What is Sea Lettuce?
Sea lettuce (Ulva species) is a beautiful light- to emerald-green seaweed with delicate, transluscent leaves. Sea lettuce belongs to the green algae group (Phylum Chlorophyta), which is comprised mostly of freshwater species; only about ten percent live in the ocean.
Sea lettuce grows rapidly and dies off in the summer. When it washes up on the beach and begins to decompose, it sometimes smells like sewage, and the filmy leaves can be mistaken for toilet paper.
Where does sea lettuce grow?
Sea lettuce grows in the low to mid intertidal (or midlittoral) zone, where it is exposed to the air during low tides. It is distributed around the world, and occurs on the west coast of North America from Alaska to Chile. Sea lettuce requires relatively plentiful nitrogen, therefore it can tolerate some pollution.
Sea lettuce grows primarily on rocks in shallow areas that are relatively protected from waves and have good exposure to sunlight. It is also common in estuaries, and can attach near the base of eelgrass, where it forms an “understory” of vegetation. Where water and light conditions are favourable, sea lettuce also grows on pilings and dock floats. Sea lettuce grows in moderate to dense concentrations in:
- Esquimalt Lagoon, particularly in the northern end
- Esquimalt Harbour, at the western entrance, along the southwestern shore, around Inskip Islands and along Dallas Bank
- Victoria Harbour, in Rose Bay, Lime Bay and Selkirk Waters
- Portage Inlet, in the centre at the northeast end
How does sea lettuce live & reproduce?
Unlike terrestrial plants, algae including sea lettuce do not have specialized tissues. The thin leaves of sea lettuce are composed only of two layers of cells; these allow it to easily absorb nutrients from sea water. Like other marine algae and terrestrial plants, it uses photosynthesis to produce energy and sugars.
Sea lettuce has two life stages that are isomorphic, meaning they have the same form. The sporophyte plant releases swimming spores, which establish in the sediment and grow into male and female gametophyte plants. These produce mobile male and female gametes, or sexual cells, that are released in time with lunar spring tides. The gametes fuse together, then swim to the bottom where they grow into the sporophyte plant once more (see the Lifecycle of Sea Lettuce diagram below).
Sea lettuce has high rates of photosynthesis and reproduction. This
allows it to quickly colonize areas of substrate that have recently
been cleared of other organisms, for example following a disturbance
such as a storm.
How have people used sea lettuce?
Sea lettuce is named partly for its appearance but also for its edibility. Along with many other types of seaweeds, sea lettuce has probably been eaten by various cultures around the world for many centuries; it is particularly well-known in Scotland. It can be eaten raw, dried or roasted, in salads, soups or stews. However, it should not be harvested in areas that may be subject to pollution.
Why is sea lettuce important?
Sea lettuce is an important food source for grazing animals including Brant geese, crustaceans such as amphipods, and molluscs such as sea hares, chitons, periwinkles and sea urchins.
Due to its rapid reproduction, particularly in water with plentiful nitrogen, sea lettuce can sometimes grow excessively and smother other plants and organisms. When this occurs, large amounts of decaying plant matter can rob the water of oxygen and lead to fish kills. This problem occurs mainly in areas where the water is not well mixed, and near sewage outfalls.
Sea lettuce can be useful bioindicators, meaning they can be used to make inferences about the environment. As stated above, excessive growth may indicate high nutrient concentrations (i.e. nitrogen and phosphates). The absence or poor health of sea lettuce may also indicate a problem, such as contamination of the water with heavy metals. The rapid growth and simple structure of sea lettuce makes it well suited to laboratory studies.
What threatens sea lettuce?
Algae such as sea lettuce can be damaged by pollution such as dissolved metals (e.g. lead, zinc, cadmium, copper, iron). These can enter the marine environment through runoff from roads and industrial areas. Some metals are common ingredients in pesticides and herbicides. Sea lettuce may also be damaged by oil contamination.
Although sea lettuce can thrive in high nutrient concentrations, excessive algae growth can negatively affect other species, through crowding or depletion of dissolved oxygen. Nutrients such as nitrogen and phosphates can be washed into estuaries and harbours from sewage (e.g. leaking septic systems) or fertilizers.
How can I help protect sea lettuce?
For information on protecting sea lettuce, please visit our How Can I Help Section.