Red Tides


Red tides, toxic algae blooms harmful to sea life and humans alike, have been sited worldwide since the 1800's ( 2001) but in recent years have been occurring more frequently and more densely than ever before. Red tide is not caused by any single organism, although some are more common than others. In one tide bloom off of Rhode Island, at least 18 species of harmful algae have been recorded in a single location. ( 2002) It is believed that between 60 and 80 species total are toxic enough to adversely affect the world around them. (Smayda, Theodore. 1997).

Red tides are primarily a phenomenon of tropical and subtropical areas, but they also occur in the temperate zone in late spring and summer. (van den Hoek, 1995, 271.) While these organisms build up every year in warmer weather, a visible "red tide" does not always occur. Wind and water patterns must clump the organisms together more densely than they naturally live before a color (be it a murky brown, an off-orange, or a blood red) appears. Currents can produce the tide, as can upwellings of water. The tide occasionally occurs naturally without artificial movement in places where the water is exceptionally nutrient-rich.

Red Tide organisms

Red tide organisms are quite adaptive and have figured out many tricks to survive over time. The sheer number of species that are capable of producing a similar response in the environment alone is impressive. The two main types of toxic red tide creatures are certain phytoplanktons which produce mostly chemical toxins harmful to fisheries and the environment and a group of dinoflagellates that produce mostly neurotoxins harmful to humans and marine mammals. ( 2001) Dinoflagellates make up the vast majority of the most toxic.

Many of these species are regional. Chattonella antiqua is most prevalent off of Japan, while Gymnodinium breve/ Karenia brevis is the most common organism in the Gulf Coast region. (Smayda, Theodore 1997 / Tester, Patricia 1997). They have all adapted differently to survive. One species, a cyanobacteria called Trichodesmium, has developed the fairly amazing ability to run two previously incompatible processes at the same time. These bacteria can process nitrogen to yield ammonia, and is also capable of making oxygen by photosynthesis. (Pennisi, 2001, 1435) Many red tide algae are flagellates, most of those dinoflagellates, which means they have rudimentary ?tails? to propel themselves with instead of being at the total mercy of the tides and winds. These animals by and large practice simple division as their manner of reproduction. Red tide populations are generally not rapidly growing populations, unless a large source of nutrients and proper warmth is found, at which point a "bloom" occurs. (Smayda, Theodore 1997).

While tide blooms have been occurring for many many years, and there is no absolutely direct proof that humans are responsible for the entire problem, the excess use of fertilizers on farmland, sending nutrient-rich runoff to the oceans, and the overall trend of global warming do provide more suitable and more frequent environments for red tides than occurred in the past. Growth of many species is stimulated by the presence of nitrogen and phosphorous and to lesser extents ammonium and nitrate (Amano, Kunihiko et all, 1998, 118) products found in many commercial fertilizers. Reducing fertilizer runoff is suspected to help control blooms, but even with no artificial waste there is no way to prevent them as they are a natural phenomenon.

Affects on sea life and ecosystem

The most visible affect of red tides is the fish kill. Certain types of organisms (Ptichodiscus brevis in particular) secrete toxins that are absolutely lethal to fish. Even where these organisms are not present in enough numbers to be seen as a ?tide?, they can poison local fish. There are very few preliminary symptoms until a massive die off occurs and dead fish are found floating and washed ashore everywhere. While it was previous thought that fish kills could be due to oxygen depletion or suffocation through clogging the animals? gills, lab tests were run and it was finally determined that the cause of the kills was the secreted toxins. (Van den Hoek, 1992. p.271-273)

The massive fish die-off affects two main populations higher in the food chain. Humans, and other marine life. Red tide-poisoned fish are being blamed for various die offs of dolphins, porpoises, whales, manatees, and large sea birds.( 2001) Even if the toxin level is not high enough to kill the fish straight off, if a bird or mammal eats too many lightly-poisoned fish, the toxins build up more than enough to kill the larger animal. Other species of red tides also affect sea-going mammals that dine mainly on shellfish laden with neurotoxins.

Die offs also affect humans, of course, because they have time and again devastated both the fishing industry and commercial fisheries, causing entire crops of fish to be lost. The excessive rotting of fish and animal carcasses pollutes the body of water and those surrounding it, disturbing other ecosystems as well. This is just yet another way it is visible that when these delicate ecological balances are damaged and multiple layers of the food chain affected, the entire ecosystem remains very unstable until it can find a way to balance itself.

Affects on humans

Red tide has very few permanently devastating effects on humans who encounter it in the wild. It gives many itchy eyes and a raw throat, but the symptoms usually clear up within hours of the end of exposure. Merely contacting the algae in the water does not affect most people; it?s in rough serf conditions when ocean spray is being thrown up that it causes respiratory distress. ( However, some people will develop a skin rash after swimming in heavy red tide-laden waters.

The far more serious cause of worry for most people is eating infected shellfish which have built up red tide toxins in their flesh. Eating toxic oysters causes serious cases of up to five different toxic shellfish-related diseases.

Amnesic Shellfish Poisoning (ASP)
  • Caused by species of Pseudo-nitzschia
  • Life-threatening illness caused by a buildup of domoic acid from eating infected shellfish.
  • Mild cases appear within 24 hours and only affect the gastro-intestinal system with cramps, diarrhea, nausea, and vomiting.
  • Serious cases, apparent after about 48 hours, affect the neurological system as well. Memory loss, dizziness, trouble breathing, headaches, seizures, and comas can occur.
  • Four fatalities in 1987 off of Prince Edward Island, Canada, led to closer monitoring of toxins and safety levels of edible sea life.
  • Ciguatera Fish Poisoning (CFP)
  • Caused by Gambierdiscus toxicus, multiple species of Prorocentrum, Thecadinium and Ostreopsis,, Coolia monotis, and Amphidinium carterae. These species affect fish as well as shellfish.
  • Rarely-fatal illness as a result of poisoning by Ciguatoxin and Maitotoxins.
  • This disease often feels like a sudden, severe case of the flu. Gastrointestinal problems are the first wave of illness, followed by aches, dizziness, inability to control body temperature, and numbness.
  • Recovery is generally slow (weeks or months), and although it?s suspected very early treatment by manitol can help, there is no real antidote and the disease must run its course.
  • Diarrhetic Shellfish Poisoning (DSP)
  • Caused by species of Dinophysis, which attack shellfish.
  • Non-fatal disease caused by overdose of Okadaic Acid
  • This non-lethal but incapaciting disease runs its course in about three days. It produces severe irritation of the gastrointestinal system and diarrhea, vomiting, chills, and strong cramps are its symptoms.
  • Neurotoxic Shellfish Poisoning (NSP)
  • Caused by Karenia brevis (formerly Gymnodinium breve)
  • Non-fatal disease caused by a buildup of Brevetoxins found in shellfish.
  • Short-term incapaciting but non-deadly illness that affects gastrointestinal and neurological systems. This toxin may also attack the respiratory system and produce asthma-like symptoms.
  • Paralytic Shellfish Poisoning (PSP)
  • Caused by species of Alexandrium, Gymnodinium catenatum, and Pyrodinium bahamense.
  • Often deadly disease caused by Saxitoxins.
  • One of the most dangerous red tide diseases, this toxin attacks only the neurological symptoms without the gastrointestinal upset of its counterparts. Respiratory failure is the most severe worry, other symptoms include dizziness, giddiness, drowsiness, actual fever (as opposed to trouble regulating temperature perception) and a visible rash. There is no treatment, but once out of the danger period, full recovery is normal.

  • All illness data from

    In most parts of the world, it is considered safe to eat the ?hard? flesh of fish without undue worry about red tide illnesses. It must be noted, however, that as more species of red tide organisms become more widespread, more species of fish are affected. Needless to say, wherever there is visible evidence of a fish die off, it is considered unsafe to consume even freshly caught fish. Crustaceans such as shrimp, crabs, and lobster are also as a whole considered safe. It is not recommended to eat the organs of any of these creatures, however, just what is commercially sold as meat.

    The real danger to humans during red tide events is shellfish, especially filter-feeding bivalves. As they take in water and plankton and algae to feed, they naturally ingest red tide organisms, and the toxins build up in the soft flesh of the animals. Commercially harvested shellfish are carefully regulated both by national, regional, and statewide organizations through careful monitoring red tide activity in the region.

    It is important to note that red tide toxins can remain in shellfish for quite some time after the bloom is no longer visible. "Urban legend" and "old wives tales" have for years stated that it was safe to eat shellfish during any month that has an R in the name. These were the cooler winter months, September through April. But research has proven this false. These dates had to do more with the weather outside and attempts to prevent spoiling of shellfish than red tides. ( Checking with local agencies is the only way to be truly safe.


    Red tides are a very large, complex phenomenon involving many different types of creatures with different characteristics. But no matter what type of organism involved, the trends and impacts of red tides remain the same. Fish kills and extreme disruptions of the ecosystems, destruction of fishing industries, and poisoning of humans from infected shellfish occur throughout the world wherever red tides are found.

    Red tides also have in common that they require warm waters and ample nutrients, especially nitrogen and phosphorous, to thrive and bloom enough to cause these ill effects. They are natural phenomena that cannot be stopped, but perhaps with enough study and enough care can be controlled.

    Works Cited

    Amano, Kunihiko (and others). "Conditions necessary for chattonella Antiqua red tide outbreaks." Limnol Oceanogr, v.43 (1998) p.117-128. This article focuses on the forms of red tide most common in Japanese seas. It talks about water and nutrient and weather conditions necessary for the normal bloom to get strong enough to be considered a ?red tide?

    Franks, Peter J. R. "Models of Harmful Algal Blooms" Limnol Oceanogr, v.42 (1997) p.1273-1282. Article on scientific models of the tides. These are being researched to compare to data to gain a better understanding of how the tide actually works.

    Mote Marine Laboratory (2001) "About Red Tide"
    This article provides a good overview of the red tides in general, focusing on the blooms and history of problems in Florida.

    National Science Foundation (2002) "The Harmful Algae Page"
    Run by the National Science Foundation, this is an entire website devoted to the red tides. It has many very useful and informative sub-pages full of data.

    Pennisi, Elizabeth. "Multitasking is this Plankton?s Trademark" Science, V. 294 Nov 2001, p. 1435.
    Short article on the biological oddities of one species of tide organism capable of running logically incompatible processes.

    Smayda, Theodore J. "Harmful algal blooms" Limnol Oceanogr, v. 42 (1997) p. 1137-1153. This article talks about a wide number of species around the world and some of their odder biological adaptations to survive. Very mathematical and hard to read.

    Tester, Patricia A; Steidinger, Karen A. "Gymnodinium breve red tide blooms: initiation, transport, and consequences of surface circulation" Limnol Oceanogr, v 42 (1997) p.1039-1051.
    Focuses on the red tide organism most common in the Gulf of Mexico. Long, technical article on the cycles of the tide.

    Texas Parks and Wildlife (2001) "Red Tide in Texas"
    Overview of red tides in general with the history and affects in Texas. Has many useful links to other pages of other local agencies.

    University of Rhode Island (2002) "URI oceanographers study effects of harmful blooms and red tides on Narragansett Bay" Short article on a grant to research red tides in Narragansett Bay.

    Van den Hoek, C. ; Mann, D.G.; Jahns, H. M. Algae: an introduction to phycology. (Cambridge University press, 1992.) This is mostly a textbook overview, but provides good information on the red tide phenomenon without getting so bogged down in technical detail it?s incomprehensible.

    Washington State Department of Health (2002 ) "Biotoxin program"
    This site is mostly on the human health issues and regulation of harmful shellfish. It provides data on what beaches in Washington it is safe to harvest and information on illness, as well as a 'common misconceptions' page.