Description of the Role of Zooplankton in a Food Web

Zooplankton as Primary Consumers

• Most zooplankton consume bacteria and phytoplankton (i.e., plant plankton)--the primary producers. Therefore, these zooplankton occupy the role of primary consumer in the aquatic food web. Primary consumers eat organisms called primary producers or autotrophs, which obtain their energy from inorganic sources. For example, many of the phytoplankton obtain energy from the sun through photosynthesis. In some areas, grazing by zooplankton can be so extreme that it temporarily eliminates phytoplankton from the food web.

Zooplankton as Secondary Consumers

• Some zooplankton prey on other zooplankton--the primary consumers that eat bacteria or phytoplankton. Because these predatory zooplankton rely on herbivores rather than primary producers for their source of nutrition, they are considered secondary consumers and occupy a higher position in the food web.

Other Secondary Consumers

• Filter-feeding fishes, including manta rays and whale sharks, consume zooplankton and are also considered to be secondary consumers. Other secondary consumers include baleen whales, which also are filter feeders. These secondary consumers occupy a position in the food web that usually is considered to be higher overall than zooplankton but can also be considered equal to that of zooplankton species that are secondary consumers.

Tertiary Consumers

• Carnivorous fishes, mammals and reptiles in aquatic food webs prey on secondary consumers. These predators occupy the position of tertiary consumer or top predator in the food web and do not rely directly on zooplankton for sustenance. However, the majority of their prey consume zooplankton, without which the top predators could not survive.