What are the primary herbivores found in the open water zone?

Zooplankton are the primary herbivores found in the open water zone of aquatic ecosystems. They play a critical role in these environments as they feed on phytoplankton, which are microscopic plants that perform photosynthesis and are a fundamental component of the aquatic food web. By consuming phytoplankton, zooplankton help to regulate their populations and maintain a balance in the ecosystem. Their presence also serves as a food source for larger predators, such as fish, contributing to the overall health and productivity of the aquatic environment. In contrast, other groups like fish and crustaceans can be herbivorous or carnivorous depending on the species and life stage, while insects are generally more associated with terrestrial or benthic habitats. Thus, zooplankton stand out for their role as primary herbivores in open water settings.