Final answer:
Dead zones are typically worst in shallow water due to factors like nutrient levels and oxygen availability. The Gulf of Mexico has one of the worst dead zones caused by fertilizer runoff.
Step-by-step explanation:
Dead zones, areas depleted of normal flora and fauna, are typically worst in shallow water. This is because water near the surface of lakes and the ocean usually has more dissolved oxygen than deeper water. Surface water absorbs oxygen from the air above it, while water near the bottom may contain more nutrients due to decomposition of dead organisms. Factors such as eutrophication, oil spills, and dumping of toxic chemicals can contribute to the creation of dead zones. One of the worst dead zones is off the coast of the United States in the Gulf of Mexico, where fertilizer runoff from the Mississippi River basin has created a dead zone of over 8,463 square miles.