In situ grazing measurements using fluorescent particles of 0.5, 2.4 a
nd 6.3 mu m diameter in eutrophic Lake Loosdrecht (The Netherlands) sh
owed that Anuraeopsis fissa, a small rotifer, filtered the smallest, b
acteria sized particles as efficiently or more efficiently than the la
rger particles. In contrast, three other rotifer species (Brachionus a
ngularis, Filinia longiseta and Pompholyx sulcata) filtered the bacter
ia-sized particles less efficiently than the larger particles. Both Ke
ratella cochlearis and Conochilus unicornis only ingested the bacteria
-sized particles. Anuraeopsis fissa had a higher uptake of fluorescent
bacteria-sized particles than K.cochlearis, both in 1 mu m filtrate o
f lake water and in lake water. Within both species, uptake did not di
ffer between juveniles and adults. When cultured on three different si
ze fractions of lake water (1, 3 and 15 mu m filtrate) in July, all ro
tifer species declined in numbers on the 1 and 3 mu m filtrates, while
A.fissa and B.angularis increased in numbers on the 15 mu m filtrate.
The high abundance of small bacteria in the lake water could not supp
ort rotifer populations. It is concluded that bacteria are not a suita
ble food source of high quality for A.fissa because its population doe
s not grow even though the bacterial concentration was higher than its
estimated threshold food concentration. In August, when individually
cultured, the mortality was high for all species, but especially for F
.longiseta. The lifespan of K.cochlearis was reduced in the 1 and 3 mu
m filtrates of lake water, compared with in the 15 mu m filtrate. The
lifespan of A.fissa was similar in all filtrates, but reproduction wa
s reduced in the 1 and 3 mu m filtrates, as in Keratella. On the 15 mu
m filtrate, their ages at first reproduction and growth rates did not
differ. Individuals of A,fissa older than 4 days showed a higher surv
ival in the 15 mu m filtrate than in the other two filtrates, as did K
.cochlearis throughout its life. Hence, bacteria seem to be a more imp
ortant food source for younger individuals of A.fissa than of K.cochle
aris.