You might think that a clear blue lake is a healthy lake. But new research suggests that many such waters are, in fact, unhealthy: Excess nutrients from fertilizer runoff in some parts of the U.S. Midwest are so heavy that they choke out even the algae that thrive at lower levels of pollution. To find out why some of Iowa’s lakes appear blue, whereas others are green, scientists reviewed 13 years of sampling data from 139 lakes in the farm-rich state. Most were bright green, caused by algae feasting on nitrogen and phosphorous in the runoff from agricultural fields. Over that 13 year period, 64 lakes appeared clear and blue. But samples revealed they were actually more polluted than green-colored lakes, with some of the clear lakes showing nutrient levels as much as 30 times higher than normal, they report this week in Inland Waters. The authors hypothesize that nutrient levels are so high they that they kill the algae, much as applying too much fertilizer can “burn” your lawn. The researchers warn that regulatory agencies should no longer use water clarity as an indicator of water quality, a widespread practice.
*Correction 13 October, 12:25 p.m.: An earlier version of this story misstated the number of polluted lakes that appeared clear and blue.