Red tide is a common name for a
phenomenon known as an algal bloom (large concentrations of aquatic
microorganisms) when it is caused by a few species of dinoflagellates and the
bloom takes on a red or brown color. Red tides are events in which estuarine,
marine, or fresh water algae accumulate rapidly in the water column, resulting
in coloration of the surface water. It is usually found in coastal areas.
Certain species of phytoplankton,
dinoflagellates, contain photosynthetic pigments that vary in color from green
to brown to red. When the algae are present in high concentrations, the water
appears to be discolored or murky, varying in color from purple to almost pink,
normally being red or green. Not all algal blooms are dense enough to cause
water discoloration, and not all discolored waters associated with algal blooms
are red. Additionally, red tides are not typically associated with tidal
movement of water, hence the preference among scientists to use the term algal
bloom.
Some red tides are associated
with the production of natural toxins, depletion of dissolved oxygen or other
harmful effects, and are generally described as harmful algal blooms. The most conspicuous
effects of these kinds of red tides are the associated wildlife mortalities of
marine and coastal species of fish, birds, marine mammals, and other organisms.
Red tide is also potentially
harmful to human health. Humans can become seriously ill from eating
oysters and other shellfish contaminated with red tide toxins.
- The term red tide is being phased out among researchers for the following reasons:
- Red tides are not necessarily red and many have no discoloration at all.
- They are unrelated to movements of the tides.
- The term is imprecisely used to refer to a wide variety of algal species that are known as bloom-formers.
The occurrence of red tides in
some locations appears to be entirely natural (algal blooms are a seasonal
occurrence resulting from coastal upwelling, a natural result of the movement
of certain ocean currents) while in others they appear to be a result of
increased nutrient loading from human activities. The growth of marine
phytoplankton is generally limited by the availability of nitrates and
phosphates, which can be abundant in agricultural run-off as well as coastal
upwelling zones. Coastal water
pollution produced by humans and systematic increase in sea
water temperature have also been implicated as contributing factors in
red tides. Other factors such as iron-rich dust influx from large desert areas
such as the Saharan desert are thought to play a major role in causing red
tides.
Credit: Wikipedia
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