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

The photic zone in aquatic environments is characterized by the penetration of sunlight, which is essential for photosynthesis. The depth of the photic zone is defined as the point where light intensity diminishes to about one percent of full sunlight. This threshold is crucial because it delineates the area where sufficient light exists for photosynthetic organisms, such as phytoplankton and aquatic plants, to thrive.

At depths beyond this point, light is insufficient for photosynthesis, which significantly affects the distribution and abundance of aquatic life. Therefore, the first option, regarding fish abundance, does not directly define the photic zone, as fish can inhabit various depths depending on many factors, including species and habitat preferences. The third option, concerning aquatic plants' survival, is related, as it does describe conditions beyond the photic zone but does not specify the light intensity criterion. The fourth option about the thriving of algae is also indirectly related but again does not specify the light intensity metric necessary to accurately define the bottom of the photic zone.

Hence, identifying the depth at which light intensity drops to one percent of full sunlight provides a clear, measurable criterion for defining the lower boundary of the photic zone in aquatic ecosystems.