Eutrophication
Eutrophication occurs when large quantities of nutrients such as nitrates and
phosphates enter an aquatic environment. Sources of these nutrients include animal
wastes, agricultural runoff, and sewage. The ecosystem quickly experiences an increase
in photosynthetic and blue-green algae, as these organisms thrive in the presence of the
added nutrients. An algae bloom occurs as the algae accumulates into dense, visible
patches near the surface of the water, prohibiting light from penetrating deeper areas of
lake or stream. Some fish are unable to survive without this light, but for them an even
more serious problem arises when the algae begin to die. At this point, oxygendemanding bacteria take over the ecosystem, decomposing the algae and using up
dissolved oxygen in the process. These bacteria increase the biological oxygen demand
(BOD) of the ecosystem. BOD is the amount of oxygen required for the decomposition of
organic compounds by microorganisms in a given amount of water. It is usually
measured in milligrams of oxygen consumed per liter of water. Biological oxygen
demand is important because it affects the amount of dissolved oxygen available to all
species in an aquatic ecosystem. A higher BOD indicates a lower level of dissolved
oxygen. This lower concentration of oxygen causes many fish to suffocate, and as they
die, the number of oxygen-demanding decomposers increases even more.
Cultural Eutrophication (Larger View)
Causes of Eutrophication
The two major causes of eutrophication are excess nitrates and excess phosphates in
water. It is important to study these nutrients because oftentimes, human activity is
responsible for their negative effect on the environment.
Nitrates
Nitrates (NO3-) are water-soluble (they dissolve easily in water), and are commonly
applied to agricultural fields as fertilizer. Once applied, nitrates may leach into
groundwater or erode and end up in surface runoff. Eventually, they may enter a lake,
river, or stream and contribute to eutrophication. Nitrates can also vaporize into the
atmosphere, where they become a major source of acid rain. When ingested by organisms
in drinking water, nitrates bind to hemoglobin and reduce the oxygen-carrying capacity
of the bloodstream. This form of nitrate poisoning seems particularly prevalent among
amphibians, and may be contributing to a worldwide decline in the biodiversity of these
species.
Phosphates
Unlike nitrates, phosphates (PO43-), are not water-soluble; they do not usually
dissolve in water. However, they do adhere to soil particles, and as such often accumulate
in soil and erode along with soil into aquatic environments. Phosphates also form a major
component of most fertilizers, and in conjunction with nitrates, they have made
agriculture the largest source of nonpoint water pollution in the United States.
Eutrophication: Before and After (Larger View)
Controlling Eutrophication
The following is a list of methods that can be used to control eutrophication:



planting vegetation along streambeds to slow erosion and absorb nutrients
controlling application amount and timing of fertilizer
controlling runoff from feedlots

1.
2.
3.
4.
5.
6.
7.
researching use of biological controls; for example, the process of denitrification
uses specialized bacteria that convert nitrates to harmless molecular nitrogen
What two nutrients typically cause eutrophication.
Name three specific sources of these nutrients.
Describe the eutrophication process.
Which two organisms typically thrive in a eutrophied lake?
If you must fertilize, how can we control it from entering a stream?
How does vegetation protect lakes form eutrophication?
What is BOD and why is it measured?
8. What is meant by “water insoluble?”
9. How do nitrates affect the bloodstream?
10. What can you do to prevent eutophication?