Oceanography Questions and Answers

Answer: C

Corals are confined to low and middle latitudes because of suitable temperature conditions.

Reef-building corals need warm water that does not fall below about 20°C, with the best growth between about 23°C and 29°C. Water that warm is found only in the tropical and subtropical seas, roughly between 30°N and 30°S, and along coasts washed by warm currents. Cold currents keep reefs away even from tropical coasts, as on the western side of the continents.

The other options do not explain the pattern. Heavy rain and fresh river water dilute the sea and kill corals, so high rainfall is a hindrance. Hot and humid air describes the climate on land, not the water corals live in. Corals do need normal salinity, but suitable salinity is found in all latitudes, so it is not what limits them to warm seas.

Answer: A

The producer of the marine ecosystem is phytoplankton.

Phytoplankton are microscopic drifting algae and cyanobacteria, such as diatoms and dinoflagellates, that photosynthesise in the sunlit surface layer. They make organic matter from carbon dioxide and sunlight and so form the first trophic level on which nearly all marine food chains rest; they also supply a large share of the world's oxygen.

The others are consumers or decomposers. Zooplankton are tiny animals that graze on phytoplankton, so they are primary consumers. Small fish eat zooplankton and are higher consumers. Fungi break down dead organic matter and are decomposers. None of them makes its own food from sunlight.

Answer: B

The key's answer is isostatic rebound.

When an ice sheet melts, the land that was pressed down by its weight slowly rises again. As the land rises, the sea level relative to that coast falls, leaving raised beaches high above today's shore, as around Scandinavia and Hudson Bay.

Melting of ice sheets returns water to the oceans and raises sea level. Evaporation does not lower sea level, since the water comes back as precipitation and runoff. Hydroisostasy is the sinking of the ocean floor under the weight of added water, a side effect of a sea-level rise, not a cause of a fall.

Answer: C

The answer is (A) is true but (R) is false.

Phytoplankton are the producers of aquatic food chains, and zooplankton feed on them; ecologists describe phytoplankton populations as the grazing grounds of the zooplankton, so the Assertion is true.

The Reason is false, because zooplankton are exactly the grazers in question: copepods, krill and other small animals graze on phytoplankton much as cattle graze on grass. A statement that contradicts the Assertion's own premise cannot explain it.

Answer: A

Ocean gyres result mainly from prevailing winds and the Coriolis effect.

The trade winds drive surface water westward near the equator and the westerlies drive it eastward in mid-latitudes. The Coriolis effect deflects moving water to the right in the northern hemisphere and to the left in the southern, and the continents block the flow, so the currents join into large closed loops: clockwise gyres in the northern oceans and anticlockwise gyres in the southern.

Diversion of currents by coastlines is part of the pattern, not its cause. The global conveyor belt and salinity differences drive the deep thermohaline circulation, not the wind-driven surface gyres.

Answer: C

The sunken reef is associated with the atoll.

In Darwin's subsidence theory, a fringing reef grows around a volcanic island. As the island slowly sinks, the coral keeps growing upward and becomes a barrier reef separated from the shore by a lagoon. When the island has sunk completely below the sea, only the ring-shaped reef remains around a central lagoon: an atoll, built on a sunken foundation.

A fringing reef is attached to the coast, and a barrier reef still has land behind it, so neither stands on a fully sunken island. 'Abyssal reef' is not a recognised reef type, since reef-building corals need shallow, sunlit water.

Answer: A

Statements i, ii and iii are true, so the answer is Only (i), (ii) and (iii) are true.

Being very small gives phytoplankton a high surface area to volume ratio (i). That large surface relative to their mass increases frictional drag and so slows sinking, helping them stay in the sunlit layer (ii). The large surface also allows efficient uptake of dissolved nutrients by diffusion (iii).

Statement iv is false: phytoplankton are drifters that cannot navigate against currents, and small size does not give them navigational ability. That rules out 'all are true', and the options leaving out i or ii are incomplete.

Answer: B

The clockwise North Atlantic gyre includes the Canaries (Canary) Current.

The subtropical gyre of the North Atlantic circulates clockwise: the North Equatorial Current flows west, the Gulf Stream carries the water north along the American coast, the North Atlantic Drift carries it east, and the cold Canary Current flows south along the coast of north-west Africa to close the loop.

The Benguela Current is the cold eastern boundary current of the South Atlantic, the Agulhas Current is a warm current of the south-west Indian Ocean, and the Oyashio is a cold current of the north-west Pacific.

Answer: A

In J. Tuzo Wilson's cycle of ocean basins, the Red Sea is at the juvenile stage.

The Wilson cycle traces an ocean through its life: an embryonic rift valley (the East African Rift), a juvenile narrow sea with new ocean floor (the Red Sea), a mature wide ocean with spreading ridges (the Atlantic), a declining ocean shrinking by subduction (the Pacific), a terminal remnant (the Mediterranean), and finally a suture where continents collide.

The Arabian Sea is part of the mature Indian Ocean, the South China Sea is a marginal sea of the western Pacific, and the Dead Sea is an inland lake in a fault basin, not a young ocean.

Answer: D

The major dissolved component of sea water by weight is sodium chloride.

Of the salts dissolved in sea water, chloride makes up about 55 per cent and sodium about 30 per cent by weight, so together, as sodium chloride (common salt), they account for roughly 85 per cent of the total.

Magnesium chloride, at around a tenth of the total, comes a distant second, and sodium sulphate is smaller still. Calcium carbonate is only a minor dissolved constituent, since marine organisms remove it to build shells and coral.