Species co-occurrence patterns give significant insights into the processes shaping communities. While biotic interactions have been widely studied using co-occurrence analyses in animals and larger plants, studies about co-occurrences among micro-organisms are quite rare. We examined stream diatom co-occurrences and nestedness in France, and tested the significance of the patterns with a set of random and environmentally constrained null models. Real communities showed a higher segregation than the most conservative random null models, but a general aggregation of co-occurrences when compared to environmentally constrained null models. We did not find any evidence of limiting similarity between co-occurring species. Aggregations of species co-occurrences were associated with the high levels of nestedness. Altogether, these results suggested that biotic interactions were not structuring co-occurrences of diatom species at our study scale. Instead, the patterns were more likely to be related with colonization patterns, mass-effect, and local dynamics of diatom biofilms. We further highlight that the association of random and environmentally constrained null models may give realistic insights into the co-occurrence patterns of microbial communities data.