Importance of pigmented and colourless nano-sized protists as grazers on nanoplankton in a phosphate-depleted Norwegian fjord and in enclosures

Particulate food sizes of nano-sized colourless flagellates, ciliates, and mixotrophic dinoflagellates were investigated as well as the competition between mixotrophic and strictly heterotrophic protists. Samples were collected during 1 wk in July 1995 from the phosphate-depleted surface layer of the Hylsfjord, Norway, and from nutrient-manipulated enclosures. Grazing experiments conducted on the last day of the study period, using fluorescently labelled algae (FLA) and bacteria (FLB), suggested that phagotrophic protists, 10-20 mu m in size, had a considerable grazing impact on nanoplankton, but not on picoplankton. In contrast, smaller colourless flagellates, 5-10 mu m in size, ingested FLB significantly. In phosphate-enriched enclosures, the growth of strictly phototrophic protists and of heterotrophic bacteria was stimulated. At the end of the study period, the biomass of colourless flagellates and ciliates was also much higher in phosphate-enriched enclosures than in non-phosphate-enriched enclosures and the surface layer of the fjord. In contrast, mixotrophic dinoflagellates had a similar biomass regardless of phosphate enrichment. Mixotroph share of protist grazing on FLA-sized, 2-5 mu m prey was on average 34% in non-phosphate-enriched environments and 12% in phosphate-enriched enclosures. The additional phototrophic mode of nutrition probably gave the mixotrophs better means to compete with strictly heterotrophic protists in environments where the prey density or production was low. The finding that both pigmented and colourless nanoprotists had a considerable grazing impact on 2-5 mu m protists (part of the nano size fraction) is important for the understanding of the relationships between organisms of the microbial food web.