What distinguishes the photic zone from the aphotic zone?

The distinction between the photic zone and the aphotic zone is primarily based on the availability of light. The photic zone is the upper layer of a body of water where sunlight penetrates, allowing for photosynthesis to occur. This is important because it enables the growth of phytoplankton, which forms the base of many aquatic food webs. In contrast, the aphotic zone lies beneath the photic zone and does not receive sufficient sunlight, making photosynthesis impossible in this region. Organisms that live in the aphotic zone rely on different energy sources, such as decaying matter that sinks down from the photic zone or chemosynthesis in certain environments. This clear distinction based on light availability defines the boundaries and characteristics of these zones in aquatic ecosystems.