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. Field experiments were carried out during the ice-free period in Lago Paione Superiore (LPS, Italian Central Alps) in order to measure species-specific direct uptake of fluorescently labelled bacteria (FLB) by MxFl and Daphnia. Bacterial production of DNA and protein were measured as well as Dissolved Organic Carbon (DOC). Between ice melt and September Gymnodinium spp., Dinobryon sertularia, Chromulina spp. and Ochromonas sp. represented the most common groups of MxFl in the lake. On average they ingested 2.9, 3.7, 9.3 and 14.6 bacteria ind-1 h-1 respectively, whereas Daphnia ingested 1.9 bacteria 10^6 ind-1 h-1. However, the MxFl community ingestion rate was always higher than the rate of Daphnia except in mid August when the Daphnia population ingested 2.4 times more bacteria than MxFl. MxFl and Daphnia grazing did not show any significant correlation with bacterial abundance and biomass (ro Spearman, P > 0.05) but a significant correlation was found between MxFl grazing and Thy uptake at 9 m (ro Spearman = 0.821, P < 0.014, xy pairs = 7). From this study it appears that Daphnia actively feeds on bacteria when it first appears in the lake, but overall, the Daphnia population had a lower impact on bacteria than did mixotrophic flagellates.