Prokaryoplankton and phytoplankton community compositions in five large deep perialpine lakes

Phytoplankton (PCC) and prokaryoplankton community compositions were studied in five perialpine lakes (Garda, Maggiore, Como, Iseo, and Lugano) of different trophic conditions and mixing regimes, using inverted microscopy and CARD-FISH technique. The aim of this research was to assess, for the first time in these lakes, the relationships among PCC, bacteria, and archaea, and the effects of environmental drivers. We observed a clear difference of PCC compositions in spring and summer. All the lakes showed Bacillariophyta dominating in spring, whereas in summer, there were remarkable differences of PCC. Prokaryoplankton composition showed more pronounced differences in the vertical profile than those between spring and summer. The oligomictic lakes showed a uniform vertical gradient of prokaryotes in spring, while in the meromictic lakes, their abundances were incremented with depth. In summer, the prokaryotic community changed, and niche differentiation occurred in almost all lakes. In conclusion, our study showed a general pattern, common to all the lakes, of a first appearance of the large-sized “opportunistic” bacteria in spring, followed by ultramicrobacteria, less vulnerable to predation in summer. Significant correlations between a few PCC and bacterial groups were found, thus elucidating that functional interactions can be the key to understand plankton successions.