From: University of Minnesota
Published October 10, 2017 11:52 AM

Clear Lakes Disguise Impaired Water Quality

Looks can be deceiving.

Look at a hundred lakes in the agricultural heartland of the United States and you will likely see green lakes surrounded by green fields. The nitrogen and phosphorus in agricultural fertilizers that help crops grow also fuel the growth of algae and cyanobacteria that in excess can turn lakes the color of pea soup.

Yet when scientists looked at 13 years of data from 139 lakes in intensively agricultural areas of Iowa they saw lakes that were surprisingly clear despite extremely high nutrient concentrations.

In a study published October 9, 2017 in the journal Inland Waters scientists from the University of Minnesota Duluth and Minnesota Sea Grant report that the lakes were so excessively fertilized that most of that algae and cyanobacteria containing the green pigment chlorophyll were killed.

Read more at University of Minnesota

Image: Aerial view of a small lake near the city of Clear Lake, Iowa, which represents typical landscapes surrounding the lakes in this study. About 92 percent of land within Iowa is in production agriculture and crops on these lands receive large amendments of nitrogen as anhydrous ammonia and phosphorus. Excessive algae growth caused by these nutrient inputs have turned many of the lakes in this region bright green. Surprisingly, a number of lakes in this study were clearer and appeared bluer than expected, yet are far from healthy. The study authors hypothesize that very high nitrogen levels, often >10 mg/L, suppress high chlorophyll (algae) concentrations. (Credit: John A. Downing)

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