Eutrophication is a big word that basically means a lake is getting greener.  More specifically, eutrophication is the lake’s response to the addition of nitrates and phosphorus entering the lake either naturally or artificially (as a result of human activity).  Before human influence, lakes naturally went through the eutrophication process, but it took many hundreds of years for a lake to move from one classification to the next.  Cultural activity has sped up the eutrophication process of many lakes.
Lakes can be classified as one of four categories: Oligotrophic, Mesotrophic, Eutrophic, or Hypereutrophic.  Oligotrophic lakes have very low nutrient content and, therefore, low algal production.  Generally, oligotrophic lakes are very clear.  Mesotrophic lakes have an intermediate level of algal productivity, but are still generally clear.
Most of our lakes are Eutrophic, meaning they have high levels of nutrients and produce an abundance of aquatic vegetation.  When native aquatic plants, not algae, dominate the lake, the water tends to be clear.  However, when algae dominates the lake, the water is darker and can be low in oxygen levels.  If severe enough, the low level of oxygen can cause fish kills and dead zones at the bottom of the lake.
Hypereutrophic lakes are very nutrient-rich and have frequent algal blooms.  The water clarity is often poor and oxygen levels are low.  Recreational opportunities are very impaired in these lakes due to the nuisance algal blooms.
What drives lakes to move from clearer water to the eutrophic and hypereutrophic end of the spectrum?  The main factor is the increased availability of nutrients in the lake, specifically nitrogen and phosphorus.  These nutrients come from many sources.  Point sources include wastewater effluent, feedlot runoff, and untreated sewage.  Nonpoint sources include agricultural runoff, urban runoff, and leaking septic tanks.
The Chisago Soil and Water Conservation District is working with local partners to help reduce both point and non point sources of nutrients reaching the lakes and streams in the county.  To learn more about our Best Management Practices, visit our website at