Name three possibilities how a lake can form

Under the Eutrophication one understands unchecked aquatic plant growth (above all algae, later also animal plankton) due to an oversupply of nutrients.

Phytoplankton needs a number of inorganic substances for growth and reproduction: water and carbon dioxide, as well as the nutrients phosphate, nitrate and potassium. H2O from the lake and CO2 from the air are almost unlimited. The growth-limiting factors are therefore the nutrient salts.

If nutrients get into the lake from outside, the phytoplankton can multiply unchecked. As a result, a so-called algae bloom occurs, which gives the lake a greenish tint (see figure on the left; algae bloom in a canal). At some point the algae die (lifespan of an alga approx. 1-5 days) and are broken down by the destructors (bacteria) while consuming oxygen. When organic substances are broken down, ammonium ions (NH4+).

A distinction is now made between two processes:
1. Under aerobic conditions, i.e. as long as there is still enough oxygen available, nitrite bacteria first oxidize the NH4+ to N02-(Nitrite). Then the nitrite of nitrate bacteria becomes N03- oxidized. The nitrate also acts as a nutrient and increases the biomass production of the phytoplankton. As a result, more and more oxygen is withdrawn from the lake.
2. Under anaerobic conditions, i.e. when there is no more oxygen available, the ammonium ions are broken down into toxic substances. In most cases this is methane (CH4), Hydrogen sulfide (H.2S) and ammonia (NH3). Fish that have not yet died from the lack of oxygen at this point will now die from ammonia poisoning.