Red tide
"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, or "bloom".  These algae, more specifically phytoplankton, are microscopic, 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 color from green to brown to red, and when the algae are present in high concentrations, the water appears to be discolored or murky, varying in color from white to almost black, normally being red or brown. 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.
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 produced naturally by Karenia brevis, called brevetoxin.

# Usage
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, and is also called "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 liter of seawater, and often discolor the water a deep reddish-brown hue.
The term "Red tide" is also commonly used on the northern east coast of the United States, and particularly in the Gulf of Maine. This type of bloom is caused by another species of dinoflagellate known as Alexandrium fundyense. These blooms of organisms cause severe disruptions in fisheries of these waters as the toxins in these organism cause filter-feeding shellfish in affected waters to become poisonous for human consumption due to saxitoxin.

# Causes
It is unclear what causes red tides; their occurrence in some locations appears to be entirely natural, while in others they appear to be a result of human activities  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.  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.  Some algal blooms on the Pacific coast have also been linked to occurrences of large-scale climatic oscillations such as El Nino 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 is in fact a real increase or is due to increased observation effort and advances in species identification methods  

# Associated illnesses
Some organisms that can cause red tide naturally produce potent toxins, such as saxitoxin, domoic acid, or brevetoxin, and such red tides composed of toxic phytoplankton are commonly referred to as "harmful algal blooms" or "HABs".  The various red tide toxins each have different modes of action, such as disrupting the proper function of ion channels in neurons, mimicking of neurotransmitters, or inhibiting enzymatic activity. Domoic acid, a toxin produced by diatoms of the genus Pseudo-nitzschia, has been linked to neurological damage in certain marine mammals, and is frequently found in algal blooms on the U.S. West Coast. Some red tide toxins can become highly concentrated in various marine organisms that have the ability to filter and consume large quantities of toxic plankton directly from seawater, which include shellfish, finfish, baleen whales, and benthic crustaceans.  Frequently, shellfish collected in areas affected by algal blooms can be potentially dangerous for human consumption, leading to closures of shellfish beds for harvesting.

# Notable Occurrences
- On August 30, 2007, Japanese music producer N-Kei Enzaki was reported as being found dead in his hotel room in Osaka, Japan after eating mussels contaminated with red tide toxin.  The story was later revealed to be a hoax propagated by TV Tokyo staff.  An apology statement was aired shortly after.
- On August 23, 2007,  5 year-old boy was killed and 12 others were hospitalized after eating mussels shellfish contaminated with red tide toxin in Sorsogon, Philippines. The BFAR also raised red tide alerts in the coastal waters of Milagros Bay in Masbate; Juag Lagoon in Matnog; Bislig Bay in Bislig City in Surigao del Sur; Hinatuan Bay in Surigao del Sur; and Dumanguilas Bay in Zamboanga del Sur.
- In June, 2 children were killed and 5 others were hospitalized in Casiguran, Sorsogon, after eating red tide-contaminated mussels.
- In 1972 a red tide was caused in New England by a toxic dinoflagellate Alexandrium (Gonyaulax) tamarense.