Bad Bloom Rising
The remarkable sequence of images shown below was captured by the Moderate
Resolution Imaging Spectroradiometer (MODIS) instruments on the Terra and Aqua
platforms on August 10-13, 2003. On August 10, a very large thunderstom
complex over Algeria produced immense amounts of rain that were quite unusual
for this arid desert region. Local flooding was reported. On the following
days, MODIS observed the development of a phytoplankton bloom originating in
the semi-circular bay that is surrounded by Algiers, the capitol city of
Algeria.
Sequence of MODIS images showing the development of a
phytoplankton bloom in the Mediterranean Sea off of Algiers. Top left:
Storm over Algiers, August 10, 2003. Top right: Image of bloom,
August 11, 2003. Bottom left: Image of bloom, August 12, 2003.
Bottom right: Image of bloom, August 13, 2003. Clicking each image
will provide the full 500-meter resolution image.
Phytoplankton blooms normally occur in response to either an increased
supply of nutrients, or an increase amount of sunlight. In the southern
Mediterranean Sea off the coast of Africa, sunlight is rarely in short supply,
but nutrients usually are. The Mediterranean is actually a low-productivity
body of water, and the clarity of the Mediterranean waters is renowned for this
reason.
The thunderstorm seen in the first image must have caused a large amount of
stormwater to run off of the hills and streets of Algeria and into the Bay of
Algiers. It is also possible that sewage treatment facilities were overwhelmed
by the volume of stormwater, causing the release of untreated sewage into the
bay. (There are no reports of this happening, but similar problems can still
occur for sewage treatment plants in the United States under high flow
conditions caused by storms.) The stormwater carried with it a high nutrient
load from the dust, dirt, and debris that it received from the surrounding
land. Opportunistic phytoplankton immediately utilized this transient nutrient
supply to create the rapid population growth characteristic of a bloom. As
the water was carried out to sea, the bloom went with it, slowly dissipating
as the water dispersed and as the phytoplankton exhausted the nutrients in the
pulse of stormwater.
Most of the phytoplankton blooms occurring in the oceans are induced by
natural causes and seasonal cycles. The North Atlantic
Bloom is the largest seasonal bloom observed every year, but there are also
large blooms in the Arabian Sea due to the monsoon cycle, and increases in
productivity associated with the rainy season in South America can be seen
emanating from the mouths of the Orinoco and Amazon rivers. Because these
blooms take place due to natural processes, it's possible to call them 'good'
blooms.
However, the activities of mankind are causing an increasing incidence of
what can be termed 'bad' blooms: blooms induced by high nutrient
concentrations due to pollution, enhanced runoff due to loss of vegetative
cover and erosion, or agriculture. These blooms, rather than enhancing
oceanic productivity, tend to interrupt the natural cycle, providing an
overabundance of organic matter to the ocean bottom (see Creeping Dead Zones) and frequently enhancing the
growth of noxious or toxic phytoplankton.
This problem is particularly acute in what are called enclosed oceanic
basins. The Mediterranean Sea is one example of an enclosed oceanic basin;
the Baltic Sea is another. The Sea-viewing Wide Field-of-view Sensor (SeaWiFS)
image below shows an immense bloom of toxic phytoplankton in the Baltic Sea.
SeaWiFS image of the Baltic Sea acquired on July 24,
2003, by station HDUN at the University of Dundee, Scotland. A bloom of
Nodularia spumigena is seen covering a large portion of the southern
Baltic Sea. (Click picture for larger version)
The species which causes this bloom in the Baltic Sea, Nodularia
spumigena, is a species of blue-green algae. Nodularia produces a
hepatotoxin, a substance that can damage the liver. When such blooms are
present, swimmers are cautioned not to ingest the water, and dog owners should
keep their pets away from the water to avoid possible poisoning. Blue-green
algal blooms can also cause skin irritation.
An important aspect of enclosed oceanic basins is the limited rate of
exchange (renewal) of water that lies within them with the adjacent ocean.
The routes of water exchange from the Mediterranean are through the narrow
straits of Gibraltar to the Atlantic, the even narrower Dardanelles, leading to
the Black Sea through the Bosporus Strait, and a miniscule flow through the
Suez Canal. The Baltic Sea outflow and water exchange is restricted through
the contorted Skaggerak and the passages around the islands that are part of
the country of Denmark. (The new Oresund Bridge between
Denmark and Sweden may have further restricted the flow of water here.) In
either case, these restricted passages for the exchange of water mean that
nutrients and other forms of pollution are not purged rapidly from these bodies
of water. Increasing concentrations of nutrients foster larger and more
prolonged phytoplankton blooms; other forms of pollution may enter the
ecosystem and affect it detrimentally (such as the buildup of metals like
cadmium and mercury in the trophic levels of the oceanic food web).
Another problem with "bad blooms" is their effect on coral reefs. Many of
the world's most beautiful and extensive coral reefs lie adjacent to Third
World countries with burgeoning populations, particularly along the coast. As
excess nutrients, stormwater runoff, and untreated or poorly treated sewage
enter coastal waters, they also foster increased concentrations of
phytoplankton. These algal blooms cause the overlying water to become
increasingly turbid, and because corals require extremely clear water for their
existence, this turbid water slowly extinguishes many coral species. Excess
nutrient concentrations may also encourage the growth of encrusting macroalgae
directly on the coral, as shown below, even in waters that appear relatively
clear and pristine. Furthermore, sewage can carry with it bacteria and viruses
that can induce coral diseases, which may cause rapid die-off of living coral
formations.
Healthy coral reef environment. (Click each picture for larger version)
Coral covered with macroalgae. (Click each picture for larger version)
Remote sensing cannot, by itself, stop the processes that are causing the
rise of "bad blooms", but the data from instruments like SeaWiFS and MODIS can
be used to monitor these events, determine their likely causes, and aid in the
identification of methods to reduce their incidence and severity.
Acknowledgements
This Science Focus! article acknowledges the contributions
of the references linked below, the MODIS Rapid Response Team and
Norman Kuring of the SeaWiFS Project for the MODIS and SeaWiFS
images.
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