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ESS Topic 4: Water and aquatic food production systems and societies

Terms in this set (77)

Reservoirs can be either natural or artificially created lakes, used to collect and store water. Reservoirs are built by damming rivers and flooding suitable valleys. The aim of the reservoir is to store water during periods of high rainfall to provide a plentiful supply throughout the year.

What additional benefits are there to a reservoir?

Generation of hydropower: Some reservoirs also incorporate hydropower schemes to generate electricity.

Flood control: In some regions, reservoirs are also used to capture floodwater and reduce the risk of flooding in downstream areas.

Navigation: The reservoir can provide transport route from one site along the shore to another.

Fisheries: Commercial fisheries have been developed in some reservoirs.

Recreational, aesthetic and scenic value: Reservoirs can be used for many recreational activities, such as water sports such as canoeing and water skiing. Picnic spots are often located along the shores of the lake for their aesthetic and scenic value.

Control of water quality: Sediment load of the water can be reduced in standing water. The particles in the water precipitate out, improving the water quality.


What are the potential impacts of building reservoirs?

Change of habitat: When an area is flooded to become a reservoir, there is a change from a terrestrial to an aquatic ecosystem. The establishment of a lake ecosystems introduces new freshwater habitats to the area. However, scarce terrestrial habitats and species may be lost.

Relocation of people: People may need to be moved out of an area that is to be flooded and relocated elsewhere. Whole towns and villages may be affected. It has been estimated that China's Three Gorges Dam on the Yangtze River led to the displacement of about 1.3 million people.

Change to the flow of the water: Much of the water from the reservoir is diverted elsewhere e.g. to urban areas for industrial and domestic use. Some of the water may re-join the river further downstream as waste water, potentially polluting the river.

Loss of fish and mammal migratory routes: Dam walls can block the migratory route of some fish and dolphins. In an effort to alleviate the loss of fish, fish ladders (concrete steps filled with water) are incorporated into the dam wall. Some fish are able to find and use the ladder allowing them to travel through the wall and continue along their way.

Sedimentation in the reservoir and loss of capacity: The sedimentation of particles from the water behind the dam wall reduces the holding capacity of the reservoir. In addition, this may not always be desirable for farmers downstream who rely on the nutrients in the sediments to fertilise their fields.
Sewage effluent can be routed away from sensitive areas e.g. groundwater and waters that are sensitive to eutrophication.

The effluent can be treated to breakdown the organic material and therefore reduce the BOD and also the amount of suspended solids. During treatment the amount of a nutrients and pathogens are reduced.

Levels of sewage treatment can involve various processes.

Preliminary treatment which involves: (i) screens to remove large objects that may otherwise damage the mechanical equipment or cause blockages and (ii) grit removal to prevent abrasion and wear of equipment and deposition in pipes and channels

Primary treatment in which the piped sewage is allowed to settle within primary sedimentation tanks during which time any settlable solids are removed. This reduces the suspended solids and BOD levels.

Primary treatment in which the piped sewage is allowed to settle within primary sedimentation tanks during which time any settlable solids are removed. This reduces the suspended solids and BOD levels.

Tertiary treatment is less common than primary and secondary treatment. It can involve a variety of different processes. For example:

Nitrate removal involves biological processes in which ammonium ions are oxidised to nitrates and then using denitrifying bacteria the nitrates are converted to nitrogen gas which can be lost to the atmosphere.
Ammonium → nitrites → nitrates (nitrosomonas bacteria and nitrobacter).
Nitrates → nitrogen gas (denitrifying bacteria).
Phosphate removal involves use of chemicals such as iron and aluminium salts which react with the phosphates and precipitate it out.
Macrophyte beds can be used to treat effluent from primary of secondary treatment. The effluent is passed through the beds of growing macrophytes e.g. Phragmites australis to remove suspended solids, nitrates, phosphates, metals and pathogens.

Once effluent is discharged, DO2 levels within the receiving waters can be increased by use of weirs, steps or waterfalls to aerate the water as it flows.
Eutrophication of waters is often caused by domestic or industrial effluent discharges and run-off from farms.

Action to reduce nutrients which cause eutrophication entering aquatic ecosystems includes:

Substituting phosphates in detergents with an alternative such as Zeolite A.

Removing nitrates and phosphates from sewage effluent (as discussed above this is typically part of tertiary treatment of sewage effluent).

Diverting sewage effluent away from water systems that are vulnerable to eutrophication (e.g. may have low degree of dilution and dispersal properties).

More efficient use of fertilizers and appropriate methods of dealing with animal manure, slurry and silage effluent (as discussed above).

Using buffer zones to intercept runoff and absorb the nutrients.

Restricting access of livestock to aquatic ecosystems


Once the nutrients have entered the waters, the following approaches can be taken:

Use macrophyte channels to absorb the nutrients from the water. The macrophytes would need to be harvested to prevent nutrients re-entering the water. In some regions, macrophyte growth is seasonal and therefore its use is limited to certain times of the year.

Mix the water to aerate it and prevent anoxic conditions that will kill many aquatic organisms.
Dredge the bottom to remove sediments that contain nutrients and enhance eutrophication.
Use herbicides to control algal blooms, although this could be problematic if the aquatic system is a source of drinking water.

Mechanically remove the macrophytes and use e.g. as a fertilizer on land.

Use biological control e.g. fish such as Tilapia that feed off the algal bloom. However, the introduction of non-native species may cause changes in the community composition and threaten other species.

Following action to remove nutrients and algal blooms reintroduce native species back into the aquatic ecosystem.