*2.3.3 Peru and Bolivia*

In the Lake Titicaca, in the corresponding Bolivian basin, Fonturbel and Castaño-Villa [46] considered nutrient concentrations and phytoplankton groups as a whole to determine that the families Oscillatoriaceae and Nostocaceae (cyanobacteria) respond positively to the increase in pH (alkalinization) and negatively to the increasing nutrients, while the families Naviculaceae (diatoms), Closteriaceae, and partly Zygnemataceae (green algae) showed an inverse tendency with proliferating in acidic water enriched with nutrients. The diatoms seem to respond negatively, both to the acidification of the water and to the excessive nutrient enrichments. Studies revealed that they are the most sensitive groups to the eutrophication of waters.

Between the 1970s and 1990s, both sections of the Lake Titicaca, deep Lago Mayor and large part of shallow Lago Menor, were oligotrophic with high water transparency and strong nitrogen limitation. Chlorophyta and cyanobacteria (particularly *Anabaena* spp.) dominated the phytoplankton with low biomass and primary production, except for diatoms during the dry season [47]. Currently, the deep pelagic areas of the Lake Lago Mayor remain oligotrophic. However, shallow littoral areas of the Lake Lago Mayor and the Lago Menor turn to eutrophic from mesotrophic. In the northern littoral area of the Lago Menor, there are a lot of villages which have domestic pollution sources, while El Alto is responsible for the heavy contamination of the Cohana bay. In 2015, the extended rainy season produced the first major phytoplankton bloom event in dominance of *Carteria* sp., which is a harmless unicellular green algae in the northern part of the Lago Menor in the period of March–April. Phytoplankton blooms in the region have been spotted since the 2000s. Cyanobacteria *Limnoraphis* (syn. *Lyngbya*) predominates in the Puno bay.
