Introducing water

Water

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About this section

These articles have been made possible by a partnership between The Crown Estate and The Open University. The Crown Estate manages property in the UK within the Marine, Urban and Rural Estates and all their revenue surplus is returned to the Treasury. The Marine Stewardship Fund, which supports this project, contributes to the good management and stewardship of the marine estate. The Open University is committed to making education available to all.

This extract from Potable Water Treatment, an Open University OpenLearn unit, introduces the basics of water as a source of nutrition.

Nutrients in water

The list of necessities for the provision of life includes various nutrients and water. Water is one of the basic resources needed for the process of photosynthesis. Since it is an excellent solvent, water, even in its “natural” state, is never pure H₂O but contains a variety of soluble inorganic and organic compounds. Water can also carry large amounts of insoluble material in suspension. The amounts and types of impurities vary with location and time of year, and determine some of the characteristics of a particular watercourse.

One of the most important determining factors is the presence of organic material in solution or in suspension. Organic material can be used as “food” by the organisms living in natural water, provided the material is biodegradable. Biodegradable materials are those organic substances which can be decomposed by micro-organisms (usually bacteria and fungi) into inorganic substances.

A typical food chain

In a water environment, as on land, the primary producers (green plants and algae) are eaten by herbivores (primary consumers) and these in turn are devoured by the secondary consumers (carnivores). The interdependence of these organisms gives a complex food web within which there are many food chains, the successive links in the chains being composed of different species in a predator-prey relationship.

For a river, a typical food chain could be:

algae → microscopic animals → mayfly → small fish (minnow) → large fish (pike)

Scavengers eat bottom debris, including dead organisms. If the latter are not eaten immediately, the decomposers (bacteria and fungi) break them down, releasing nutrients which can be taken up by plants.

Ecological equilibrium

Through this cyclic movement of nutrients, the water environment achieves an ecological equilibrium. In a given stretch of water a balance occurs between total production of living material and the occurrence of death and decomposition over a period of time. The river neither becomes choked with living organisms nor devoid of them – although, depending on location and geological conditions, the numbers and varieties of the biota vary enormously.

The maintenance of equilibrium is dependent on the complexity of biota and the interlinking of food chains and webs.

A typical river has several sources in high ground which are characterised by steep gradients, swift current velocities, and erosion of the surrounding rocks and soil. As the gradient lessens, the velocity of the current decreases and the river deepens and widens.

The river then tends to deposit stones, gravel and sand. This variation in the flow downhill has a direct influence on the types of organisms and substrata to be found at different points along the river. The whole length of the river can be subdivided into different zones, each characterised by its own typical fauna and flora.