Is there enough oxygen in the deep basin under the ice?

Early winter, yes — fall turnover recharges the whole column, and 6–12 m (20–40 ft) basin flats hold grazing schools comfortably. The problem builds late: under snow-covered ice, photosynthesis stops while decomposition on the soft bottom keeps burning oxygen, so fertile lakes deplete from the bottom up through February and March at Great Lakes and Canadian latitudes. Watch your electronics — when marks that hugged bottom in December ride 1–2 m (3–7 ft) off it in late winter, or schools shift toward shallower, weedier water, that lift is usually the oxygen line, not a mood. Fish above the fish, or follow them shallow.

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