Phosphorus gain by bacterivory promotes the mixotrophic flagellate Dinobryon spp. during re-oligotrophication

Bacterivory by mixotrophic flagellates may contribute to theirnutrient supply, providing a competitive advantage in oligotrophicwaters. We hypothesized an increase in Dinobryon biomass duringthe re-oligotrophication process in the large and deep LakeConstance. To estimate whether bacterivory contributed substantiallyto the flagellates’ phosphorus supply, we determined ingestionrates. Dinobryon biomass increased with decreasing total phosphorusconcentrations in the lake over a period of 17 years (P = 0.0005).The promotion of Dinobryon biomass during re-oligotrophicationmay be explained by the increasing light availability due tothe decreasing biomass of other phytoplankton yielding a releasefrom competition. The date of the Dinobryon abundance maximumshifted to earlier time points in the year, probably becausea smaller phosphorus pool was depleted more quickly. Ingestionrates of Dinobryon ranged between 0.5 and 13 bacteria cell^–1h^–1 (0.2–5.4 fg C pg C^–1 h^–1), and clearancerates varied between 0.2 and 3.2 nL cell^–1 h^–1 (4–78pL pg C^–1 h^–1), leading to bacterial losses of upto 30% day^–1 of bacterial standing stock. The ingestionof bacteria covered 77% of the phosphorus need of the flagellateduring the period of maximum growth in 1996 (net growth rate0.34 day^–1), and it fully covered the need at all othertimes.