Biomes, Ecological Cycles, and Environmental Milestones

What does it mean when a lake is brown or blue?

When a lake is brown, it usually indicates that eutrophication is occurring. This process refers to the premature “aging” of a lake, when nutrients are added to the water, usually due to runoff, which may be either agricultural or industrial in origin. Due to this rich supply of nutrients, blue-green algae begin to take over the green algae in the lake, and food webs within the lake are disturbed, leading to an eventual loss of fish. When a lake is blue, this usually means that the lake has been damaged by acid precipitation. The gradual drop in pH caused by exposure to acid rain causes disruption of the food webs, eventually killing most organisms. The end result is clear water, which reflects the low productivity of the lake.


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