descripbe the effects of eutrophication on the chemiitry and biology of a river?

Question

chemical change and biological change

Answer 1

Eutrophication is caused by the increase in an ecosystem of chemical nutrients, typically compounds containing nitrogen or phosphorus. It may occur on land or in water. Eutrophication is frequently a result of nutrient pollution such as the release of sewage effluent into natural waters (rivers or coasts) although it may also occur naturally in situations where nutrients accumulate (e.g. depositional environments) or where they flow into systems on an ephemeral basis (e.g. intermittent upwelling in coastal systems). Eutrophication generally promotes excessive plant growth and decay, favors certain weedy species over others, and is likely to cause severe reductions in water quality . In aquatic environments, enhanced growth of choking aquatic vegetation or phytoplankton (that is, an algal bloom) disrupts normal functioning of the ecosystem, causing a variety of problems. Human society is impacted as well: eutrophication decreases the resource value of rivers, lakes, and estuaries such that recreation, fishing, hunting, and aesthetic enjoyment are hindered. Health-related problems can occur where eutrophic conditions interfere with drinking water treatment. Although traditionally thought of as enrichment of aquatic systems by addition of fertilizers into lakes, bays, or other semi-enclosed waters (even slow-moving rivers), terrestrial ecosystems are subject to similarly adverse impacts. Increased content of nitrates in soil frequently leads to undesirable changes in vegetation composition and many plant species are endangered as a result of eutrophication in terrestric ecosystems, e.g. majority of orchid species in Europe.

Answer 2

Organic and inorganic fertilisers leach nitrate into the river. Eutrophication can also occur when old soil is ploughed or nitrogen oxides from the air (pollution) enter the water via rain. Phosphorus from sewage has similar effects to eutrophication.

1. Rapid growth of surface aquatic plants (e.g. duckweed) and algae in an 'algal bloom.'
2. Light cannot penetrate under the surface of the water to other aquatic plants. They cannot photosynthesise and die.
3. Aerobic bacteria feed on the remains of these plants and (since they have a high biochemical oxygen demand) use up a fair amount of oxygen in the river. Anaerobic bacteria later produce hydrogen sulphide which is toxic.
4. Organisms in the river with a high oxygen uptake soon perish, especially fish and other invertebrates.

If such contamination occurs at a drinking source it can lead to 'blue baby syndrome' where the NO3- combines with haemoglobin in the blood reducing its oxygen carrying ability (hence babies look blue).

N.B. Eutrophication rarely happens in rivers since they are fast flowing and any chemicals are washed away. Most often occurs in stagnant pools, ponds and lakes.

Answer 3

Eutrophication ultimately reduces the dissolved oxygen in the river.

Biologically, the sequence of changes goes: algae bloom, algae die, bacteria increase as they decompose the dead algae, fish and other organisms die because the bacteria deplete the dissolved oxygen.