Red tide

Further information: Harmful algal bloom

Red tide is a common name for a phenomenon also known as an algal bloom (large concentrations of aquatic microorganisms), an event in which estuarine, marine, or fresh water algae accumulate rapidly in the water column and results in discoloration of the surface water. It is usually found in coastal areas.[1]

These algae, known as phytoplankton, are single-celled protists, plant-like organisms that can form dense, visible patches near the water's surface. 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 kind of red tides are the associated wildlife mortalities of 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.

Contents

Overview

Red tides in the Gulf of Mexico are a result of high concentrations of Karenia brevis, a microscopic marine algae that occurs naturally but normally in lower concentrations. In high concentrations, its toxin paralyzes the central nervous system of fish so they cannot breathe. Dead fish wash up on Gulf of Mexico beaches. Dense concentrations appear as discolored water, often reddish in color. It is a natural phenomenon, but the exact cause or combination of factors that result in a red tide outbreak are unknown.[2] Red tide causes economic harm and for this reason red tide outbreaks are carefully monitored. For example, the Florida Fish and Wildlife Conservation Commission provides an up-to-date status report on the red tide in Florida.[3] Texas also provides a current status report.[4]

Red tide is also potentially harmful to human health.[5] Humans can become seriously ill from eating oysters and other shellfish contaminated with red tide toxin.[6]

The cause of red tides is controversial. Red tides occur off coasts all over the world and have occurred for thousands of years. Not all red tides have toxins or are harmful.[7]

Definition

Red tide is a colloquial term used to refer to one of a variety of natural phenomena known as a harmful algal blooms or HABs. The term red tide specifically refers to blooms of a species of dinoflagellate known as Karenia brevis.[8] It is sometimes used to refer more broadly to other types of algal blooms as well.

The term red tide is being phased out among researchers for the following reasons:

  1. Red tides are not necessarily red and many have no discoloration at all.
  2. They are unrelated to movements of the tides.
  3. The term is imprecisely used to refer to a wide variety of algal species that are known as bloom-formers.

As a technical term it is being replaced in favour of more precise terminology including the generic term harmful algal bloom for harmful species, and algal bloom for non-harmful species.

The term red tide is most often used in the United States of America to refer to Karenia brevis blooms in the eastern Gulf of Mexico, also called the Florida red tide. 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 discolor the water a deep reddish-brown hue.

The term red tide is also sometimes used to describe harmful algal blooms on the northern east coast of the United States, 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.[9]

Causes of red tide

The occurrence of red tides in some locations appear to be entirely natural (algal blooms are a seasonal occurrence resulting from coastal upwelling, a natural result of the movement of certain ocean currents)[10][11] while in others they appear to be a result of increased nutrient loading from human activities.[12] 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.[13] 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,[14] 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.[15][16]

Notable occurrences

See also

References

  1. ^ "Discover NOAA's Coral Reef Data". www8.nos.noaa.gov. http://www8.nos.noaa.gov/coris_glossary/index.aspx?letter=r. Retrieved 2009-08-22. 
  2. ^ "Red Tide FAQ". www.tpwd.state.tx.us. http://www.tpwd.state.tx.us/landwater/water/environconcerns/hab/redtide/faq.phtml. Retrieved 2009-08-23. 
  3. ^ Florida Fish and Wildlife Research Institute. "Red Tide Current Status Statewide Information". research.myfwc.com. http://research.myfwc.com/features/view_article.asp?id=9670. Retrieved 2009-08-23. 
  4. ^ "Red Tide Index". www.tpwd.state.tx.us. http://www.tpwd.state.tx.us/landwater/water/environconcerns/hab/redtide/. Retrieved 2009-08-23. 
  5. ^ "Harmful Algal Blooms: Red Tide: Home". www.cdc.gov. http://www.cdc.gov/hab/redtide/. Retrieved 2009-08-23. 
  6. ^ "Red Tide FAQ - Is it safe to eat oysters during a red tide?". www.tpwd.state.tx.us. http://www.tpwd.state.tx.us/landwater/water/environconcerns/hab/redtide/faq.phtml#q9. Retrieved 2009-08-23. 
  7. ^ Gregg W. Langlois, Pamela D. Tom. "Red Tides: Questions and Answers". U.S. Government. http://www.whoi.edu/fileserver.do?id=47320&pt=10&p=18553. Retrieved 2009-08-23. 
  8. ^ "Harmful Algal Blooms (HABs): Red Tide". U.S. Centers for Disease Control and Prevention. http://www.cdc.gov/hab/redtide/default.htm. Retrieved 2 Oct 2011. 
  9. ^ "Red Tide Fact Sheet - Red Tide (Paralytic Shellfish Poisoning)". www.mass.gov. http://www.mass.gov/?pageID=eohhs2modulechunk&L=4&L0=Home&L1=Provider&L2=Guidance+for+Businesses&L3=Food+Safety&sid=Eeohhs2&b=terminalcontent&f=dph_environmental_foodsafety_p_red_tide&csid=Eeohhs2. Retrieved 2009-08-23. 
  10. ^ Trainer VL, Adams NG, Bill BD, Stehr CM, Wekell JC, Moeller P, Busman M, Woodruff D (2000) Domoic acid production near California coastal upwelling zones, June (1998). Limnol Oceanogr 45:1818–1833
  11. ^ Adams NG, Lesoing M, Trainer VL (2000) Environmental conditions associated with domoic acid in razor clams on the Washington coast. J Shellfish Res 19:1007–1015
  12. ^ Lam CWY, Ho KC (1989) Red tides in Tolo Harbor, Hong Kong. In: Okaichi T, Anderson DM, Nemoto T (eds) Red tides. biology, environmental science and toxicology. Elsevier, New York, pp 49–52.
  13. ^ Walsh et al. (2006). Red tides in the Gulf of Mexico: Where, when, and why? Journal of Geophysical Research 111, C11003, doi:10.1029/2004JC002813
  14. ^ Cabeza de Vaca, Álvar Núnez. La Relación (1542). Translated by Martin A. dunsworth and José B. Fernández. Arte Público Press, Houston, Texas (1993)
  15. ^ Sellner, K.G.; Doucette G.J., and Kirkpatrick G.J. (2003). "Harmful Algal blooms: causes, impacts and detection". Journal of Industrial Microbiology and Biotechnology 30 (7): 383–406. doi:10.1007/s10295-003-0074-9. PMID 12898390. http://www.springerlink.com/content/ptybc0qg8y4klr5c/. 
  16. ^ Van Dolah, F.M. (2000). "Marine Algal Toxins: Origins, Health Effects, and Their Increased Occurrence". Environmental Health Perspectives 108 (suppl.1): 133–141. doi:10.2307/3454638. JSTOR 3454638. PMC 1637787. PMID 10698729. http://www.ehponline.org/docs/2000/suppl-1/133-141vandolah/abstract.html. 
  17. ^ HAB 2000
  18. ^ (R/V) Oceanus, National Science Foundation
  19. ^ Moore, Kirk. "Northeast Oysters: The bigger danger, growers assert, would be the label of endangered". National Fisherman. http://www.nationalfisherman.com/month-content.asp?ItemID=1156&pcid=184&cid=196&archive=yes. Retrieved 2008-07-31. 
  20. ^ University of Florida Marine and Natural Resources, IFAS Extension

External links