Okay. The western basin is a case in point, which is perhaps the best study of the Great Lakes because it's been subject to pollution for the longest time. When conditions are good, the water mixes, there's enough oxygen there, and the mayflies come out as fish flies and do very well. They require oxygen at the bottom of the lake because they burrow into the sediments. If you have periods of calm, or you have elevated nutrients and the algae drop to the bottom, the oxygen levels drop to zero, the mayflies die, and the animals that are able to take their place are bloodworms or oligochaetes, and they're indicative of polluted conditions.
Western Lake Erie is constantly being turned over, so that's one of the examples of an extreme. You might have lots of oxygen for most of the year, but all you need is five days of zero oxygen to wipe out that population, wipe out the food base for the fish.