Effects of nutrients, planktivorous fish and water column depth on components of the microbial food web

An experimental study was conducted uslng large enclosures to examine the effects of nutrients, presence of planktivorous fish, and depth of the water colun~n 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 nl depths in Lac Croche, a small oligotrophic lake on the Canadian Shield The abundance of protozoans, including obligate heterotrophic flagellates (HNAN) and potentially mutotrophic flagellates (MIXO), was examined in relatlon to the abundance of thelr prey, both heterotrophlc bacteria (HBAC) and picocyanobactena (PPICO). HBAC levels increased over the summer and were s~gnificantly higher in the nutrient enriched enclosures. While HBAC dld 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 slgnificantly between the shallcw and deep enclosures. In contrast, PPICO were signlf~cantly dffected 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 sigruficantly lower HNAN levels, but neither the presence of fish nor depth of the water column slgniflcantly affected HNAN. However, the lowest average HNAN concentrations were in the deep nutrient enriched enclosures without fish, where the highest populatlons 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 m a consistent manner by the presence of fish. Experimental results did not demonstrate that Increases in nutnent levels would lead to Increases in HNAN as current empirical models imply. There was no increase in HNAN abundance related to Increased bacterial abundance.