Lake turnover

Why does a deep lake suddenly turn over from top to bottom at certain times of year?

A deep lake in high summer is two lakes. Swim down and at some depth you pass a boundary sharp enough to feel on your legs: warm above, shockingly cold below. It sits there for months while wind rakes the surface daily. Then, over a few autumn days, it is gone, and the lake is one temperature from top to bottom. Nothing was added and nothing stirred harder than usual. What changed — and why all at once rather than gradually?

Start with what holds the boundary in place. Warm water is less dense than cold, so the sun-warmed surface layer floats on the cold deep water, and floating is stable — to mix them the wind must lift dense water up and push light water down, which costs energy. In midsummer the surface may be 24 degrees and the bottom 5, and that density difference is large enough that ordinary wind cannot pay the bill.

Now the fact everything hinges on, and it is a genuinely odd one. Water is densest not at its freezing point but at about 4 degrees Celsius. Below 4 it gets lighter again as it cools, which is why ice floats and why a lake freezes from the top down. It also means the coldest water a lake can stack at its own bottom is 4-degree water. That is not a rule about lakes; it is a property of water molecules, and it sets the floor of the whole system.

With that in hand, follow a year. In autumn the air cools and the surface temperature falls toward the temperature of the deep water, so the density difference between the layers shrinks. The crucial point is what happens to the cost of mixing: it falls faster than you might guess, because water's density changes very little per degree near 4 degrees and much more per degree up in the twenties. The resistance does not decline steadily to zero — it collapses.

At that point the wind, blowing all along at no unusual strength, is suddenly sufficient, and the whole column mixes from surface to bed. That is autumn turnover, and it is abrupt for the reason a jammed drawer opens abruptly: the force was always there, and what changed was what it had to overcome.

Winter continues the story. The surface keeps cooling past 4 degrees, and now the cold water is lighter, so the lake stratifies again upside down, with near-freezing water and often ice on top and 4-degree water below. In spring the ice goes, the surface warms back toward 4, the inversion vanishes, and the lake mixes a second time. Two mixings a year makes it dimictic, which is what most deep temperate lakes are.

Should we expect that everywhere? No, and here the tidy story needs qualifying. Shallow, wind-exposed lakes mix repeatedly all summer. Warm lakes that never cool below 4 degrees mix once a year, in winter; cold polar lakes mix once, in summer. And some never fully mix at all — if the deep water is saltier or richer in dissolved minerals, that chemical density difference does not disappear when the temperatures match, so the bottom stays sealed off permanently.

Three refinements connect the reasoning.

First, the wind is not a spectator. It is easy to read turnover as something the density does by itself, but the density change only removes the obstacle; the energy that mixes the column comes from wind stress on the surface, helped by cold surface water sinking. A sheltered lake overturns sluggishly and sometimes incompletely.

Second, the layering is what makes a lake seasonally habitable or not. During stratification the deep water is cut off from the atmosphere while organisms and decaying matter keep consuming its oxygen, so by late summer the deep water of a productive lake can go anoxic. Fish are squeezed into a shrinking middle band: too warm above, no oxygen below. Anoxic sediments also release stored phosphorus, which turnover then delivers to the sunlit surface — one reason for autumn algal blooms.

Third, that is why turnover matters ecologically: it carries oxygen down and nutrients up in a single event. The winter counterpart explains winterkill — under a long-lasting cover of ice and snow, light is blocked, photosynthesis stops, oxygen is consumed with no resupply, and fish suffocate in a lake that looks healthy from above. Nor is turnover purely benign: an overturn can bring anoxic, sulphide-rich bottom water to the surface, and in some lakes that itself kills fish.

• Why the summer thermocline sits at the depth it does, and what sets it.
• How meromictic lakes keep a permanently sealed bottom layer, and what gets preserved down there.

Every term the collection defines is gathered in the glossary.