Eutrophication of coastal waters leads most directly to which condition?
- A.A rise in dissolved oxygen that helps fish
- B.Depletion of dissolved oxygen and the formation of dead zones
- C.A fall in water temperature
- D.An increase in water salinity
Correct answer
Explanation
The correct answer is B. Eutrophication begins with nitrogen and phosphorus from fertiliser, detergent and sewage. The nutrients feed an algal bloom; the bloom is short-lived, and when it dies the bacteria that decompose the mass consume large amounts of dissolved oxygen. The surface mat also blocks light to plants below. Oxygen falls to hypoxic levels and the area becomes a dead zone, where fish leave or suffocate and bottom-dwelling life dies. Option A is wrong because, although living algae release oxygen by day, the net effect of the bloom-and-decay cycle is a loss of oxygen. Option C is wrong because eutrophication is a nutrient process and does not cool water; thermal pollution raises temperature. Option D is wrong because salinity is governed by evaporation, freshwater inflow and ice formation, not by nutrient loading.
Read the full article: Marine Pollution, Oil Spills and Ocean Acidification
Practice Questions
View allOcean acidification is caused mainly by the absorption of which gas by seawater?
- A.Nitrogen
- B.Carbon dioxide
- C.Methane
- D.Sulphur dioxide
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Carbon dioxide dissolves in seawater and reacts with water to form carbonic acid, which dissociates into bicarbonate and hydrogen ions; the extra hydrogen ions lower the pH, and they also combine with carbonate ions, leaving less carbonate for shells and coral skeletons.
Read the full explanation →Minamata disease, first recognised in Japan, is caused by poisoning from which element?
- A.Cadmium
- B.Arsenic
- C.Mercury
- D.Lead
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Methylmercury discharged with factory effluent into Minamata Bay entered fish and shellfish, biomagnified up the food chain and poisoned the people who ate them, producing the nervous disorder named after the bay and recognised in 1956.
Read the full explanation →Particles of plastic smaller than which size are classified as microplastics?
- A.5 millimetres
- B.5 centimetres
- C.5 micrometres
- D.5 nanometres
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Microplastics are conventionally defined as plastic particles less than five millimetres across.
Read the full explanation →The Exxon Valdez oil spill of 1989 took place in which waters?
- A.Gulf of Mexico
- B.Prince William Sound, Alaska
- C.Persian Gulf
- D.Bay of Bengal
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The tanker Exxon Valdez struck a reef in Prince William Sound in 1989, releasing crude oil that coated a long stretch of a cold, sheltered coastline where oil breaks down slowly, and killing large numbers of seabirds, sea otters and salmon.
Read the full explanation →MARPOL, the convention of 1973 with its Protocol of 1978, deals with which subject?
- A.Prevention of pollution from ships
- B.Trade in endangered species
- C.Protection of wetlands of international importance
- D.Transboundary movement of hazardous waste
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MARPOL is the International Convention for the Prevention of Pollution from Ships, adopted in 1973 and modified by the Protocol of 1978, and it is the main pollution instrument of the International Maritime Organization.
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