The aquatic photosynthetic community in Lake Waiau (elev. 3980 m, lat. 19° 48′ N) is exposed to intense solar ultraviolet radiation (UVR) due to the high altitude and low latitude. There is a strong negative effect of UVR on photosynthesis both in the planktonic and benthic mat communities in the lake. The mats apparently receive some protection from UVR-absorbing mycosporine-like amino acids (MAAs), while the planktonic primary producers do not appear to have such protection and exhibit no net photosynthesis at levels of UVR characteristic of the lake surface at midday. These forms may be meroplanktonic or rely on vertical mixing for protection from extended periods of exposure to UVR.