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Why can algal blooms lead to oxygen depletion and fish kills in lakes?
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Lake Ann in North Cascades National Park, Washington, U.S., viewed from the park's Maple Loop Trail. The North Cascades National Park is a large wilderness area that preserves majestic mountain scenery, snowfields, glaciers, and other unique natural features.
Algal blooms lead to oxygen depletion and fish kills in lakes primarily through the processes of biological growth and decay. When excessive nutrientsâsuch as nitrates and phosphates from agricultural fertilizers or wastewaterâenter a lake, they trigger the rapid, dense growth of phytoplankton and other algae.123
This process, known as eutrophication, impacts oxygen levels in several ways:
- Biological Decay: As the massive amount of algae dies, it sinks and undergoes decomposition. This biological decay consumes vast quantities of dissolved oxygen, often leading to severe depletion in the water column.12
- Physical Obstruction: Dense blooms can block sunlight, preventing submerged plants from performing photosynthesis, which is a primary source of oxygen production in lakes.12
- Suffocation: In addition to oxygen depletion, the physical density of a bloom can clog the gills of fish, directly causing suffocation.1
- Toxicity: Some algal species produce toxins that are lethal to fish and other aquatic organisms, further contributing to mass die-offs.415