Benthic fauna and bioturbation on the Mediterranean continental slope: Toulon Canyon

The major bioturbation features in an oligotrophic canyon, macrobenthic assemblage and biogenic structures have been studied simultaneously at a station of 1000 m depth in the channel of Toulon Canyon. Two different techniques of quantifying biogenic structures have been employed: X-rays of the sediment column and photographs of the sea floor. In this deep environment, a macrobenthic community dominated by 64.4% of subsurface deposit feeders provides slow bioturbation with intensity proportional to low macrobenthos density (13.1 ± 5.6 individuals dm− Z. Subsurface feeding appears to be the best feeding strategy as there are numerous turbidite deposits observed in this canyon. Burrow records in the sediment column due to macrobenthos are very scarce. Sediment mixing processes do not exceed 3 cm as 85% of the macrobenthos is encountered in the top 3 em. From photographs of the sediment surface, the density of biogenic features induced by megabenthos is estimated to be 3.3 ± 1.1 structures m−2. Organisms responsible for the more frequently encountered biogenic structures, groups of six, nine or 12 holes, are identified as the Thalassinidean shrimpsCalocaris macandreae. In the Toulon Canyon, the relatively low biological mixing associated with low sedimentation rates provide good maintenance of biogenic structures at the sediment surface. The density of biogenic structures at the sediment-water interface is inversely proportional to macrobenthos abundance which is in agreement with Kitchellʹs hypothesis. This canyon represents an inverse example to the Atlantic Ocean where macrobenthos is more abundant and biogenic structure density is smaller.