In the last lesson you watched Lake Geneva stop turning over. Its surface no longer cools enough in winter for the whole lake to mix, so its deepest water has been losing oxygen for more than a decade. You also saw that the ocean carbon sink stalls when warm surface water stops sinking. Both stories rest on one idea: water sorts itself into layers by density.
This lesson looks at those layers directly. Why do they form, what do they keep apart, and why are they getting stronger? What can break them open? And how do the same differences in density drive a current that loops around the whole planet?
Two clear cups hold the same amount of water at room temperature. One is fresh water. The other has salt dissolved in it, about as salty as seawater. An identical ice cube, coloured with a drop of food colouring, goes into each cup at the same moment.
Watch where the coloured meltwater goes in each cup, and which ice cube disappears first. You will explain the result later in the lesson.
https://www.youtube.com/watch?v=soef2g6WEF4
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4.1.10 The temperature of water varies with depth, with cold water below and warmer water above. Differences in density restrict mixing between the layers, leading to persistent stratification.
The phenomenon of water being at its densest at 4°C means that colder water will float above it and a body of water freezes from the surface downwards, allowing freshwater ecosystems to survive beneath an insulating layer of ice.
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Sunlight heats a lake or sea from the top. Warm water is less dense than cold water, so it stays at the surface while colder, denser water sits beneath it. You met this last lesson as one of water's unusual properties.
Moving dense water upwards takes energy. The larger the density difference between two layers, the more energy it takes to mix them, so a big difference keeps them apart. When the layers last for months or longer, the water body shows stratification: persistent layers of different density that resist mixing.
Fresh water has one twist. It is densest at about 4 °C, and below that it becomes less dense again as it cools. In winter, water colder than 4 °C floats on top of the 4 °C water beneath it, so the surface is the first place to reach freezing point. Ice is less dense still, so it floats.
The result is that a lake freezes from the surface downwards. The ice works as an insulating lid, slowing the loss of heat from the water below, so fish, invertebrates and plants survive the winter in liquid water under the ice.
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Lac de Joux: the Jura's biggest frozen lake
Lac de Joux sits in the Vallée de Joux, in the Jura mountains of canton Vaud. Swiss Tourism promotes it as the largest lake in the Jura that freezes over. In a cold winter, thousands of people walk and skate on it.
The ice forms on top because, as the lake cools, the coldest water stays at the surface. Underneath the ice, the lake stays liquid.
Safe ice is not guaranteed, and it seems to be getting harder to come by. In January 2022, skating on the lake was treated almost as an event, because it had not been possible for three years. On 7 February 2026, the regional ice bulletin reported a thin layer of ice over most of the lake that was not solid at all, with walking and skating allowed only on a few marked sections.
Sources: Swiss Tourism | 24 heures, via Journal de Morges, 28 January 2022 | Jura Glace ice bulletin, 7 February 2026
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Task
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4.1.11 Stratification occurs in deeper lakes, coastal areas, enclosed seas and open ocean, with a thermocline forming a transition layer between the warmer mixed layer at the surface and the cooler water below.
Warmer surface water and cold deep water also differ in the concentrations of dissolved oxygen and mineral nutrients.
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Stratification forms in deeper lakes, coastal areas, enclosed seas and the open ocean. In very shallow water, wind can often stir the whole depth, so layers rarely last.
A stratified water body has three parts:
The thermocline works like a barrier. Whatever is in one layer tends to stay there, and that matters for two things living organisms need.
Dissolved oxygen. The mixed layer is in contact with the air, and phytoplankton photosynthesise in its sunlit water, so it holds plenty of dissolved oxygen. Below the thermocline, decomposers use up oxygen as they break down dead organisms that sink from above, and there is no contact with the air to replace it. This is exactly what happened in the deep water of Lake Geneva once complete mixing stopped.
Mineral nutrients. Phytoplankton in the mixed layer take up nitrates and phosphates as they grow. When they and the animals that eat them die, their remains sink through the thermocline, carrying those nutrients down. Decomposition releases the nutrients into the cold water below, where they build up, but it is too dark down there for phytoplankton to use them.
The result is a sunlit surface with oxygen but short of nutrients, sitting on dark deep water rich in nutrients but short of oxygen. Stratification keeps the two apart. Anything that mixes them brings nutrients back up to the light.
