Excess nutrients, such as phosphorus and nitrogen, are a major threat to lake water quality. These nutrients occur naturally in the environment, but their concentrations can increase beyond natural levels due to human (anthropogenic) activity such as decreasing or eradicating natural lakeside and streamside vegetation, over-fertilizing lawns and raising livestock close to lakes and streams. These higher nutrient concentrations can quickly overload lakes and negatively impact their aquatic life.
Since the early 20th century, lakes have been classified according to their trophic state. Trophic means nutrition or growth. Although lakes are arranged into a few trophic classes, each lake has a unique set of attributes that contribute to its trophic status. The three main factors that regulate the trophic state of a lake are rate of nutrient supply, climate, and shape of the lake basin. Common trophic categories include:
- Oligotrophic: Deep clear lakes with low nutrients levels, little organic matter, and a high dissolved-oxygen level.
- Mesotrophic: Moderate nutrients levels and moderately productive in aquatic animal and plant life.
- Eutrophic: High nutrient levels and highly productive in aquatic animal and plant life.
- Hypereutrophic: Extremely rich in nutrients and minerals.
- Dystrophic: Brown or tea-colored water that is acidic due to organic matter input. Nutrient levels can vary but low dissolved oxygen levels affect aquatic life.
Nutrients feed microscopic plant-like organisms called algae. Algae are typically aquatic, photosynthetic organisms that lack true stems, roots, and leaves. Algae grow when they have adequate light, pH, temperature and nutrient conditions. Healthy lakes need algae since they are important primary producers for the lake. Algae are then eaten by zooplankton grazers, such as, daphnia (water fleas), copepods, and rotifers. The zooplankton are eaten by small fish, which are eaten by bigger fish, which again are eaten by animals higher in the food chain.
A productive lake that provides food and habitat for wildlife and waterfowl starts with a solid base to the food web, in this case algae. However, excessive nutrients accelerate algal growth, which reduces water clarity and can lead to unpleasant odors. As algae die, they fall to the bottom of the lake and are decomposed by bacteria. As the bacteria consume the dead algae they deplete the oxygen levels in the surrounding water. Decreased oxygen levels make it difficult for fish and other aquatic animals to survive.
Many seemingly harmless activities added together can cause nutrient overloads. For example, disturbance of the natural landscape during the construction of a new home can increase nutrient concentrations in run-off. These high concentrations can continue if the surrounding landscape is not returned to its original condition. Removing natural vegetated buffers that filter and soak up nutrients allows an increase in nutrients of up to 10 times. Hard surfaces, such as sidewalks, driveways and rooftops, also increase nutrient loads by preventing run-off from soaking into the ground.
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