问答题 Imagine fishermen walking down to the seashore, ready to carry out their early morning routine of preparing their boats and net. As usual they hope for a good catch of fish. But to their astonishment, a horrible sight meets their still sleepy eyes. Thousands of fish have been washed ashore dead. The cause of this mass destruction? A red tide!
Red tides are a global phenomenon, They have been observed on both the Atlantic and the Pacific coasts of the United States and Canada. They have also occurred in many other places. Though relatively few people are aware of them, red tides are not new.
In the Philippines, a red tide was first seen in the province of Bata an in 1908. Since then, red tides have been seen in many other coastal areas A Philippines red tide expert told us that "besides the fish kills, the Philippines has documented 1,926 eases of dead shellfish poisoning caused by red tides."
The term "red tide" applies to the discoloration of water that sometimes occurs in certain areas of the ocean or sea. Although the color is often red, it may also be shades of brown or yellow. The World Book Encyclopedia reports that "the discolored areas may range from less than a few square yards to more than 2,600 square kilometers."
What causes such discoloration? Red tides are generally caused by several species of single-celled organisms. These tiny organisms have hair-like projections which they use to propel them-selves in water. There are about 2,000 varieties of these organisms, 30 of which carry poisonous substances these minute organisms usually stay in warm waters with high content of salt.
A red tide occurs when there is a sudden and rapid bloom of these organisms. The concentration of these organisms may swell to 50,000,000 per quart of water! Although scientists do not fully understand why this happens. It is known that these organisms accumulate when certain conditions simultaneously affect the water. These include abnormal weather, optimum temperatures, an over-supply of nutrients in the water, a generous amount of sunlight, and favorable water currents. When a heavy rainfall occurs, minerals and other nutrients are sometimes washed from the land into coastal water. These nutrients can contribute to the breeding of the organisms. The result? Red tides!