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The word 'seaweed' generally refers to simple marine algae that also are sometimes misnamed 'plants'. About 6,000 species of seaweed are considered 'red' seaweed; 2,000 are 'brown'; and 1,200 species fall into the 'green' category, which includes Ulva lactuca, the sea lettuce commonly found throughout the brackish and higher salinity waters of the Chesapeake Bay. No species of seaweed yet identified has been found to be poisonous, and some are considered delicacies. Byproducts of seaweed, such as agar and carrageenan, are used to process other foods. Some forms of seaweed, however, in excessive quantities, can deprive other plants and organisms of the oxygen required to sustain life.
Ulva lactuca, or sea lettuce, is a widespread macroalgae occurring at all levels of the intertidal zone, and in northern climates also is found in brackish habitats in the sublittoral zone. It grows in salt or brackish waters, particularly in those that are nutrient-rich or polluted. In some areas it has been known to form mats 2- to 3-feet deep. In areas that do become infested with sea lettuce, these algaflora can virtually clog the waterway, depriving it of oxygen, causing the algae to die off and release yet greater quantities of nitrogen into the water column. Sea lettuce grows along rocky or sandy coasts of oceans and estuaries. It flourishes in brackish waters rich in organic matter or sewage and can survive in a range of temperatures near and below the low-tide mark, to a depth of 10 feet. Like some terrestrial lettuce plants, which it resembles only superficially, sea lettuce appears bright green and grows in thin, crumpled membranous fronds two cells thick. Fragments often litter the Bay’s shorelines, and in other coastal areas around the world they are gathered for food, mulch or as fodder for animals. Sea lettuce growing in moderation has many uses. In some parts of Britain and Asia it is consumed by humans and livestock and is considered valuable to human nutrition. It can be eaten in salads, or dried and later added to soups. Its many nutrients include iron; protein (15 percent); iodine; vitamins A, B1, and C; potassium, magnesium and calcium, and nickel; and other trace minerals.
This species has been used in many countries for years to feed domestic animals, and is cultivated for the purpose in some cultures. Its antibacterial properties recommend it for treating skin irritations topically, including burns; and it also is gathered for use as packing material for shipping more precious edible seaweeds or to preserve fish. It works well in ordinary gardens as a nutrient-rich, if pungent, mulch. Under the best conditions sea lettuce also provides food and nursery areas for aquatic species and fodder for waterfowl. Unfortunately, its success also threatens the health of other species in some systems, including parts of the Bay. In seasons with above-average rainfall or winter precipitation, storm water and agricultural runoff send enormous quantities of suburban wastewater and fertilizer into the Bay, producing suffocating quantities of sea lettuce and other algae, contributing to ‘dead zones’ in which fish and other aquatic organisms and plants, including benthic organisms and eelgrass, cannot survive. A 250-square-mile section of the Chesapeake Bay—from the Bay Bridge south to the James River—was characterized as an oxygen-depleted zone in 2003. Such overgrowth also chokes other important marsh plants such as Spartina, and blocks the sunlight necessary to underwater bay grasses, or SAV. Some studies have also shown that thick mats of sea lettuce can suffocate sessile organisms and slow-moving polychaetes (worms) and mollusks. Decaying, unharvested sea lettuce along the Bay’s shorelines can produce odors that attract nuisance insects and discourage human activity.
Not all consider sea lettuce a noxious plant, however. For culinary risk-takers, the Monterey Bay Aquarium Research Institute (MBARI) recommends a couple of sea lettuce recipes including: Sea Lettuce Soup and Toasted Sea Lettuce.
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