Zooplankton are the primary consumers of phytoplankton in lakes, the ocean and in deep slow-moving rivers where the principal energy pathway is through a predator-prey oriented food web. In highly eutrophic or organically polluted waters, where energy is diverted through detritus pathways, zooplankton may also be the transporter of that energy by consuming bacteria and detritus. Assuming that temperature is optimum and there is minimal inhibition by low DO or toxicity, the effectiveness of zooplankton consumption and production will usually be controlled by the size, quality and abundance of food (the phytoplankton), as well as by planktivory-the predation by planktivorous fishes. Control of their consumption and production effectiveness by those factors has often been related to the resulting change in their size structure. The role of planktivory can be manipulated to increase consumption of phytoplankton, improve water clarity and reduce nutrients. Results of such attempts at ‘biomanipulation’ are covered in Chapter 10, but the ecological principals supporting that technique will be discussed in this chapter.