Dried kelp
Dried sheets of brown seaweed (Korean dashima, Japanese kombu), used above all to give broth a deep, savoury base.
Caution: Dried kelp is very high in iodine, so it should be used only in small amounts and not eaten in large quantities.
Dried kelp is sold as firm, dark sheets, often with a white bloom that carries flavour and should be wiped at most, not washed off. It is found in Asian grocers, sometimes in health food or organic shops under the name kombu, otherwise by mail order.
Substitute:
There is no equivalent substitute; a pinch of glutamate seasoning or a little soy sauce adds savouriness but not the rounded, faintly oceanic taste, and the broth will be flatter.