Izembek Lagoon covers 150 square miles (388 square km) of the Izembek State Game Refuge and is the site of one of the world’s largest eelgrass beds. The eelgrass Zostera marina grows in dense beds here, both subtidally and on intertidal flats, where it is grazed by wading birds at low tide. Over half a million geese, ducks, and shorebirds stop over at the lagoon during migration to refuel on the eelgrass. On the rocky, open coasts outside the lagoon, kelp forests thrive in the cold water. The most common forest-forming kelp here is bull kelp (Nereocystis luetkeana), which can grow to 130 ft (40 m) in length. Bull kelp is an annual, which means that it reaches maturity within a single year. It grows quickly, at a rate of up to 5 in (13 cm) per day. The huge fronds, which have many long, strap-shaped blades, are supported by gas-filled bladders (pneumatocysts) that are up to 6 in (15 cm) in diameter.