What defines the bottom of the photic zone in aquatic environments?

The depth at which light intensity is equivalent to one percent of full sunlight is crucial for defining the bottom of the photic zone in aquatic environments. The photic zone is the upper layer of a water body where sufficient sunlight penetrates to allow photosynthesis to occur. This layer is typically well-lit, extending from the surface down to where light begins to diminish substantially.

At approximately one percent of full sunlight, most aquatic plants and algae can no longer perform photosynthesis effectively. This threshold marks the lower boundary of the photic zone, beyond which light becomes too weak to support the primary production necessary for sustaining most aquatic life forms reliant on these photosynthetic organisms.

In contrast, the presence and abundance of fish species, the ability of aquatic plants to survive, or the thriving of algae do not directly determine this boundary; rather, they are influenced by the availability of light within the photic zone. Hence, the correct understanding of the photic zone's lower limit is directly related to light intensity, making this choice the correct answer.