Dead zone (ecology)

Tipping over a body of water is a colloquial term for a sudden, catastrophic state change with the death of the most colonizers (eg fish kills ), often associated with the emergence of harmful and malodorous gases.

The effect of chain

The immediate cause of upset is always the sudden, drastic drop in oxygen concentration in the open water, often to zero ( " anoxia " ), which removes all aerobic organisms livelihoods. Cause of the oxygen shrinkage itself is usually too high biomass, especially of unicellular algae ( phytoplankton) in the open water body, the water escapes in the event of their natural dying by microbial degradation of oxygen consumption to oxygen. This excessive algal biomass is usually not a natural state, but is triggered by unnatural initiated or enhanced by man enrichment of water by nutrients that are essential for the growth of algae, so have a fertilizing effect; this process is called eutrophication of the water body. This one nutrient is essential for the unnaturally enhanced algae growth, had the most limited by its low concentration previously to the growth in unaffected waters ( minimum law ), this is in natural inland waters of exceptional cases apart, almost always phosphorus. So you can abbreviate the causal chain in which one says: For overturning a body of water, it comes by increasing the phosphate concentration in the open water body. The phosphate enrichment is mostly a slow, continuous process running, which initially appears to be no or only minor effects. The state of the water but it is imperceptibly becoming increasingly unstable. Finally, a minor offense, such as sufficient particularly warm weather conditions to set the chain reaction. These are then just the trigger, not the actual cause of upset.

Tipping over a body of water is not only a drastic and irreversible in shorter periods of time changing its community of life, it also limits its usefulness to humans. Technical measures to stabilize the condition of the water as far as to prevent this from happening are combined as " lake restoration ." The most important agent of a lake restoration must therefore always be to reduce the phosphate concentration in the free water body permanently. This task is in practice often very difficult to achieve, since waters are complex systems with numerous internal control loops, and interactions. Therefore, a reduction of phosphorus inputs is not always a linear decrease of the concentration in the water result because here complex interactions, especially with the aquatic organisms and deposited on the stream bottom sediment lakes exist.

That it can come to the overturning of the water body by oxygen depletion, is favored or prevented by various circumstances. The process counteracts in particular the subsequent delivery of atmospheric oxygen into the water body, which replaces and subsequently supply the oxygen consumed there. To upset this causes especially in stagnant water, so in smaller lakes or inland seas, and parts thereof, for example the Baltic Sea, since the flow in rivers through the mixing process which counteracts. Even in very shallow standing water occurs because of the large surface rarely tip over. Depth of standing water are also temporarily stacked in the course of the year, so do not completely mixed, whereby the Sauerstoffnachlieferung is interrupted in the deep water layers. But the process in shallow waters and rivers is ultimately the same, and can also, albeit rarely, cause it to tip over.

Role of the phosphate

As shown above, is based excessive eutrophication of stagnant water, which can lead to upset almost exclusively on the influence of a single factor, the increase in the phosphate content. It is this knowledge that in the late 1960s and was achieved against fierce opposition, allowed the remediation of polluted lakes. The progress of knowledge was initially delayed by lobbying of large detergent companies who wanted to phosphate additions in their products renounce reluctant (comparable to the present-day role of oil companies in support of so-called climate skeptics ). Surprising is the low contribution of nitrogen, which is essential complicit in Eutrophierungsvorgängen in terrestrial ecosystems and in marine coastal waters ( estuaries ), or even crucial. The fact that nitrogen eutrophication in inland waters, except for a few special cases, plays practically no role has been impressively demonstrated by manipulation experiments with around lakes.

The phosphate content in seawater arises mainly as a simple function of its supply from the catchment area, it will depend on factors such as the volume of the lake, the ratio of its surface to its depth, the relationship between lake water volume and inflows and outflows (residence time ) and the influenced alkalinity of the sea water, so that some lakes are more resistant to tipping over with the same phosphate intake than others. When modeling the relationships the main contributions from the OECD Program on Lake Eutrophication under long-term direction of Richard A. Vollenweider come, the models are therefore usually called " OECD models " or " Vollenweider models ". The phosphate entry generated by the people in the lake is called " phosphate load ". Increasing the phosphate load shifts the waters in a predictable manner more or less continuously from oligotrophic to mesotrophic over the eutrophic state, the phosphate load is known, so the fate of the lake predictable, even if the change has not yet occurred. Designated shift of phosphate intake above the threshold for eutrophic condition as " critical load " ( or English. Critical load), so predictably leads to deterioration, ultimately to tip over.

The phosphate into surface waters from the catchment area ( now rarely from direct discharges into a lake ) comes in industrialized countries about half of what are called point sources and the other half on so-called diffuse sources back. Point sources are wastewater discharges or runoff from sewage treatment plants, which are loaded with phosphate from sewage, industrial effluents and from from washing and cleaning agents. The content of polyphosphates in detergents to water conservation has been greatly reduced, but so far not in dishwasher means. Diffuse sources derive mainly from agricultural fertilizers, which passes through direct runoff with rain water or by soil erosion into the water. Also the natural phosphorus entry was diffuse, but he makes today usually only a few percent of the total entry.

If the phosphorus inputs reduced later in a body of water, rarely the former condition occurs immediately again. This hysteresis is mainly due to the fact that a large part of the phosphate is set sometime in the seabed sediment in lakes and it can be remobilized later, so that the phosphate content at first hardly falls in the open water. This factor is referred to as " internal load " ( engl. internal load). Although in the case of equilibrium more and more phosphate is deposited in the sediment, as is mobilized from there. However, if the influx is reduced, water and sediment are no longer in balance. The recovery will be delayed.

Background: phosphate redissolution

The simple correlation between the phosphate content in the feeds and the concentration in water of a water body is complicated by the role of lakes sediments. A portion of the supplied phosphate is determined in the sediments. Later, depending on the conditions at the lakes reason, a more or less large part of this fixed percentage can be redissolved. This feedback solution can be long, possibly decades, to reverberate when the external inflows have been reduced again. By incorporating the phosphorus in living organisms, sedimentation of biomass at the water bottom and redissolution from the sediment also forms an internal nutrient cycle by which the condition of the water body is very long lasting effect, possibly irreversible, deteriorated in some cases. Many researchers in this case from two metastable states that can "tip" depending on the phosphorus content, so that the water could hardly be the new state, which then would the new equilibrium state, without drastic interventions from the outside ever leave again. In this case, the overturning of the water body and the upset between these two states of the sediment would be more or less the same. Other researchers started with more gradual transitions between the states of.

Conditions at the seabed eutrophic lakes

In lakes formed in the summer and usually also in winter a temperature-induced density stratification with intervening full circulations in the fall and in the spring of. This leads to a clear separation of primary production and decomposition of biomass., The structure of biomass with binding of inorganic nutrients and the production of oxygen takes place in the light- rich layers near the surface, so 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, depending on lake depth in the hypolimnion or lower metalimnion is concentrated. There, oxygen is consumed and released inorganic nutrients again. The water of the epilimnion is mixed by wind and convection daily. In this case, the oxygen content is adjusted to the equilibrium with the air. Also found here because of light offer the largest part instead of oxygen -producing photosynthesis. The water in the metalimnion and hypolimnion receives from the outside oxygen. Rather fall dying algae and plankton animals and their droppings from above. Biodegradation of their substance as much oxygen is consumed, was formed as in the development of their biomass by photosynthesis in the epilimnion. This is the bottom of a lake eutophen often all of the oxygen consumed, the sediment itself, often layered over the water, is oxygen-free. Even in shallow lakes and ponds, in which does not form a thermal stratification and the light can reach the bottom of the water, is often anaerobically at least the sediment itself by biomass degradation.

Storage and release of the phosphate

Phosphate can be stored on the lakes reason in the sediment. This store is reversible partly permanent, partly depending on external conditions, so that the phosphate can be released later. This release is controlled primarily through the alkalinity and the redox potential on the river bottom.

In calcareous, highly basic waters, a portion of the phosphorus known as calcium phosphate, the mineral phase of hydroxyapatite, precipitated. More important is the partial incorporation into precipitated calcium carbonate ( " marl " ), known as the mineral calcite. In strongly acidic lakes, a portion of the phosphate with free aluminum ions precipitate as aluminum phosphate or aluminum hydroxide Al ( OH) 3 co-precipitated. However, both processes play in most waters not very important, since they only occur in very hard or strongly acidified waters.

More significant is the precipitation of phosphate with iron ions. Case, the phosphate is effectively determined only by oxidized, trivalent iron. If the weight of trivalent iron at the sediment surface fifteen times the mass of phosphorus, phosphate is very effectively removed from the open water. This mechanism is referred to as " phosphate trap". The phosphate is first and foremost of amorphous iron ( III) oxide-hydroxide (FeO (OH ) ) bound, precipitated only under special conditions as defined iron phosphate.

Under anaerobic conditions, the more there Any trivalent iron is reduced to divalent in the sediment:

Since the compounds of divalent iron are much more soluble, wherein the iron oxides are dissolved and released the bound phosphate thereto again under reducing conditions. Shortage of oxygen, therefore, the phosphate is dissolved in the water and is distributed with the next circulation again fertilizing effect on the entire lake. This reaction is, however, strongly influenced by other ions in the lake water. If the nitrate content of the water high, the redox potential can be kept high after consumption of free oxygen by nitrate respiration. High sulfate levels are, however, reduced by bacterial Desulfurikation to sulfide, with the divalent iron as iron sulfide ( referred to as crystalline mineral phase pyrite ) are precipitated, so that the iron may be permanently removed from the open water. High pH values ​​with Seekreidefällung reduce the effect of the phosphate case, since a part of the phosphate can be replaced by the hydroxide ions. Predicting the actual P release, so is extremely difficult.

Role of macrophytes in shallow lakes

In shallow lakes and ponds, in which a large part of the ground waters lies in the exposed zone, it may potentiate the growth of "higher" water plants instead of the proliferation of algae. These include reed species and submerged (or submerged ) living vascular plants, but also on the bottom ( benthic ) growing, larger algae such as the Chara spp. The scattering and the residues of macrophytes are biologically degradable heavier than planktonic algae, so a macrophyte -dominated waters is stable against overturning. The makrophytenreiche condition could be an alternative (meta- ) stable state with the same nutritional content. Key factor for the transition between plankton - dominated and macrophyte - dominated states appears to be the grazing pressure of zooplankton on unicellular algae to be, with the Zooplanktonbestand of fish is regulated ( " trophic cascade ").

Attempts to prevent the overturning of eutrophic lakes by promoting macrophytes are combined as " biomanipulation ".