Bacterial grazing by phagotrophic phytoflagellates in a deep humic lake in northern Sweden |
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Authors: | Isaksson A; Bergstrom A; Blomqvist P; Jansson N |
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Institution: | Institute of Limnology, Uppsala University, Norbyvagen 20, S-752 36 Uppsala, Sweden; Department of Physical Geography, Umea University, 901 87 Umea, Sweden |
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Abstract: | Bacterial grazing was measured from June to August 1995 in Lake
Ortrasket, a deep brown-water lake in northern Sweden.
Mixotrophic chrysophytes were the dominating bacterivores at all times,
grazing 3-14% of bacterial standing stock daily. The effects of altered
nutrient supply and light availability 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. Furthermore, clearance rates were
not reduced after alleviation of nutrient limitation conditions. Rather,
phagotrophy may work as a relatively fixed attribute of the mixotrophic
community in this lake. When availability of dissolved nutrients is
restricted, phagotrophy permits the mixotrophs to outcompete other
phytoplankton, but they become less competitive at high nutrient
concentrations. The relative share of mixotrophs in relation to total
phytoplankton decreased considerably after enrichment with nitrogen +
phosphorus.
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