Fig. 1

The Blanes Bay Microbial Observatory and the variation of its resident microbiota and measured environmental variables over ten years. A Location of the Blanes Bay Microbial Observatory. B All possible correlations between the measured environmental variables including the richness and abundance of resident OTUs (NB: only 709 resident OTUs are considered, see Table 1). Only significant Pearson correlation coefficients are shown (p < 0.01). The p values were corrected for multiple inference (Holm's method). C Unconstrained ordination (NMDS based on Bray Curtis dissimilarities) of communities including resident OTUs only, to which environmental variables were fitted. Only variables with a significant fit are shown (p < 0.05). Arrows indicate the direction of the gradient, and their length represents the strength of the correlation between resident OTUs and a particular environmental variable. The color of the samples (circles) indicates the season to which they belong. The bottom-left arrow indicates the direction of the seasonal change. PNF = photosynthetic nanoflagellates. D Constrained ordination (Distance-based redundancy analyses, dbRDA, using Bray Curtis dissimilarities) including only the most relevant variables after stepwise model selection using permutation tests. Each axis (i.e., dbRDA1 and dbRDA2) indicates the amount of variance it explains according to the associated eigenvalues (both dbRDA1 and dbRDA2 are significant [p < 0.01]). The color of the samples (circles) indicates the season to which they belong. Arrows indicate the direction of the gradient, and their length represents the strength of the correlation between resident OTUs and a particular environmental variable. The bottom-left arrow indicates the direction of the seasonal change. E, F Resident OTUs displaying different niche preferences (blueish areas) in terms of the two most important abiotic variables: Temperature (E) and Daylength (F). The red dots indicate the randomization mean, and the orange curves represent the confidence limits. Black dots indicate individual OTUs for which temperature or daylength preferences are significantly (p < 0.05) higher or lower than a random distribution over 10 years. At least two assemblages with different niches become evident: one preferring higher temperature and longer days (summer/spring), and another one preferring lower temperature and shorter days (winter/autumn). Note that several OTUs associated with Spring or Autumn are not expected to be detected with this approach, as their preferred temperature or daylength may not differ significantly from the randomized mean