A case history of a eutrophic lake

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Strathclyde Park Loch is situated between Hamilton and Motherwell in South Lanarkshire, Scotland. It is an artificial loch and was created from an area of derelict and subsided ground which was a coal mine at one time. The original pond in the subsided ground was extended and contoured to make an attractive amenity lake and became the central feature of Strathclyde Country Park.

The loch is very popular in the summer months with anglers, dinghy sailors, canoeists and wind surfers. Unfortunately it also suffers from algal

Maleficent Horns Template
Figure 8. Stratification of lakes in calm summer weather

blooms in the summer because the water is eutrophic. The loch is filled by the South Calder Water and the overflow goes to the River Clyde.

The South Calder Water receives the effluents from three sewage treatment works at different places along its length, and all of which contribute nitrates and phosphates to the river and these eventually enter Strathclyde Park Loch. Table 8 illustrates the concentration of these nutrients in different seasons.

From these results it can be seen that, in winter months, the concentrations of nitrate and phosphate for the sampling points at the inlet and outlet to the loch are almost the same because the lack of sunlight and the cold water are unsuitable for algae to develop. By contrast, in the summer months, there is a marked reduction in the concentrations of the nutrients at the outlet in comparison to the inlet because they are being used up by the fast-growing algae within the lake. The overall concentrations of the nutrients in the samples taken in the summer months are higher than the winter values because the larger flows in the river in the winter dilute the effluents to a greater degree.

The excess algae in the loch in summer make the loch look very green and turbid - on windy days, the wave action blows the algae into a froth

Table 8. Concentrations of nutrients in the inlet and outlet of Strathclyde

Park Loch in winter and summer (mg/l)

Inlet

Outlet

1995

Winter

Phosphate phosphorus

0.19

0.12

Nitrate nitrogen

1.68

1.70

Summer

Phosphate phosphorus

0.42

0.16

Nitrate nitrogen

1.89

0.53

1996

Winter

Phosphate phosphorus

0.22

0.22

Nitrate nitrogen

1.74

1.63

Summer

Phosphate phosphorus

0.70

0.41

Nitrate nitrogen

2.35

1.22

which accumulates on the shore line where it is a hazard to people and their pets. An example of algal froth is shown in Cover Illustration 2.

One particular type of algae, known as blue-green algae, is a danger to the public and to people who use the water. This is because the algae produce a poison as they photosynthesize, and this enters the water. People or animals that swallow the water can suffer from a variety of illnesses ranging from ear infections and sore throats to diarrhoea.

The occurrence of toxic algae is regularly monitored by staff from the Scottish Environment Protection Agency. The biologist who specializes in identifying algae, an algologist, collects samples of the loch water at regular intervals from spring to autumn and, on return to the laboratory, looks for the presence of blue-green algae. If the numbers exceed a safe level, a warning notice is put up at the lochside warning of the danger and advising people to prevent their dogs from drinking the water.

Figure 9 shows when the blue-green algae appeared in 1993-96 and how long the numbers exceeded the safe level. During one of these periods, the rowing events for the Commonwealth Games were due to be held but they were postponed because of the presence of the toxic algae.

Strathclyde Park Loch is not the only freshwater lake to suffer from poisonous algae. In 1989, sheep and dogs died after drinking algae-infected water at Rutland Water whilst soldiers who had been swimming in Rudyard Lake in Staffordshire later became ill.

In the example described above, of Strathclyde Park Loch, the nutrients that gave rise to the algal problem originated from the sewage works

Figure 9. The appearance and intensity of blue-green algae in Strathclyde Park Loch, 1993-96

that discharged into the feeder river. In other lakes, the nutrients can enter the water in drainage from agricultural land. Farmers add fertilizers to increase the growth rate or size of their crops (see Chapter 4). This particularly applies to arable (vegetables) farming, where the amounts added to the crops are greater than for stock (animal) farming. Fertilizer usually contains three nutrients, nitrogen, phosphorus and potassium, usually called 'NPK'.

Not all the nutrients stay in the soil and some are washed into streams by rain. The amount of the nutrients that enter the streams depends on many factors such as the type of soil and crop, the slope of the land, how much fertilizer was applied, etc. In general, the nitrate is more readily washed out (leached) from the soil than phosphate whilst the potassium is most strongly attached to the soil. The loss of nitrate can vary from 5 to 50 per cent of the amount applied whereas the phosphate losses are in the range 0.1 to 5 per cent.

However, although the amount of phosphate lost is relatively small, after a rain storm it can still reach a concentration in a stream of 3 mg/l which is well in excess of the 0.1 mg/l (100 pg/l) which gives rise to hyper-trophic water. It has been estimated that leaching of phosphate from agricultural land can account for 55 per cent of the total amount of phosphorus that enters fresh waters.

Fertilizers are costly and farmers should not waste them by applying too much or at the wrong time. In an effort to reduce the amount of phosphorus entering rivers and lakes, farmers are now given advice by the Agricultural Advisory Service on how much fertilizer their fields require for the appropriate crop and when is the best time to apply it. They are also encouraged to leave a so-called buffer strip next to any rivers so that fertilizers are not spread onto the ground immediately adjacent to the water. Figure 10 shows an ideal buffer strip which protects a stream from receiving excess nutrients from the cultivated fields.

There has been an interesting new development in the control of algae in freshwater lakes and that is the use of barley straw. It had been noticed, when barley straw had accidentally fallen into a small lake that was usually green with algae in the summer, that excessive growths of algae did not occur during the following summer. Since this accidental discovery, many authorities have been using barley straw to control algal blooms. It is not known yet how this works but it appears that, as the straw decomposes in the water, some substance is released that is toxic to the algae. This algicidal chemical has yet to be identified but it seems to be released after microbial decomposition of the straw; research is still being carried out to find the active ingredient. However, at this stage the findings of this research are not important because the straw is readily available and it is not necessary to obtain anyone's approval to put it into a lake or pond. If the active ingredient is identified, it will take years before

Impermeable layer

Figure 10. Buffer strip between cultivated fields and a stream

Impermeable layer

Figure 10. Buffer strip between cultivated fields and a stream it can be marketed as a treatment chemical because it will have to have the approval of many different organizations involved in checking its toxicity and safety before it is licensed for use. Meanwhile, throughout the UK and elsewhere, problems of excessive algae, particularly blue-green algae, are being controlled by putting either bales or long tubes of netting filled with straw into fresh water early in the spring so as to start the decomposition process by the summer. It has been used with some success, for example, in Strathclyde Park Loch where it seems to have reduced the intensity of the algal bloom.

A more systematic experiment was conducted in the Press Top Reservoir in Derbyshire which had been used for water supply storage but is now a recreational fishery.2 There was a noticeable decline in the fish stocks in 1990-93 which was blamed on the development of blooms of blue-green algae. In April 1994 the reservoir was treated with barley straw at a rate of 50 g/m3 and the extent of the algal growth was monitored and compared with earlier years when no straw had been added. The results are shown in Figure 11 and seem to suggest that the straw has successfully reduced the extent of the algal problem.

Much less is known about eutrophication in sea water because it has not been studied as extensively as fresh water.3 Algal blooms do occur though and can be harmful at times.

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  • AMELIA
    What lakes are historically eutrophic in the US?
    3 years ago

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