrel (Halobaena carulea) on the Kerguelen Islands is depressed by episodic, warm sea-surface temperatures (SSTs) in the winter before breeding. Lagged cross-correlations between SSTs in the Kerguelen sector and the Southern Oscillation Index indicate that warm SSTs were found south of the Kerguelen Islands within a year of, and between 4.2 and 5.4 years after an El Nin¬ o event took place. These results can be discussed with respect to the recently described Antarctic Circumpolar Wave that drives climatic anomalies eastward around the Southern Ocean.

Global warming is expected to increase the frequency and intensity of inter-annual variation in Sea-Surface Temperatures (SST) associated with a latitudinal shift of frontal structures in the Southern Ocean. However, the long-term consequences of these major climatic events on the biotic environment remain poorly understood. We studied the effect of SST anomalies in the southern Indian Ocean on the breeding success of eight seabird species, and found these temperature anomalies to have different effects depending on the foraging habitat of the species. The breeding success of four seabird species foraging mainly south of the Polar Front in Antarctic waters was significantly depressed by warm SST occurring mainly in winter and spring, prior to breeding. Conversely, warm SST anomalies were associated with a higher breeding success for species foraging mainly north of the Polar Front, while no significant effect was found for two species that forage on the Kerguelen plateau. These different responses to changes in the SST were also observed for two closely related species (sooty albatross Phoebetria fusca and light-mantled sooty albatross P. palpebrata) breeding at Kerguelen. These observations highlight the importance of multi-species long-term monitoring programs for understanding the ecological consequences of environmental variability. Our results suggest that the predicted southward shift of the Polar Front caused by oceanic warming could lead to an important decrease in the breeding performance of top predator seabirds depending on the location and changes of their foraging habitat in relation the Polar Front. P. Inchausti