A 3-hectare lake in the American Midwest suddenly has succumbed ...

[penza]

A 3-hectare lake in the American Midwest suddenly has succumbed to an algal bloom. What is the likely cause of eutrophication in freshwater ecosystems, such as this one?
A) increased solar radiation
B) introduction of non-native tertiary consumer fish
C) nutrient runoff
D) accidental introduction of a prolific culture of algae
E) iron dust blowing into the lake

[plus1]

I recall doing this last year, it's C, bye bye!

[penza]

Incredible, thanks for your help!