Freshwater eutrophication

Algal blooms have increased markedly in many freshwater lakes and ponds; they also occur in very-slow moving water, in canals and rivers such as the Norfolk Broads. The problem can be attributed to the increasing amount of phosphorus entering the affected waters. We have already seen that phosphates are a major component in modern detergents, but they are also present in human sewage, animal excreta, industrial effluents and agricultural fertilizers.

In most freshwater lakes and rivers, phosphorus is known as a limiting nutrient. In other words, all the other nutrients required by the plants and algae are in excessive amounts and the growth rate is decided by the amount of phosphorus present. If phosphorus is added to oligotrophic water, algal and plant growth is triggered and the trophic status shifts towards mesotrophic. Figure 5 shows the relationship between the concentration of phosphorus in the water and the growth of algae.1 The algal growth is measured by the concentration of chlorophyll which is the green pigment in the algal cell.

When phosphorus enters a lake from either a discharge or in an inflowing stream, it can be present in a number of different forms:

• soluble inorganic phosphate

• soluble organic phosphorus compounds

• phosphorus absorbed onto suspended particles in the water (partic-ulate phosphorus)

When present as particulate phosphorus, these particles slowly settle to the bottom of the lake and become part of the sediment. Sometimes

Eutrophication Pollution

1 10 100 1000 Phosphorus (mg/m3)

1 10 100 1000 Phosphorus (mg/m3)

Figure 5. The relationship between the amount of phosphorus and the development of algae, as measured by the concentration of chlorophyll Source: Phytoplankton Ecology, G.P. Harris, 1986. With kind permission from Kluwer Publishers the particles are eaten by fish or microscopic animals. The phosphorus then passes through their bodies and is excreted in their faeces. This movement of the element in different sectors of the water is known as phosphorus cycling and is summarized in Figure 6.

The algae require soluble inorganic phosphate for their growth and, as we see from Figure 6, this can be present in the inflowing stream but is also formed from the phosphorus cycling within the lake. For this reason, the nutrient status of water quality is classified by the concentration of total phosphorus in the water.1

Table 7. Classification of standing waters

Trophic category

Concentration of

total P (pg/l)

Ultra-oligotrophic

<4

Oligotrophic

4-10

Mesotrophic

10-35

Eutrophic

35-100

Hypertrophic

>100

Source: Eutrophication of Waters: Monitoring, Assessment and Control, Organisation for

Economic Cooperation and Development, OECD, Paris, 1982

Freshwater Pollution
Figure 6. The phosphorus cycle in a freshwater lake Source: Ecological Effects of Wastewater. Second edition. E.B. Welsh. With kind permission from Kluwer Publishers

Oligotrophic lakes and ponds are usually found in upland areas where the surrounding soils are lacking in minerals (usually because they have not been treated with fertilizers by farmers). Eutrophic lakes are found in lowland areas; the surrounding fields may be well fertilized to encourage crop or grass growth and the inflowing stream may contain effluent from the sewage works of a nearby community.

In the summer months, the rapid growth of algae in a eutrophic lake removes the soluble phosphorus and nitrogen and the concentration of these nutrients in the water declines. They are replenished in the winter by inflowing water or by resuspension from the sediment. The annual cycle of algal growth and changes in phosphorus concentration is illustrated in Figure 7.

There is an additional factor to consider in the eutrophication of lakes and the sea in the temperate climates of the southern and northern hemispheres, and that is stratification.

In winter months, the air and water are cold, and the air is often colder than the water. Also, it tends to be windier in winter months than in the summer. The result of these weather factors is that the temperature of the water in winter is almost the same at whatever depth you sample it.

Winter Lake Eutrophication

Months

Figure 7. Changes in phosphorus and phytoplankton in a lake over a year Source: The Chemistry of Water and Water Pollution. Dojlido and Best, 1993. E & F.N. Spon. Reproduced with permission

Months

Figure 7. Changes in phosphorus and phytoplankton in a lake over a year Source: The Chemistry of Water and Water Pollution. Dojlido and Best, 1993. E & F.N. Spon. Reproduced with permission

It also has the same density because it is well mixed throughout by the wind.

In late spring and summer, the water is heated by the sun and, in calm conditions, the surface layers get warmer and less dense than the deeper water. If the calm weather persists, the water becomes stratified. This means that the surface layers do not mix with the deeper layers because there is a marked difference in temperature and, consequently, density between the two parts. This is illustrated in Figure 8.

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