Occurrence and diversity of bacterioplankton in drinking water tropical reservoirs of contrasting trophic state

Abstract Bacteria communities play key roles within freshwater reservoirs, affecting their functioning and services. Understanding their occurrence in the reservoirs will help predict how these ecosystems change in response to human activities and climate change. In this study, 16S rRNA gene sequencing was used to investigate the composition and structure of bacterioplankton in two drinking water tropical reservoirs with different trophic state in Puerto Rico: Cerrillos (mesotrophic) and Lucchetti (eutrophic). The dominant lineages were typical of freshwaters such as Actinobacteria, Gammaproteobacteria, Alphaproteobacteria, Firmicutes, and Cyanobacteria. However, the bacterioplankton composition and diversity were variable throughout the water column in both reservoirs, indicating that environmental gradients such as temperature, dissolved oxygen, and nutrient create a variety of habitats that support different bacterial assemblages in these reservoirs. For example, Frankiales and Micrococcales were dominant in the metalimnion, whereas Bacillales were common in the deep layers, mainly in the anoxic zone of Lucchetti. The trophic state of these reservoirs also affects the bacterioplankton structure. Picocyanobacteria were represented mainly by Cyanobium PCC-6307, with higher abundance in Cerrillos compared to Lucchetti, indicating its preference for less eutrophic reservoirs. The detection of potentially pathogenic species, such as Acinetobacter lwoffii and A. Schindleri, and of cyanobacteria ecotypes as possible indicators of eutrophication indicate the importance of monitoring these tropical ecosystems to implement mitigation programs in eutrophic reservoirs and conservation strategies in oligotrophic reservoirs used as drinking water source.