AME 19:67-80 (1999)  -  doi:10.3354/ame019067

ABSTRACT: An experimental study was conducted using large enclosures to examine the effects of nutrients, presence of planktivorous fish, and depth of the water column on the components of the microbial food web. Enclosures (8 m in diameter) were installed, open to the sediments, at both 3.5 and 11 m depths in Lac Croche, a small oligotrophic lake on the Canadian Shield. The abundance of protozoans, including obligate heterotrophic flagellates (HNAN) and potentially mixotrophic flagellates (MIXO), was examined in relation to the abundance of their prey, both heterotrophic bacteria (HBAC) and picocyanobacteria (PPICO). HBAC levels increased over the summer and were significantly higher in the nutrient enriched enclosures. While HBAC did not respond in a consistent manner to the presence of fish, the effect of nutrients was greater when fish were present. HBAC levels did not differ significantly between the shallow and deep enclosures. In contrast, PPICO were significantly affected by all 3 independent variables; nutrients had a strong overall negative effect on abundance and the presence of fish tended to increase their abundance particularly in the shallow enclosures. Similarly, nutrient additions resulted in significantly lower HNAN levels, but neither the presence of fish nor depth of the water column significantly affected HNAN. However, the lowest average HNAN concentrations were in the deep nutrient enriched enclosures without fish, where the highest populations of Daphnia middendorffiana developed. MIXO were most abundant early in the season (exceeding HNAN) but declined significantly by mid-summer and were generally not as abundant as HNAN. MIXO levels were significantly lower in the nutrient enriched enclosures. Depth of the water column had a significant effect on MIXO, but MIXO levels were not affected in a consistent manner by the presence of fish. Experimental results did not demonstrate that increases in nutrient levels would lead to increases in HNAN as current empirical models imply. There was no increase in HNAN abundance related to increased bacterial abundance.

KEY WORDS: Microbial food web · Picoplankton · Nutrients · Zooplankton · Top-down control