The distribution of OTUs linked to a monospecific diet was investigated by measuring proportions of OTUs shared between two consecutive sampling dates. For instance, in the digestive microbiota associated with the P. palmata diet, there were 29–40% OTUs common to abalone sampled in February 2012 and those sampled in April 2012, meaning that there were about 60–71% new OTUs after 2 months of monospecific diet (Additional file 1: Figure S2A). Over the year, the proportions of OTUs common to two consecutive sampling dates varied between 16–59%, 18–70%, 23–61%, and 10–71% for the P. palmata, U. lactuca, L. digitata, and S. latissima diets, respectively. These common OTUs, however, accounted for 40–99% of the sequence reads (Additional file 1: Figure S2). About 3% OTUs were present the whole year for each monospecific diet and corresponded to a large proportion of sequence reads (from 88–97% in each cage). Altogether, this indicates that the rarest OTUs fluctuate with time while the most abundant OTUs remain in the digestive gland during the year, indicating the presence of a dominant core microbial community associated with each algal diet.