Seaweeds, or marine algae, are generally divided into three groups: green, red, and brown. The brown algae include the kelps, which form dense underwater forests in the nutrient-rich, cold waters of the high latitudes. These forests provide habitat and food for countless other organisms (many of which are economically important), as well as ecosystem services such as carbon sequestration and the regulation of CO2 exchange between the atmosphere and the oceans. Kelp itself is an important food and fertilizer in coastal cultures all over the world, from Japan to South Africa and from Chile to Ireland.
Saccharina latissima is a species of yellowish-brown kelp that grows at depths of up to 30 meters and is widely distributed in the coastal waters of the temperate Northern Hemisphere, including along the coast of eastern North America from Long Island Sound, through the Gulf of Maine, to the Arctic. Its common names include sea belt, devil’s apron, and sugar kelp. The latter name derives from the fact that, when this seaweed washes up on the shore and dries, a coating of sweet white powder, made of a molecule called mannitol, forms on its surface. (Mannitol is used as an alternative sweetener.) Sugar kelp grows to a length of up to 4 meters and has a short stipe (the equivalent of a stem) and undivided blades or fronds (the equivalent of leaves) with no midrib, a blistered surface, and a wavy edge. It is a close relative of Saccharina japonica, the species most often used in Japan to make the umami broth known as dashi; several species of Saccharina are referred to in Japanese as kombu. Sugar kelp is rich in vitamins and minerals and can be added to soups and stews as a vegetable, made into tisane, or dried as crisps. Because sugar kelp does not store well, it should be consumed soon after it has been harvested, or else dried for later use. In New England in the 19th century, in addition to being used as a vegetable, sugar kelp was often collected from the beach after storms and then dug into gardens and farm fields to enrich the soil. In the early 20th century, people began drying and milling kelp to sell it as fertilizer, but this business died out with the introduction of synthetic fertilizers in the middle of the century.
Seaweeds in general, and sugar kelp in particular, are becoming increasingly popular as a health food in the United States, but many people are unaware that this resource grows close to home, and imported kelp satisfies most of the current demand. At the same time, populations of S. latissima in other parts of the world, most notably Norway, have recently experienced sharp declines. This is due in part to eutrophication from nutrient pollution. Some of this nutrient pollution comes, of course, from agricultural runoff, including synthetic fertilizers—in other words, the chemicals that replaced kelp as a fertilizer now threaten the survival of kelp forests. Another major contributing factor is ocean warming, and particularly alarming is the fact that the Gulf of Maine is warming more rapidly than 99% of the world’s oceans. Along with warming come invasive species and ocean acidification, which threaten kelp’s ability to grow and reproduce. Coincidentally, the decline of kelp forests, which are huge carbon sinks, accelerates ocean acidification.
Recently, the incorporation of sugar kelp into aquaculture systems on the coasts of Long Island and New England has begun to provide economic and environmental benefits. There is great potential for domestic, sustainably grown sugar kelp to meet local demand while simultaneously cleaning coastal waters, storing carbon, and providing habitat for other species that are important in aquaculture. In order for these new ventures to succeed, it is critical that more people become aware of the gastronomic and health value of kelp, which is, after all, a traditional coastal New England food.