Bacteria, cyanobacteria, protozoa and small eukaryotic phytoplankton are now known to be fundamental components of marine planktonic ecosystems (Azam et al., 1983; Sieburth, 1984). These organisms have high potential growth rates, are efficient converters of material and energy, and usually constitute most of the plankton biomass (Davis et al., 1985; Fuhrman et al., 1989). Their different trophic levels are often closely coupled. These characteristics require that experimental and sampling designs for studying the microbial populations incorporate appropriate (i.e., small) temporal and spatial scales. The corollary to this is that large numbers of samples must be taken. If these can be processed rapidly, sampling designs can be modified to existing conditions. Quantifying the abundance and biovolume of these microbial cells is difficult because these organisms are extremely small and cell biovolume is a cubic function of linear dimensions. These difficulties are major obstacles to developing realistic trophodynamic models.