What primary role do most planktonic algae play in aquatic ecosystems?

Planktonic algae primarily play a crucial role in aquatic ecosystems by converting sunlight and nutrients into organic matter through the process of photosynthesis. This production of food is foundational for the food web in aquatic environments; planktonic algae serve as primary producers, meaning they form the basis of the diet for many marine and freshwater organisms, such as zooplankton, fish, and other aquatic species. This energetic input is vital for sustaining a diverse array of life forms and maintaining the health of aquatic ecosystems. In this way, planktonic algae not only support higher trophic levels but also help in regulating the overall productivity of the aquatic environment.