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 growth of bacteria and algae from a highly humic lake were studied in a batch 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 the initial dissolved organic carbon (DOC) pool was consumed, and the growth efficiency of bacteria varied between 18-56% depending on nutrient and DOM concentration in the cultures. The addition of phosphate and other extra nutrients resulted in substantially higher growth of bacteria and algae than without extra phosphate. In the presence of algae and protozoa, the cell numbers ail bacteria declined, demonstrating bacterial grazing by protists. Allochthonous DOM generally also enhanced the growth of algae. In particular, two chlorophyceans (Dictyosphaerium and Chlamydomonas) and a diatom (Synedra berolinensis) grew better when DOM concentration increased. However, DOM had no detectable effect on the growth of Cryptomonas (Cryptophyceae). The results of this study support the hypothesis that allochthonous DOM may strongly influence the structure and functioning of plankton food chains in humic lakes. (C) 1998 Elsevier Science Ltd.