Oceanography Questions and Answers

Answer: D

Estuarine crabs keep the salt concentration of their bodies more or less constant by active transport.

Estuaries swing between near-fresh and near-marine salinity with the tides and the seasons. Crabs living there, such as the shore crab Carcinus, are osmoregulators: their gills actively pump ions in when the water is dilute and out when it is salty, so the body fluid stays near a fixed concentration.

Letting the body fluid follow the water (osmoconforming) is the strategy of many open-sea invertebrates, which cannot survive the wide swings of an estuary. Moving towards the fresh or the sea-water side is not an adaptation to changing salinity but an escape from it, and the tide brings the change to the crab wherever it is.

Answer: A

This community indicates a rocky inter-tidal environment.

Seaweeds, mussels and barnacles all need a hard surface to attach to and tolerate being exposed at low tide; they form the classic zones of a rocky shore. Lichens such as the black Verrucaria mark the splash zone at the top of the same shore.

A soft-bottom (sandy or muddy) intertidal flat has no stable surface for attached organisms, so its fauna is burrowers such as worms and clams. A coral reef is dominated by corals, and the continental slope lies in deep water, below the depth where seaweeds can photosynthesise and far from any lichen.

Answer: B

That level is the calcite compensation depth (CCD).

Carbonate shells sinking through the ocean dissolve faster at depth, because cold water under high pressure holds more carbon dioxide. The CCD is the depth at which the rate of dissolution equals the rate at which calcium carbonate is supplied, so below it no carbonate accumulates in the sediment. In today's Pacific it lies at about 4,000 to 5,000 m.

The lysocline lies above the CCD and marks where dissolution first becomes noticeably faster; carbonate still accumulates below it. A transition layer is a general term for a zone of change in the water column, and carbonate critical depth is not the standard name for this balance level.

Answer: B

Deep circulation is driven by density differences due to temperature and salinity.

This is the thermohaline circulation. Seawater becomes denser when it is cooled and when its salinity rises, for example as sea ice forms and rejects salt. In the North Atlantic and around Antarctica, cold, salty surface water sinks and spreads through the deep basins, slowly driving the global overturning circulation.

Water depth is not a property of the water that can make it sink, and the difference between sea surface and air temperature drives heat exchange and winds rather than deep circulation directly. Only the pair temperature and salinity sets seawater density.

Answer: D

The correct matching is (a)-(iv), (b)-(iii), (c)-(ii), (d)-(i).

1. A layer where salinity changes sharply with depth is the halocline.
2. A layer where density changes sharply with depth is the pycnocline.
3. Water much saltier than normal seawater is brine.
4. Clay particles clumping together in seawater is flocculation, which happens where river water meets the sea and the salts neutralise the particles' charges.

Options A and B start by matching the salinity layer to flocculation, and option C to brine; each confuses a water layer with a substance or a process.

Answer: A

An ebb current is the seaward flow of tidal current.

As the tide falls, water drains out of bays, estuaries and tidal inlets back towards the open sea. That outgoing flow is the ebb current. Its opposite, the landward flow while the tide rises, is the flood current.

So option B describes the flood current, option C wrongly denies that the ebb is a tidal current, and option D is wrong because tidal currents are a feature of the sea and the waters connected to it, not of inland lakes.

Answer: A

The correct matching is (a)-(iii), (b)-(ii), (c)-(iv), (d)-(i).

1. The flat accumulation of sand above the high tide line is a berm.
2. A wave that becomes unstable and collapses at the shore is a breaker.
3. A steep wave front that travels up a river on the flooding tide is a tidal bore.
4. The zig-zag movement of sand along the beach, up the beach obliquely with the swash and straight down with the backwash, is beach drift.

Option B swaps breaker and bore, option C starts with beach drift for the berm, and option D swaps bore and beach drift.

Answer: C

The keyed answer is the lining of the new moon and the sun on the same side of the Earth.

Syzygy is the near-straight alignment of three celestial bodies, here the Sun, the Earth and the Moon. It happens at new moon, when the Moon lies between the Earth and the Sun (conjunction), and at full moon, when the Earth lies between them (opposition). Of the options, only C describes such an alignment; it is the new-moon case.

The Moon at right angles to the Sun-Earth line is quadrature, which gives neap tides, the opposite of syzygy. Spring and neap tides never occur together, and the Earth is not 'below' the Sun in any defined sense, so options A, B and D do not describe syzygy.

Answer: B

Both statements are true but the Reason does not explain the Assertion.

The Assertion is the definition of tides: the periodic rise and fall of the sea caused by the gravitational pull of the Moon and the Sun on a rotating Earth.

The Reason is also true: the actual tide at a place, its range and whether it is semi-diurnal, diurnal or mixed, depends on the Earth's rotation, the shape and size of the ocean basin and coast, and local conditions. But these factors modify the local tide; they do not explain why tides occur, which is the gravitational interaction the Assertion already states. So the answer is not the first option, and neither statement is false.

Answer: C

The factors are (1), (2) and (3): wind velocity, wind duration and fetch.

Wind-generated waves grow by taking energy from the wind, so their size depends on how strong the wind is (velocity), how long it blows (duration) and the distance of open water over which it blows (fetch). A strong wind blowing for a long time over a great fetch, as in the Southern Ocean, produces the largest waves.

Wave height is not a cause but a measure of the result: it is the size of the wave itself. That is why the option including (4) is wrong, and options with only two of the three true factors are incomplete.