Evidence for strong selective forces structuring a subtropical bacterioplankton metacommunity along 800 km from freshwater reservoir to the sea

ABSTRACT Testing ecological theory on microbial communities could shed light on the mechanisms behind general biodiversity patterns. Here, we tested explicit a‐priori predictions from a coalescent‐based simulation model of community structure, macroecological patterns and 16S rRNA gene metabarcoding to evaluate bacterioplankton diversity ( α , β , γ diversity and species abundance distribution (SADs)) along a large region (∼800 km) in 6 sites from a subtropical freshwater reservoir in the Uruguay river to the Atlantic Ocean. A test based on the genetic distances among sequences suggests species sorting as the determinant of community structure. Stronger selective pressure was evidenced towards the outer estuary, with a decrease in species diversity and changes in the dominant classes, while keeping a similar power‐law SAD structure. Distance decay in similarity matched theoretical patterns suggested by the replacement (turnover) of species and not to those generated by the loss of species from a larger pool. Each site along the continuum strongly contributes to the γ bacterioplankton diversity, highlighting the relevance of local species pool to the bacterioplankton metacommunity.