Microbial planktonic communities in lakes from a Patagonian basaltic plateau: influence of the water level decrease

In this study we analyzed the structure of the microbial planktonic communities in 16 lakes located in one of the Patagonian plateaus. In these environments picoplankton constitute a key component of the food webs. We compared lakes with different regimes (clear vegetated and turbid), in periods with contrasting hydrological conditions. Samplings were conducted during late summer in 2015 (higher water levels), 2016 and 2017 (lower water levels). In each lake we measured limnological variables and quantified picoplankton by flow cytometry. Differences in picoplankton structure among lakes were associated with their regime and hydrological conditions. We found higher abundances of heterotrophic bacterioplankton (HB) in more turbid and eutrophic lakes; these lakes also showed higher abundances of both bacterial fractions analyzed (high and low nucleic-acid content). Clear vegetated oligotrophic lakes showed the lowest photosynthetic picoplankton abundances and higher densities of PE-rich picocyanobacteria (Pcy). All lakes presented PE-rich cells and picoeukaryotes (Peuk), the latter being the most frequent group in all the environments and the most abundant in turbid ones. PC-rich Pcy appeared only in clear vegetated, organic turbid and inorganic turbid, showing one or two cytometric populations. Clear vegetated lakes with water level reduction show an increase of HB, PC-rich cells and Peuk abundances. We provided evidences that picoplankton structure is influenced by lake regime, and in some lakes the shift from clear vegetated to turbid states strongly affects the structure of these communities.