Oxygen Depleting can cause Warm water. During summer, The upper layer of the water is lighter therefore it does not mix with the cooler water below. Which soon becomes stagnant. (Oxygen becomes depleted and toxic compounds may produce).
That is one of them. I hope it helps. I'm not so good when it comes to this type of science. But if you ever need help with Anatomy Or anything similar to that. I will gladly help.