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HOME / WATER / STREAMS, PONDS, WETLANDS, AND ESTUARIES / WATER QUALITY /


Sea Lettuce

Sea lettuce is a nuisance species of green algae found all over the world. It grows very well in PEI’s estuaries. In the past, sea lettuce was kept in check by lower nutrient levels, grazing by marine life, and shorter grower seasons. Today, it has become an issue for cottage owners, recreational boaters, shellfishers, and aquaculture operators. In many Island estuaries the presence of sea lettuce is linked to low levels of oxygen in the water.
 
Learn more about sea lettuce by following the links below:
 
What can we do about the Sea Lettuce Problem?

What is Sea Lettuce?
Sea lettuce is a kind of green algae. It is formed by two layers of cells, which together create a broad flat blade. The plants can get as large as 30 cm (12 inches) wide and 75 cm (30 inches) long. Sea lettuce grows in Prince Edward Island from April to October. During mild winters with little ice cover, sea lettuce can survive year-round.
 
Sea lettuce is not an invasive plant. It has always grown here in Prince Edward Island. Sea lettuce can be found loosely attached to rocks, shells, sand, or mud bottoms, or it can grow free-floating in the water. It is most common in the shallow upper reaches of bays and estuaries.    
 
Sea lettuce grows well in Prince Edward Island for a number of reasons. These include:
 
  • Nutrients: Human activity has made our estuaries and bays even more productive, over the past 50 years, by adding large amounts of nutrients to them. 
  • Shallow and Warm Coastal Waters: PEI’s bays and estuaries are relatively shallow, so there are plenty of areas where sea lettuce can grow. As well, coastal waters are warm and receive large amounts of sunlight. 
  • Flushing: Relatively small tidal ranges, especially on the north shore of PEI, limit flushing.
  • Climate Change: A changing climate may lead to even more growth as our winters get milder and summers get warmer and longer.
 
Why is Sea Lettuce a Problem?
Sea lettuce can grow (or ‘bloom’) very fast. It can double in size in two or three days. Sea lettuce can form dense mats and cover the bottom of an estuary. These mats can grow in depths of up to 3 m 
(11 feet) if the water is clear enough. 
 
Once a floating or bottom-attached mat of sea lettuce gets think enough, the amount of light reaching the underside of the mat is reduced (this is called self-shading). The shaded sections of sea lettuce begin to die due to a lack of light. What appear to be healthy, growing mats of sea lettuce on the water’s surface often have layers of dying or dead material below. 
 
As the sea lettuce decays, it uses oxygen from the water until there is none left.  Once this happens, anaerobic bacteria (bacteria which work in the absence of oxygen) start to grow. They produce a milky white–cloudy green–grey discoloration of the water and produce a rotten turnip or rotten egg smell. This is referred to as an anoxic event. This growth–death cycle will often repeat itself several times over the summer months.
 
Sea lettuce used to be kept in check by lower nutrients, grazing by marine life, and shorter growing seasons. However, as early as the 1980's, increased levels of sea lettuce growth were noticed in some Island estuaries. 

Heavy growth of sea lettuce can have the following impacts:
 
·         loss of habitat for aquatic life as sea lettuce out competes sea grasses for space,
·         loss of other aquatic life (eel grass, other sea weeds, some invertebrates, and fish),
·         smothering of shellfish by heavy mats growing over them,
·         enrichment of estuary sediments with organic matter (black mayonnaise),
·         loss of mud or sand bottoms, and
·         retention of nutrients within the system, as organic matter settles to the bottom of the estuary.
 
Sea lettuce in an estuary An anoxic event
Sea lettuce in an estuary                                        An anoxic event

What can we do about the Sea Lettuce Problem?
With growing concerns about sea lettuce growth and anoxic events, people often ask what can be done to solve these problems. Unfortunately, in most cases, there are no short-term ways to solve these problems. Many ‘solutions’ have addressed only a symptom or have not worked at all.
 
The Ultimate Solution
 
Nutrient Reduction: The only way to fully ‘fix’ the issue of excess growth of sea lettuce is to reduce the amount of nutrients entering an estuary.
 
A Temporary Measure
 
Harvest of sea lettuce:  Harvesting sea lettuce will reduce the amount of rotting material.  This could eliminate problems associated with low dissolved oxygen and improve conditions for shellfish beds.  A pilot sea lettuce harvest project was conducted during the summer of 2011.  This project showed that the harvest of sea lettuce is possible in PEI. However, almost continuous harvesting would be needed throughout the summer to stop the anoxic events. Harvesting sea lettuce is very expensive, and currently there is no use for the material that will help to offset the cost of its removal.

What Will Not Work
 
Dredging: Even with significant dredging, PEI’s estuaries would still be relatively shallow. Since sea lettuce can grow in depths of 3 m (11 feet) or more in clear water, dredging will not solve the issue of prolific sea lettuce growth. The creation of deep-water areas in an upper estuary can create new water quality issues or make current problems worse.

Removal of causeways and bridges at the mouth of estuaries
: If bridges and causeways are restricting tidal movement in the estuary, then removing them may help. However, on PEI, most problem structures have already been fixed.
 
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