ABSTRACT
Organisms in the plankton are generally assigned to one of
three compartmental groups: bacterioplankton, phyto-
plankton, and zooplankton. These groups are further
subdivided into trophic groups on the basis of taxonomic
categories well above the species level. Unfortunately, this
causes organisms with differing modes of nutrition to be
grouped together, e.g., non-photosynthetic flagellates are
grouped with the algae and are considered to be phyto-
plankton, while other protozoan groups like ciliates and
sarcodinians are assigned to the zooplankton as micro-
plankton. To overcome these and other problems, Siebruth
et al. (1978) proposed a scheme based on the level of
organization, i.e., the ultrastructure, and the mode of nutri-
tion (Figure 2.1).
