Less oxygen dissolved in the water is often referred to as a “dead zone” because most marine life either dies, or, if they are mobile such as fish, leave the area. Habitats that would normally be teeming with life become, essentially, biological deserts.
Factors responsible for creation of dead zones
Hypoxic zones can occur naturally, but scientists are concerned about the areas created or enhanced by human activity. There are many physical, chemical, and biological factors that combine to create dead zones, but nutrient pollution is the primary cause of those zones created by humans. Excess nutrients that run off land or are piped as wastewater into rivers and coasts can stimulate an overgrowth of algae, which then sinks and decomposes in the water. The decomposition process consumes oxygen and depletes the supply available to healthy marine life.
Dead zones in the world
Dead zones occur in many areas of the world, particularly along the East Coast, the Gulf of Mexico, and the Great Lakes region of USA . The largest dead zone in the world is roughly 165000 km² area in the Gulf of Oman.The second largest dead zone in the world is located in the U.S., in the northern Gulf of Mexico.
Other areas are in Scandinavia’s strait called the Kattegat, which is the mouth of the Baltic Sea and in other important Baltic Sea fishing grounds, in the Black Sea, in the northern Adriatic , coastal waters of South America, China, Japan and New Zealand.
Effects of Dead zones
Most marine life either dies, or, if they are mobile such as fish, leave the area . Also low oxygen levels recorded along the Gulf Coast of North America have led to reproductive problems in fish involving decreased size of reproductive organs, low egg counts and lack of spawning.