The factors controlling heterotrophic flagellate abundance in temperate lakes.

Abstract

The "microbial loop" of aquatic systems consists of picoplankton (heterotrophic and photosynthetic), nanoflagellates (heterotrophic and mixotrophic forms), and ciliates. Heterotrophic flagellates (HNAN) are considered to be the principal grazers of both photosynthetic picoplankton (PICO) and heterotrophic bacteria (HBAC). The factors controlling PICO and HBAC abundances have been studied extensively. However, the factors which regulate HNAN remain less clear. An experimental study was conducted to examine the effects of nutrients and planktivorous fish additions on HNAN in freshwater systems using large enclosures (8m in diameter) installed at both 4 m and 11 m depths in a small oligotrophic lake. In addition to obligate heterotrophs, mixotrophic flagellates (MIXO) were also examined as were their prey, HBAC and PICO. Experimental results did not demonstrate that increases in nutrient levels would lead to increases in HNAN as current empirical models imply. The relationship between HBAC and HNAN was therefore examined within the range of trophy manipulated in the enclosures. A lake survey was conducted in dimictic lakes in Ontario and Western Quebec. (Abstract shortened by UMI.)