The following considerations relate to a freshwater lake in the temperate climate zone , a stagnant body of water without direct drainage to the sea, which is more than 10 meters deep and whose evaporation rate and inflow and outflow volume is low compared to the total amount of water (see body of water # types of water ) .
The open water zone ( pelagial ) is divided into:
- Trophogenic zone (nutrient layer): In the water layers close to the surface there is enough light for the photosynthesis of the primary producers (aquatic plants, algae and above all phytoplankton ). Through photosynthesis, these produce more oxygen and biomass than are consumed by their own cell respiration and the respiration of the heterotrophic , aerobic organisms (zooplankton, destructors and nectons). The light intensity decreases almost exponentially with the depth, depending on the density of the "turbid substances", especially the algae and zooplankton.
- Compensation level : The compensation level forms the boundary between the trophogenic and tropholytic zone. It lies roughly in the depth at which the photosynthetically active radiation still has a residual intensity of 1% of the value directly below the surface. A positive energy balance between photosynthesis and cell respiration (positive net production) is only possible above this limit . The depth of the level is dynamic: Photoautotrophic organisms have species-specific compensation depths and the depth fluctuates with the optical properties of the water and the current lighting conditions.
- Tropholytic zone ( depletion layer): Below the compensation depth, photosynthesis is almost impossible. The heterotrophic organisms consume oxygen and biomass, which sinks down from the trophogenic zone. Sinking algae breathe in their supplies and die.
The pelagial can also be zoned using thermal analysis. The following division of the lake into different depth zones is formed by the temperature stratification during the so-called summer stagnation . This stratification is removed again during the circulation phases in spring and autumn.
- Epilimnion (surface water): The temperature is subject to relatively large daily and seasonal fluctuations between 4 ° C and over 30 ° C, depending on the solar radiation, the air temperature and the wind conditions. The oxygen content is high because oxygen is constantly diffusing into the water. The oxygen content is also dependent on the water temperature, the mixing with the air by wind and waves and on the predominant organisms. During stagnation, only the epilimnion is mixed by the wind and the metalimnion remains poor in nutrients, which decisively limits primary production.
- Metalimnion ( thermocline ): The metalimnion is characterized by a rapid temperature decrease (steep temperature gradient ) with depth to 4 ° C ( thermocline , temperature incremental layer ). The oxygen content decreases rapidly, depending on the number of aerobic organisms.
- Hypolimnion (deep layer): In this layer there is a mostly constant temperature of 4 ° C, independent of the time of day and the season, and a relatively low oxygen concentration. The layer is due to the density anomaly of the water , due to which the density of water is highest at 3.98 ° C. Furthermore, the density is influenced by the pressure and the amount of dissolved substances.
- Littoral - bank zone (Latin litus, litoris , bank): The littoral comprises the bank area up to the compensation depth. It is mostly overgrown by higher plants.
- Profundal - deep zone (lat. Profundus , profound): No photoautotrophic producers live in Profundal. Consumers are therefore dependent on the biomass that forms the littoral and trophogenic zone.
Communities in the individual zones
The littoral is divided into individual zones or belts based on the communities :
- Epilitoral, alder belt, willow bush zone
- The epilitoral is the uppermost edge of the bank that is no longer affected by the waves. Due to the high groundwater level, it represents a habitat for plants that can cope with moist soil ( hygrophytes ). The sparse alder forests with black alder ( Alnus glutinosa ), downy birch ( Betula pubescens ) and willow ( Salix spec. ) Have a dense undergrowth consisting of mosses, ferns and sour or sedge grasses ( rushes ). The swamp iris ( Iris pseudacorus ) can also be found in more open places and the swamp marigold ( Caltha palustris ) near streams .
- The animal world is initially composed of detritus eaters ( earthworms (e.g. Lumbricus terrestris ), woodlice , land snails ), spiders and insects, which in turn represent the foodstuff for birds that use trees, bushes and the ground as protected breeding grounds.
- Supralitoral, splash zone
- This strip of shore is not reached by the waves themselves, but is soaked through by the splash of the waves breaking on the shore.
- Eulitoral, surf zone
- In the surf zone there are strong mechanical forces that do not allow larger plants to grow. However, firmly adhering, oxygen-loving organisms such as strudelworms ( Turbellaria spec. ) And crust-forming cyanobacteria settle here.
- Infralitoral or sublitoral
- This zone is home to larger plants that are adapted to a constantly flooded soil. They have an aerenchyma, a coherent system of large intercellular spaces, so that the roots can also be supplied with oxygen. This bank area serves as a spawning and breeding area for many fish, birds and insects. The infralitoral is divided into several sections:
- Great Sedge Zone
- It lies in the area between the high and low water mark. Characteristic plants are the sedges ( Carex spec. ). In addition, there are also water mint ( Mentha aquatica ), purple loosestrife ( Lythrum salicaria ) and marsh horsetail ( Equisetum palustre ).
- Reed zone
- Here, too, you can still find emersed plants, the stems and leaves of which mostly protrude above the water level. These are mainly reeds ( Phragmites australis ), cattails ( Typha spec. ) And frog spoons ( Alisma spec. ). Coot ( Fulica atra ) and common pond rail ( Gallinula chloropus ) nest here.
- Floating leaf zone
- In parts of the lake that are sheltered from the wind , floating leaf plants can settle which are completely submerged except for the leaves. These leaves float on the surface of the water and have stomata for gas exchange on the upper side of the leaves. (In land plants, the stomata are usually on the underside of the leaf). In addition to the water knotweed ( Persicaria amphibia ), the representatives of the water lily family , water lily ( Nymphaea alba ) and pond rose ( Nuphar lutea ) are most noticeable .
- Spawning herb zone
- Almost completely submerged aquatic plants live here, which also have leaves below the waterline, which are then often strongly dissected in order to enlarge the surface for the exchange of substances. It is named after the pondweed ( Potamogetum spec ).
- Characeae zone
- The plants in this zone are completely submerged. Flowering plants such as the rootless horn leaf ( Ceratophyllum spec. ), Milfoil ( Myriophyllum spec. ), Waterweed ( Elodea canadensis ) and water screw ( Vallisneria spec. ) Cannot penetrate to a depth of more than 10 m because the water pressure would destroy their aerenchyme. If there is enough light, however, mosses such as spring moss ( Fontinalis antipyretica ) and algae can be found up to 30 meters deep. The chandelier algae ( Characeae ) form the lowest zone of the undersea meadows.
- The mallard ( Anas platyrhynchos ) can only be found in the shallower bank areas when searching for food, as it does not submerge when it is found. The Mute Swan ( Cygnus olor ) can, with its long neck to the bottom in deeper water for food search, while the crested grebe ( Podiceps sp. ) At greater depths hunting for fish makes.
- The night heron ( Nycticorax nycticorax ) finds its food (small mammals, amphibians, insects, worms) in the epilitoral, the purple heron ( Ardea purpurea ) with a similar food spectrum goes hunting in the reed zone.
Open water zone (pelagic)
In the light-flooded open water zone , phytoplankton can be found , on the surface there are also free-floating plants ( Neuston and Pleuston ) such as duckweed ( Lemna spec. ) Or the swimming fern ( Salvinia natans ).
The entire open water zone is a habitat for zooplankton , nectons and destructors .
The most important plankton of a European freshwater lake (example: Lake Constance ).
- Phytoplankton: golden algae ( Dinobryon ), Rhodomonas , diatoms ( Tabellaria , Diatoma , Fragilaria , Asterionella ), green algae ( Chlorella , Scenedesmus )
- Zooplankton: Heterocopae , copepods ( Eudiaptomus , Cyclops , Mesocyclops ), leaf-pods ( Daphnia ), rotifers
In a lake there is regularly a partial or complete mixing (circulation) of the water body, with oxygen and nutrients being evenly distributed over the mixed area. The driving forces for the circulations are wind and density differences (cold, i.e. denser water sinks down, warm, i.e. less dense, rises). The circulation can also be promoted or hindered by differences in density of the water (due to temperature or salt content differences). If a thermocline (a temperature jump) has formed, the metal imnion acts like a barrier layer, the cold, dense deep water can no longer participate in the circulation, there is only a partial circulation and thus only a thorough mixing of the epilimnion. This condition is known as summer stagnation .
This has consequences for the oxygen supply of the aerobic organisms of the hypolimnion and for the nutrient salt and carbon dioxide supply of the primary producers of the epilimnion:
It is true that the aerobic organisms of the hypolimnion still receive enough nutrients when dead bodies of animals and plants and other organic material ( detritus ) sink to the bottom. The supply of oxygen from the epilimnion is interrupted. The hypolimnion is increasingly depleted of oxygen.
The aerobic destructors of the hypolimnion, especially in the soil zone, remineralise the organic material, producing mainly water-soluble nitrates and phosphates, which would be necessary as nutrient salts for the producers in order to carry out chemosynthesis. Due to the lack of exchange with the epilimnion, these nutrient salts and the carbon dioxide produced during dissimilation cannot get into the epilimnion.
Therefore, after the formation of a thermocline, a decrease in the phytoplankton mass in the epilimnion is observed.
If the thermocline collapses due to the cooling of the surface water, full circulation is possible again. Nutrient salts and carbon dioxide get into the epilimnion, which leads to an increase in producers. If the phytoplankton multiply very strongly, one speaks of algal bloom .
Classification of lakes according to the number of full circulations per year
- Amictic : Arctic and Antarctic waters are covered by permanent ice, which completely blocks circulation.
Monomictic table : full circulation only takes place in one season:
- Polar and sub-polar lakes: full circulation during the summer months; Winter stagnation with ice cover.
- Subtropical lakes: full circulation in winter with sufficient cooling of the surface water, stagnation in summer. Examples: Lake Maggiore , Lake Garda , Lake Constance as the northernmost of this type.
- Dimiktisch : full circulation in spring and autumn , in temperate lakes of temperate latitudes.
- Spring circulation (full circulation): In spring the surface water warms up. Spring storms ensure that the lake is completely mixed.
- Summer stagnation (partial circulation): In summer the surface water warms up significantly more than the deep water. A clear temperature gradient forms , which characterizes the area of the metalimnion. Below that, if the sea depth is sufficient, there is an area homogeneous at 4 ° C, the hypolimnion. The constant circulation through wind and nocturnal convection is limited to the resulting epilimnion, the depth of which fluctuates depending on the weather.
- Autumn circulation (full circulation): In autumn, the surface water cools down, condenses and sinks. With it, the increasingly narrow temperature jump layer also sinks. Supported by the autumn storms, there is full circulation.
- Winter stagnation (no circulation): In winter the temperature of the surface water drops below 4 ° C and thus loses density. An unstable inverse temperature distribution develops (below 4 ° C cold surface water above 4 ° C cold deep water). When ice covers the lake surface, the temperature stratification is stabilized.
- Pleiomictic (Gr. Pleion = increased): In shallower lakes of temperate latitudes, the metalimnion can reach to the bottom and a hypolimnion that is homogeneous at 4 ° C does not form. In such lakes, with changing weather conditions, several full circulations are possible through convection (when the surface water cools down considerably, especially at night) or through wind.
- Oligomictic : In tropical lowland lakes, the surface water is strongly heated, only rarely do irregular full circulations break through permanent stagnation.
- Tropical high mountain lakes: They have sustained full circulation all year round, caused by the wind and night cooling. Example: Lake Titicaca at an altitude of 3,810 m
- Lakes in the hill country of the tropics: The strong warming during the day leads to stagnation, the strong nocturnal cooling to full circulation.
- Shallow lakes of temperate latitudes, where stratification cannot occur due to the shallow water depth. The water circulates all year round, driven by the wind. However, due to the density anomaly of the water, phases of stagnation occur when the ice cover is closed.
Classification according to the range of full circulation
- Pleiomictic or holomictic (gr. Pleio = increased; gr. Holos = whole) The full circulation covers all water masses of the lake. In shallow waters in the middle latitudes, the water is mixed by convection and wind.
Meromiktisch (Gr. Meros = part): The full circulation is not possible to the bottom of the lake. The water masses not recorded in full circulation are called monimolimnion . Reasons for this can be:
- a sheltered location of the lake: several Carinthian lakes .
- In relation to the depth, the water surface offers too small an area for the wind to attack. Example: Königssee
- A salt crack layer ( halocline ) forms because the more salty and therefore denser water sinks into the depths. This happens when a lake has a relatively high salt load due to its inflows and a high rate of evaporation. The more salty water also makes the water appear darker (examples: Hallstätter See , Toplitzsee ). At the transition from the optically denser medium (high salt content) to the optically thinner medium (low salt content) in all water layers, there is largely total reflection of the light rays; reflected or scattered light rays remain “trapped” (like a solar pond ). Since less light is reflected back into the atmosphere, more light is available to the phytoplankton, which can then thrive more abundantly and cause more water turbidity (Lake Toplitz) than in lakes with less salinity.
The nutritional content of a lake has an impact especially during summer stagnation:
Effects of nutrient content
The more nutrient salts that get into the epilimnion through full circulation in spring, the more phytoplankton (floating algae) can grow. Consumers also multiply depending on this. In eutrophic lakes, the high supply of nutrients, especially phosphate, makes the biomass so large that so-called algae blooms can occur.
Example: Change in abiotic factors over the course of the year in the epilimnion of a dimictic eutrophic lake in Europe
|The oxygen maximum in May corresponds to the lowering of the other factors: Due to the rising temperatures and the improved lighting conditions as well as the high nutrient salt supply due to the full circulation in spring, there is a mass increase in photosynthesis organisms. Since this means that there is no longer enough light in the lower layers of the epilimnion, the phytoplankton there dies and sinks to the bottom.||The rise in carbon dioxide in March and November is due to the spring and autumn full circulation. The formation of carbon dioxide through the respiration of consumers is overcompensated for by the consumption through photosynthesis during summer stagnation.|
|The increase in the phosphate content occurs in March and November due to the spring and autumn full circulation. A mass increase in producers in April and May consumes almost all of the phosphate.||The increase in the nitrate content in March and November is due to the spring and autumn full circulation. A mass increase in producers in April and May consumes a great deal of nitrate, which is partially replaced by the fixation of atmospheric nitrogen by cyanobacteria.|
Classification according to the amount of nutrients
- Oligotrophs (gr. Oligos = little; gr. Trophē = food) lakes contain only a few nutrient salts; this limits the replication of phytoplankton and consumers. Since there is little dead, organic material, these lakes are well supplied with oxygen all year round in all water depths. Due to the low density of phytoplankton, the depth of vision is great (up to 10 m), the water appears clear, the lake is blue or green in color. Digested sludge does not collect on the bottom, as there is enough oxygen for the aerobic destructors to rapidly break down the organic material. Oligotrophic lakes are characterized by a great diversity of species. These are mostly stenoxybionts that tolerate only slight fluctuations in oxygen. Occasional nitrogen deficiency iscompensatedfor by nitrogen-fixing, photoautotrophic cyanobacteria . The small, mostly organically bound amount of phosphate comes mainly from tributaries. The free iron (III) ions (Fe 3+ )present in oxygen-rich waterbind the phosphate as poorly soluble FePO 4 and remove it from the lake's material cycle. The shore zone is narrow andovergrownwith a few macrophytes . Undisturbed, subalpine lakes are usually oligotrophic (example: Königssee ).
- Mesotrophic (gr. Mesos = the middle) lakes contain more nutrient salts and thus a larger amount of producers, consumers and destructors. The oxygen content in the hypolimnion during summer stagnation is temporarily low.
- Eutrophe (Gr. Eu = good, solid) Lakes contain many nutrient salts. This leads to a strong increase in phytoplankton and consumers after the full circulation in spring and autumn. However, a lack of light and a lack of nutrient salt soon cause the algae to die off again, so that the biomass in the epilimnion is lower again during summer stagnation. Since a lot of dead, organic material is now produced, there is an oxygen deficit in the hypolimnion. Due to the high density of phytoplankton, visibility is low, the water appears cloudy, the lake is brown-green in color. A thick,semi- rotten sludge layer ( Gyttja ),consistingof organic plankton detritus ,collects on the bottomwhen the aerobic destructors are replaced by anaerobic ones due to the lack of oxygen. During full circulation, this digested sludge layer can largely be broken down again by aerobic destructors. If there is insufficient oxygen for this,the lake will overturn due to the creation of a positive feedback between the permanent solubility of phosphate (with Fe 2+ ) and the increased primary production . This is then the transition to the hypertrophic state (see below). Eutrophic lakes are characterized by a lower biodiversity but a high density of individuals. These are mostly Euroxybionts , which tolerate the temporal and spatial strong oxygen fluctuations. The shore zone is wide and densely overgrown.
- Hypertrophic (Greek hypér = large, excessive) lakes contain a great deal of dead organic material, which is deposited on the bottom as a layer of digested sludge, sometimes several meters thick. The hypolimnion contains hardly any oxygen all year round. Instead of being aerobic, anaerobic destructors break down the organic material. However, this degradation is much slower and incomplete. In addition, instead of carbon dioxide and mineral salts such as nitrate and sulphate, toxic substances such as methane , ammonia and hydrogen sulphide are produced . These substances and the lack of oxygen make the lake an unstable body of water in which only a few species, but these in z. T. huge numbers of individuals, can live. Although these species are more or less adapted to conditions of lack of oxygen, they are nevertheless constantly threatened by catastrophic collapse of the population under unfavorable conditions. Hypertrophic lakes silt up from the shore, as detritus accumulates more quickly in the vegetation of the littoral.
In the ideal case, a food web represents all food relationships between the organisms of an ecosystem. In order to maintain clarity, only a section of the food web of a European lake is shown in the following example. Scavengers and destructors were avoided.
Food chains in the pelagic (open water area in a lake):
|herbivorous zooplankton||Primary consumer|
|carnivorous zooplankton||Secondary consumer|
|pelagic coarse fish||Tertiary consumer|
|pelagic predatory fish||End consumer|
|aerobic and anaerobic bacteria||Destructors|
The gross primary production (GPP) of all lakes worldwide has been estimated at 0.65 petagrams of carbon per year (1 petagram = 1 billion tons). That is not much in relation to the total gross primary production of 100 to 150 Pg C / year. Lakes are also obviously net sources, and not sinks, of carbon dioxide. At the same time, considerable amounts of carbon are captured in lake sediments and thus withdrawn from the carbon cycle. This apparent contradiction is resolved by the substantial influx of biomass from terrestrial ecosystems into lakes. In lakes that are surrounded by carbonate rocks, the influx in the form of dissolved (hydrogen) carbonate also plays a major role, which can even exceed that of the lake's own respiration. Lakes are only isolated ecosystems when viewed ideally; in reality, they are closely linked and interwoven with terrestrial systems. The organic carbon influx in lakes also takes place predominantly in dissolved form, only to a very minor extent in solid form. The global net emission from lakes is discussed in the review by Tranvik et al. estimated at 0.53 petagrams of carbon per year. This is also a significant figure from a global perspective, which is astonishing when one considers that less than 3% of the continental area is taken up by lakes; the value is more than half the net export of carbon dioxide to the oceans. Lakes, especially eutrophic shallow water lakes, also contribute to methane production, but their share is estimated at only 6 to 16% of total natural emissions (i.e. excluding human sources). Carbon storage in lake sediments is also considerable. The global supply of organically bound carbon has been estimated at 820 petagrams, three times what is stored in the ocean sediments. However, one must bear in mind that the entire deep sea with its sediments does not contribute anything to carbon storage. The net annual storage rate of lake sediments could be around 0.03-0.07 PgC. Depending on the specific design of the ecosystem, lakes can be both sources and sinks for carbon dioxide. Predicting the effects of future changes is very difficult because of the numerous processes with conflicting results.
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