A red strip was seen floating in waters off Dameisha Beach on Monday. Strips of a red substance appeared off the city s east coast near Yantian Port and Dameisha Beach on Monday morning. Red tide, also known as algal bloom, usually occurs in Shenzhen in spring and summer, an environmental protection activist said. Staff with the Dameisha beach management said they were yet to issue a warning notice because few people had been swimming in the sea recently.