In a body of fresh water, a dissolved oxygen concentration of 0.262 or higher is usually needed to maintain a healthy aquatic ecosystem. For a lake to maintain this concentration, what is the maximum water temperature?

Answer 1

0.262 WHAT? Here is a solubility chart for oxygen at three pressures. I would choose the one at 1 atm (or 1 bar).
http://docs.engineeringtoolbox.com/documents/841/oxygen_solubility_fresh_sea_water.pdf

Answer 2

Also, note that air is only 20% oxygen; therefore, the oxygen content will be 1/5 that which you read from the curve for 1 atm.