Eutrophication in marine waters

Figure 11. General and blue-green algal numbers in Press Top Reservoir

1990-94

Reprinted from Water Research 30, 2, Everall and Lees, 'Use of barley straw to control general and blue-green algal growth in a Derbyshire reservoir'. With permission from

Elsevier Science

Figure 11. General and blue-green algal numbers in Press Top Reservoir

1990-94

Reprinted from Water Research 30, 2, Everall and Lees, 'Use of barley straw to control general and blue-green algal growth in a Derbyshire reservoir'. With permission from

Elsevier Science

Most coastal towns discharge their sewage into the sea with minimal treatment. Usually, the sewage is collected together at a central discharge point and then piped into the sea some distance from the coast after it has been 'screened' - this means that it has been filtered through a wire-mesh screen that retains all particles above a certain size, usually 6 mm. As a result, the amount of nutrients in the sewage discharged from the coastal communities is greater than that from inland ones because none has been removed by the treatment process.

In the sea, the marine algae have the same requirements as those of fresh water, namely, adequate sunlight and the nutrients nitrate and phosphate. However, one particular type of marine algae (or phyto-plankton, also known as diatoms) also requires silica, SiO2, in order to make a small skeletal structure (diatoms also occur in fresh water: see Chapter 8 dealing with acid rain).

Phosphate is not in short supply in sea water and, in most situations, the nitrate concentration is the limiting factor for algal growth. Nitrate enters the sea from sewage discharges and from rivers. We learned earlier that nitrate is readily leached from the soil by rain and as much as 50 per cent of the fertilizer applied to the land can end up in the river.

The development of an algal bloom in the sea depends on many factors, but one of the most important is calm weather because this encourages the development of stratification. The still waters allow the algae to multiply but also to become concentrated in a small area.

Just as we learned about the hazards of blue-green algae in fresh water, so there are toxic algae in sea water. One of the commonest is called a 'red tide' because the vast number of brown single-cell organisms (Gyrodinium aureoleum) give a distinct red colour to the water. Red tides have been responsible for the deaths of thousands of salmon in fish farms in sea lochs in Scotland, Norway and elsewhere. In August 1996, such a bloom was responsible for the deaths of thousands of marine invertebrates in the west and north of Scotland. Reports were received from Islay, Coll, Tiree and Orkney that dead lugworms, sea urchins and shellfish were washed up on the beaches. It appears that the deaths were caused by decaying algae being washed into shallow waters in these areas and smothering the animals that lived in the sediments.

Another problem caused by toxic marine algae is contamination of shellfish which gives rise to a condition known as 'paralytic shellfish poisoning'. Shellfish such as mussels and scallops are filter feeders. They suck in the sea water surrounding them, extract any edible particles and eject the rest. If there are algae in the water they become incorporated into the shellfish flesh. In many places, shellfish are collected for food but this has to be halted if the shellfish have ingested toxic algae. Eating the infected shellfish can cause paralysis or even death, and such poisoning has occurred every year since 1990 off the North and East coasts of Scotland and England and toxic algal blooms have been reported off the South coast of Wales and in the English Channel.

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