Asked February 15, 2016, 6:31 PM EST
Dauphin County Pennsylvania
That's a great question. Without knowing too much about the specific lake in question, I can tell you that a lack of oxygen in some bodies of water is not uncommon. I will explain why, but first let me elaborate on what that means.
When referring to oxygen in water, what is really being discussed is dissolved oxygen molecules. This is the oxygen that allows aquatic organisms to respire, through gills or through their skin and other mechanisms. It is different than the element oxygen that you are picturing, as part of H2O. The oxygen in the chemical formula for water molecules is the element oxygen (#8 on the periodic table) bound to the element hydrogen. Elements, by themselves, can sometimes have the same name as a molecule made up of only that element, but they do not act the same. We can not breathe oxygen the element, it has to be found as a pair of oxygen atoms bound together, O2, forming the oxygen molecule - the gas you are most familiar with.
So when tests show "no oxygen" in the lake below 20m, there are still oxygen atoms bound to hydrogen atoms to make the water molecules, but there is no molecule oxygen (O2) dissolved in the water.
Dissolved oxygen is incredibly important to life in the water, all living things that respire need oxygen. Oxygen molecules need to be dissolved in the water for the majority of aquatic animals, since they don't have lungs that can absorb oxygen from the atmosphere. Dissolved oxygen is produced by plants growing in the water, as a byproduct of photosynthesis. Some dissolved oxygen also comes from contact between the surface of the water and the atmosphere.
In some cases, a lack of oxygen is a natural condition of deep, still lakes. Sunlight can only penetrate so far into the water, so plants generally only grow in the shallower waters. If the water is still, the oxygen doesn't get dispersed to the deepest parts of the lake.
In other cases, human influence has exaggerated the lack of oxygen in bodies of water that historically had a sufficient supply to support life. A good example of that is the Chesapeake Bay. There is a large section of the bay that can no longer support the living things that used to inhabit those waters, like oysters, crabs, and many types of fish. Though there are several causes of this, one major reason why the oxygen levels have dropped is because of algal blooms. For many years, nutrients from agriculture, city wastewater treatment plants, and home landscaping (among other sources) have been running off into the streams and rivers that feed the bay. Those nutrients act like fertilizers to the algae growing in the bay, and huge dense layers form on top of the water. As the bottom layers die off, do to lack of sunlight, they sink to the bottom of the bay. There, large numbers of decomposing bacteria reproduce and use up all of the oxygen in those areas.
I hope this helps you better understand what may be happening in Lake Toplitz and other bodies of water around the world.