Hydrosphere, Biosphere and Ecosystems

The last topic of physical geography gives 2 questions in the Part-III paper, covering the oceans, ocean currents and the biosphere. Ocean currents are the most examinable part of it, because they explain coastal climates that latitude alone cannot account for.

The Oceans

• About 71 per cent of the earth's surface is water, and about 97 per cent of that is salt water.
• The Pacific is the largest and deepest ocean; the Mariana Trench is the deepest point.
• Ocean depth is measured by echo sounding.
• The continental shelf is the shallow margin, richest in fish and often holding oil.
• Average ocean salinity is about 35 parts per thousand; it is highest in enclosed seas with high evaporation.

The continental shelf is the item asked most often, because its shallow sunlit water supports the plankton on which fisheries depend — a fact that connects the physical and economic sides of the subject.

Ocean Currents

Two effects are asked. The North Atlantic Drift keeps north-west European ports ice-free far above the latitude where they should freeze. And where a warm and a cold current meet, the mixing produces the world's richest fishing grounds — Newfoundland being the classic example — along with dense fog.

✗ Ocean currents have little effect on the climate of coasts  |  ✓ They are a major control — the North Atlantic Drift keeps European ports ice-free

Currents are driven mainly by prevailing winds, modified by the earth's rotation and the shape of the coastline. Tides, by contrast, are caused by the gravitational pull of the moon and to a lesser extent the sun — a distinction the options confuse.

The Biosphere and Ecosystems

The biosphere is the narrow zone where land, water and air meet and where life exists. Within it, an ecosystem is a community of living things interacting with their non-living surroundings.

1. Producers — green plants, which make their own food.
2. Consumers — herbivores, carnivores and omnivores.
3. Decomposers — bacteria and fungi, which return nutrients to the soil.

Energy flows in one direction and diminishes at each level, which is why food chains are short. Nutrients, by contrast, cycle — the carbon, nitrogen and water cycles return them for reuse. The contrast between one-way energy and cycling matter is the reasoning question.

Major Biomes

The temperate grasslands are called the granaries of the world because their deep soils and moderate rainfall suit wheat, and the taiga supplies most of the world's softwood. Both are asked as one-line associations, and both link forward to the agriculture and industry topics in the next chapter.

Frequently Asked Questions

How do ocean currents affect climate?

Warm currents raise the temperature of the coasts they pass, so the North Atlantic Drift keeps north-west European ports ice-free at latitudes where they would otherwise freeze. Cold currents lower coastal temperatures and often bring fog where they meet warm water.

What causes ocean currents and what causes tides?

Currents are driven mainly by prevailing winds, modified by the earth's rotation and the shape of coastlines. Tides are caused by the gravitational pull of the moon, with a smaller contribution from the sun. The two are regularly confused in options.

Why are food chains short?

Because energy flows in one direction and a large part of it is lost at each level, so there is not enough left to support many further consumers. Nutrients behave differently: they cycle through the ecosystem and are returned to the soil by decomposers.

Why are temperate grasslands called the granaries of the world?

Because their deep fertile soils and moderate rainfall suit large-scale wheat cultivation, and mechanised farming across the prairies, steppes and pampas produces much of the world's grain. The taiga, by contrast, supplies most of the world's softwood timber.