INFLUENCE OF LIGHT ON BACTERIOPLANKTON IN A TROPICAL LAKE

Bacterial abundance and production in a vertical profile in Lake Karib a (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 a t 10 m (2.5 x 10(9) bacteria l(-1)), then decreasing with depth. Bacte rial production at the surface (0.145 mu g C l(-1) h(-1)) was nearly f our times less than production at 10 m although bacterial numbers were only 26% less. Thus, bacterial production per cell was lower at the s urface than deeper down, suggesting that bacterial production is inhib ited at the surface. Bacterial production in GF/F filtered lake water in Whirl Pack(TM) bags showed an exponential decrease down to 3 m dept h. 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 phototran sformation of recalcitrant DOM, which substitutes enzyme activity. Hyp olimnetic enzyme activity was less affected by solar light than epilim netic.