EFFECTS OF ALLOCHTHONOUS DISSOLVED ORGANIC-MATTER AND INORGANIC NUTRIENTS ON THE GROWTH OF BACTERIA AND ALGAE FROM A HIGHLY HUMIC LAKE

The effects of allochthonous dissolved organic matter (DOM) on the gro wth of bacteria and algae from a highly humic lake were studied in a b atch culture experiment. The growth rates, cell numbers, and biomasses of bacteria were substantially higher in the presence of DOM than in cultures without DOM. At the stationary phase of growth, 3.4-5.9% of t he initial dissolved organic carbon (DOC) pool was consumed, and the g rowth efficiency of bacteria varied between 18-56% depending on nutrie nt and DOM concentration in the cultures. The addition of phosphate an d other extra nutrients resulted in substantially higher growth of bac teria and algae than without extra phosphate. In the presence of algae and protozoa, the cell numbers ail bacteria declined, demonstrating b acterial grazing by protists. Allochthonous DOM generally also enhance d the growth of algae. In particular, two chlorophyceans (Dictyosphaer ium and Chlamydomonas) and a diatom (Synedra berolinensis) grew better when DOM concentration increased. However, DOM had no detectable effe ct on the growth of Cryptomonas (Cryptophyceae). The results of this s tudy support the hypothesis that allochthonous DOM may strongly influe nce the structure and functioning of plankton food chains in humic lak es. (C) 1998 Elsevier Science Ltd.