Why can photosynthesis occur only in the photic zone of a lake?

In an inland water ecosystem, photosynthesis is restricted to the photic zone because this upper layer of the lake provides the necessary light required for the photochemical stage of the process.12

Within this zone, also known as the trophogenic zone, the intensity of light is sufficient to allow primary producers—such as algae, cyanobacteria, and various macrophytes—to produce biochemical energy through photosynthesis at a rate that exceeds their consumption through respiration and decomposition.12

Conversely, in the deeper aphotic zone (or tropholytic zone), light penetration is insufficient to support these processes. The boundary between these two regions is defined by a plane of compensation, where the rates of primary production and consumption are equivalent. Below this plane, the lack of adequate light makes the net production of energy through photosynthesis impossible, limiting the biological activity of primary producers to the upper, illuminated layers of the water column.1