ITALY, Po Delta. Adriatic Sea, Goro Lagoon In response to the algae bloom that occured in 1989 which covered 4000 square kilometres of the Adriatic, the authorities have introduced special dredgers which scrape algae off the sea bottom. This method of control badly damages the sea bottom. Algae blooms may be caused by the addition of plant nutrients to water, particularly phosphates or nitrates in sewage or fertilizer run off from farmland.