Bacterial grazing by mixotrophic flagellates and Daphnia longispina: a comparison in a fishless alpine lake

We compared the bacterivory of mixotrophic flagellates (MxFl) and of Daphnia longispina in a fishless high altitude lake with a simple food web. During the ice-free period of Lago Paione Superiore (LPS, Italian Central Alps) we measured species-specific direct uptake of fluorescently labelled bacteria (FLB) by MxFl and D. longispina. Bacterial production of DNA and protein and dissolved organic carbon (DOC) were also measured. Between ice melt and September, the most common groups of MxFl in the lake were Gymnodinium spp., Dinobryon sertulaiia, Chromulina spp. and Ochromonas sp. On average they ingested 2.9, 3.7, 9.3 and 14.6 bacterial cells ind.(-1) h(-1), respectively, whereas D. longispina ingested 1.9 x 10(6) bacterial cells ind.(-1)h(-1). The MxFl community ingestion rate was always higher than the rate of D. longispina, except in mid August when the D. longispina population ingested 2.4 times more bacteria than MxFl. Rates of MxFI and D. longispina grazing did not show any significant correlation with bacterial abundance and biomass (p Spearman, p > 0.05), but a significant correlation was found between MxFI grazing and thymidine uptake at 9 m (p Spearman = 0.821, p < 0.014, xy pairs = 7). It appears that D. longispina actively feeds on bacteria when it first appears in the lake, but that overall the D. longispina population had a lesser impact on bacteria than did MxFl.