Final answer:
A "dead zone" is an area in aquatic environments with very low dissolved oxygen, leading to the death of flora and fauna. The correct answer to the fill-in-the-blank question is A) Dead Zones.
Step-by-step explanation:
A "dead zone" occurs when the aquatic environment is very low on dissolved oxygen. The correct answer is A) Dead Zones. A dead zone is an area in lakes and oceans usually near the mouths of rivers, where large areas are depleted of their normal flora and fauna.
These zones can result from a process known as eutrophication, along with other contributing factors such as oil spills, dumping of toxic chemicals, and various human activities. The number of dead zones has been increasing, and as of 2008, there were more than 400 known dead zones globally.
The process whereby oxygen is depleted by the growth of microorganisms due to excess nutrients in aquatic systems is called eutrophication.
These excessive nutrients, particularly nitrogen (N) and phosphorus (P), often come from sources like agricultural runoff, including fertilizers, sewage, and phosphorus-based detergents.
When excessive algae grow and then decay, they consume large amounts of oxygen, creating low-oxygen conditions (< 2 ppm O2), a state referred to as hypoxia. This leads to the formation of dead zones where aquatic life may suffocate and die due to a lack of oxygen. Option A.