The Red Tide Phenomena

"Red tide" is a common name for a phenomenon known as an algal bloom,
an event in which estuarine, marine, or fresh water algae accumulate rapidly in
the water column. These algae, more specifically phytoplankton, are single-celled protists, plant-like organisms that can form dense, visible patches near the water's surface. Certain species of phytoplankton contain photosynthetic pigments that vary in colour from green to brown to red.

When the algae are present in high concentrations, the water appears to be discoloured or murky, varying in colour from purple to almost pink, normally
being red or green. Not all algal blooms are dense enough to cause water
discolouration, and not all discoloured 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 impacts, and are generally described
as harmful algal blooms. The most conspicuous effects of red tides are the associated wildlife mortalities among marine and coastal species of fish, birds, marine mammals and other organisms.

In the case of Florida red tides, these mortalities are caused by exposure to a
potent neurotoxin called brevetoxin which is produced naturally by the marine algae Karenia brevis.

"Red tide" is a term used to refer to a natural phenomenon known as a "harmful algal bloom" The term "red tide" is being phased out among researchers for the following reasons:

Red tides are not necessarily red and many have no discolouration at all.

They are unrelated to movements of the tides.

A wide variety of algal species are known bloom-formers.

It is being replaced in favour of the more accurate "harmful algal bloom" for harmful species, or simply "algal bloom" for non-harmful species.

The term "red tide" is often used in the United States of America to describe a particular type of algal bloom common to the eastern Gulf of Mexico, also called the
"Florida red tide". This type of bloom is caused by a species of dinoflagellate known as Karenia brevis, and these blooms occur almost annually along Florida
waters. The density of these organisms during a bloom can exceed tens of millions of cells per litre of seawater, and often discolour the water a deep
reddish-brown hue.

It is unclear what causes red tides; their occurrence in some locations appears to be entirely natural,[1] while in others they appear to be a result of human activities[2] The frequency and severity of algal blooms in some parts of the world have been linked to increased nutrient loading from human activities. In other areas, algal blooms are a seasonal occurrence resulting from coastal upwelling, a natural result of the movement of certain ocean currents.

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[citation needed].

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.[4] Some algal
blooms on the Pacific coast have also been linked to occurrences of large-scale climatic oscillations such as El Niño events.

While red tides in the Gulf of Mexico have been occurring since the time of early
explorers such as Cabeza de Vaca, it is unclear what initiates these blooms and how
large a role anthropogenic and natural factors play in their development. It is also
debated whether the apparent increase in frequency and severity of algal blooms in
various parts of the world.

In fact a real increase or is due to increased observation effort and advances in species identification methods.

Impacts include human illness and mortality following consumption of or indirect exposure to HAB toxins, substantial economic losses to coastal communities and commercial fisheries, and HAB-associated fish, bird and mammal mortalities. To the human eye, blooms can appear greenish, brown, and even reddish- orange depending upon the algal species, the aquatic ecosystem, and the concentration of the
organisms.

No deaths of humans have been attributed to Florida red tide, but people may experience respiratory irritation (coughing, sneezing, and tearing) when the red tide organism (Karenia brevis) is present along a coast and winds blow its toxic aerosol onshore. Swimming is usually safe, but skin irritation and burning is possible in areas of high concentration of red tide.

-Info reprinted from Wikipedia