Overturning

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Red circles indicate dead zones (2008). External influences can reverse this, the water comes to life.

Overturning is a sudden, catastrophic change in the state of a body of water due to a lack of oxygen.

A noticeable sign is the death of many colonists, e.g. B. fish death , and the one-sided prevalence of other colonists, especially blue-green algae and other bacteria. In portions of larger waters and bays can thus dead zones are formed ( engl. Dead zones ).

The chain of effects

Overturning due to algae decomposition during algal bloom ; here in La Jolla , California

The immediate trigger of the tipping over is always the drastic drop in the oxygen concentration in the water, often to zero (" anoxia "), which deprives all aerobic organisms of the basis of life. The cause of the oxygen depletion itself is usually too high a biomass , in particular from unicellular algae ( phytoplankton ) in the free water body, which, in the event of their natural death, deprive the water of oxygen through microbial degradation with oxygen consumption. The formation of this excessively high algae biomass can be a natural process, but is often caused by unnaturally increased enrichment of the water with nutrients that are essential for algae growth, i.e. have a fertilizing effect. This process is called eutrophication of the water. For the unnaturally increased algae growth, the essential nutrient is that which, due to its previously insufficient concentration, had the greatest limitation on growth in unaffected water ( minimum law ); this is almost always phosphorus in natural inland waterways, apart from rare exceptional cases .

The causal chain can be shortened by saying: A body of water overturns due to an increase in the phosphate concentration in the free water body. The accumulation of phosphate is usually a slow, continuous process that initially only manifests itself in the ever denser growth of the floating algae (phytoplankton). Finally, a slight push is enough, e.g. B. particularly warm weather conditions to set the chain reaction in motion. However, these are then only the trigger, not the actual cause of the tipping over.

The overturning of a body of water not only means a drastic, inherently irreversible change in its community, it also restricts its usability for humans. Technical measures that are intended to restore the state of the water body to a usable state are referred to as "lake restoration". The most important means of lake restoration must always be to permanently reduce the phosphate concentration in the free water body. This task is often very difficult to achieve in practice, as bodies of water are complex systems with numerous internal control loops and interactions. Therefore, a reduction in phosphate inputs does not always result in a linear decrease in the concentration in the water, because there are complex interactions, above all with the aquatic organisms and the lake sediment deposited on the water floor .

The fact that the water can tip over due to oxygen depletion is favored or prevented by various circumstances. This process is counteracted in particular by the subsequent delivery of atmospheric oxygen into the water, which replaces and replenishes the oxygen used there. Overturning occurs primarily in stagnant waters, i.e. in lakes or smaller inland seas and parts of them, for example the Baltic Sea , since the flow in flowing waters counteracts the process by mixing. Even in very shallow standing waters, overturning rarely occurs due to the large surface area. Deep, stagnant waters are also temporarily stratified over the course of the year, i.e. not completely mixed, which interrupts the supply of oxygen to the deeper water layers. The process in shallow water and flowing water is ultimately the same and can also lead to tipping over here, albeit less often.

Role of phosphate

As shown above, excessive eutrophication of stagnant waters, which can lead to overturning, is almost entirely due to the influence of a single factor, the increase in phosphate levels. Only this knowledge, which was achieved at the end of the 1960s and against strong opposition, made the rehabilitation of polluted lakes possible. The progress of knowledge was initially delayed by lobbying of large detergent companies, which were reluctant to forego the addition of phosphate in their products (comparable to the current role of oil companies in supporting climate change deniers ). What is surprising is the small contribution of nitrogen , which is significantly involved or even decisive in eutrophication processes in terrestrial ecosystems and in coastal waters ( estuaries ). The fact that nitrogen over-fertilization in inland waters, apart from a few special cases, plays practically no role, has been impressively demonstrated by manipulation experiments with entire lakes.

The phosphate content in lake water essentially results as a simple function of its intake from the catchment area, it is determined by factors such as the volume of the lake, the ratio of its surface to its depth, the ratio between lake water volume and inflows and outflows (length of stay) and the The alkalinity of the lake water is influenced so that some lakes are more resistant to tipping than others with the same phosphate supply. When it comes to modeling the relationships, the main contributions come from the OECD Program on Lake Eutrophication, under the long-standing leadership of Richard A. Vollenweider, which is why the models are usually called “OECD models” or “Vollenweider models”. The man-made input of phosphate into the lake is known as the “phosphate load”. An increase in the phosphate load shifts the water body more or less continuously from the oligotrophic to the mesotrophic to the eutrophic state in a predictable manner ; if the phosphate load is known, the further fate of the lake can be predicted, even if the change has not yet occurred. Displacement of the phosphate feed over the threshold value for eutrophic state as "critical load" (or engl. Critical Load hereinafter), and therefore predictable leads to deterioration, ultimately to tip over.

The input of phosphate into waters from their catchment area (nowadays only rarely from direct discharges into a lake) in industrialized countries is about half due to so-called point sources and the other half to so-called diffuse sources. Point sources are wastewater discharges or outflows from sewage treatment plants that are contaminated with phosphate from faeces , from industrial wastewater and from detergents and cleaning agents. The content of polyphosphates in detergents has been greatly reduced to protect water, but not so far in dishwasher detergents. Diffuse inputs mainly originate from agricultural fertilizers, which enter the water body through direct runoff with rainwater or through soil erosion . The natural input of phosphorus was also diffuse, but today it usually only accounts for a few percent of the total input.

If the phosphate inputs into a body of water are later reduced again, the previous state only rarely occurs immediately. This hysteresis is mainly due to the fact that a large part of the phosphate is fixed at some point on the water bed in the lake sediment and can be remobilized from it later, so that the phosphate content in the free water hardly drops at first. This factor is known as the "internal load". In the case of equilibrium, more and more phosphate is deposited in the sediment than is mobilized from there. But if the influx is reduced, the water and sediment are no longer in equilibrium. This will delay recovery.

Background: phosphate redissolution

The simple relationship between the phosphate content in the tributaries and the concentration in the water body of a body of water is complicated by the role played by lake sediments. Part of the supplied phosphate is set in the sediments. Later, depending on the conditions on the lake floor, a more or less large part of this fixed portion can be redeemed. This redissolution can have an effect for a long time, possibly decades, if the external inflows have already been reduced again. The uptake of phosphorus in living organisms, sedimentation of biomass on the bottom of the water and redissolving from the sediment also form an internal nutrient cycle that worsens the condition of the water for a long time, possibly even irreversibly in some cases. Many researchers assume that there are two metastable states that can "tilt" depending on the phosphorus content, so that the water could hardly ever leave the new state, which would then become the new state of equilibrium, without drastic external intervention. In this case the overturning of the water and the overturning between these two states of the sediment would be more or less the same. Other researchers assume that the transitions between the states are more gradual.

Conditions at the bottom of eutrophic lakes

In lakes, a temperature- related density stratification forms in summer and usually also in winter, with full circulations in between in autumn and spring. This results in a clear separation between primary production and the breakdown of biomass. The build-up of biomass by binding inorganic nutrients and generating oxygen takes place in the light-rich layers near the surface, i.e. in the epilimnion and often also in the upper metalimnion . In contrast, the remineralization of the sunken remains of biomass above the bottom of the lake is concentrated in the hypolimnion or lower metalimnion , depending on the depth of the lake . There, oxygen is consumed and the inorganic nutrients are released again. The water of the epilimnion is mixed daily by wind and convection. The oxygen content is brought into equilibrium with the air. Because of the light available, most of the oxygen-producing photosynthesis takes place here. The water in the metalimnion and hypolimnion does not receive any oxygen from the outside. Rather, dying algae and plankton animals and their droppings sink from above. During the biological degradation of their substance, as much oxygen is consumed as was created in the epilimnion during the formation of their biomass through photosynthesis. As a result, all of the oxygen at the bottom of a eutrophic lake is often used up; the sediment itself, and often the water layered above it, becomes oxygen-free. Even in shallow lakes and small bodies of water, in which no thermal stratification forms and the light can reach the bottom of the body of water, at least the sediment itself is often anaerobic due to biomass degradation.

Storage and release of the phosphate

Phosphate can be stored in the sediment on the lake floor. This storage is partly permanent, partly reversible depending on the external conditions, so that the phosphate can be released again later. This release is controlled primarily via the alkalinity and the redox potential at the bottom of the water.

In lime-rich, strongly alkaline waters, some of the phosphorus can precipitate as calcium phosphate, known as hydroxylapatite as the mineral phase . Partial storage in precipitated calcium carbonate (" sea ​​chalk "), known as the mineral calcite , is more important. In strongly acidic lakes, part of the phosphate can precipitate with free aluminum ions as aluminum phosphate or it can also be precipitated with aluminum hydroxide Al (OH) 3 . However, both processes do not play a major role in most bodies of water, as they only take place in very hard or highly acidic waters.

The precipitation of phosphate with iron ions is more important. The phosphate is effectively only fixed by oxidized, trivalent iron. If the mass of trivalent iron on the sediment surface is fifteen times the mass of phosphorus, the phosphate is very effectively removed from the free water. This mechanism is known as the " phosphate trap ". The phosphate is mainly bound to amorphous iron (III) oxide hydroxide (FeO (OH)), only precipitated as defined iron phosphate under special conditions.

Under more anaerobic conditions, the trivalent iron present in the sediment is reduced to bivalent:

Since the compounds of divalent iron are much more soluble, the iron oxides are dissolved under reducing conditions and the phosphate bound to them is released again. In the case of a lack of oxygen, the phosphate therefore remains dissolved in the water and is again distributed over the entire lake with the next circulation . However, this reaction is strongly influenced by other ions in the lake water. If the nitrate content of the water is high, the redox potential can be kept high after the free oxygen has been consumed by nitrate breathing . On the other hand, high sulphate contents are reduced to sulphide by bacterial desulphurication , which is precipitated with the divalent iron as iron sulphide (called pyrite as a crystalline mineral phase ), whereby the iron can be permanently removed from the free water. High pH values ​​with sea chalk precipitation reduce the effectiveness of the phosphate trap, because some of the phosphate can be displaced by hydroxide ions. Predicting the actual P release is therefore extremely difficult.

Role of macrophytes in shallow lakes

In shallow lakes and ponds, in which a large part of the water bed lies in the exposed zone, instead of the mass multiplication of algae, increased growth of "higher" aquatic plants can occur. These include reed beds and submerged (or submerged) vascular plants, but also larger algae that grow on the ground (benthic), e.g. B. the candelabrum algae . The litter and the residues of the macrophytes are more difficult to biodegrade than planktonic algae, so a body of water dominated by macrophytes is more stable against tipping over. The macrophyte-rich state could be an alternative (meta-) stable state with the same nutrient content. The key factor for the transition between plankton-dominated and macrophyte-dominated states seems to be the feeding pressure of the zooplankton on the unicellular algae, whereby the zooplankton population is regulated by fish (“ trophic cascade ”).

Attempts to prevent eutrophic lakes from tipping over by promoting macrophytes are summarized as " biomanipulation ".

Individual evidence

  1. ^ A b David W. Schindler: Recent advances in the understanding and management of eutrophication. In: Limnology and Oceanography. 51, No. 1, 2006, pp. 356-363.
  2. ^ DW Schindler: Evolution of Phosphorus Limitation in Lakes. In: Science. 195, No. 4275, 1977, pp. 260-262, doi : 10.1126 / science.195.4275.260 .
  3. ↑ David W. Schindler, RE Hecky, DL Findlay, MP Stainton, BR Parker, MJ Paterson, KG Beaty, M. Lyng, SEM Kasian: Eutrophication of lakes cannot be controlled by reducing nitrogen input: Results of a 37-year whole- ecosystem experiment. In: Proceedings of the National Academy of Sciences. 105, No. 32, 2008, pp. 11254-11258, doi : 10.1073 / pnas.0805108105 , PMID 18667696 .
  4. ↑ Eutrophication of Waters (OECD) Monitoring, Assessment and Control
  5. ↑ cf. z. B. Richard A. Vollenweider: Input-output models. In: Swiss Journal of Hydrology. 37, No. 1, 1975, pp. 53-84, doi : 10.1007 / BF02505178 .
  6. ↑ Brigitte Osterath: Wash phosphate-free - rinse with phosphate-packed. In: News from chemistry. 59, No. 9, 2011, pp. 828-830, doi : 10.1002 / nadc.201190015 .
  7. ↑ Erik Jeppesen et al .: Lake responses to reduced nutrient loading - an analysis of contemporary long-term data from 35 case studies. In: Freshwater Biology. 50, No. 10, 2005, pp. 1747-1771, doi : 10.1111 / j.1365-2427.2005.01415.x .
  8. ^ Martin Sondergaard, Peder Jens Jensen, Erik Jeppesen: Retention and Internal Loading of Phosphorus in Shallow, Eutrophic Lakes. In: The Scientific World Journal. 1, 2001, pp. 427-442, doi : 10.1100 / tsw.2001.72 , PMID 12806078 .
  9. ^ SR Carpenter, D. Ludwig, WA Brock: Management of eutrophication for lakes subject to potentially irreversible change. In: Ecological Applications. 9, No. 3, 1999, pp. 751-771, doi : 10.1890 / 1051-0761 (1999) 009 [0751: MOEFLS] 2.0.CO; 2 .
  10. ↑ a b Michael Hupfer, Jörg Lewandowski: Oxygen Controls the Phosphorus Release from Lake Sediments - a Long-Lasting Paradigm in Limnology. In: International Review of Hydrobiology. 93, No. 4-5, 2008, pp. 415-432, doi : 10.1002 / iroh.200711054 .
  11. ^ René Gächter, Bernhard Wehrli: Ten Years of Artificial Mixing and Oxygenation: No Effect on the Internal Phosphorus Loading of Two Eutrophic Lakes. In: Environmental Science & Technology. 32, No. 23, 1998, pp. 3659-3665, doi : 10.1021 / es980418l .
  12. ↑ Jiřı́ Kopáček, Kai-Uwe Ulrich, Josef Hejzlar, Jakub Borovec, Evžen Stuchlı́k: Natural inactivation of phosphorus by aluminum in atmospherically acidified water bodies. In: Water Research. 35, No. 16, 2001, pp. 3783-3790, doi : 10.1016 / S0043-1354 (01) 00112-9 .
  13. ↑ HS Jensen, P. Kristensen, E. Jeppesen, A. Skytthe: Iron: phosphorus ratio in surface sediment as an indicator of phosphate release from aerobic sediments in shallow lakes. In: Hydrobiologia. 235-236, No. 1, 1992, pp. 731-743, doi : 10.1007 / BF00026261 .