A. Ojala et al., GROWTH OF DAPHNIA-LONGISPINA L IN A POLYHUMIC LAKE UNDER VARIOUS AVAILABILITIES OF ALGAL, BACTERIAL AND DETRITAL FOOD, Hydrobiologia, 315(2), 1995, pp. 119-134
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.