Inhibition of phytoplankton photosynthesis by solar ultraviolet radiation:: studies in Lake Titicaca, Bolivia

1. Lake Titicaca is a large, high altitude (3810 m a.s.l.) tropical lake (16 degrees S, 68 degrees W) that lies on the border of Bolivia and Peru, receiving high fluxes of ultraviolet radiation (UVR) throughout the year. Our studies were conducted during September of 1997 with the main objective of studying the impact of solar UVR upon phytoplankton photosynthesis. 2. Water samples were taken daily and incubated in situ (down to 14 m depth) under three radiation treatments to study the relative responses to PAR (Photosynthetic Available Radiation, 400-700 nm), UV-A (320-400 nm), and W-B (280-320 nm) radiation. 3. Photosynthetic inhibition by UVR in surface waters was about 80%, with W-A accounting for 60% and UV-B for 20%; the inhibition by high levels of PAR was less than 20%. The inhibition due to UVR decreased with depth so that there were no significant differences between treatments at 8.5 m depth. 4. The amount of inhibition per unit energy received by phytoplankton indicates that even though there was a significant inhibition of photosynthesis due to UVR, species in Lake Titicaca seem to be better adapted than species in high latitude environments. 5. The cellular concentration of UV-absorbing compounds, a possible mechanism of photoadaptation, was low in phytoplanktonic species. However, they were abundant in zooplankton, suggesting a high rate of bioaccumulation through the diet.