Influence of light on bacterioplankton in a tropical lake

Bacterial abundance and production in a vertical profile in Lake Kariba (17 degrees S), Zimbabwe, were affected by solar irradiance. At the surface, 1.87 x 10(9) bacteria l(-1) were found and abundance peaked at 10 m (2.5 x 10(9) bacteria l(-1)), then decreasing with depth. Bacterial production at the surface (0.145 mu g C l(-1) h(-1)) was nearly four times less than production at 10 m although bacterial numbers were only 26% less. Thus, bacterial production per cell was lower at the surface than deeper down, suggesting that bacterial production is inhibited at the surface. Bacterial production in GF/F filtered lake water in Whirl Pack(TM) bags showed an exponential decrease down to 3 m depth. The inhibition was well in accordance with light extinction in the UV region. Phosphatase activity was low in light exposed bags compared to dark, indicating photolysis of extracellular enzymes, or phototransformation of recalcitrant DOM, which substitutes enzyme activity. Hypolimnetic enzyme activity was less affected by solar light than epilimnetic.