Turnover

Through summer a lake is layered: warm oxygenated water on top, a thermocline in the middle, cold oxygen-poor water below. As the surface cools in autumn it becomes denser than the layer beneath, sinks, and the whole water column overturns.

The consequences are visible. Water often goes cloudy or takes on a musty smell, debris rises from the bottom, and the fish that had been predictable scatter completely because the depth restrictions that were organising them have vanished.

It lasts a week or two and then fishing improves sharply, because fish can now use the whole water column. Smaller ponds turn over faster than big lakes, and deep reservoirs may only partly turn.

You can usually tell it is happening without a thermometer. Water that had been clear goes off-color, bits of decaying weed and leaf appear at the surface, and the lake takes on a distinctly sulphurous smell.