Phytoplankton diversity and cyanobacterial dominance in a hypereutrophic shallow lake with biologically produced alkaline pH.

In this work, we have characterized the diversity of phytoplanktonic species in a highly alkaline and hypereutrophic shallow lake, Santa Olalla (southwestern Spain), the evolution of their relative abundances, and that of several physicochemical parameters over 2 years. In the absence of an external input of alkaline water, Santa Olalla's stable high pH (average pH 9.52, with several maxima >10.5) is explained by an extremely high photosynthetic primary productivity. A variety of phytoplankton species was observed even during pH maxima. These included several species of green algae, diatoms, and euglenoids and several cyanobacteria from the orders Nostocales and Chroococcales. Quantitatively, cyanobacteria dominated. A blooming event due to Aphanothece clathrata was observed at one pH maximum, during which the diversity as measured by the Shannon-Weaver index was extremely low. Santa Olalla's cyanobacteria are alkaliphilic and/or extremely alkalitolerant and appear to be responsible for the generation and maintenance of stable high-pH conditions in their environment.