A. Ocean dead zones, technically known as hypoxic zones, are areas in the ocean or large lakes where the concentration of dissolved oxygen in the water is so low that it can no longer support most aquatic life. These zones can occur naturally, but their frequency and size have been dramatically increased by human activities, making them a significant and growing threat to marine ecosystems and coastal economies worldwide.
B. The primary cause of modern dead zones is a process called eutrophication, which is triggered by an excess of nutrients, primarily nitrogen and phosphorus, entering a body of water. These nutrients come from sources like agricultural fertiliser runoff, sewage discharge, and industrial waste. The influx of nutrients fuels a massive bloom of algae and phytoplankton on the water''s surface.
C. When these algae die, they sink to the bottom and are decomposed by bacteria. This decomposition process consumes large amounts of oxygen from the water. In areas with poor water circulation, the oxygen is used up faster than it can be replenished from the atmosphere or surrounding waters. This leads to the creation of a hypoxic or, in extreme cases, an anoxic (completely without oxygen) zone. Fish and other mobile marine animals can flee the area, but slower-moving or stationary organisms like crabs, oysters, and corals are unable to escape and die.
D. The Gulf of Mexico, at the mouth of the Mississippi River, contains one of the largest and most well-known dead zones in the world, largely due to agricultural runoff from the vast US Midwest. The expansion of these zones disrupts marine food webs and can have a devastating impact on commercial fishing industries. Reducing the size and number of dead zones requires a significant effort to manage nutrient pollution at its source, such as improving agricultural practices to reduce fertiliser runoff and upgrading wastewater treatment facilities.
IELTS Practice Tests Practice Test / Part 1 #195
IELTS Practice Tests Practice Test / Part 1 #208
