Assessment of the spatial variability of intertidal benthic communities by axial tomodensitometry: importance of fine-scale heterogeneity Florian Mermillod-Blondin a,b , Sabrina Marie a , Gaston Desrosiers a, * , Bernard Long c , Laure de Montety a , Emma Michaud a,d , Georges Stora d a ISMER, Universite ´ du Que ´bec a ` Rimouski, 310 Alle ´e des Ursulines, C.P. 3300, Rimouski, Quebec, Canada G5L 3A1 b UMR 5023, LEHF, Universite ´ Lyon I, Campus de la Doua, 69622 Villeurbanne cedex, France c INRS-Ge ´oressources, 880 chemin Sainte-Foy, Quebec, Quebec, Canada G1S 2L2 d UMR 6535, LOB, Campus de Luminy, 13009 Marseille, France Received 14 May 2002; received in revised form 5 November 2002; accepted 14 November 2002 Abstract At the water – sediment interface of aquatic ecosystems, the presence of biogenic structures produced by benthic invertebrates strongly affects biogeochemical processes. The quantification of these structures and the assessment of the vertical distribution of fauna are essential for determining the impact of communities in sediments. In the present study, computer axial tomodensitometry (CAT-scan) was used to measure the space occupied by an intertidal community of the St. Lawrence estuary. Three cores were sampled at a site that was considered homogeneous according to surface sediments. The vertical distribution of biogenic structures and gravel were measured in the three cores using CAT-scan; the vertical distribution of fauna was also analysed for each core. The biogenic structures were highest at the water – sediment interface and decreased with depth in the three cores. The number of invertebrates also decreased with depth. We observed similar distributions of biogenic structures in cores 1 and 2. However, fewer biogenic structures were observed below 90 mm in core 3. This result was correlated with a high quantity of gravel from 90 to 140 mm in core 3 whereas the other cores had lower quantities of coarse material. We found relationships among the distributions of biogenic structures, fauna, and sediment characteristics (gravel quantity) that can affect species distribution. The vertical distributions of Macoma balthica, Mya arenaria, Nereis virens, and small-sized gallery-producing species (nematodes and oligochaetes) could also be recorded with the CAT-scan method. Thus, CAT-scan is an excellent 0022-0981/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-0981(02)00548-8 * Corresponding author. www.elsevier.com/locate/jembe Journal of Experimental Marine Biology and Ecology 287 (2003) 193 – 208