Trophic control of benthic foraminiferal abundance and microhabitat in the bathyal Gulf of Lions, western Mediterranean Sea

Surface sediment was sampled at two bathyal sites in the southwestern Gulf of Lions in the western Mediterranean Sea in February and August 1997 to study the distribution and microhabitat of living (Rose Bengal stained) deep sea benthic foraminifera. Both standing stock and diversity of the faunas, and the microhabitat of distinct species mirror the trophic situation and the depth of the oxidised layer at the different sites. Our results suggest that the faunas do not comprise highly opportunistic species and are adapted to rather stable environments. In the axial channel of the Lacaze-Duthiers Canyon, organic matter fluxes are enhanced due to advective transport of organic matter resulting in elevated oxygen consumption rates in the surface sediment and a rather thin oxidised layer. The corresponding benthic foraminiferal fauna is characterised by rather high standing stock and diversity, and a well-developed deep infauna. In addition to freshly deposited phytodetritus, more degraded organic matter seems to be an important food source. In contrast, at the open slope, organic matter fluxes and oxygen consumption rates in the surface sediment are lower and the oxidised layer is much thicker than inside the canyon. The corresponding benthic foraminiferal fauna comprises mainly epifaunal and shallow-infaunal species with much lower standing stocks and clear differences between February and August. In August standing stocks are higher and the average living depths of most species shift towards the sediment surface. These differences can be attributed to patchiness or represent a seasonal trophic signal

Relationship between sedimentary organic matter and benthic fauna within the Gulf of Lion: synthesis on the identification of new biochemical descriptors of sedimentary organic nutritional value

The abilities of several biochemical characteristics of sedimentary organic matter to describe quantitative changes in benthic fauna were assessed based on several surveys carried out within the Gulf of Lion during the last decade. Two sets of high frequency samplings were carried out both inshore and offshore, together with an assessment of seasonal variability along a depth gradient, and a synoptic assessment of mesoscale spatial variability. The considered biochemical parameters were: total organic matter, organic carbon, nitrogen, total proteins, available proteins, carbohydrates, lipids, total and available amino acids. The considered faunal parameters were the abundance and the biomass of both meio- and macrofauna. Our results contribute to unravel the relative importance of food digestibility and composition in controlling particulate organic matter nutritional value. Spectra of both total and available amino acids were almost constant irrespective of the seasons and/or the environments, which were studied during these surveys. This underlines the role of particulate organic matter digestibility relative to its ability to meet specific nutritional requirements in controlling benthic fauna. Multivariate analysis used to relate biochemical and faunal parameters suggest that available amino acids and lipids are the best descriptors of food nutritional value. In contrast with carbohydrates, these two parameters are both associated with the most labile fraction of particulate organic matter. This result suggests that the use of the sum of proteins, carbohydrates and lipids as a proxy for labile organic matter is precluded at least in areas similar to the Gulf of Lion.Les aptitudes de plusieurs des caractéristiques biochimiques de la matière organique particulaire sédimentée à décrire les changements quantitatifs de la faune benthique ont été comparées à partir des résultats de plusieurs campagnes de prélèvements ayant eu lieu dans le golfe du Lion au cours de ces dix dernières années. Ces travaux ont consisté en deux suivis haute fréquence conduits à une station littorale et à une station du large, en une évaluation de la variabilité saisonnière le long d’un gradient côte large, et, enfin, en une étude synoptique de la variabilité spatiale à mesoéchelle. Les paramètres biochimiques considérés sont la matière organique totale, le carbone organique, l’azote, les protéines totales, les protéines disponibles, les sucres, les lipides, les acides aminés totaux et les acides aminés disponibles. Les paramètres faunistiques considérés sont l’abondance et la biomasse de la meio- et de la macrofaune. Les résultats obtenus contribuent à délimiter l’importance relative de la digestibilité et de l’aptitude à satisfaire les besoins nutritionnels dans la définition de la valeur nutritive de la matière organique particulaire sédimentée. Ainsi, la constance des spectres en acides aminés totaux et disponibles quels que soient les environnements et les saisons étudiés, suggère-t-elle un plus grand rôle de la digestibilité de la matière organique particulaire que de son aptitude à satisfaire des besoins nutritionnels spécifiques dans le contrôle de la faune benthique. Par opposition aux sucres, les lipides et les acides aminés disponibles sont associés à la fraction la plus labile de la matière organique particulaire. Ceci rend inopérant l’usage de la somme des concentrations en protéines, sucres et lipides en tant qu’indice de la fraction labile de la matière organique, au moins dans les zones directement comparables au golfe du Lion