Much of the following information was taken from the website "Harmful Algae: Red Tide". This website has numerous useful links to scientific information, human health impacts, and impacts to animals from harmful algal blooms.
Harmful algae are microscopic, single-celled plants that live in the sea. Most species of algae or phytoplankton (plant plankton) are not harmful and serve as the energy producers at the base of the food web, without which higher life on this planet would not exist. Phytoplankton are believed to generate as much as 80% of the world's oxygen supply. They absorb nutrients and carbon dioxide from the water and produce oxygen through photosynthesis. The two most common types of marine phytoplankton are diatoms and dinoflagellates.
Occasionally, the algae grow very fast or "bloom" and accumulate into dense, visible patches near the surface of the water. "Red Tide" is a common name for such a phenomenon where certain phytoplankton species contain reddish pigments and "bloom" such that the water appears to be colored red. The term "red tide" is thus a misnomer because they are not associated with tides; they are usually not harmful; and those species that are harmful may never reach the densities required to discolor the water. During these blooms, most of the phytoplankton eventually die and sink to the bottom, where they are decomposed by bacteria. In addition, at night when photosynthesis stops, algae and other aquatic plants produce carbon dioxide and consume oxygen. These processes deplete the dissolved oxygen necessary for the survival of fish and other organisms.
Unfortunately, a small number of species also produce potent neurotoxins that can be transferred through the food web where they affect and even kill the higher forms of life such as zooplankton, shellfish, fish, birds, marine mammals, and even humans that feed either directly or indirectly on them. Scientists now prefer the term, HAB, to refer to bloom phenomenon that contain toxins or that cause negative impacts.
Only a few dozen of the many thousands of species of microscopic and macroscopic algae are repeatedly associated with toxic or harmful blooms. Some species, such as the dinoflagellate Alexandrium tamarense and the diatom Pseudo-nitzschia australis produce potent toxins, which are liberated when the algae are eaten.
People are exposed principally to the toxins produced by harmful algae through the consumption of contaminated seafood products. Dinoflagellate algal blooms create "red tides" which can release strong neurotoxins, such as saxitoxin, that can be ingested by shellfish and passed on to humans who eat the infected shellfish. The most significant public health problems caused by harmful algae are:
Each of these types of health problems are caused by different species of toxic algae which occur in various coastal waters of the US and the world. For instance, Karenia brevis found in the Gulf of Mexico, produces brevetoxins which can cause neurotoxic shellfish poisoning. With the inhalation of the aerosolized Red Tide toxins, especially the brevetoxins, from the sea spray exposure associated with Gulf of Mexico Red Tides with and without accompanying fish kills, respiratory irritation and possibly other health effects in humans and other mammals occur. More on this.
With the increase in interstate and international transport of seafood, as well as international travel by seafood consumers, there are virtually no human populations that are free of risk. Since 1978, illnesses in the US due to natural algal toxins have included PSP, NSP, CFP, and ASP. No incidents of DSP have yet been verified in this country. Although records are incomplete because reporting to the Centers for Disease Control (CDC) is voluntary, evidence indicates that ciguatera was responsible for about half of all seafood intoxications. A growing body of evidence indicates that incidents of ASP are on the increase and that DSP may shortly make its debut in the United States, since the causative organisms occur throughout the temperate coastal waters of the US.
In the last few years there have been several reported instances of mass deaths of birds and marine mammals associated with the diotom pseudonitzschia and domoic acid, which is produced by pseudonitzschia. In September 1991, more than 100 brown pelicans and cormorants died in Monterey Bay, California. The cause was traced to domoic acid found in anchovies, which is the birds' food source. Psuedonitzcschia australis dominated the phytoplankton in Monterey Bay at that time and was also abundant in the stomachs of anchovies taken from the same area. More recently, domoic acid was identified as the cause of death of many sea lions throughout central and southern California. See Domoic Acid and Sea Lions for more information.
Many reports of mass abundance of pseudonitzschia have been listed by local/state coastal and health authorities around the world and relationships between algal blooms and the uptake of these diatoms and domoic poison in the food chain have been documented. Large losses to both commercial and personal coastal catch have been caused by these algae.
The following information was adapted from correspondence from Peter Franks at Scripps Institute of Oceanography in La Jolla, California:
A periodic occurrence along the California coast is the appearance of water that has a red, brown or purple hue, commonly referred to as "red tide". This is caused by dense accumulations near the surface of dinoflagellates (think: tiny cellulose-covered balls with two little whips for propulsion). Many dinoflagellates are photosynthetic (make sugars from light and carbon dioxide), and bioluminescent (make their own light). Each cell is about 30-40 microns across (there are 1000 microns in a millimeter, which is about the thickness of a dime). The cells have tiny sacs of enzymes that react when the cell is jostled (for example by the breaking surf). When the enzymes react, they give off a bluish flash of light. So our waves (and your footsteps on the beach; your hands and feet when you swim) will be adorned with gorgeous flashes of light at night.
The bioluminescence of these cells is on a circadian rhythm - they don't bother making light during the day because no one could see it. Try this to amaze your friends and children - get a clear jar or bottle, and fill it with some water from the surf zone (ask a friendly neighborhood surfer to fill it for you in deeper water, to get less sand in it). Take your bottle home, and wait until after the sun has gone down. Then take your bottle into a dark room (bathroom or closet with no windows). Wait for your eyes to adjust to the darkness (a minute or so), and then give your bottle a swirl. You should see a really amazing light show. For extra added excitement, add some vinegar to the bottle. You'll get a particularly bright flash (but then all the cells will die, so it only works once). The acid of the vinegar makes the enzymes react inside the cell, even without stirring.
Unfortunately, although scientists understand what red tides are, often they don't know why they form at particular times. Red tides are natural occurrences; the plankton community becomes dominated by one or a few species at extraordinarily high concentrations. The net growth rate of the cells (including accumulation by swimming) is higher than their net loss rate (due to grazing, etc.). The growth can be fueled by nutrients brought to the surface by upwelling. There is some evidence that development and growth of red tides can be facilitated by discharges of treated sewage from municipal sewage treatment plants (containing nitrates, urea and other nutrients) and also from "urban runoff" that contains fertilizers washed off of landscaped areas.
The organisms that make red tides (at least in southern California) tend to be able to swim. They might be swimming upward to photosynthesize, and downward to take up nutrients. If they are all doing the same thing, then they can form dense layers that are visible from the surface. It's possible that the total amount of nutrients in the red-tide organisms is higher than the nutrients that were in the water before the bloom. This means that the cells must have moved relative to the water, in order to accumulate nutrients.
Two thoughts on this: One is that the organisms eating the red-tide cells don't like lights flashing in their face, and so don't graze on the flashing cells. The other thought is that the cell's flashing acts like a burglar alarm: an organism eating a red-tide cell causes it to flash, attracting the notice of a visual predator (the police) who then eats the grazer (burglar). Swift justice.
To really know why red tides appear and to be able to predict their appearance, dense and continuous sampling all up and down the Southern California Bight for several months prior to a bloom would have to be done. Scientists would have to measure the growth rates of the red-tide organisms, their swimming behaviors, and all their sources of mortality. Furthermore, they would have had to measure all the water velocities, since the water motions move the blooms around. Unfortunately, none of this sampling is currently being done. On the upside, there are plans to put observation networks in place all along the coast that would help generate data that might provide clues about how and why these blooms form. From a scientific point of view, these red tides are wonderful examples of physical-biological interactions leading to a massive perturbation of the marine ecosystem. If we could understand why they occur, we'd learn a lot about how the planktonic ecosystem works the rest of the time.
The foam is probably degradation products and exudates from a phytoplankton bloom. According to Lihini Aluwihare of Scripps, the foam is mostly protein (a lot like egg whites).
In general, water containing "red tide" is not harmful to recreate in. Showering after swimming or surfing in water that contains an algal bloom is advised. In fact, showering after swimming or surfing is always a good idea. In some very unusual circumstances, due to coastal wind and wave action, HAB biotoxins can be transported by and through the air, causing severe eye, nose, and throat irritation. In other instances, obnoxious odors and smells can emanate from blooms, especially in confined tidewaters and bays.
In July and August 2010 and during the same period in 2012 a very noticeable iridescent green foam covered large areas of the ocean in southern California from Long Beach to San Diego. It was determined to be Tetraselmis (see report for 26 July 2012 and scroll down to reports from July-August 2010), which is very small flagellated chlorophyte. There are no documented health hazards from this organism. Here's another discussion of tetraselmis after this species of phytoplankton reappeared in 2012.
As noted above, HABs include different types of algal taxa such as dinoflagellates, diatoms, and cyanobacteria. Cyanobacteria, also known as blue-green algae, are of special concern because of their potential impacts on drinking and recreational waters. In freshwaters, cyanobacteria can produce unsightly conditions along the shoreline and in open waters degrading aquatic habitats and posing a health risk to humans, pets or wildlife. Increasingly, water managers and the public have expressed concerns about public health and environmental quality from HABs toxins in recreational and drinking waters have become an increasingly serious public health and environmental concern in the United States. EPA has compiled information on freshwater HABs and their effects to help inform the public about potential impacts of toxic algal blooms in freshwater.
Harmful Algal Blooms - by NOAA
Harmful Algae and Red Tides - by Woods Hole Oceanographic Institution
Harmful Algae & Red Tide Regional Monitoring Program - by Southern California Coastal Ocean Observing System (SCCOOS)
Toxic Algae Blooms KQED radio report
Red Tide (Surfline)
The Beach Manager's Manual - Harmful Algal Blooms (Great Lakes)