Although the increase in the rate of algal growth can clearly be seen, measured, and correlated to increases in the use of fertilizers (especially nitrogen-based fertilizers) in farming practices around the world, one might wonder how algal growth could have a negative effect. After all, algae are a naturally evolved and essential part of any fresh or salt water ecosystem. Algae convert the sun’s energy to sugars, and many fish and other organisms feed on algae to survive. The fish and other organisms then become prey to other animals, indirectly making algae an important source of energy for predators as well. One might even draw the conclusion that an increase in algae would induce an increase in the growth and populations of many other organisms as well.
However, the implications of rapid algal growth are much more complex that a simple increase in food. Algal blooms can create hypoxic conditions in marine ecosystems. This decrease in oxygen levels kills fish and other organisms that rely on higher concentrations of oxygen dissolved in the water. A potentially more devastating effect in higher algal populations is the increase in algae that produce harmful toxins. One such variety of algae comes from the genus Pseudo-nitzschia, which is one type of algae that blooms recurrently off the coast of California.
Pseudo-nitzschia produces the neurotoxin domoic acid. In humans, domoic ac
id induces vomiting, nausea, diarrhea, abdominal cramps, headache, dizziness, confusion, disorientation, loss of short-term memory, motor weakness, seizures, cardiac arrhythmias, coma, and can even cause death in high enough concentrations. Humans who suffer from domoic acid poisoning typically acquire the elevated levels of domoic acid from mussels and other shellfish that eat the Pseudo-nitzschia in their environment. Because of the tendency for short-term memory loss after eating the contaminated shellfish, domoic acid poisoning is also referred to as Amnesic Shellfish Poisoning (ASP). Because of the dangers of domoic acid and the recurring blooms of Pseudo-nitzschia off the coast of California, the Department of Health Services requires shellfish growers in areas with a history of blooms to screen their harvests and test phytoplanton for levels of the neurotoxin. If the concentrations become too high, monitoring increases and admonitions against eating—and even quarantines on—the sea life in the area are set in place. In the spring of 2007, California saw a dramatic increase in the concentrations of domoic acid off their coast.
While seriously affected, humans are not the only organisms affected by the neurotoxin. The fish and other organisms, such as shellfish, that eat Pseudo-nitzschia do not generally receive high enough concentrations of domoic acid to suffer observable negative consequences; however, the species that eat the fish and shellfish are frequently affected. Much like DDT becomes more likely to kill as it is consumed by higher-level predators, domoic acid accumulates in higher and higher concentrations as it moves up an ecosystem’s food web, affecting the topmost predators the most. For example, domoic acid affects the brains of birds—such as pelicans—that eat fish. As a result, the many of the birds start having seizures. When the seizures occur during flight, the birds can end up flying to unfamiliar areas, or simply falling out of the sky and even drowning. When sea lions eat fish containing domoic acid during pregnancy, the domoic acid tends to accumulate in the amniotic fluid. As a result, the fetuses’ brains develop differently and the offspring tend to have epileptic seizures and strange behavioral habits later in life.
While the algae are a naturally occurring organism in the aquatic environment, the high levels of fertilizers in the water have increase their populations, causing detrimental effects to the aquatic ecosystem and affecting all levels of the food web, including humans. If the growth is not curbed by decreasing the amount of fertilizer released into the ocean, the affects could even spread to future generations of both humans and aquatic species.
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References
http://www.cop.noaa.gov/stressors/extremeevents/hab/features/ca_pn_050807.html
http://www.sciencedaily.com/releases/2008/06/080609103232.htm
http://www.sciencedaily.com/releases/2007/04/070427084149.htm
