The USGS National Wildlife Health Center has a website that lists wildlife mortality reports. If large numbers of animals die and are reported, they are documented here.
There is a report of a mortality event involving turtles–64 snapping, painted, and Blandings turtles were found dead between May 22 and May 26, 2014 at Dunbar Slough Wildlife Management Area in Greene County, Iowa. The cause of death was listed as emaciation, which I believe means that they appeared to be in poor health prior to dying, but the exact cause of death is unknown.
So what caused this large number of turtles to die? I can only speculate. However, Dunbar Slough is surrounded by agricultural areas. There are leased agricultural areas within the wildlife management area.
Modern agriculture uses very toxic pesticides. In Iowa, we continue to use neonicotinoids that other nations and groups have banned. Insecticides, herbicides, nematocides, and fungicides are routinely applied. We don’t track what was applied or how much was applied.
I like turtles. I don’t like to see them killed off.