Algal Blooms
by Pearson Prentice Hall Textbooks
Over a five week period one year, the bodies of fourteen humpback whales washed up along beaches on Cape Cod, Massachusetts. The whales showed no outward signs of sickness. Their stomachs were full of food. Their bodies contained plenty of blubber to insulate them from changes in water temperature. What caused such healthy-looking animals to die?
When biologists examined the dead whales' tissues, they identified the cause of the puzzling deaths. The whales' cells contained a deadly toxin produced by a dinoflagellate called Alexandrium tamarense. For reasons that scientists don't fully understand, the population of these algae grew rapidly in the ocean waters through which the whales were migrating. When the whales fed on the toxin-producing algae or on the fishes that had eaten the algae, the toxins reached a deadly level and killed the whales.
Algae are common in both saltwater and freshwater environments on Earth. They float on the surface of the waters and use sunlight to make food. The rapid growth of a population of algae is called an algal bloom. The deaths of the humpbacks is one example of the damage that an algal bloom can cause.
Saltwater Blooms
Saltwater algal blooms are commonly called red tides. This is because the algae that grow rapidly often contain red pigments and turn the color of the water red. But red tides do not always look red. Some red tides are brown, green, or colorless depending on the species of algae that blooms. Dinoflagellates and diatoms are two algae that frequently bloom in red tides.
Scientists are not sure why some saltwater algal populations increase rapidly at times. But red tides occur most often when there is an increase in nutrients in the water. Increases in ocean temperature due to climate changes also affect the occurrence of red tides. Some red tides occur regularly in certain seasons. The cold bottom layers of the ocean contain a lot of nutrients. When the cold water mixes with surface waters, more nutrients become available to surface organisms. With excess nutrients present in the surface waters, blooms of algae occur.
Red tides are dangerous when the toxins that the algae produce become concentrated in the bodies of organisms that consume the algae. Shellfish feed on large numbers of the algae and store the toxins in their cells. Fishes may also feed on the algae and store the toxins. When people or other large organisms eat these shellfish and fishes, it may lead to serious illness or even death. Public health officials close beaches in areas of red tides and prohibit people from gathering shellfish or fishing.
Integrating Technology
Red tides occur more frequently worldwide today than they did a decade ago. Scientists cannot yet predict when red tides will occur. They use images taken by satellites in space to track how red tides move with ocean currents. Satellite images can also detect increases in ocean temperatures, which may put an area at risk for red tide.