Phytoplankton are typically found in the epilimnion zone of a lake, which is the uppermost layer of water. This zone is well-lit by sunlight, providing the necessary conditions for photosynthesis. Because phytoplankton are microscopic plants that rely on sunlight to grow and reproduce, they thrive in areas where sunlight penetrates sufficiently.
In addition to favorable light conditions, the epilimnion also has warmer temperatures compared to deeper layers, which further supports phytoplankton growth. The combination of abundant sunlight and warmer water makes the epilimnion the most productive zone for these vital organisms, contributing significantly to the aquatic food web and overall ecosystem health.