ABSTRACT
Nonphotosynthetic protists with cell diameters less than 20 µm are known to be important grazers of both phytoplankton and bacteria. These organisms are usually referred to as heterotrophic nanoplankton. To evaluate their position in planktonic food webs, efforts have been made to measure their abundance and growth rates, and to discover their fate in natural waters. Some investigators have measured changes in heterotrophic nanoplankton abundance in situ to estimate growth rates, especially to evaluate diel or other variations in population dynamics. Since most evidence indicates that heterotrophic nanoplankton populations are closely controlled by grazers, however, net population growth rates may be much smaller than cell division rates. Advantages of the method include: growth is observed directly as change in cell abundance rather than in some biochemical surrogate such as adenosine triphosphate or carbon; it is not technically difficult to perform; and much additional information can be obtained from the microscopy.
