One of the great things about the Great Lakes is the twice-yearly turnover. There is no 'stagnant' old water at the bottom. The water at 0C is the least dense of the cold water. It then reaches maximum density at 4C. When you see the map at 4, you know that section has completely turned over. The sections below that are in the process of turnover. The bottom water is always at 4, so all the fishies live. The water from the top at 4 comes crashing down, and gives the fishies a tickle.
As it gets warmer, the temps go above 4 and then they stay on top. I always found it amazing that all of life depends on this little thing. But then, if it didn't happen, we wouldn't be here to talk about it. :)
You'll also notice there is still some ice. My cottage lake is frozen solid to maybe a foot or two, and the boats may not be in this year until the end of May. During our lovely warm cycle, the ice went out middle of April or earlier, and we could make a visit at the beginning of May. Not going to happen this year.
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