The coastline of Falmouth Bay includes two drowned river valleys (rias), the Fal and Helford, which are now long, sheltered sea inlets. Because of their rich marine life, these inlets, together with part of Falmouth Bay, have been designated as a European marine Special Area of Conservation. Beds of the eelgrass Zostera marina and maerl in the inlets are home to a wide variety of animals, including the rare Couch’s goby.
On the wave-exposed rocky coasts outside the inlets, the kelp Laminaria hyperborea grows in dense forests that support many associated seaweeds and animals. This kelp has a stiff stipe, which raises the frond off the sea bed and means that the forest has developed well vertically. On the rock beneath the kelp, there is competition for space among anemones, sponges, and smaller seaweeds, while other animals hide in kelp holdfasts. The kelp stipes have a rough surface and provide effective attachment points for red seaweeds, bryozoans, soft corals, and other types of encrusting animals. On the fronds, tiny blue-rayed limpets graze, and colorful sea slugs eat small hydroids and lacy bryozoans.
In deeper water, the kelp Laminaria ochroleuca grows, close to its northern limit in Europe. This kelp is similar to Laminaria hyperborea, but has a smooth stipe on which little can grow. Two other kelps are found in the area, the sugar kelp (Laminaria saccharina), which has a crinkled frond, and furbelows (Saccorhiza polyschides), which has a large, hollow holdfast and grows up to 13 ft (4 m) long in just one season.