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RED TIDES: Causes, Consequences and Control of Algal Blooms
Marie Manandise and Tom Sumner
CoMPLEX, University College London, 4 Stephenson Way, London, NW1 2HE
What are Red Tides?
When Do They Occur?
“Red tides” are massive blooms of a single phytoplankton species which
change the colour of the sea (fig. 1). The term red tide is confusing
because the colour depends on the pigmentation of the species that
blooms. It can also be orange (fig. 2), brown or even bright green.
In contrast to common seasonal algal blooms, their occurrence is not
predictable; the conditions required for a red tide to appear are not yet
fully understood. Besides, seasonal blooms involve a whole community
of algae and not a single species.
Red tides are a natural phenomenon which can be without any
consequence. However, some blooms can temporarily disturb the marine
ecosystem.
a.
b.
Figure 1. Two examples of red tides a) Cape Rodney, New Zealand b)
a bloom of Noctiluca scintillans, La Jolla, California.
The Effects of Red Tides
The appearance of red tides seems largely unpredictable. The species
causing the blooms are always present at low concentrations and are a
fundamental part of the pelagic ecosystem.Their growth is limited by:
• the availability of light
• the availability of nutrients, namely N, P and Fe
• competition, mainly with diatoms
• predation
When one or more of these factors are modified, a bloom can occur.
These blooms are often part of an annual cycle (for example: spring
blooms of phytoplankton in the north sea as a result of increased
availability of light).
In the case of red tides, the most likely explanation is a change in the
concentration of nutrients. Dinoflagellates are usually outcompeted by
diatoms. Two changes can reverse the situation and consequently lead to
a red tide:
1. An increase in N  growth of the diatom population  consumption
of all the Si  collapse of the diatom population  blooms of
dinoflagellates
2.Drop of N:P ratio towards the optimum ratio for the growth of
dinoflagellates  dinoflagellates outcompete the diatoms (fig. 3)
•Red tides can cause oxygen depletion
resulting in massive fish kills (fig. 4)
•The spines and protusions on many
dinoflagellates can obstruct fish gills
preventing them from breathing
•Some phytoplankton species (for
example Gymnodium breve) produce
toxins which can be fatal to marine
veterbrates
•Toxins can accumulate in molluscs
and
crustaceans
making
them
poisonous to humans and other
animals which consume them
Due to the environmental and economic
impacts of red tides it is necessary to be
able to monitor and control their
occurrence.
Figure 4. Fish kill due to
oxygen depletion,
Narrangasett Bay, USA.
Monitoring and Control
Dinoflagellates
Approximately 200 species are known to cause red tides. The majority of
these species belong to the phylum dinoflagellates.
Dinoflagellates are autotrophic protozoans, i.e. unicellular algae. They
are primary producers at the basis of the marine food chain. Not all of
them are strictly autotrophic as they can also phagocyte organic matter.
Dinoflagellates typically have two flagella which are used for propulsion.
Each lies in a groove on the cell surface. The sulcus trails freely, while
the cingulum is wrapped around the body (Fig 2).
Most dinoflagellates have a cell wall made out of cellulose plates trapped
in intracellular vesicles or alveoli. Xantophophyll pigments are
responsible for their brown to golden-brown colour.
Figure 2.
Dinoflagellate
anatomy
Red tides are short lived phenomenon but
can have a considerable impact on the
marine ecosystem.
Attempts
to
monitor
the
occurrence of red tides include
satellite imaging of sea surface
temperature
and
pigment
concentration (fig. 5).
Figure 3. Change in N:P
ratio in Tolo Harbour
between 1982 and 1989,
and the occurrence of
red tides during the
same period.
Reproduced from Hodgkiss and
Ho, 1997.
Figure 5. Concentration of
chlorophyll a monitored by
satellite.
Research is also ongoing to
develop methods to control or
prevent red tides. Possible
solutions include the release of
clay particles and the introduction
of new predators into the sea.
References
Barnes R. D., and E. E. Ruppert, 1996. Invertebrate zoology, sixth edition. Ed. Harcourt, Orlando; Dubois P. 2005. Marine biology, course notes (Free University of Brussels); Hodgkiss I. J., and
K. C. Ho,1997. Are changes in N:P ratio in coastal waters the key to increased red tieds blooms? Hydrobiologia 352: 141-147; Millie D. M., Schofield O. M., Kirkpatrick G. J., Johnsen G., Tester
P. A., and B. T. Vinyard, 1997. Detection of harmful algal blooms using photopigments and absorption signatures: a case study of the Florida red tide dinoflagellate, Gymnodinium breve.
Limnology and Oceanography 42(5): 1240-1251; Tester P. A., and K. A. Steidinger, 1997. Gymnodinium breve red tide blooms: initiation, transport, and consequences of surface circulation.
Limnology and Oceanography 42(2): 1039-1051; Fig. 1a. N. Godfrey www.niwascience.co.nz; Fig. 1b. Peter Franks, Scripps intstitution of oceanography; Fig. 2.
www.geo.ucalgary.ca/~macrae/palynology/dinoflagellates/; Fig. 4. www.geo.brown.edu/georesearch/insomniacs; Fig. 5. sg.geocities.com/myredtide/250m.htm