Nutrient limitation of algal growth and nitrogen fixation in a tropical alpine lake, Lake Titicaca (Peru/Bolivia)

Nutrient limitatioti of phytoplankton growth and nitrogen fixation was studied during 1981-82 in tropical, high altitude Lake Titicaca, Peru/Bolivia (16° S longitude; 3812 m ASL). 2. In eleven 5 day in sitti growth bioassays nitrogen was the most frequently limiting nutrient. NH4^ significantly stimulated carbon fixatioti or chlorophyll production in the main lake basin in three experiments and twice in a shallower bay. PO^^" and Fe"^ stimulated carbon fixation once in the main basin. SiO2 and a micronutrient mixture never stimulated algal growth. Nitrogen fixation by blue-green algae was stimulated 40-130% by PO4-^" and Fe^^ and inhibited by NH4^. Dissolved nutrients measured at the beginning of each bioassay indicated that nitrogen was in short supply relative to phosphorus and silicate. 3. Nitrogen limitation in Lake Titicaca is discussed relative to the geography of its drainage basin.