Why do trout lakes need to be deep or spring-fed?

Because a trout needs cold water and oxygen in the same place at the same time, and summer tries to separate them. A stratified lake in July offers warm oxygen-rich water on top and cold water below — but that cold bottom layer is sealed off from the atmosphere, so its oxygen only gets consumed, never replenished, until fall turnover. Only deep, relatively infertile lakes keep enough oxygen banked in the cold layer to carry trout through; in fertile green lakes, decomposition burns the deep oxygen out by midsummer. Springs solve the problem locally, injecting cold oxygenated water year-round — the reason tiny spring ponds can hold brook trout that a much larger warm lake cannot.

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