Contextual parameters were measured in a parallel study, and the best combinations of these to explain the variation in the microbial community matrix were determined by forward selection (Additional file 8 and Roussel, personal communication). Parameters were log10-transformed, and the community matrix was standardized by Hellinger transformation . Variation partitioning was then used on the community matrix to investigate the effects of the selected parameters (abalone characteristics, algal composition, sampling date) and of their covariation on the structure of the microbiota [88, 89].