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Top Down vs. Bottom Up
The Trophic Cascade

The Lakes of Missouri Volunteer Program tests for nitrogen and phosphorus, the nutrients primarily responsible for algae growth. We tell anyone who will listen that if you want to control algae, you must control the nutrients. However, it is possible to alter the amount of algae in a lake by manipulating fish populations.

To understand how this works, it’s important to understand the role of various organisms in the food web.

 

Top Down vs. Bottom Up

The Trophic Cascade

Each box in the graphic to the right represents a “trophic” level, or food group from nutrients to piscivores. Altering the higher trophic levels (e.g. piscivores) can lead to a “cascade” effect on the lower trophic levels. For example, a lack of piscivores usually results in an abundance of zooplanktivores and algae (top row), while an increase in the number of piscivores eventually results in a decrease in the number of zooplanktivores and, theoretically, algae (bottom row). By adding piscivores, the grazing pressure is removed from the zooplankton and their numbers should increase. As a result, the algae population is more thoroughly grazed. The nutrients are generally not affected by this “biomanipulation.”

The Trophic Cascade

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