| Red tide. View of an algal bloom known as a red
tide. Red tides consist of vast numbers of red dinoflagellate
single-celled algae. They result from a surge in the level of nutrients
in coastal waters through the upwelling of cold nutrient-rich water from
deep within the ocean. In some cases red tides may be due to untreated
sewage or agricultural fertilizers entering the ocean. Certain dinoflagellates
produce toxins which become concentrated in animals who feed on them. As
a result it is dangerous to eat clams, mussels and certain fish after a
red tide. Photographed on the Bountiful Islands, Gulf of Carpentaria in
Queensland, Australia. |
Ponds or lakes containing a
rich supply of organic matter also often develop
huge populations ("blooms") of green algae or cyanobacteria, giving
the water an opaque, green appearance. The water may become so green with these
organisms that objects even a few centimeters
below the surface are invisible. In this image, A
scientist monitoring water pollution fills a container with freshwater discolored
by an algal bloom. It occurs due to abnormal growth of algae, usually blue-green
algae (as here). This
growth may have natural causes such as strong sunlight, or more usually is due
to surplus phosphorus or nitrogen in the water derived from sewage or other polluting
human activities. Some algae liberate toxins harmful to fish, aquatic birds and
humans. Algal blooms may also cause eutrophication (oxygen depletion) in bodies
of water resulting in large scale death of animal and plant species. This sample
was taken at Griffith in New South Wales, Australia. |
