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Eutrophication

An estuary has been choked with sea lettuce for a number of years. Each year there appears to be more and more of the sea lettuce.  It is a nuisance that people have almost gotten used to.  One summer things suddenly get much worse.  The sea lettuce has died and is rotting, washing up on the shore and clogging small inlets.  The water turns milky white and a very disgusting smell is produced.  There have been reports of dead fish and clams. Local residents become concerned.  They ask “What is going on here?”.


The conditions described above are symptoms of a process called eutrophication.

What is Eutrophication?

Eutrophication refers to the growth of aquatic plants over and above what would be considered normal.  In this example, the increased growth occurs in the form of sea lettuce. However, microscopic algae or phytoplankton, can also be a problem.  So much sea lettuce has grown that another, more severe, symptom has appeared. As the sea lettuce died the decay process has used up most of the oxygen dissolved in the water.  As a result, fish and shellfish have suffocated.  Bacteria associated with the decay process also multiplied, producing a milky white or greenish discolouration of the water.  A distinctive rotten egg smell, which people often confuse with the smell of raw sewage, has been produced. And, an anoxic event has occurred.

What Causes Eutrophication?

Eutrophication is caused by a surplus of the key nutrients (nitrogen and phosphorus) that control plant growth.  Nitrate (a form of nitrogen) is a concern for Island estuaries. Normally, nitrate levels are low enough to limit plant growth. When nutrient loads go up, however, plant growth can increase dramatically.  Two factors can lead to an increase in nutrient levels in an estuary:


Where Does Eutrophication Occur?

Eutrophication can occur in freshwater ponds.  However, it is the eutrophication of our coastal areas (salt water ponds, bays, and estuaries) that is of greatest concern on PEI.  The problem is most common along the Island's north shore.

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