Hypolimnion (english version)

The sea was calm on the English Channel, even though the weather was a little cloudy. The "Herald of Free Enterprise" slid calmly through the water while he sat in one of the passenger seats by the window looking out at the waves. Had he known that this ship would sink just a few days later off the Belgian coast, he would certainly have enjoyed the trip less. But he didn't know it and so he looked cheerfully into the clouds above the water and thought of what he would do the next days in London. He had an important appointment with a professor at London University College, for he was about to graduate from grammar school in Germany and wanted to study at London University College. Although the most important questions were settled, it was not yet clear whether he would be granted admission to a British university.
While he was hanging on to his thoughts, an elderly gentleman took the seat next to his's. At first he ignored him, but at some point the elderly gentleman began a conversation about the weather and the sea.
"Deep down the water it must be incredibly cold," the gentleman said and looked at him questioningly.
"No," he replied. "At a certain depth the water temperature is constant plus four degrees centigrade.
That's what he had learned in biology class. Biology was to become one of his A-level subjects, so he knew quite well what he was talking about.
The gentleman looked at him with surprise and asked: "But if, for example, a lake is frozen over?
"We distinguish three different layers of water," he began to explain. "In the lowest layer, the hypolimnion, the temperature is constant plus four degrees. In the uppermost layer, the epilimnion, the temperature varies according to the season. In summer it gets really warm, while in winter ice can form. In between lies the transition layer of the metalimnion. It is the transition from the temperature of the uppermost layer to plus four degrees in the lowermost layer.
"And how thick are these layers?" asked the gentleman.
"That depends above all on the depth of the water and on the sun's rays."
"You seem to know your way around," the Gentleman said. "Are you in water research?
Now he had to grin. Should he tell him that this would be the subject matter for his A-level examination?
"I happened to read something about it recently," he said.
The gentleman was impressed. They chatted about trifles for a while, then the gentleman continued his round through the ship.
He looked out at the sea again. He had visibly impressed the elderly gentleman with his knowledge. With a little luck he would be able to do the same with the A-level examiners. Then nothing would stand in the way of him studying in London.