IELTS Academic Reading Practice Test 26: Passage 1

On a hot summer afternoon, anyone swimming in a deep lake may notice something curious. Near the surface the water feels pleasantly warm, yet a swimmer who lets their legs hang down may suddenly meet water that is startlingly cold. This contrast is not imagined. During the summer months, many lakes in temperate parts of the world separate into distinct layers that hardly mix with one another, a phenomenon known as thermal stratification. It was studied in detail in the late nineteenth century by the Swiss scientist François-Alphonse Forel, whose investigations of Lake Geneva helped to establish limnology, the scientific study of lakes.

The explanation lies in an unusual property of water. Most liquids become steadily denser as they cool, but fresh water is at its densest at about 4°C. Water warmer than this is lighter, and so, surprisingly, is water that is colder, which is why ice forms on the surface of a lake rather than on the bottom. Lighter water floats on top of heavier water, much as cream rises to the top of a jug of milk, unless something supplies the energy needed to mix them.

In spring, once any ice has melted, the whole lake is at roughly the same temperature, and therefore the same density, from top to bottom. In this state even a moderate wind can set the entire body of water circulating. As the days lengthen, however, the sun warms the surface faster than the wind can carry the heat downwards, and by early summer the lake has divided into three layers.

The uppermost layer, the epilimnion, is warm and well lit, and because the wind keeps it constantly stirred, its temperature is much the same throughout. Its thickness varies: in a small lake sheltered by hills or trees it may be only a metre or two deep, whereas in a large lake exposed to strong winds it can extend down for ten metres or more. Beneath it lies the metalimnion, a zone in which the temperature falls rapidly with depth, sometimes by several degrees within a single metre; the level at which this fall is steepest is called the thermocline. At the bottom is the hypolimnion, a mass of cold, dark, dense water whose temperature changes little until the autumn.

The metalimnion is more than a line on a temperature chart. Because the water on either side of it differs so much in density, the energy of the wind is exhausted before it can push warm water down through it, so the zone functions as a barrier to mixing. For several months the upper and lower parts of the lake are effectively sealed off from each other. The epilimnion absorbs oxygen from the air, and it receives enough sunlight for microscopic algae to photosynthesise, releasing still more. The algae, however, steadily consume nutrients such as phosphorus and nitrogen, and since these can no longer be brought up from the depths, their growth may slow as the summer goes on.

Conditions in the hypolimnion are very different. Cut off from the atmosphere and, in most lakes, too dark for photosynthesis, it has no means of replacing its oxygen. Meanwhile, dead algae are continually drifting down into it from above, and bacteria break this material down, using up oxygen as they do so. In lakes rich in nutrients, the deepest water may lose all of its oxygen by late summer. Chemical changes in the sediments on the lake bed may then release phosphorus that had been held there, together with hydrogen sulphide, a gas that is poisonous to fish. Cold-water species such as trout can be caught in a 'squeeze', finding the surface too warm and the depths too short of oxygen.

The arrangement does not last. In autumn the surface loses heat, becomes denser and sinks, while the wind mixes the upper water ever more deeply. Eventually the whole lake returns to a uniform temperature, the barrier disappears and the water mixes completely, a process known as turnover. Oxygen is carried down to the bottom, and nutrients that have accumulated in the depths are brought back to the surface. In lakes that freeze over, a second turnover follows the melting of the ice, so complete mixing occurs twice a year. Not every lake follows this pattern, however: shallow lakes may be stirred to the bottom by the wind even in summer, while some very deep lakes, particularly in the tropics, may remain stratified for years.

This annual cycle is now changing. Researchers in many countries have found that, as air temperatures rise, lakes are stratifying earlier in spring and remaining stratified later into the autumn. A longer period of separation gives bacteria more time to exhaust the oxygen in the deep water, and some lakes that once froze every winter now stay open, so they mix only once a year. The layers that a swimmer can feel with their feet have become a sensitive indicator of a warming world.