Freshwater biome - most important to us

In our series on biomes, this week's topic is the freshwater biome.

As you know, about three quarters of the Earth's surface is covered by water. The water biome is divided into freshwater (water with little salt in it, such as in ponds, rivers and streams and marine, or salt water in oceans).

Ponds and lakes

A pond is a shallow hole where water accumulates (collects). Plants generally grow around the edges of the pond, and often right across the whole pond. The temperature of the water is similar at the surface and at the bottom. The bottom is generally covered with mud. There are usually no waves. In places where winters are very cold, the whole pond can freeze solid. Some ponds are seasonal.

A lake is bigger than a pond, and could sometimes spread over thousands of square metres. Plants can only grow along the shoreline, because of the depth of the water.

Some lakes are so big that they give rise to waves too, an example is the waves forming in the 'Parakramabahu Samudraya' in Polonnaruwa.

During the hot season, the temperature of the water at different levels of the lake is not the same. The top part of the water gets more of the Sun's heat and is therefore warmer. The middle layer is cooler because it gets less of the sunlight, and the bottom layer, which does not get any sunlight, is quite cold. Most creatures live in the warmer water.

Even in the coldest places, most lakes are so big and deep that they don't freeze solid during the winter season. However, a layer of ice can form across the surface. The ice blocks out the sunlight, and oxygen levels in the water drops, killing some of the plants and animals. This is called 'winter kill.'

Lake Baikal, in central Asia, is the biggest lake on Earth. It contains about one fifth of the Earth's freshwater and has a maximum depth of 5,315 feet! Lake Tahoe is a huge lake on the border of California and Nevada, USA, that originates from mountain stream run-off.

Algae

In all freshwater around the world, algae can be found. Algae are not plants, animals or fungi. They are simple organisms. Scientists place most algae in the protist kingdom.

There are different kinds of algae, most are single celled, but some have more than one cell. As they make their food through photosynthesis, algae release oxygen into the water. Photosynthesis uses sunlight as a source of energy to combine carbon dioxide, water and salts to form chlorophyll.

This means algae are most abundant during the hot season. Too much algae also poses a problem. They can block out sunlight into the rest of the water and could even lead to pollution.

Rivers and streams

Rivers and streams are moving bodies of water. Rivers are large and flow into the ocean. Rivers and streams account for about three per cent of the Earth's freshwater.

They drain the Earth of excess water. Precipitation may fall as rain, hail, snow or sleet. Once it reaches the ground, some of the water is absorbed by trees and other plants.

Water, which is not absorbed, filters down into the ground. Over time, as rivers travel, they change course and carve a path through the land. This causes oxbow lakes, caverns and canyons. The place where fresh and salt water meets is called an 'estuary (we featured estuaries in an earlier issue). A unique place where two habitats come together is called an ecotone.

Most water runs off the surface of the earth and eventually joins a stream. Small streams join together to form rivers. Sometimes, small rivers join together to become large rivers.

A watershed is an area of land where rain water, streams and small rivers all drain into one large area of water such as a large river, lake or ocean. Because ground water also drains into watersheds, all kinds of pollutants can find their way to the ocean from many kilometres away.

The biggest rivers have the biggest watersheds. Therefore, biomes with lots of consistent rain have more rivers and streams. Deserts have little rain and fewer rivers. Desert rain is short, but very intense. So, dry rivers and lakes suddenly become full for a short period of time.

Importance

Without freshwater biomes, people would not be alive. Freshwater ecosystems are important because they provide us water for drinking, energy and transportation, recreation like boating and fishing, and many jobs. People use rivers to produce hydroelectric power. Wetlands are also an important type of freshwater eco-system.

They may be soggy and stinky, but they provide critical habitat for tons of plants and animals, help clean our water, control floods, and provide food for humans. So, we have to conserve water, and be very careful not to pollute water. We must be careful to preserve the biome that is most important to us.

Compiled by Janani Amarasekara