Herbivory is an important process in planktonic ecosystems. Small, rapidly growing microalgae are nutritious and often poorly defended; herbivorous zooplankton populations can thus develop. These can remove a relatively great fraction of primary production. Feeding modes in the freshwater zooplankton include phagocytosis, filter feeding, and individual particle selection. Each of these modes has different relationships between the size of the predator and the size of the prey. Zooplankton are a trophic bridge between the remainder of the ecosystem. Herbivorous zooplankton in lakes sometimes greatly reduce the abundance of planktonic algae; this occurs regularly in late spring in many lakes, resulting in a several-week period called a clear water phase. It also occurs at times throughout the summertime in other habitats such as productive shallow lakes. Such high levels of herbivory also affect the penetration of heat and thus thermal stratification patterns. Zooplankton also play a role in recycling nutrients into the water column; these relations have a strong stoichiometric component because zooplankton species vary in their nutrient content, which affects their nutrient recycling patterns. The identity of an herbivore trophic level in lake ecosystems is apparent when we look at many studies involving how alterations in fish communities affect lake ecosystems.