We are actually in the middle of a blog series. However, I would like to use the current news about fish dying in a lake in our region to discuss the biological oxygen demand (BOD). The processes in the lake and in the water are directly related to our work, however, in a different context than we usually use the BOD. The article is intended to supplement the glossary entry and also to deal with the problems and solutions for the domestic pond.
Biological oxygen demand (BOD) in Lake Max-Eyth
Max-Eyth-See in Stuttgart is an artificial lake that was created by gravel mining at the beginning of the 20th century. In the 1930s, the area around the lake was transformed into a recreational area. It is under landscape protection. In summer the lake can be used for boating, but swimming is not allowed.
The lake does not have its own fresh water supply and is therefore an ecosystem in itself, which gets out of balance summer after summer. Ongoing warm temperatures favour in particular the growth of blue-green algae, which spread rapidly. As the lake is shallow, it warms up particularly quickly and is also highly endangered. Plants that have been able to grow in the lake are cut off from the sunlight by the blue-green algae and die. They gradually sink to the ground, where they are decomposed by bacteria. Similar to us, these bacteria need oxygen for their metabolism. They receive this oxygen from the water. The more dead biomass there is, the better for the bacteria. This increases the BOD, i.e. the need for oxygen to break down the biomass.
Innocent victims - fish deaths
At a certain point in the lake, more oxygen is consumed than can get into the water through the surface of the water. If this development is not detected and stopped in time, no oxygen remains for the fish in the lake. To a certain extent, they suffocate in the water.
This year the death of fish was particularly devastating. Within a few days, about 50,000 fish died. The lake is "tilted", as the vernacular says.
Measures against the lack of oxygen in the water
Firstly, aquatic plants can be specifically used to filter nutrients out of the lake and release oxygen again. This works in a similar way to the indoor plant that provides good air in the living room at home.
Secondly, it is important to circulate the water in the lake or pond. That means nothing else than to increase the water surface so that more oxygen gets into the water. For example, fountains or small waterfalls serve this purpose. In this area we are also present with our Merus rings. If you have already read on our homepage, you may be familiar with the following sentence: "The Merus rings increase the solubility of substances soluble in water". This also includes oxygen. If the water is treated with our technology, it is theoretically able to absorb more oxygen. Of course, this is only useful in practice if there are sufficient possibilities for oxygen to enter the water.
The third possibility is actually the most obvious, but it is difficult to implement, especially in large lakes: If fresh water is constantly supplied, the low-oxygen water is diluted. In this way an increased BSD can be compensated.
Julika Wolf (2019a): Anglerverein zum Fischsterben im Max-Eyth-See. „Größte Naturkatastrophe, die es in Stuttgart je gegeben hat“ In: Stuttgarter Nachrichten, 04.09.2019. Available online at https://www.stuttgarter-nachrichten.de/inhalt.fischsterben-in-stuttgart-max-eyth-see-wird-noch-bis-samstag-belueftet.17f728eb-d3ee-4c41-a000-d8263d65281c.html, last checked on 11.09.2019.
Julika Wolf (2019b): Fischsterben in Stuttgart. Was passiert mit dem Max-Eyth-See? In: Stuttgarter Nachrichten, 06.09.2019. Available online at https://www.stuttgarter-nachrichten.de/inhalt.fischsterben-in-stuttgart-was-passiert-mit-dem-max-eyth-see.007eafe8-a8d3-4a01-ac90-cae835a07d26.html?utm, last checked on 11.09.2019.
Stuttgart-Marketing GmbH (ed.): Naherholungsgebiet Max-Eyth-See Stuttgart. Available online at https://www.stuttgart-tourist.de/a-max-eyth-see, last checked on 11.09.2019.