The shallowest section of seawater is called the eutrophic zone.

• A. Eutrophic zone
• B. Photic zone
• C. Mesopelagic zone
• D. Bathyal zone

Answer: (A)

The concept here is ocean layering by light and depth. The shallowest part of the seawater is the photic (also called the euphotic) zone—the upper layer where enough sunlight penetrates for photosynthesis, so organisms like phytoplankton can thrive. The depth of this zone varies with water clarity but is generally up to about 200 meters.

Eutrophic describes nutrient richness, not depth, so it isn’t a depth zone. The other two options refer to deeper parts of the ocean: the mesopelagic zone is the twilight layer roughly from 200 to about 1000 meters, and the bathyal zone sits even deeper, from about 1000 to 4000 meters. That makes the photic zone the shallowest.