Bacterial grazing by phagotrophic phytoflagellates in a deep humic lake innorthern Sweden

Bacterial grazing was measured from June to August 1995 in Lake Ortrasket, a deep brown-water lake in northern Sweden. Mixotrophic chrysophytes were t he dominating bacterivores at all times, grazing 3-14% of bacterial standin g stock daily. The effects of altered nutrient supply and light availabilit y on grazing activity and growth were studied in two mesocosm experiments. Incubation in the dark did not stimulate phagotrophy, which would otherwise be expected if bacteria were mainly being used as an energy source. Furthe rmore, clearance rates were not reduced after alleviation of nutrient limit ation conditions. Rather, phagotrophy may work as a relatively fixed attrib ute of the mixotrophic community in this lake. When availability of dissolv ed nutrients is restricted, phagotrophy permits the mixotrophs to outcompet e other phytoplankton, but they become less competitive at high nutrient co ncentrations. The relative share of mixotrophs in relation to total phytopl ankton decreased considerably after enrichment with nitrogen + phosphorus.