Canonical correspondence analysis of the relative abundance of the 1,000 most abundant OTUs (a) for all samples, (b) for all samples without depth and latitude as explanatory variables, (c) for bathypelagic samples and (d) for mesopelagic samples. Monte Carlo permutation tests for the first and all axes were highly significant for all the four CCA analyses (p < 0.002). Abbreviations of the environmental and bacteria-related variables: lat, latitude; O2, oxygen concentration; AOU, apparent oxygen utilization; DOC, dissolved organic carbon; DON, dissolved organic nitrogen; Sal, salinity; NH4, ammonium concentration.