What are the consequences of spreading of ‘Dead Zones’ on marine ecosystem?
Subtopic: Geography · ocean dead zones and marine ecology
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Detailed model answer
189 words · target 150 words · 9 min
What dead zones are
Dead zones are hypoxic areas of the ocean where dissolved oxygen falls too low to sustain most marine life. They arise mainly from eutrophication - excess nitrogen and phosphorus from fertiliser runoff, sewage and industrial effluent triggering algal blooms whose decomposition strips the water of oxygen. The Gulf of Mexico, the Baltic Sea and the naturally occurring but expanding Arabian Sea oxygen-minimum zone are prominent examples.
Consequences for marine ecosystems
- Mass mortality of fish and bottom-dwelling (benthic) organisms, and a sharp loss of marine biodiversity.
- Collapse of fisheries and the coastal livelihoods and food security that depend on them.
- Disruption of food chains, with hardy jellyfish and microbes replacing fish; habitat compression forces mobile species to migrate to oxygen-rich waters.
- Altered biogeochemistry - release of the greenhouse gas nitrous oxide and toxic hydrogen sulphide.
Climate change worsens the problem, since warmer water holds less dissolved oxygen and strengthens stratification. Curbing nutrient pollution, restoring wetlands and improving coastal management are essential to reverse the spread of over 400 dead zones now recorded worldwide. International cooperation and stricter effluent norms are equally important to protect ocean health and coastal communities.
What an examiner expects to see
- Dead zones are hypoxic ocean areas where dissolved oxygen is too low for most marine life.
- Cause: eutrophication - nitrogen and phosphorus from fertiliser runoff and sewage feed algal blooms whose decomposition consumes oxygen.
- Examples include the Gulf of Mexico, the Baltic Sea and the expanding Arabian Sea oxygen-minimum zone.
- Consequences: mass mortality of fish and benthic organisms and loss of marine biodiversity.
- Fisheries and dependent livelihoods and food security collapse; food chains are disrupted as jellyfish and microbes replace fish.
- Biogeochemical shifts release nitrous oxide (a greenhouse gas) and toxic hydrogen sulphide; habitat compression forces species to migrate.
- Climate change worsens hypoxia as warmer water holds less oxygen; over 400 dead zones now exist worldwide.
Concrete cases, schemes and judgments
- Gulf of Mexico dead zone from Mississippi nutrient runoff
- Arabian Sea oxygen-minimum zone, among the world's largest and expanding
- Baltic Sea dead zones