UPSC Mains 2018 GS1 Q7: Consequences of Spreading Dead Zones on Marine Ecosystems

A dead zone is an area of water with too little oxygen to support most marine life. This condition is called hypoxia.

Excess nutrients from fertilizer runoff and sewage trigger algae blooms. When the algae die and decompose, the process consumes dissolved oxygen faster than it can be replenished.

Fish and other mobile species can flee an expanding dead zone. This displaces them into unfamiliar habitats, disrupting normal feeding and breeding patterns.

Bottom-dwelling species like crabs, clams, and worms often cannot escape. Many die en masse once oxygen drops below survivable levels.

Losing bottom-dwelling species removes a key food source for larger predators. This disruption ripples up through the marine food web, not just at the point of oxygen loss.

Repeated or chronic hypoxia events reduce local species diversity over time. Areas that once supported varied marine life can become dominated by only the most oxygen-tolerant species.

Dead zones directly cut commercial fish and shellfish catches. Coastal fishing communities that depend on these fisheries bear this economic cost most directly.

Dead zones are spreading globally, with the UN estimating over 500 worldwide, roughly doubling every decade since the 1960s. Chronic zones, like the Baltic Sea’s, no longer clear seasonally the way some others do.

Warmer water holds less dissolved oxygen and strengthens the water-layer stratification that blocks oxygen mixing. Both effects worsen dead zones independent of any change in nutrient pollution.

Briefly define dead zones and their cause, then work through consequences in order: species displacement and die-off, food web disruption, biodiversity loss, and the economic hit to fisheries, noting the compounding effect of climate change.