A massive red tide has swamped the eastern coast of south China's Shenzhen since September 1, and so far covering more than 15 square kilometers of water.

According to the Shenzhen Ocean and Fishery Observation Station (SOFOS), this red tide is non-toxic and is currently doing no harm to fish or the marine food industry. The local government has notified nearby shrimp ponds to strengthen their water oxygenation systems and to stop draining ocean water.

People have been warned not to swim in the Dameisha area, where the highest density of the algae that cause the red tide is found.

It is the second time that red tides have bloomed along Shenzhen's east coast within a month, and the sixth the area has experienced so far this year. In the middle of August, severe red tides swept over several areas along the coast in succession but ebbed about a week later.

Professor Zhou Kai, of SOFOS, said that the primary cause of the frequent outbreaks is the release into the sea of large amounts of untreated sewage. Also, overfishing has resulted in a sharp decrease of shallow-water fish and shrimp stocks that are able to feed on algae.

Shenzhen has established 31 coastal observation stations to watch for red tide outbreaks. There are 33 red tide supervision centers nationwide, with 13 along the South China Sea coast.

In late August, the Chinese Society of Oceanography set up the country's first national red tide research and prevention committee. Also in August, the National Marine Environmental Monitoring Center completed a national red tide information management system. The center has recorded data concerning marine problems since 1993.

Red tides are caused by several species of microscopic marine algae that may produce potent chemical toxins. Such tides actually vary in color depending on the kind of algae involved, but they share the potential to harm or destroy aquatic life, either by poisoning the water or depleting its oxygen supply. Toxic forms contaminate shellfish and cause severe respiratory irritation in humans along the shore.

Chinese scientists are experimenting with ways of containing or dispersing red tides, but no completely effective method is yet available for preventing red tides or for stopping them when they start.

(China Daily, China.org.cn September 6, 2004)