Are bacteria an important food source for rotifers in eutrophic lakes?

In situ grazing measurements using fluorescent particles of 0.5, 2.4 and 6.3 mu m diameter in eutrophic Lake Loosdrecht (The Netherlands) showed that Anuraeopsis fissa, a small rotifer, filtered the smallest, bacteria sized particles as efficiently or more efficiently than the larger particles. In contrast, three other rotifer species (Brachionus angularis, Filinia longiseta and Pompholyx sulcata) filtered the bacteria- sized particles less efficiently than the larger particles. Both Keratella 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 of lake water and in lake water. Within both species, uptake did not differ between juveniles and adults. When cultured on three different size fractions of lake water (1, 3 and 15 mu m filtrate) in July, all rotifer 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 support rotifer populations. It is concluded that bacteria are not a suitable food source of high quality for A.fissa because its population does 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 was 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 survival 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 important food source for younger individuals of A.fissa than of K.cochlearis. [KEYWORDS: Fresh-water zooplankton; keratella-cochlearis;population-dynamics; resource utilization; natural-populations; planktonic rotifers; filinia-terminalis; seasonal patterns; clearance rates; size selection]