ARE BACTERIA AN IMPORTANT FOOD SOURCE FOR ROTIFERS IN EUTROPHIC LAKES

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.