As the pH of seawater falls, the availability of which ion needed by shell-building organisms decreases?
- A.Chloride
- B.Carbonate
- C.Sodium
- D.Sulphate
Correct answer
Explanation
The correct answer is B, carbonate. The hydrogen ions released when carbonic acid dissociates combine with carbonate ions to form bicarbonate, so the pool of free carbonate shrinks. Corals, molluscs, sea urchins, coralline algae and the planktonic pteropods build their skeletons and shells from calcium carbonate, and they must spend more energy to do so as carbonate becomes scarce. Where water turns undersaturated in aragonite, the form corals use, existing structures begin to dissolve. Option A is wrong because chloride, paired with sodium, is the major ion of sea salt and is not consumed in this reaction. Option C is wrong for the same reason: sodium is abundant and unaffected by the carbonate chemistry. Option D is wrong because sulphate, though a major seawater ion, plays no part in building calcium carbonate skeletons.
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 →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
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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.
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.
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