Oceans and the Water Cycle

Water covers about 71 percent of Earth's surface and almost all of it - roughly 97 percent - is in the oceans. Most of the rest is frozen. That is why the ocean is the reservoir the whole cycle runs from.

The water cycle

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Evaporation and condensation are the pair to get right.

• Evaporation: liquid becomes gas. It takes heat in.
• Condensation: gas becomes liquid. It releases heat.

Dew on grass is condensation. A puddle disappearing is evaporation. If a question describes water appearing on a cold surface, that is condensation, and the answer choice saying evaporation is there for anyone who has the pair the wrong way round.

The sun is the energy source for the whole cycle. Gravity does the rest, pulling precipitation down and runoff downhill.

The amount of water on Earth does not change. It is a closed cycle - the water moves and changes state, and none of it is created or destroyed. That is a question.

Where the water is

About 97 percent of Earth's water is salt water in the oceans. Of the 3 percent that is fresh, most is locked in glaciers and ice caps, and most of the rest is underground. Rivers and lakes are a sliver of a sliver.

Groundwater fills the spaces in soil and rock; a layer of rock or sediment that holds enough of it to supply a well is an aquifer. Pumping an aquifer faster than rain refills it - typically for irrigation - lowers the water table.

A watershed is all the land that drains into one river or lake. What happens anywhere in a watershed ends up in its water.

Eutrophication is what happens when fertilizer runoff overloads a lake with nutrients: algae bloom, then die, and the bacteria decomposing them use up the oxygen, suffocating the fish.

Salinity

Ocean water averages about 3.5 percent dissolved salt, mostly sodium chloride.

Where it comes from: rivers carry dissolved minerals from weathered rock into the sea, and evaporation removes water while leaving the salt behind, so the salt concentrates over time.

Salinity varies. It is higher where evaporation is high and rainfall low, and lower near river mouths and melting ice.

Salt water is denser than fresh water, which is why objects float more easily in the sea and why fresh water from a river floats on top of salt water where they meet.

Currents

Surface currents are driven by wind and are deflected by Earth's rotation. They move enormous amounts of heat.

The Gulf Stream carries warm water from the tropics north along the east coast of North America and across toward Europe, which is why western Europe is milder than its latitude would suggest. That is the current the test names.

Deep currents are driven by density differences - cold, salty water sinks and warm, fresher water rises. Because both temperature and salt are involved, the circulation is called thermohaline.

Currents moderate climate. A warm current makes a coast warmer; a cold current makes it cooler and often drier.

Upwelling happens where wind pushes surface water away from a coast and cold, nutrient-rich water rises from below to replace it. Those nutrients feed plankton, which is why the world's richest fishing grounds sit over upwelling zones, such as off Peru and California.

The ocean is layered by temperature. A warm, sunlit upper layer floats on cold, dense deep water, and the zone between, where the temperature drops fast, is the thermocline.

Tides

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Tides are caused by the gravitational pull of the moon, with the sun contributing about half as much despite being far more massive, because it is very much further away.

There are two high tides and two low tides each day. The moon raises a bulge of water on the near side, and a second bulge forms on the far side. Earth rotates through both, so any coast passes through two highs and two lows.

That second bulge is the part worth understanding, because it is the one that makes the count come out at two rather than one.

Spring and neap tides

"Spring" has nothing to do with the season. It refers to the water springing up, and spring tides happen roughly twice a month all year.

Ocean features

The continental shelf is the shallow, gently sloping seafloor extending from a continent, and it is where most marine life and most fishing are.

Trenches are the deepest points, formed at convergent plate boundaries by subduction. The Mariana Trench is the deepest.

Mid-ocean ridges are underwater mountain chains at divergent boundaries where new crust forms.

Waves are caused by wind, not by tides - which are two different phenomena that a question may offer together. Wave size depends on wind speed, how long it blows, and the distance it blows over. A tsunami is the exception: a wave caused by an undersea earthquake or landslide, not by wind.

The Pacific is the largest and deepest ocean.

Ocean zones and ocean life

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Zones are named by depth and by light, and the questions give you a description and ask for the name.

By light: the photic (sunlight) zone is the top 200 meters or so, where there is enough light for photosynthesis; below that, the aphotic zone gets none. With no sunlight, deep-sea food chains around hot vents start with chemosynthetic bacteria, which make food from chemicals instead of light.

Ocean life falls into three groups by how it moves: plankton drift with the currents (and phytoplankton are the ocean's producers), nekton swim freely (fish, whales), and benthos live on the bottom (crabs, sea stars).

An estuary is where a river meets the sea and fresh water mixes with salt - brackish water, rich in nutrients, and nursery ground for many fish. Coral reefs are built by tiny animals in warm, shallow, clear tropical water, and hold some of the greatest biodiversity on Earth.

What you can skip

The ocean zones are the one area of this topic with a long tail of jargon. Across all 2,104 General Science questions in our bank, these come up once or twice:

• The finer depth zones: mesopelagic, bathypelagic, hadal, and the twilight and disphotic zones. The photic and aphotic split carries the recurring questions.
• Freshwater ecology terms such as lotic and profundal, one question each.
• Named currents other than the Gulf Stream. The California Current comes up once; the Gulf Stream is the one the test reaches for.

Where people lose points

Swapping evaporation and condensation.

Saying the water cycle creates or destroys water. It moves it.

Attributing tides to the sun. The moon dominates.

Saying there is one high tide a day. Two.

Linking spring tides to the season.

Saying waves are caused by tides. Wind causes waves.

Work one in under a minute

It is a full moon. Are tides at their largest range, smallest range, or average?

Full moon means Sun, Earth and Moon are aligned, so the two pulls act along the same line and add.

Largest range - a spring tide.

New moon gives the same answer, for the same reason.

Where this leads

The evaporation and condensation here are the phase changes of the physical science block, and the clouds this cycle forms are the weather lesson's subject.