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For Green Lake, the obvious signs of water quality decline are increased weed and algae growth. These visible nuisances are symptoms of much larger watershed-wide causes: Excessive phosphorus loading and in-lake legacy phosphorus


Phosphorus is a naturally-occurring element and a fertilizer. While it is necessary for plant growth, too much wreaks havoc on waterways. The same fertilizer that causes plants to grow big and green on land also has the same effect in the lake—causing our weeds and algae to grow similarly big and green. Preventing nuisance levels of weeds and algae requires keeping phosphorus on the land and out of our lake.


Phosphorus enters the lake from a variety of sources and through a variety of mechanisms, including attached to sediments or soil in runoff and dissolved in water. Here are a few specific phosphorus sources: ​

  • Soil erosion: Bare soil is more likely to wash away into the lake along with its nutrient-packed particles. Eliminate bare soil spots by considering cover crops, use native plants to slow the flow of runoff, and reduce or eliminate soil tillage to limit disturbance.

  • Fertilizer: Fertilizer on land helps things grow, but it also does the same in the water. Limiting fertilizer use on your yard, garden, or field directly helps the lake's water quality. 

  • Changing weather patterns: 2016-2018 have been the wettest on record since rainfall measurements began in our area in the 1990s. More intense rain events produce bursts of rushing stormwater runoff that overwhelm local streams. This runoff increases streambank erosion and ultimately phosphorus pollution to Green Lake. (Learn how Project Clean Streams is dedicated to stream restoration.)

  • Leaves: Leaves raked into the road can act like a brewing cup of phosphorus tea. Water that makes its way through those leaves carries weed fueling phosphorus that flows directly into the lake through storm drains

  • Animal waste: Animal waste contains nutrients like phosphorus. If not cleaned up properly, those nutrients make their way to the lake during rainstorms where it fuels weed and algae growth.



Watershed-originated phosphorus pollution has a negative effect on Green Lake. Though the lake itself is not impaired for high phosphorus loading, the following bodies of water ARE impaired for phosphorus: Roy Creek, Wuerches Creek, Big Twin Lake, Hill Creek, and Silver Creek and some of its smaller feeder streams.

When phosphorus from eroding streams makes its way into Big Green Lake, it pollutes the water and fuels weed and algae growth. Just one pound of phosphorus is enough to fuel the growth of 500 pounds of weeds and algae.

1lb of phosphorus = 500 lbs of weeds & algae


We all learned that "matter is not created nor destroyed," and the same principle applies to phosphorus once it enters the lake. Phosphorus is "used" not "consumed" by weeds and algae, but instead, it converts back and forth from one form of phosphorus to another.


Once phosphorus enters Green Lake, it does not just disappear. It cycles from one form to another, causes algae and plants to grow, gets temporarily trapped in sediment, and stays in dissolved form in the water. The accumulation of phosphorus in the bottom of Green Lake (called "legacy phosphorus") is of growing concern. Even if we eliminate all the phosphorus from the watershed, it will take decades for various natural processes to "burn-through" the lake-bottom phosphorus.

Legacy Phosphorus


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