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.