GROWTH OF DAPHNIA-LONGISPINA L IN A POLYHUMIC LAKE UNDER VARIOUS AVAILABILITIES OF ALGAL, BACTERIAL AND DETRITAL FOOD

The availability and importance of food sources for growth of Daphnia longispina L. from a highly coloured fishless lake with anoxic hypolim nion were assessed by combining in situ and laboratory experiments. In in situ experiments populations were enclosed in tubes with natural t emperature stratification and with or without anoxic hypolimnion. In t he laboratory experiments the importance of food source (littoral zone vs pelagic epilimnion) was assessed by enclosing moss thalli and a na tural zooplankton population in a large-scale flow-through system supp lying food for experimental Daphnia. Growth of juveniles of Daphnia in epilimnetic water was determined in batch culture experiments and the importance of increasing concentrations of bacteria and algae for the ir growth and development was investigated with a small-scale flow-thr ough system. Access to the anoxic hypolimnion enhanced the growth of D aphnia by 23-24%. Growth rates in the tubes with anoxic hypolimnion we re 0.36 and 0.16 d(-1) in July and August respectively In tubes withou t anoxia the corresponding values were 0.29 and 0.13. In batch-culture s the highest growth rate determined was 0.16 and the overall rates we re lower than in in situ experiments. In batch culture Daphnia was abl e to grow in darkness for 10 days with a rate of 0.16. In the large-sc ale flow-through system Daphnia population fed with littoral water rep roduced well despite the low concentration of algae and increased its number by a factor of c. 32 in 10 days. However, the animals were smal l and net production of Daphnia population thus lower under the littor al influence than in the control treatment. Population could survive a nd grew slowly on pelagial water processed by a natural zooplankton co mmunity and with very little algae left. It is thus possible that bact eria serve as a 'life-support system' enabling the population survival over periods of algal shortage. Small-scale flow-through experiments revealed that Daphnia longispina is able to mature and reproduce on a bacterial diet if the food concentration is high enough and Daphnia on bacterial food could achieve growth rates similar to those on an alga l diet. The threshold food level for Daphnia longispina was estimated to be c. 18.5 mu g C l(-1). Detrital material is of limited value in n utrition of Daphnia even in a lake where more than 75% of carbon is bo und in particulate detritus.