Eutrophication of a lake
Under the eutrophication One understands an unrestricted water plant growth (especially algae, later also animal plankton) due to an oversupply of nutrients.
Phytoplankton requires a range 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 available. Growth limiting factors are therefore the nutrient salts.
If nutrients enter the lake from outside, the phytoplankton can multiply unchecked. As a result, there is a so-called algae blooms, which greenens the lake (see picture on the left, algae blooms in a canal). At some point, the algae die (life of an alga about 1-5 days) and are degraded by the destroyers (bacteria) under oxygen consumption. The decomposition of organic substances produces ammonium ions (NH4+).
Now one differentiates between two processes:
1. Sub aerobic conditionsSo, as long as there is enough oxygen, nitrite bacteria first oxidise the NH4+ to N02-(Nitrite). Then the nitrite of nitrate bacteria becomes N03- oxidized. The nitrate additionally acts as a nutrient and enhances the biomass production of the phytoplankton. This removes more and more oxygen from the lake.
2. Sub anaerobic conditionsSo if there is no more oxygen, the ammonium ions are degraded to toxic substances. This is in most cases methane (CH4), Hydrogen sulfide (H2S) and ammonia (NH3). Fish that have not yet died from the lack of oxygen at this time, now die of ammonia poisoning.
Causes of eutrophication / algae blooms
Untreated sewage: Wastewater contains an extremely high amount of phosphates and thus ensures a high excess of nutrients.
Dьnger: Flooding of agriculturally used and fertilized areas leads to nutrient-rich nitrates entering the lake.
Feeding of fish and water birds: Fish food and other foods are nutritious and are degraded by oxygen consumption of bacteria again.
urine: Visitors experience a high "entry" of urine, and thus an increase in the natural nutrient concentration, especially in the case of much-used bathing lakes.
Overriding the phosphate trapUnder aerobic conditions, phosphate is incorporated into ferric phosphate in the sediment of the lake. It is thus temporarily deprived of its natural cycle and can no longer be used by phytoplankton as a nutrient. In this way, considerable amounts of phosphate can be deposited on the bottom of the lake. If it lacks oxygen, the iron (III) phosphate is reduced again to phosphate and thus returns in large quantities back into the nutrient cycle of the lake. Accordingly, an algae blooms follows.
Consequences of eutrophication / algal blooms
Extreme propagation of algae and other aquatic plants (e.g.
Grьnfдrbung of water through the phytoplankton
Poisoning of the water with ammonia, methane and hydrogen sulfide
mass extinction almost all organisms in the lake. Either by suffocation or poisoning
Initiation of oxygen into the lake: Thus, at least in the short term, the living creatures there can be prevented from suffocating.
Reduction of light radiation on epilimnion: To slow down the photosynthesis rate of phytoplankton
Removal of the sludge at the bottom of the lake: This can prevent poisoning in the context of a spring circulation / autumn circulation.
Removal of biomass from the lake: When degrading dead biomass, there would be an oxygen-depleting process.
Removal of ferric phosphate bound by phostrate trap At the bottom of the lake: If an anaerobic condition occurs in the hypolimnion, at least no additional phosphate would be released.
Chemicals to reduce algae growth: has the disadvantage that it would harm the other organisms in the lake as well
Tipping over a lake
Procedure: After nutrients have been introduced into the lake from outside, a strong algae blooms will occur. The mass propagation of the phytoplankton is followed by an increase of the animal plankton. At some point, algae and plankton die off and are broken down by bacteria (destructors) while consuming oxygen. The oxygen concentration drops to a critical level below which aerobic organisms such as e.g. Fish can no longer breathe and die. This leads to an additional increase in the dead biomass to be degraded and the oxygen concentration drops to 0. The tipping over of the lake is no longer (naturally) prevented at this time. Bound ferric phosphate dissolves in the hypolimnion back to phosphate and thicken the lake additionally. In addition, anaerobic bacteria begin to convert ammonium ions to toxic ammonia. In addition, gases such as methane and hydrogen sulfide are formed. The lake is "overturned" within a very short time.