A tremendous
variety of microscopic plants and plant-like microorganisms
are referred to collectively as "algae." Although
algae serve as the base of the food chain in most freshwater
habitats and throughout the oceans, they can and often do
cause environmental calamities involving fish, birds, marine
mammals, and even humans.
Fish kills
in bayous, streams, and other freshwater habitats are often
caused by oxygen depletion associated with excessive levels
of fertility and algae. As nutrients are introduced into an
aquatic environment, some are tied up directly in the bottom
mud but most dissolve in the water. Most nutrients in fresh-
and saltwater are taken up by algal communities, collections
of microscopic single-celled plants and plant-like microorganisms
suspended in the water.
Like most
plants, many algae produce oxygen during the daylight as a
by-product of photosynthesis. At night these algae consume
oxygen, but usually much less than was produced during the
daylight. Many common situations, however, can reduce the
amount of oxygen a bloom produces without reducing its nighttime
oxygen demand. Extremely calm or cloudy days may reduce photosynthesis
and oxygen production. This type of oxygen depletion may kill
fish directly or weaken their immune systems through prolonged
stress.
Temperature
also influences algae and oxygen dynamics. As water becomes
warmer, its capacity to hold dissolved oxygen decreases. During
warm weather algae populations usually increase but much of
the oxygen they produce during daylight may be lost to the
atmosphere, resulting in insufficient carry- over to meet
the night-time needs of fish and algae. The freshwater algae
most associated with oxygen depletion are known as blue-greens.
Blue-green algae generally thrive in shallow, calm, fertile,
warm, brightly lit waters. Blue-greens are poor oxygen producers
with significant nighttime oxygen demands. They often exhibit
population explosions and abrupt die-offs, also resulting
in oxygen deficits.
Apart
from killing aquatic life indirectly through oxygen depletion,
blue-green algae-and some other types of algae as well-can
cause more direct harm. While some blue-greens are currently
being touted for their nutritional benefits and medicinal
value in health food markets, other varieties are known to
be poisonous to fish, animals, and humans through toxic impacts
on the nervous system, skin, respiratory tract and liver.
Occasional livestock losses are attributable to blue-green
toxins in stock ponds, and some people may have extreme allergic
reactions to toxins produced by these algae.
In marine waters, fish, manatees, sea lions, dolphins, whales,
pelicans, and many other forms of wildlife are targets of
other toxic algae. One widespread group is known as the dinoflagellates.
Although they generally fall under the broad heading of "algae,"
dinoflagellates are single-celled organisms with characteristics
somewhere between those of plants and animals, belonging in
a group scientists call the "protists." While most
are important components of aquatic food chains, a small number
of the thousands of dinoflagellate species (including those
responsible for red tides) are toxic to fish and many other
animals.
Toxins
produced within these microorganisms can become concentrated
by animals that feed on them, such as shellfish and other
filter-feeders. This "bioconcentration" can affect
humans if contaminated shellfish are accidentally consumed.
Monitoring programs and regular shellfish inspections, however,
generally prevent such occurrences. Toxins can also be released
directly into the environment when a dinoflagellate bloom
dies back. An estimated 14 million fish were killed during
a red tide outbreak in Texas in September and October 1997.
Airborne toxins in sea spray can irritate the eyes and respiratory
systems of humans.
Noxious
dinoflagellate blooms appear to be occurring more frequently
in poorly flushed bays and lagoons in many parts of the world,
although some have been occurring regularly for hundreds of
years. The red tide organism relies on upwelling of fertile
off-shore waters to initiate a bloom, and subsequent currents
and winds to push it into near-shore areas. Nutrient run-off
from urban and agricultural sources can sustain a red tide
outbreak in near-shore areas for an extended period until
weather and current conditions can dissipate the bloom.
Another
toxic dinoflagellate in the news recently is the particularly
sinister Pfiesteria (pronounced "Fi-steer-ia")
piscicida. Rather than producing its food through
photosynthesis, Pfiesteria prefers to feed directly
on tissues from fish and other aquatic animals. Pfiesteria
produces two distinct toxins-one damages the nervous system
of fish passing nearby, partially or completely stunning them.
Another causes lesions, producing blood and sloughed tissue
for the protists to feed on. Both these toxins can adversely
affect people who come into contact with them.
Diatoms
are another group of algae with several toxic species, indirectly
killing anything from pelicans to humans that may consume
shellfish containing their toxins. Of roughly 4,400 known
species of marine algae, only about 50 produce toxins, but
many can produce localized oxygen depletions similar to those
caused by blue-green algae in freshwater habitats. Heavy blooms
in coastal waters in Texas following the 1989 freeze were
determined to cause a 20 percent loss of sea grass beds simply
shaded because they shaded the sea bottom. Decomposition of
fish and other animals killed by the freeze was suspected
as the source of fertilizer for the bloom.
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