SHORT COMMUNICATION Grazing of planktonic diatoms by microflagellates

Abstract. Colorless microflagellates (6—14 jim in diameter) were isolated from an oligotrophic lake in Western Canada, and shown to consume diatoms up to six times longer than the diameter of the flagellate. Observations were also made of morphologically similar flagellates attached to diatoms from the Great Lakes, indicating that this phenomenon may occur in nature. It is suggested that predation by microflagellates on much larger algal cells could potentially affect the pathway of nutrient and energy transfer in aquatic foodwebs. Considerable evidence has accumulated in recent years demonstrating that in aquatic ecosystems heterotrophic microflagellates (HMF) are important consumers of bacteria and other similarly sized planktonic organisms, mainly cyanobacteria (for review see: Sherr and Sherr, 1984). It has also been suggested that these flagellates may be more important than bacteria as remineralizers of nutrients in some aquatic foodwebs (Azam et al., 1983). The 'classic ' pathway of nutrient and energy flow, from relatively large photoautotrophs such as diatoms and dinoflagellates into metazoan grazers such as copepods, is not the only significant one; the 'microbial loop ' through HMF (Figure 1) may be equally important (Pomeroy, 1974; Azam et al., 1983; Sherr and Sherr, 1984). In this report we provide preliminary evidence that HMF capable of consumin